Could an object block out the Sun from Earth's view for 3 hours?

Could an object block out the Sun from Earth's view for 3 hours?

how large, massive, far away (from earth), and fast would an object (planet-like) have to be to block out the Sun for 3 hours and not disturb Venus, Earth, or the Moon?

Is it possible? Not really. Not with your criteria. Could some mad scientists make it happen, like if we make the Moon bigger? Er, maybe.

Types of eclipses.

Nasa's eclipse page (scroll down for maps over 20 year periods).

2001-2020 map below.

The map has a notation for hybrid eclipses (see here), not all that relevant to the question.

Total solar eclipses (blue lines on the map), are quite small because the umbra or fully shaded region that the Moon casts on Earth is small, maybe 150 km in diameter on average, 267 km max. All planets and objects that orbit the sun cast an Umbra that extends behind them. The Moon (diameter 3,474 km), is always casting an umbra somewhere in space, being about 1/400th the diameter of the sun, it's umbra is about 1/399th the distance between the sun and the moon, or roughly 375,000 km on average. Eclipses happen on Earth when the Earth is lined up below the Moon's Umbra. Mostly they don't line up which is why eclipses are rare, but the Umbra is always somewhere in space, not all that far from the Earth, following the moon's path.

Rogue Planet

Your scenario mentioned planet-like. Now, if it's a planet from our solar-system, that would imply disturbing the orbits of Earth/Venus/Moon, which you specified you didn't want. There's also real danger if you were to have a planet like object in a near Earth orbit that passes from time to time, close enough to cast it's Umbra over Earth. For safety reasons "just passing through" is the way to go.

A rogue planet on a hyperbolic trajectory past the sun that, by very rare coincidence, passes between the Earth and the Sun, but close enough to cast it's Umbra over Earth would be very rare to say the least, but a theoretically possible scenario to cast a total eclipse over Earth, but it would need to pass very close and at just the right angle to block the 1/2 of 1 degree of arc that the Sun takes up in the sky.

How close?

Mars is roughly twice the diameter of the Moon, so would need to pass within about 750,000 km to the earth, and line up just right, to cast a tiny umbra that moved across the Earth.

Venus, about 3.5 times the diameter, about 1.3 million km

Jupiter, about 41 times the diameter of the moon, would need to pass within about 15 million KM to cast an umbra on the Earth of similar size to the typical Moon's eclipse Umbra.

And those are the minimum distances. You'd need the theoretical planet to be pretty well inside those limits to have an Umbra of significant size and for a 3 hour eclipse, bigger is better.

What would a rogue planet passing that close look like?

Consider that it's passing close enough that it looks larger than the Moon, at least for a 3-plus hours. How close it would need to be depends on the size, as noted above.

The minimum velocity for an object on a hyperbolic trajectory just passing through the solar system but near Earth distance would be Earth's escape velocity, or the orbital velocity of the Earth (108,000 kph) times the square root of 2, and if it approached on the same plane and in the same direction as the Earth, the relative velocity between this Rogue planet and Earth, the minimum relative velocity would be about 45,000 kph, give or take. That's absolute bare minimum as a theoretical rogue planet passing inside the Earth's orbit would usually be traveling a fair bit faster than that.

So, for a planet like that to remain between the Earth and the Sun for 3 hours, it would need to be about 3 times 45,000 km in diameter, or, nearly the size of Jupiter.

A small rogue planet couldn't cast an Umbra over Earth anywhere close to 3 hours because it would pass over the Earth much too quickly. A Jupiter-sized Rogue planet could, theoretically, if it was closer than closer than 15 million km, cast it's umbra over Earth for 3 hours or so, but that would be close enough to change the Earth's orbit and, at the very least, give the Earth significant climate change by pushing the Earth a few million km either closer or further from the sun. Not an ideal scenario.

So there's no scenario that does what you ask. You'd need a planet the size of Jupiter, with the mass of Neptune or perhaps less to leave the Earth/Moon undisturbed. A Neptune mass planet with a large ring system… just maybe. But given how rare your scenario is to begin with, I'm going to say basically no. You could do it, but it would change the Earth's orbit, or you could do it with a smaller planet but for much less than 3 hours. In any case, it would be a preposterously rare scenario no matter what.

What if we move the Moon?

With rogue planets, I just looked at the relative velocity between the planets, which is close enough for a near-sighted umpire. When you look at the Moon's umbra, the Moon's motion is much slower, so you also have to take into account the Earth's rotation too.

The Moon (Synodic period), takes 59 minutes to cross the sun, that's tip to full cover, and another 59 minutes back to tip. (based on a 29.5 day synodic orbit, 29.5 days to cover 360 degrees, the sun, roughly 1/2 of 1 degree in the sky). So, 158 minutes of partial eclipse, and a record, of 7 minutes total eclipse in one spot on Earth (7 minutes, the record for a total eclipse is possible when the Moon is slightly larger than the sun, close to it's perigee).

So if our mad scientists pull the moon closer to the Earth, the period of time for a total eclipse increases but the rate that the Moon moves across the sun also increases, so you're effectively increasing the 7 minutes of total but decreasing the 158 minutes of partial. If you pull the Moon all the way into a geostationary orbit, some 10.5 times closer, the Moon would span an enormous 5.25% of the sky, but it wouldn't move in the sky while the sun would and the sun, which crosses about 1 degree of sky in 4 minutes would pass behind the enormous geo-stationary moon in just 21 minutes. You can't create a 3 hour eclipse by pulling the Moon closer.

This is where the Earth' rotation speed comes in. The Moon moves over the Earth in it's orbit at about 3,683 kph, and relative to the sun, but because the Earth is in orbit to, the effective velocity relative to the sun (Synodic not sidereal) is about 8% less.

The Moon's umbra effectively moves with the Moon's relative velocity - at least, close enough.

The Earth rotates in the same direction that the Moon moves, peak velocity at the equator of a bit over 1,600 kph. In effect, the rotation of the Earth tries to keep up with the movement of the Umbra during an eclipse, but the Umbra moves West to East about twice as fast as the Earth turns West to East.

So the solution for your 3 hour eclipse? Make the Moon bigger and push it further away so it orbits the Earth more slowly and it's Umbra moves closer to the Earth's rotation speed, making a 3 hour eclipse, at mid day, close to the equator… possible.

If we replace the Moon with Venus (3.5 times the diameter of the Moon) and we push it some 3 times further away (which would be pushing the limits of the stability of the Earth's hill sphere), then you'd be a lot closer, as our new "Venus Moon" would move across the sky and across the sun still to fast, but it would move more slowly and closer to Earth's rotational velocity leading to eclipses that stayed in the sky much longer. If you did that, you might get your 3 hours of total eclipse. You wouldn't want to push Venus too far, cause you'd want it a big bigger than the Sun in the sky, but 3 times the Moons distance, it would be about 15% larger and close to the limit of orbital stability.

Other silly ideas.

Now if you were to Move the Planet Uranus into Earth's L1 Trojan point, which… well, the Trojan point would be further away due to the larger planet's mass dominating so… that wouldn't work and because L1 points aren't stable, it would also be highly dangerous. Moving on.

If you were to move Uranus to where the Earth is and make the Earth a satellite of Uranus… we might get longer than 3 hour eclipses every time we were on the far side of Uranus making an orbit. Seems a lot of work though just for an eclipse… but that would work too.

OK, I'll stop now.

Could an object block out the Sun from Earth's view for 3 hours? - Astronomy

Lesson 3) Three in a Row

Professor Turing enters the classroom. In his hand, there are copies of old newspapers about eclipses and other astronomical phenomena. As he enters the classroom, he reveals a few images, each of which features two astronomical bodies.

Syzygy Types

Good evening, students, and welcome back to Astronomy. Last week, we discussed the concept of syzygy. What do you recall about a syzygy? Yes, a syzygy is an event that occurs when three astronomical bodies line up with each other. In the case of spring tides, we are talking about the Sun, the Earth, and the Moon. But a syzygy can also be two planets and a moon, two moons and a planet, or any combination of astronomical bodies.

Syzygy is quite rare in Astronomy. As I mentioned in the previous lesson, spring tides occur during syzygy or near syzygy. We do not often get true syzygys during most Full Moons and New Moons. Because of the angles of orbits, as well as the timing involved, it is hard to obtain a full syzygy, as it would require the three astronomical bodies to be fully aligned with each other. When we do achieve a full syzygy, though, we will know because we will experience a rather unique event - an eclipse.

When you are standing on the Earth (or any astronomical body - but for simplicity, I will use the Earth as a reference point), there are a few ways to know that the Earth and two other bodies are in syzygy. Three of these situations are transits, occultations, and eclipses. Please note that in the definitions below, the words &ldquosmaller&rdquo and &ldquolarger&rdquo refer to observed size from one&rsquos point of view.

Transit: Io Transits Jupiter
Image Source: NASA

When a smaller astronomical body moves in front of a larger astronomical body, this event is called a transit. The above picture shows Io transiting Jupiter. When referring to transits, please use the form transits . During a transit, you should be able to see the larger body behind the smaller body. Please note that while the picture above happens to be taken from a Muggle spacecraft, and since the spacecraft is not an astronomical body, it is not an example of syzygy. However, if the picture had been taken from Earth, such a transit would show a syzygy between the Earth, Io, and Jupiter.

Occultation: Just Before Jupiter is Occulted by the Moon
Image Source: Wikipedia

When a larger astronomical body moves in front of a smaller astronomical body, the event is called an occultation. During an occultation, you may only be able to see the larger body, as the smaller body may be hidden behind it, but just before the occultation, you will be able to see both the larger and smaller body. The image above was taken a few moments before Jupiter is occulted by the Moon. When referring to occultations, please use the form is occulted by . As stated previously, the words &ldquosmaller&rdquo and &ldquolarger&rdquo refer to observed size, not actual size. Jupiter is the smaller object in the upper right corner, while the Moon is the larger object in the image. Since this image was taken from the Earth, the occultation of Jupiter by the Moon also resulted in a syzygy between the Earth, the Moon, and Jupiter.

When the interaction between two astronomical bodies results in one being momentarily hidden from view, the event is a special kind of occultation called an eclipse. Eclipses can occur either when an astronomical body directly blocks another from view, such as a solar eclipse, or when a shadow of an astronomical body blocks another from view, such as a lunar eclipse.

Solar and Lunar Eclipses from Earth

Daily Prophet Article on an Eclipse
Image Source: Harry Potter Wiki

When we talk about eclipses seen from the Earth, we are usually talking about solar eclipses and lunar eclipses. I will talk a little about both in this lesson, as it is important to know the difference between them. However, while this year will have more of a focus on lunar eclipses, I will explain more about solar eclipses next year. You will also learn about eclipses and their effects in other classes.

Total Solar Eclipse
Image Source: Wikipedia

Solar eclipses are an example of an occultation. In a solar eclipse, the Moon moves in between the Earth and the Sun. When seeing a total solar eclipse from the Earth, we would see the Moon covering all but the corona, or outer edge, of the Sun. Based on the definition, we can also see that a solar eclipse happens during New Moons and that a solar eclipse is also an occultation in which the Sun is occulted by the Moon. The image of the eclipse above depicts a solar eclipse. In particular, the image above shows a total solar eclipse. Partial and annular solar eclipses are also possible. In these cases, the Moon does not completely block out the Sun. Solar eclipses are a syzygy of the Sun, Moon, and Earth.

Total Lunar Eclipse
Image Source: Wikipedia

In lunar eclipses, the Earth&rsquos shadow covers the view of a Full Moon. In other words, a lunar eclipse is a syzygy between the Sun, the Earth, and the Moon. A total lunar eclipse is distinguished by its distinctive red color, as seen in the picture above. In this situation, the shadow of the Earth completely falls over the Moon. Partial lunar eclipses are also possible when the Sun, the Earth, and the Moon are not completely aligned.

If you want to watch an eclipse, please refer to NASA&rsquos page on eclipses. On this page, you can see when and where to view future solar and lunar eclipses. When you do view a solar eclipse, please only do so through indirect means, as looking at a solar eclipse with your magical telescope or with your naked eye could cause irreversible damage. Lunar eclipses are Full Moons and thus cannot be viewed through your magical telescope. However, they are safe to view with your naked eye, and if you get some friends to watch the eclipse with you, they make great social events.

Thank you for your time. Class is dismissed, and see you next Wednesday evening.

Total solar eclipses won't be around forever!

The Moon's orbit is changing. In fact, the Moon's orbit grows about 1.5 inches (3.8 cm) larger every year. As the Moon's orbit takes it farther and farther away from Earth, the Moon will appear smaller and smaller in our sky.

This occasionally happens now. The Moon's orbit isn't perfectly round. That means that sometimes the Moon is slightly farther away from Earth than it is at other times. Sometimes the Moon is far enough away that it doesn't create a total solar eclipse. In this case, the Moon obscures most of the Sun, but a thin ring of the Sun remains visible around the Moon.

However, once the Moon's growing orbit takes it approximately 14,600 miles (23,500 km) farther away from Earth, it will always be too far away to completely cover the Sun. That won't happen for a long time though. If the Moon's orbit grows only 1.5 inches every year, it will take more than 600 million years for total solar eclipses to completely disappear!

What we don&rsquot know

What does it look like?

All that astronomers have seen of 'Oumuamua is a single point of light. But because of its trajectory and small-scale accelerations, it must be smaller than typical objects from the Oort Cloud, the giant group of icy bodies that orbit the solar system roughly 186 billion miles (300 billion kilometers) away from the Sun. Oort Cloud objects formed in our own solar system, but were kicked out far beyond the planets by the immense gravity of Jupiter. They travel slower than 'Oumuamua and will forever be bound by the gravity of our Sun. But besides its elongated nature, scientists do not know what kinds of features 'Oumuamua has on its surface, if any. An elongated shape would explain its rotation behavior, but its exact appearance is unknown.

What is it made of?

Comets from our solar system have a lot of dust, but because none is visible coming off 'Oumuamua, scientists conclude it may not have very much at all. It is impossible to know what materials make up 'Oumuamua, but it could have gases such as carbon monoxide or carbon dioxide coming off the surface that are less likely to produce a visible coma or tail.

Where did it come from?

'Oumuamua came into our solar system from another star system in the galaxy, but which one? Scientists observe that its incoming speed was close to the average motion of stars near our own, and since the speed of younger stars is more stable than older stars, 'Oumuamua may have come from a relatively young system. But this is still a guess -- it is possible the object has been wandering around the galaxy for billions of years.

What is it doing now?

After January 2018, 'Oumuamua was no longer visible to telescopes, even in space. But scientists continue to analyze it and crack open more mysteries about this unique interstellar visitor.

Ask Ethan: Can We Build A Sun Screen To Combat Global Climate Change?

Normally, structures like IKAROS, shown here, are viewed as potential sails in space. But a . [+] different application, placed at the right point, could block out some of the sunlight, helping cool the Earth.

Wikimedia Commons user Andrzej Mirecki

Global climate change is one of the most pressing long-term issues facing humanity today. The science is abundantly clear on what's happening and why: the Earth is getting warmer, human-caused emission of heat-trapping greenhouse gases is the reason, and the concentration of these gases only continues to rise, unabated, over time. While there are a great many calls to reduce emissions, capture carbon, and move away from fossil fuels, there's little that's effectively been done. The Earth continues to warm, sea levels continue to rise, and the global climate continues to change. Could we take a different approach, and partially block the light coming from the Sun? That's Tony De La Dolce's question, as he asks:

[W]hy don't we evaluate building a "sun screen" in space to alter the amount of light (energy) earth receives? Everybody who did feel a total eclipse knows temperature goes down and light dims. So the idea is to build something that would stay between us and sun all year long.

This is one of the most ambitious, but also one of the sanest, options we could possibly consider when it comes to combatting global climate change.

In general, it's well-understood that increased concentrations of greenhouse gases in the atmosphere are driving global warming, which in turn is driving the Earth's climate and weather patterns to change in a number of ways. Most (but not all) of these ways are generally recognized as bad for the majority of humans on this world, and so there is a global movement underway to combat these changes. If the most popular solution, to return Earth's atmospheric gas concentrations to pre-industrial revolution levels, isn't chosen, the only options left to humanity will be to either adapt to the changes, or to attempt geoengineering solutions.

The SPICE project will investigate the feasibility of one so-called geoengineering technique: the . [+] idea of simulating natural processes that release small particles into the stratosphere, which then reflect a few percent of incoming solar radiation, with the effect of cooling the Earth. But there may be thoroughly unwanted side effects.

Wikimedia Commons user Hughhunt

This final option, of geoengineering, is not without risk. Most of the solutions involve altering Earth's surface or atmosphere further, with largely unknown, unpredictable consequences. Of all the geoengineering options, however, the least risky is the one put forth by Tony: to fly something in space, far from Earth, to simply block a portion of the Sun's light. With less solar irradiance, the temperatures can be controlled, even if the atmospheric greenhouse gas concentrations continue to rise. If we wanted to completely counteract the effects of all the global warming that has happened since the industrial revolution, we'd have to block out approximately 2% of the Sun's light on a continuous basis.

Solar eclipses are possible on Earth, and occur whenever the Moon aligns with the Earth-Sun plane . [+] during a new Moon. An object could be smaller or farther, and cast no shadow onto our planet, but still reduce the amount of sunlight reaching Earth.

But this is easier, at least theoretically, than you might intuit. There's a gravitationally quasi-stable point, in between the Earth and the Sun, which will always effectively dim the light from the Sun. Known as the L1 Lagrange point, it's the ideal location for a satellite that you wish to remain directly between the Earth and the Sun. As the Earth orbits the Sun, an object at L1 will constantly remain in between the Earth and Sun, never straying at any point throughout the year. Its physical location is in interplanetary space: approximately 1,500,000 kilometers closer to the Sun than the Earth is.

A contour plot of the effective potential of the Earth-Sun system. Objects can be in a stable, . [+] lunar-like orbit around the Earth or a quasi-stable orbit leading-or-trailing (or alternating between both both) the Earth. The L1 point is ideal for blocking sunlight continuously.

At that distance, even an Earth-sized object wouldn't cast a shadow on our planet, as its shadow-cone would come to an end well before it reached our world. But a single shade, or a series of smaller shades, would effectively block enough light to reduce the amount of sunlight reaching the Earth. To achieve the reduction we'd want to counteract global warming, i.e., to reduce the received solar irradiance by 2%, we'd need to cover a surface area of 4.5 million square kilometers at the L1 Lagrange point. That's the equivalent of an object that takes up half the surface area of the Moon. But unlike the Moon, we could divide that up into as many smaller components as necessary.

The graphic shows the 2 foot-diameter flyers at L1. They are transparent, but blur out transmitted . [+] light into a donut, as shown for the background stars. The transmitted sunlight is also spread out, so it misses the Earth. This way of removing the light avoids radiation pressure, which would otherwise degrade the L1 orbit.

University of Arizona / Steward Observatory

One proposal, put forth by University of Arizona astronomer Rogel Angel, propsed flying a constellation of small spacecrafts at the L1 Lagrange point. Instead of a large, heavy structure, an array of approximately 16 trillion structures, each one a thin circle about 30 centimeters (one foot) in radius, could block enough light to provide us with exactly the reduction of irradiance that we require. It wouldn't create a shadow anywhere on Earth, but would rather reduce the total amount of sunlight striking the entire surface of our planet by an even amount, similar to an enormous array of tiny sunspots placed on the surface of the Sun.

Principle of a space lens. The basic function of a space lens to mitigate global warming, refracting . [+] sunlight away from the Earth. The actual lens needed would be smaller and thinner than what's shown here.

Mikael Häggström / Wikimedia Commons

Another proposal, dating as far back as 1989, when James Early proposed it, would be to put a very large lens in space. A glass shield could be created to act as a lens, diffusing a large amount of sunlight away from the Earth. An enormous space lens, or a series of smaller space lenses, would only need to be a few millimeters thick to refract the sunlight where much of the light that would have been incident on Earth instead gets shunted into interplanetary space. At the L1 Lagrange point, the lens (or series of lenses) would have to cover right around a million square kilometers to reduce the solar energy reaching Earth by about 2%.

In principle, this sounds like an easy strategy, and potentially a low-risk, high-reward solution to our global warming problem. But there are two problems with it.

The very first launch of the Falcon Heavy, on February 6, 2018, was a tremendous success. The rocket . [+] reached low-Earth-orbit, deployed its payload successfully, and the main boosters returned to Cape Kennedy, where they landed successfully. The promise of a reusable heavy-lift vehicle is now a reality, and could lower launch costs to

Jim Watson/AFP/Getty Images

1.) Launch costs. To send any object to the L1 Lagrange point is well within the scope of what humanity's spaceflight program is capable of. We've done it numerous times: it's where the majority of our Sun-observing satellite missions go. But even for a series of very thin, very light spacecrafts, the launch costs would be tremendous. If Angel's proposal of a transparent, thin film were flown, with each flyer only 1/5000th of an inch thick and weighing no more than a gram, the total mass required would still add up to 20 million metric tonnes. Even if next-generation launch technologies like the Falcon Heavy can get costs down to under $1000-per-pound (a factor of 10 improvement over what they presently are), we're still looking at hundreds of billions of dollars to launch an array like this. And that's not even getting to the second problem.

NASA conceived of a Solar Power Satellite back in the 1970s. If a series of solar power satellites . [+] were placed at L1, they could not only block some of the sunlight, but could provide usable power for other purposes. L1, however, is not stable.

2.) Orbital stability. The L1 Lagrange point is only quasi-stable, meaning that either everything we launch there needs to be maintained (with rocket boosts) in order to remain in its current orbit, or it will eventually drift away, ceasing to block the sunlight from reaching Earth. This happens, unfortunately, way too quickly for our comfort: on the timescales of years-to-decades, depending on how well the initial orbital insertion works. This means, for the light-blocking approach, we'd need to have an ongoing cost hovering in the tens of billions of dollars per year just for maintenance launches alone: comparable to NASA's entire annual budget. And that's if the launch costs are lowered by the factor of 10 over what they are today.

Just as shade here on Earth can lower the temperature by reducing the incoming sunlight, a series of . [+] light-blocking apparatuses in space could lower the incident sunlight here on Earth.

Wikimedia Commons user Mattinbgn

The big advantage of blocking the incoming sunlight from afar is that there's no risk of long-term negative effects on planet Earth from geoengineering solutions. Other ideas, such as large-scale modification of the atmosphere, a constellation of satellites in low-Earth orbit, or the injection of cloud-forming materials or reflective particulates into the skies or oceans, have potentially hazardous unforeseen consequences. But the big problems of costs and long-term instability, right now, are the largest barriers to implementing such a solution.

The concentration of carbon dioxide in Earth's atmosphere can be determined from both ice core . [+] measurements and atmospheric monitoring stations. The increase in atmospheric CO2 since the mid-1700s is staggering, drives global warming, has since passed 410 ppm, and continues unabated.

In the meantime, the planet continues to warm, CO2 levels continue to rise, and there are no effective strategies in place to change the course of events. Ideas for a screen like this, usually called a Space Sunshade, may become our best option. While the cost is prohibitively expensive, it may, in the long run, be the cheapest option we're willing to implement. As the years, decades, centuries, and millennia tick by, our descendents will be dealing with the consequences of our actions or inactions today for generations to come.


Global catastrophic vs. existential Edit

A global catastrophic risk is one that is at least global in scope and is not subjectively imperceptible in intensity. Those that will affect all future generations and are "terminal" (meaning that they cause a complete end to the object or organism in question) in intensity are classified as existential risks. While a global catastrophic risk may kill the vast majority of life on earth, humanity could still potentially recover. An existential risk, on the other hand, is one that either destroys humanity entirely or prevents any chance of civilization's recovery. [6]

Similarly, in Catastrophe: Risk and Response, Richard Posner singles out and groups together events that bring about "utter overthrow or ruin" on a global, rather than a "local or regional", scale. Posner singles out such events as worthy of special attention on cost–benefit grounds because they could directly or indirectly jeopardize the survival of the human race as a whole. [7] Posner's events include meteor impacts, runaway global warming, grey goo, bioterrorism, and particle accelerator accidents.

Studying human near-extinction directly is not possible, and modelling existential risks is difficult, due in part to survivorship bias. [8] However, individual civilizations have collapsed many times in human history. While there is no known precedent for a complete collapse into an amnesic pre-agricultural society, civilizations such as the Roman Empire have ended in a loss of centralized governance and a major civilization-wide loss of infrastructure and advanced technology. Societies are often resilient to catastrophe for example, Medieval Europe survived the Black Death without suffering anything resembling a civilization collapse. [9]

Some risks are due to phenomena that have occurred in Earth's past and left a geological record. Together with contemporary observations, it is possible to make informed estimates of the likelihood such events will occur in the future. For example, an extinction-level comet or asteroid impact event before the year 2100 has been estimated at one-in-a-million. [10] [11] [ further explanation needed ] Supervolcanoes are another example. There are several known historical supervolcanoes, including Mt. Toba, which may have almost wiped out humanity at the time of its last eruption. The geologic record suggests this particular supervolcano re-erupts about every 50,000 years. [12] [13] [ further explanation needed ]

Without the benefit of geological records and direct observation, the relative danger posed by other threats is much more difficult to calculate. In addition, it is one thing to estimate the likelihood of an event taking place, but quite another to assess how likely an event is to cause extinction if it does occur, and most difficult of all, the risk posted by synergistic effects of multiple events taking place simultaneously. [ citation needed ]

The closest the Doomsday Clock has been to midnight was in 2020, when the Clock was set to one minute forty seconds until midnight, due to continued relations troubles between the North Korean and United States governments, as well as rising tensions between the US and Iran. [14]

Given the limitations of ordinary calculation and modeling, expert elicitation is frequently used instead to obtain probability estimates. [15] In 2008, an informal survey of experts on different global catastrophic risks at the Global Catastrophic Risk Conference at the University of Oxford suggested a 19% chance of human extinction by the year 2100. The conference report cautions that the results should be taken "with a grain of salt" the results were not meant to capture all large risks and did not include things like climate change, and the results likely reflect many cognitive biases of the conference participants. [16]

Risk Estimated probability
for human extinction
before 2100
Overall probability 19%
Molecular nanotechnology weapons 5%
Superintelligent AI 5%
All wars (including civil wars) 4%
Engineered pandemic 2%
Nuclear war 1%
Nanotechnology accident 0.5%
Natural pandemic 0.05%
Nuclear terrorism 0.03%
Table source: Future of Humanity Institute, 2008. [17]

The 2016 annual report by the Global Challenges Foundation estimates that an average American is more than five times more likely to die during a human-extinction event than in a car crash. [18] [19]

There are significant methodological challenges in estimating these risks with precision. Most attention has been given to risks to human civilization over the next hundred years, but forecasting for this length of time is difficult. The types of threats posed by nature have been argued to be relatively constant, though this has been disputed, [20] and new risks could be discovered. Anthropogenic threats, however, are likely to change dramatically with the development of new technology while volcanoes have been a threat throughout history, nuclear weapons have been an issue only since the 20th century. Historically, the ability of experts to predict the future over these timescales has proved very limited. Man-made threats such as nuclear war or nanotechnology are harder to predict than natural threats, due to the inherent methodological difficulties in the social sciences. In general, it is hard to estimate the magnitude of the risk from this or other dangers, especially as both international relations and technology can change rapidly.

Existential risks pose unique challenges to prediction, even more than other long-term events, because of observation selection effects. Unlike with most events, the failure of a complete extinction event to occur in the past is not evidence against their likelihood in the future, because every world that has experienced such an extinction event has no observers, so regardless of their frequency, no civilization observes existential risks in its history. [8] These anthropic issues can be avoided by looking at evidence that does not have such selection effects, such as asteroid impact craters on the Moon, or directly evaluating the likely impact of new technology. [5]

In addition to known and tangible risks, unforeseeable black swan extinction events may occur, presenting an additional methodological problem. [21]

In his book The Precipice: Existential Risk and the Future of Humanity, Toby Ord gives his overall estimate at the likelihood of human extinction risk this century as 1 in 6. [22] [23]

Some scholars have strongly favored reducing existential risk on the grounds that it greatly benefits future generations. Derek Parfit argues that extinction would be a great loss because our descendants could potentially survive for four billion years before the expansion of the Sun makes the Earth uninhabitable. [24] [25] Nick Bostrom argues that there is even greater potential in colonizing space. If future humans colonize space, they may be able to support a very large number of people on other planets, potentially lasting for trillions of years. [6] Therefore, reducing existential risk by even a small amount would have a very significant impact on the expected number of people who will exist in the future.

Exponential discounting might make these future benefits much less significant. However, Jason Matheny has argued that such discounting is inappropriate when assessing the value of existential risk reduction. [10]

Some economists have discussed the importance of global catastrophic risks, though not existential risks. Martin Weitzman argues that most of the expected economic damage from climate change may come from the small chance that warming greatly exceeds the mid-range expectations, resulting in catastrophic damage. [26] Richard Posner has argued that humanity is doing far too little, in general, about small, hard-to-estimate risks of large-scale catastrophes. [27]

Scope insensitivity influences how bad people consider the extinction of the human race to be. For example, when people are motivated to donate money to altruistic causes, the quantity they are willing to give does not increase linearly with the magnitude of the issue: people are roughly as concerned about 200,000 birds getting stuck in oil as they are about 2,000. [29] Similarly, people are often more concerned about threats to individuals than to larger groups. [28]

There are economic reasons that can explain why so little effort is going into existential risk reduction. It is a global good, so even if a large nation decreases it, that nation will enjoy only a small fraction of the benefit of doing so. Furthermore, the vast majority of the benefits may be enjoyed by far future generations, and though these quadrillions of future people would in theory perhaps be willing to pay massive sums for existential risk reduction, no mechanism for such a transaction exists. [5]

Some sources of catastrophic risk are natural, such as meteor impacts or supervolcanoes. Some of these have caused mass extinctions in the past. On the other hand, some risks are man-made, such as global warming, [30] environmental degradation, engineered pandemics and nuclear war. [31]

Anthropogenic Edit

The Cambridge Project at Cambridge University says the "greatest threats" to the human species are man-made they are artificial intelligence, global warming, nuclear war, and rogue biotechnology. [32] The Future of Humanity Institute also states that human extinction is more likely to result from anthropogenic causes than natural causes. [5] [33]

Artificial intelligence Edit

It has been suggested that if AI systems rapidly become super-intelligent, they may take unforeseen actions or out-compete humanity. [34] According to philosopher Nick Bostrom, it is possible that the first super-intelligence to emerge would be able to bring about almost any possible outcome it valued, as well as to foil virtually any attempt to prevent it from achieving its objectives. [35] Thus, even a super-intelligence indifferent to humanity could be dangerous if it perceived humans as an obstacle to unrelated goals. In Bostrom's book Superintelligence, he defines this as the control problem. [36] Physicist Stephen Hawking, Microsoft founder Bill Gates, and SpaceX founder Elon Musk have echoed these concerns, with Hawking theorizing that such an AI could "spell the end of the human race". [37]

In 2009, the Association for the Advancement of Artificial Intelligence (AAAI) hosted a conference to discuss whether computers and robots might be able to acquire any sort of autonomy, and how much these abilities might pose a threat or hazard. They noted that some robots have acquired various forms of semi-autonomy, including being able to find power sources on their own and being able to independently choose targets to attack with weapons. They also noted that some computer viruses can evade elimination and have achieved "cockroach intelligence". They noted that self-awareness as depicted in science-fiction is probably unlikely, but there are other potential hazards and pitfalls. [38] Various media sources and scientific groups have noted separate trends in differing areas which might together result in greater robotic functionalities and autonomy, and which pose some inherent concerns. [39] [40]

A survey of AI experts estimated that the chance of human-level machine learning having an "extremely bad (e.g., human extinction)" long-term effect on humanity is 5%. [41] A 2008 survey by the Future of Humanity Institute estimated a 5% probability of extinction by super-intelligence by 2100. [16] Eliezer Yudkowsky believes risks from artificial intelligence are harder to predict than any other known risks due to bias from anthropomorphism. Since people base their judgments of artificial intelligence on their own experience, he claims they underestimate the potential power of AI. [42]

Biotechnology Edit

Biotechnology can pose a global catastrophic risk in the form of bioengineered organisms (viruses, bacteria, fungi, plants or animals). In many cases the organism will be a pathogen of humans, livestock, crops or other organisms we depend upon (e.g. pollinators or gut bacteria). However, any organism able to catastrophically disrupt ecosystem functions, e.g. highly competitive weeds, outcompeting essential crops, poses a biotechnology risk.

A biotechnology catastrophe may be caused by accidentally releasing a genetically engineered organism from controlled environments, by the planned release of such an organism which then turns out to have unforeseen and catastrophic interactions with essential natural or agro-ecosystems, or by intentional usage of biological agents in biological warfare or bioterrorism attacks. [43] Pathogens may be intentionally or unintentionally genetically modified to change virulence and other characteristics. [43] For example, a group of Australian researchers unintentionally changed characteristics of the mousepox virus while trying to develop a virus to sterilize rodents. [43] The modified virus became highly lethal even in vaccinated and naturally resistant mice. [44] [45] The technological means to genetically modify virus characteristics are likely to become more widely available in the future if not properly regulated. [43]

Terrorist applications of biotechnology have historically been infrequent. To what extent this is due to a lack of capabilities or motivation is not resolved. [43] However, given current development, more risk from novel, engineered pathogens is to be expected in the future. [43] Exponential growth has been observed in the biotechnology sector, and Noun and Chyba predict that this will lead to major increases in biotechnological capabilities in the coming decades. [43] They argue that risks from biological warfare and bioterrorism are distinct from nuclear and chemical threats because biological pathogens are easier to mass-produce and their production is hard to control (especially as the technological capabilities are becoming available even to individual users). [43] In 2008, a survey by the Future of Humanity Institute estimated a 2% probability of extinction from engineered pandemics by 2100. [16]

Noun and Chyba propose three categories of measures to reduce risks from biotechnology and natural pandemics: Regulation or prevention of potentially dangerous research, improved recognition of outbreaks and developing facilities to mitigate disease outbreaks (e.g. better and/or more widely distributed vaccines). [43]

Cyberattack Edit

Cyberattacks have the potential to destroy everything from personal data to electric grids. Christine Peterson, co-founder and past president of the Foresight Institute, believes a cyberattack on electric grids has the potential to be a catastrophic risk. She notes that little has been done to mitigate such risks, and that mitigation could take several decades of readjustment. [46]

Environmental disaster Edit

An environmental or ecological disaster, such as world crop failure and collapse of ecosystem services, could be induced by the present trends of overpopulation, economic development, and non-sustainable agriculture. Most environmental scenarios involve one or more of the following: Holocene extinction event, [47] scarcity of water that could lead to approximately half the Earth's population being without safe drinking water, pollinator decline, overfishing, massive deforestation, desertification, climate change, or massive water pollution episodes. Detected in the early 21st century, a threat in this direction is colony collapse disorder, [48] a phenomenon that might foreshadow the imminent extinction [49] of the Western honeybee. As the bee plays a vital role in pollination, its extinction would severely disrupt the food chain.

An October 2017 report published in The Lancet stated that toxic air, water, soils, and workplaces were collectively responsible for nine million deaths worldwide in 2015, particularly from air pollution which was linked to deaths by increasing susceptibility to non-infectious diseases, such as heart disease, stroke, and lung cancer. [50] The report warned that the pollution crisis was exceeding "the envelope on the amount of pollution the Earth can carry" and "threatens the continuing survival of human societies". [50]

Experimental technology accident Edit

Nick Bostrom suggested that in the pursuit of knowledge, humanity might inadvertently create a device that could destroy Earth and the Solar System. [51] Investigations in nuclear and high-energy physics could create unusual conditions with catastrophic consequences. For example, scientists worried that the first nuclear test might ignite the atmosphere. [52] [53] Others worried that the RHIC [54] or the Large Hadron Collider might start a chain-reaction global disaster involving black holes, strangelets, or false vacuum states. These particular concerns have been challenged, [55] [56] [57] [58] but the general concern remains.

Biotechnology could lead to the creation of a pandemic, chemical warfare could be taken to an extreme, nanotechnology could lead to grey goo in which out-of-control self-replicating robots consume all living matter on earth while building more of themselves—in both cases, either deliberately or by accident. [59]

Global warming Edit

Global warming refers to the warming caused by human technology since the 19th century or earlier. Projections of future climate change suggest further global warming, sea level rise, and an increase in the frequency and severity of some extreme weather events and weather-related disasters. Effects of global warming include loss of biodiversity, stresses to existing food-producing systems, increased spread of known infectious diseases such as malaria, and rapid mutation of microorganisms. In November 2017, a statement by 15,364 scientists from 184 countries indicated that increasing levels of greenhouse gases from use of fossil fuels, human population growth, deforestation, and overuse of land for agricultural production, particularly by farming ruminants for meat consumption, are trending in ways that forecast an increase in human misery over coming decades. [3]

Mineral resource exhaustion Edit

Romanian American economist Nicholas Georgescu-Roegen, a progenitor in economics and the paradigm founder of ecological economics, has argued that the carrying capacity of Earth—that is, Earth's capacity to sustain human populations and consumption levels—is bound to decrease sometime in the future as Earth's finite stock of mineral resources is presently being extracted and put to use and consequently, that the world economy as a whole is heading towards an inevitable future collapse, leading to the demise of human civilization itself. [60] : 303f Ecological economist and steady-state theorist Herman Daly, a student of Georgescu-Roegen, has propounded the same argument by asserting that ". all we can do is to avoid wasting the limited capacity of creation to support present and future life [on Earth]." [61] : 370

Ever since Georgescu-Roegen and Daly published these views, various scholars in the field have been discussing the existential impossibility of allocating earth's finite stock of mineral resources evenly among an unknown number of present and future generations. This number of generations is likely to remain unknown to us, as there is no way—or only little way—of knowing in advance if or when mankind will ultimately face extinction. In effect, any conceivable intertemporal allocation of the stock will inevitably end up with universal economic decline at some future point. [62] : 253–256 [63] : 165 [64] : 168–171 [65] : 150–153 [66] : 106–109 [67] : 546–549 [68] : 142–145 [69]

Nanotechnology Edit

Many nanoscale technologies are in development or currently in use. [70] The only one that appears to pose a significant global catastrophic risk is molecular manufacturing, a technique that would make it possible to build complex structures at atomic precision. [71] Molecular manufacturing requires significant advances in nanotechnology, but once achieved could produce highly advanced products at low costs and in large quantities in nanofactories of desktop proportions. [70] [71] When nanofactories gain the ability to produce other nanofactories, production may only be limited by relatively abundant factors such as input materials, energy and software. [70]

Molecular manufacturing could be used to cheaply produce, among many other products, highly advanced, durable weapons. [70] Being equipped with compact computers and motors these could be increasingly autonomous and have a large range of capabilities. [70]

Chris Phoenix and Treder classify catastrophic risks posed by nanotechnology into three categories:

  1. From augmenting the development of other technologies such as AI and biotechnology.
  2. By enabling mass-production of potentially dangerous products that cause risk dynamics (such as arms races) depending on how they are used.
  3. From uncontrolled self-perpetuating processes with destructive effects.

Several researchers say the bulk of risk from nanotechnology comes from the potential to lead to war, arms races and destructive global government. [44] [70] [72] Several reasons have been suggested why the availability of nanotech weaponry may with significant likelihood lead to unstable arms races (compared to e.g. nuclear arms races):

  1. A large number of players may be tempted to enter the race since the threshold for doing so is low [70]
  2. The ability to make weapons with molecular manufacturing will be cheap and easy to hide [70]
  3. Therefore, lack of insight into the other parties' capabilities can tempt players to arm out of caution or to launch preemptive strikes [70][73]
  4. Molecular manufacturing may reduce dependency on international trade, [70] a potential peace-promoting factor may pose a smaller economic threat to the aggressor since manufacturing is cheap and humans may not be needed on the battlefield. [70]

Since self-regulation by all state and non-state actors seems hard to achieve, [74] measures to mitigate war-related risks have mainly been proposed in the area of international cooperation. [70] [75] International infrastructure may be expanded giving more sovereignty to the international level. This could help coordinate efforts for arms control. International institutions dedicated specifically to nanotechnology (perhaps analogously to the International Atomic Energy Agency IAEA) or general arms control may also be designed. [75] One may also jointly make differential technological progress on defensive technologies, a policy that players should usually favour. [70] The Center for Responsible Nanotechnology also suggests some technical restrictions. [76] Improved transparency regarding technological capabilities may be another important facilitator for arms-control.

Grey goo is another catastrophic scenario, which was proposed by Eric Drexler in his 1986 book Engines of Creation [77] and has been a theme in mainstream media and fiction. [78] [79] This scenario involves tiny self-replicating robots that consume the entire biosphere using it as a source of energy and building blocks. Nowadays, however, nanotech experts—including Drexler—discredit the scenario. According to Phoenix, a "so-called grey goo could only be the product of a deliberate and difficult engineering process, not an accident". [80]

Warfare and mass destruction Edit

The scenarios that have been explored most frequently are nuclear warfare and doomsday devices. Mistakenly launching a nuclear attack in response to a false alarm is one possible scenario this nearly happened during the 1983 Soviet nuclear false alarm incident. Although the probability of a nuclear war per year is slim, Professor Martin Hellman has described it as inevitable in the long run unless the probability approaches zero, inevitably there will come a day when civilization's luck runs out. [81] During the Cuban Missile Crisis, U.S. president John F. Kennedy estimated the odds of nuclear war at "somewhere between one out of three and even". [82] The United States and Russia have a combined arsenal of 14,700 nuclear weapons, [83] and there is an estimated total of 15,700 nuclear weapons in existence worldwide. [83] Beyond nuclear, other military threats to humanity include biological warfare (BW). By contrast, chemical warfare, while able to create multiple local catastrophes, is unlikely to create a global one.

Nuclear war could yield unprecedented human death tolls and habitat destruction. Detonating large numbers of nuclear weapons would have an immediate, short term and long-term effects on the climate, causing cold weather and reduced sunlight and photosynthesis [84] that may generate significant upheaval in advanced civilizations. [85] However, while popular perception sometimes takes nuclear war as "the end of the world", experts assign low probability to human extinction from nuclear war. [86] [87] In 1982, Brian Martin estimated that a US–Soviet nuclear exchange might kill 400–450 million directly, mostly in the United States, Europe and Russia, and maybe several hundred million more through follow-up consequences in those same areas. [86] In 2008, a survey by the Future of Humanity Institute estimated a 4% probability of extinction from warfare by 2100, with a 1% chance of extinction from nuclear warfare. [16]

World population and agricultural crisis Edit

The 20th century saw a rapid increase in human population due to medical developments and massive increases in agricultural productivity [88] such as the Green Revolution. [89] Between 1950 and 1984, as the Green Revolution transformed agriculture around the globe, world grain production increased by 250%. The Green Revolution in agriculture helped food production to keep pace with worldwide population growth or actually enabled population growth. The energy for the Green Revolution was provided by fossil fuels in the form of fertilizers (natural gas), pesticides (oil), and hydrocarbon-fueled irrigation. [90] David Pimentel, professor of ecology and agriculture at Cornell University, and Mario Giampietro, senior researcher at the National Research Institute on Food and Nutrition (INRAN), place in their 1994 study Food, Land, Population and the U.S. Economy the maximum U.S. population for a sustainable economy at 200 million. To achieve a sustainable economy and avert disaster, the United States must reduce its population by at least one-third, and world population will have to be reduced by two-thirds, says the study. [91]

The authors of this study believe the mentioned agricultural crisis will begin to have an effect on the world after 2020, and will become critical after 2050. Geologist Dale Allen Pfeiffer claims that coming decades could see spiraling food prices without relief and massive starvation on a global level such as never experienced before. [92] [93]

Since supplies of petroleum and natural gas are essential to modern agriculture techniques, a fall in global oil supplies (see peak oil for global concerns) could cause spiking food prices and unprecedented famine in the coming decades. [94] [95]

Wheat is humanity's third-most-produced cereal. Extant fungal infections such as Ug99 [96] (a kind of stem rust) can cause 100% crop losses in most modern varieties. Little or no treatment is possible and infection spreads on the wind. Should the world's large grain-producing areas become infected, the ensuing crisis in wheat availability would lead to price spikes and shortages in other food products. [97]

Non-anthropogenic Edit

Asteroid impact Edit

Several asteroids have collided with Earth in recent geological history. The Chicxulub asteroid, for example, was about six miles in diameter and is theorized to have caused the extinction of non-avian dinosaurs at the end of the Cretaceous. No sufficiently large asteroid currently exists in an Earth-crossing orbit however, a comet of sufficient size to cause human extinction could impact the Earth, though the annual probability may be less than 10 −8 . [98] Geoscientist Brian Toon estimates that while a few people, such as "some fishermen in Costa Rica", could plausibly survive a six-mile meteorite, a sixty-mile meteorite would be large enough to "incinerate everybody". [99] Asteroids with around a 1 km diameter have impacted the Earth on average once every 500,000 years these are probably too small to pose an extinction risk, but might kill billions of people. [98] [100] Larger asteroids are less common. Small near-Earth asteroids are regularly observed and can impact anywhere on the Earth injuring local populations. [101] As of 2013, Spaceguard estimates it has identified 95% of all NEOs over 1 km in size. [102]

In April 2018, the B612 Foundation reported "It's a 100 per cent certain we'll be hit [by a devastating asteroid], but we're not 100 per cent sure when." [103] Also in 2018, physicist Stephen Hawking, in his final book Brief Answers to the Big Questions, considered an asteroid collision to be the biggest threat to the planet. [104] [105] [106] In June 2018, the US National Science and Technology Council warned that America is unprepared for an asteroid impact event, and has developed and released the "National Near-Earth Object Preparedness Strategy Action Plan" to better prepare. [107] [108] [109] [110] [111] According to expert testimony in the United States Congress in 2013, NASA would require at least five years of preparation before a mission to intercept an asteroid could be launched. [112]

Cosmic threats Edit

A number of astronomical threats have been identified. Massive objects, e.g. a star, large planet or black hole, could be catastrophic if a close encounter occurred in the Solar System. In April 2008, it was announced that two simulations of long-term planetary movement, one at the Paris Observatory and the other at the University of California, Santa Cruz, indicate a 1% chance that Mercury's orbit could be made unstable by Jupiter's gravitational pull sometime during the lifespan of the Sun. Were this to happen, the simulations suggest a collision with Earth could be one of four possible outcomes (the others being Mercury colliding with the Sun, colliding with Venus, or being ejected from the Solar System altogether). If Mercury were to collide with Earth, all life on Earth could be obliterated entirely: an asteroid 15 km wide is believed to have caused the extinction of the non-avian dinosaurs, whereas Mercury is 4,879 km in diameter. [113]

If our universe lies within a false vacuum, a bubble of lower-energy vacuum could come to exist by chance or otherwise in our universe, and catalyze the conversion of our universe to a lower energy state in a volume expanding at nearly the speed of light, destroying all that we know without forewarning. Such an occurrence is called vacuum decay. [115] [116]

Another cosmic threat is a gamma-ray burst, typically produced by a supernova when a star collapses inward on itself and then "bounces" outward in a massive explosion. Under certain circumstances, these events are thought to produce massive bursts of gamma radiation emanating outward from the axis of rotation of the star. If such an event were to occur oriented towards the Earth, the massive amounts of gamma radiation could significantly affect the Earth's atmosphere and pose an existential threat to all life. Such a gamma-ray burst may have been the cause of the Ordovician–Silurian extinction events. Neither this scenario nor the destabilization of Mercury's orbit are likely in the foreseeable future. [117]

A powerful solar flare or solar superstorm, which is a drastic and unusual decrease or increase in the Sun's power output, could have severe consequences for life on Earth. [118] [119]

Astrophysicists currently calculate that in a few billion years the Earth will probably be swallowed by the expansion of the Sun into a red giant star. [120] [121]

Extraterrestrial invasion Edit

Intelligent extraterrestrial life, if existent, could invade Earth [122] either to exterminate and supplant human life, enslave it under a colonial system, steal the planet's resources, or destroy the planet altogether.

Although evidence of alien life has never been proven, scientists such as Carl Sagan have postulated that the existence of extraterrestrial life is very likely. In 1969, the "Extra-Terrestrial Exposure Law" was added to the United States Code of Federal Regulations (Title 14, Section 1211) in response to the possibility of biological contamination resulting from the U.S. Apollo Space Program. It was removed in 1991. [123] Scientists consider such a scenario technically possible, but unlikely. [124]

An article in The New York Times discussed the possible threats for humanity of intentionally sending messages aimed at extraterrestrial life into the cosmos in the context of the SETI efforts. Several renowned public figures such as Stephen Hawking and Elon Musk have argued against sending such messages on the grounds that extraterrestrial civilizations with technology are probably far more advanced than humanity and could pose an existential threat to humanity. [125]

Pandemic Edit

There are numerous historical examples of pandemics [126] that have had a devastating effect on a large number of people. The present, unprecedented scale and speed of human movement make it more difficult than ever to contain an epidemic through local quarantines, and other sources of uncertainty and the evolving nature of the risk mean natural pandemics may pose a realistic threat to human civilization. [20]

There are several classes of argument about the likelihood of pandemics. One stems from history, where the limited size of historical pandemics is evidence that larger pandemics are unlikely. This argument has been disputed on grounds including the changing risk due to changing population and behavioral patterns among humans, the limited historical record, and the existence of an anthropic bias. [20]

Another argument is based on an evolutionary model that predicts that naturally evolving pathogens will ultimately develop an upper limit to their virulence. [127] This is because pathogens with high enough virulence quickly kill their hosts and reduce their chances of spreading the infection to new hosts or carriers. [128] This model has limits, however, because the fitness advantage of limited virulence is primarily a function of a limited number of hosts. Any pathogen with a high virulence, high transmission rate and long incubation time may have already caused a catastrophic pandemic before ultimately virulence is limited through natural selection. Additionally, a pathogen that infects humans as a secondary host and primarily infects another species (a zoonosis) has no constraints on its virulence in people, since the accidental secondary infections do not affect its evolution. [129] Lastly, in models where virulence level and rate of transmission are related, high levels of virulence can evolve. [130] Virulence is instead limited by the existence of complex populations of hosts with different susceptibilities to infection, or by some hosts being geographically isolated. [127] The size of the host population and competition between different strains of pathogens can also alter virulence. [131]

Neither of these arguments is applicable to bioengineered pathogens, and this poses entirely different risks of pandemics. Experts have concluded that "Developments in science and technology could significantly ease the development and use of high consequence biological weapons," and these "highly virulent and highly transmissible [bio-engineered pathogens] represent new potential pandemic threats." [132]

Natural climate change Edit

Climate change refers to a lasting change in the Earth's climate. The climate has ranged from ice ages to warmer periods when palm trees grew in Antarctica. It has been hypothesized that there was also a period called "snowball Earth" when all the oceans were covered in a layer of ice. These global climatic changes occurred slowly, near the end of the last Major Ice Age when the climate became more stable. However, abrupt climate change on the decade time scale has occurred regionally. A natural variation into a new climate regime (colder or hotter) could pose a threat to civilization. [133] [134]

In the history of the Earth, many ice ages are known to have occurred. An ice age would have a serious impact on civilization because vast areas of land (mainly in North America, Europe, and Asia) could become uninhabitable. Currently, the world is in an interglacial period within a much older glacial event. The last glacial expansion ended about 10,000 years ago, and all civilizations evolved later than this. Scientists do not predict that a natural ice age will occur anytime soon. [ citation needed ] The amount of heat-trapping gases emitted into Earth's Oceans and atmosphere will prevent the next ice age, which otherwise would begin in around 50,000 years, and likely more glacial cycles. [135] [136]

Volcanism Edit

A geological event such as massive flood basalt, volcanism, or the eruption of a supervolcano [137] could lead to a so-called volcanic winter, similar to a nuclear winter. One such event, the Toba eruption, [138] occurred in Indonesia about 71,500 years ago. According to the Toba catastrophe theory, [139] the event may have reduced human populations to only a few tens of thousands of individuals. Yellowstone Caldera is another such supervolcano, having undergone 142 or more caldera-forming eruptions in the past 17 million years. [140] A massive volcano eruption would eject extraordinary volumes of volcanic dust, toxic and greenhouse gases into the atmosphere with serious effects on global climate (towards extreme global cooling: volcanic winter if short-term, and ice age if long-term) or global warming (if greenhouse gases were to prevail).

When the supervolcano at Yellowstone last erupted 640,000 years ago, the thinnest layers of the ash ejected from the caldera spread over most of the United States west of the Mississippi River and part of northeastern Mexico. The magma covered much of what is now Yellowstone National Park and extended beyond, covering much of the ground from Yellowstone River in the east to the Idaho falls in the west, with some of the flows extending north beyond Mammoth Springs. [141]

According to a recent study, if the Yellowstone caldera erupted again as a supervolcano, an ash layer one to three millimeters thick could be deposited as far away as New York, enough to "reduce traction on roads and runways, short out electrical transformers and cause respiratory problems". There would be centimeters of thickness over much of the U.S. Midwest, enough to disrupt crops and livestock, especially if it happened at a critical time in the growing season. The worst-affected city would likely be Billings, Montana, population 109,000, which the model predicted would be covered with ash estimated as 1.03 to 1.8 meters thick. [142]

The main long-term effect is through global climate change, which reduces the temperature globally by about 5–15 degrees C for a decade, together with the direct effects of the deposits of ash on their crops. A large supervolcano like Toba would deposit one or two meters thickness of ash over an area of several million square kilometers.(1000 cubic kilometers is equivalent to a one-meter thickness of ash spread over a million square kilometers). If that happened in some densely populated agricultural area, such as India, it could destroy one or two seasons of crops for two billion people. [143]

However, Yellowstone shows no signs of a supereruption at present, and it is not certain that a future supereruption will occur there. [144] [145]

Research published in 2011 finds evidence that massive volcanic eruptions caused massive coal combustion, supporting models for the significant generation of greenhouse gases. Researchers have suggested that massive volcanic eruptions through coal beds in Siberia would generate significant greenhouse gases and cause a runaway greenhouse effect. [146] Massive eruptions can also throw enough pyroclastic debris and other material into the atmosphere to partially block out the sun and cause a volcanic winter, as happened on a smaller scale in 1816 following the eruption of Mount Tambora, the so-called Year Without a Summer. Such an eruption might cause the immediate deaths of millions of people several hundred miles from the eruption, and perhaps billions of death worldwide, due to the failure of the monsoons, [147] resulting in major crop failures causing starvation on a profound scale. [147]

A much more speculative concept is the verneshot: a hypothetical volcanic eruption caused by the buildup of gas deep underneath a craton. Such an event may be forceful enough to launch an extreme amount of material from the crust and mantle into a sub-orbital trajectory.

Planetary management and respecting planetary boundaries have been proposed as approaches to preventing ecological catastrophes. Within the scope of these approaches, the field of geoengineering encompasses the deliberate large-scale engineering and manipulation of the planetary environment to combat or counteract anthropogenic changes in atmospheric chemistry. Space colonization is a proposed alternative to improve the odds of surviving an extinction scenario. [148] Solutions of this scope may require megascale engineering. Food storage has been proposed globally, but the monetary cost would be high. Furthermore, it would likely contribute to the current millions of deaths per year due to malnutrition. [149]

Some survivalists stock survival retreats with multiple-year food supplies.

The Svalbard Global Seed Vault is buried 400 feet (120 m) inside a mountain on an island in the Arctic. It is designed to hold 2.5 billion seeds from more than 100 countries as a precaution to preserve the world's crops. The surrounding rock is −6 °C (21 °F) (as of 2015) but the vault is kept at −18 °C (0 °F) by refrigerators powered by locally sourced coal. [150] [151]

More speculatively, if society continues to function and if the biosphere remains habitable, calorie needs for the present human population might in theory be met during an extended absence of sunlight, given sufficient advance planning. Conjectured solutions include growing mushrooms on the dead plant biomass left in the wake of the catastrophe, converting cellulose to sugar, or feeding natural gas to methane-digesting bacteria. [152] [153]

Global catastrophic risks and global governance Edit

Insufficient global governance creates risks in the social and political domain, but the governance mechanisms develop more slowly than technological and social change. There are concerns from governments, the private sector, as well as the general public about the lack of governance mechanisms to efficiently deal with risks, negotiate and adjudicate between diverse and conflicting interests. This is further underlined by an understanding of the interconnectedness of global systemic risks. [154] In absence or anticipation of global governance, national governments can act individually to better understand, mitigate and prepare for global catastrophes. [155]

Climate emergency plans Edit

In 2018, the Club of Rome called for greater climate change action and published its Climate Emergency Plan, which proposes ten action points to limit global average temperature increase to 1.5 degrees Celsius. [156] Further, in 2019, the Club published the more comprehensive Planetary Emergency Plan. [157]

The Bulletin of the Atomic Scientists (est. 1945) is one of the oldest global risk organizations, founded after the public became alarmed by the potential of atomic warfare in the aftermath of WWII. It studies risks associated with nuclear war and energy and famously maintains the Doomsday Clock established in 1947. The Foresight Institute (est. 1986) examines the risks of nanotechnology and its benefits. It was one of the earliest organizations to study the unintended consequences of otherwise harmless technology gone haywire at a global scale. It was founded by K. Eric Drexler who postulated "grey goo". [158] [159]

Beginning after 2000, a growing number of scientists, philosophers and tech billionaires created organizations devoted to studying global risks both inside and outside of academia. [160]

Independent non-governmental organizations (NGOs) include the Machine Intelligence Research Institute (est. 2000), which aims to reduce the risk of a catastrophe caused by artificial intelligence, [161] with donors including Peter Thiel and Jed McCaleb. [162] The Nuclear Threat Initiative (est. 2001) seeks to reduce global threats from nuclear, biological and chemical threats, and containment of damage after an event. [163] It maintains a nuclear material security index. [164] The Lifeboat Foundation (est. 2009) funds research into preventing a technological catastrophe. [165] Most of the research money funds projects at universities. [166] The Global Catastrophic Risk Institute (est. 2011) is a think tank for catastrophic risk. It is funded by the NGO Social and Environmental Entrepreneurs. The Global Challenges Foundation (est. 2012), based in Stockholm and founded by Laszlo Szombatfalvy, releases a yearly report on the state of global risks. [18] [19] The Future of Life Institute (est. 2014) aims to support research and initiatives for safeguarding life considering new technologies and challenges facing humanity. [167] Elon Musk is one of its biggest donors. [168] The Center on Long-Term Risk (est. 2016), formerly known as the Foundational Research Institute, is a British organization focused on reducing risks of astronomical suffering (s-risks) from emerging technologies. [169]

University-based organizations include the Future of Humanity Institute (est. 2005) which researches the questions of humanity's long-term future, particularly existential risk. It was founded by Nick Bostrom and is based at Oxford University. The Centre for the Study of Existential Risk (est. 2012) is a Cambridge-based organization which studies four major technological risks: artificial intelligence, biotechnology, global warming and warfare. All are man-made risks, as Huw Price explained to the AFP news agency, "It seems a reasonable prediction that some time in this or the next century intelligence will escape from the constraints of biology". He added that when this happens "we're no longer the smartest things around," and will risk being at the mercy of "machines that are not malicious, but machines whose interests don't include us." [170] Stephen Hawking was an acting adviser. The Millennium Alliance for Humanity and the Biosphere is a Stanford University-based organization focusing on many issues related to global catastrophe by bringing together members of academic in the humanities. [171] [172] It was founded by Paul Ehrlich among others. [173] Stanford University also has the Center for International Security and Cooperation focusing on political cooperation to reduce global catastrophic risk. [174] The Center for Security and Emerging Technology was established in January 2019 at Georgetown's Walsh School of Foreign Service and will focus on policy research of emerging technologies with an initial emphasis on artificial intelligence. [175] They received a grant of 55M USD from Good Ventures as suggested by the Open Philanthropy Project. [175]

Other risk assessment groups are based in or are part of governmental organizations. The World Health Organization (WHO) includes a division called the Global Alert and Response (GAR) which monitors and responds to global epidemic crisis. [176] GAR helps member states with training and coordination of response to epidemics. [177] The United States Agency for International Development (USAID) has its Emerging Pandemic Threats Program which aims to prevent and contain naturally generated pandemics at their source. [178] The Lawrence Livermore National Laboratory has a division called the Global Security Principal Directorate which researches on behalf of the government issues such as bio-security and counter-terrorism. [179]

Astronomers find 'blinking giant' star at the center of the Milky Way

June 11 (UPI) -- Astronomers have discovered a giant, blinking star in the middle of the Milky Way, roughly 25,000 light-years from Earth.

The star's blinking pattern, dimming by a factor of 30, was first identified by the VISTA Variables in the Via Lactea survey, which utilizes the European Southern Observatory's VISTA telescope in Chile.

Unlike pulsing stars, which brighten and dim across short time-scales, the newly discovered VVV-WIT-08 star -- described Friday in the journal Monthly Notices of the Royal Astronomical Society -- dimmed, nearly disappeared and brightened over a period of months.

Scientists suspect the star won't dim again for a long time.

In fact, astronomers estimate the VVV-WIT-08 star is member of a new class of "blinking giant" binary star systems, featuring a giant star, 100 times bigger than the sun, and an unidentified companion with a decades-long orbit.

Though scientists don't know whether it's a star or planet, observational data suggests VVV-WIT-08's mystery companion has a large opaque disk that helps block out much of the giant star's light.

"It's amazing that we just observed a dark, large and elongated object pass between us and the distant star and we can only speculate what its origin is," study co-author Sergey Koposov, astronomer at the University of Edinburgh, said in a press release.

Though astronomers don't yet know for certain what's circling VVV-WIT-08, galactic models showed it's highly unlikely that the giant star's light was dimmed by an unrelated passing object.

Astronomers have only a couple of examples of similar binary systems.

The star Epsilon Aurigae is dimmed 50 percent every 27 years by a passing disk of dust.

And the recently discovered TYC 2505-672-1 is an eclipsing binary star system with an orbital period of 69 years -- a record that could be eclipsed by VVV-WIT-08.

"Occasionally we find variable stars that don't fit into any established category, which we call 'what-is-this?', or 'WIT' objects," project co-leader Philip Lucas said in the release.

"We really don't know how these blinking giants came to be. It's exciting to see such discoveries from VVV after so many years planning and gathering the data," said Lucas, a professor at the University of Hertfordshire.

Ongoing surveys and followup investigations have turned up a number of other blinking giant candidates, but it may be some time before astronomers are able to shed any light on the companions responsible for feature that make these systems unique in the first place -- the blinking.

"There are certainly more to be found, but the challenge now is in figuring out what the hidden companions are, and how they came to be surrounded by discs, despite orbiting so far from the giant star," lead study author Leigh Smith said in a press release.

"In doing so, we might learn something new about how these kinds of systems evolve," said Smith, a researcher at Cambridge University's Institute of Astronomy.

5. Moon Causes the Ocean&aposs Tides

High tide always occurs when Moon is right overhead or underneath. Low tide happens when Moon is near the eastern or western horizon. This explains how the gravitational pull of the Moon causes the ocean&aposs tides.

The high and low tides become more pronounced during New Moon or Full Moon days. Sun&aposs gravitational pull also has an influence on tides, though Moon has a more decisive effect on tides. During a full moon or new moon, Sun, Earth, and Moon are all aligned in a straight line. The gravitational force of Sun and Moon adds up to make the tides more pronounced.

One can observe the tidal movements by visiting a nearby beach and making periodic observations at a gap of 3 hours. One would then understand the link between Moon&aposs position and the height of the tides.

The transit of Venus across the Sun, 2004

10 Ways to Stop a Killer Asteroid

If you were being stalked by a killer, you would you try to stop him or her, right? Now let's say your killer is a space rock shaped like an Idaho spud. What would you do about that? Interestingly enough, the odds of you being murdered at the hands of a madman are about one in 210 [source: Bailey]. The odds of being killed by a cosmic potato are a tad lower -- about one in 200,000 to 700,000 over your lifetime, depending on who's making the calculation [sources: Bailey, Plait]. But here's the rub: No single person -- not even someone as evil as Hitler -- could wipe out the entire human race. An asteroid could. If a rock just 6 miles (10 kilometers) across struck our beautiful, blue world, it would be adiós muchachos for every last one of us [source: Plait].

So, stopping an asteroid from blindsiding Earth makes sense, but is it even possible? And if it's possible, can we afford it? The answer to the first question might surprise you, because there are, in fact, many different ways to thwart a space rock. (No one ever said they were smart.) How much it might cost remains uncertain at best. Money, however, shouldn't be the main concern when you're talking about the survival of the human race. So let's throw that question out the window and focus on the top 10 ways to stop a killer asteroid, no matter how crazy (or costly) they seem on paper.

Up first, we have a solution based on tried-and-true Cold War technology: nuclear weapons.

10: Drop the Big One on the Big One

Nuclear weapons may not be original, but they're a known entity and, as a result, a logical choice if you need to blast a boulder to smithereens. This supermacho approach involves slamming a nuclear warhead into an approaching asteroid. There's only one problem: A direct hit on a large object might only break it into several smaller pieces (remember "Deep Impact"?). A better option might be to detonate a warhead near the asteroid, letting heat from the explosion sear one side of the rock. As material vaporizes from its surface, the asteroid would accelerate in the opposite direction -- just enough (fingers crossed) to steer it away from Earth.

If explosions aren't your thing, but you still want to hit something, then you'll appreciate another technique known as kinetic impactor deflection. The "kinetic" in this case refers to kinetic energy, which all moving objects have and the universe conserves. But we're getting ahead of ourselves. Turn the page to learn how the behavior of billiard balls just might save our planet.

9: Speak Softly and Carry a Big Thwack

If you've ever played pool, then you know about kinetic energy, which is the energy possessed by any moving object. The kinetic energy of a struck cue ball is what gets transferred to other balls on the table. Astronomers believe the same principle could deflect an earthbound asteroid. In this case, the cue ball is an unmanned spacecraft similar to the probe used in NASA's Deep Impact mission (not to be confused with the movie). The mass of the Deep Impact vessel was only 816 pounds (370 kilograms), but it was moving really, really fast -- 5 miles (10 kilometers) per second [source: NASA].

Kinetic energy depends on both the mass and speed of an object, so a small object moving fast still has a lot of energy. When mission engineers slammed the Deep Impact probe into the surface of the Tempel 1 comet in 2005, it was slated to deliver 19 gigajoules of kinetic energy. That's the equivalent of 4.8 tons of TNT, enough to shift the comet ever so slightly in its orbit [source: NASA].

Astronomers weren't looking to alter Tempel 1's trajectory, but they know now it could be done, should an asteroid or comet set its sights on Earth. Even with a success under their belt, scientists acknowledge the enormous challenge of such a mission. It's sort of like hitting a speeding cannonball with a speeding bullet. One wrong move, and you could miss your target completely or hit it off-center, causing it to tumble or crack into pieces. In 2005, the European Space Agency came up with the Don Quijote concept to improve the odds of a kinetic impactor mission (see sidebar).

You might classify nuclear weapons or kinetic impactors as instant-gratification solutions because their success (or failure) would be immediately apparent. Many astronomers, however, prefer to take the long view when it comes to asteroid deflection.

Leave it to Europe to merge great literature with great impact. The European Space Agency's take on a kinetic impactor is dubbed Don Quijote and calls for two spacecraft -- an orbiter named Sancho and an impactor named Hidalgo. Sancho would arrive at the killer asteroid first, get the lay of the land and transmit details back to Hidalgo. Trailing behind its companion, Hidalgo would arrive with all the intelligence it needed to make a pinpoint strike.

8: Throw a Few Photons at the Problem

Electromagnetic energy produced by the sun applies pressure to any object in the solar system. Astronomers like to call it solar, or radiation, pressure and have long thought this stream of energy could be a source of propulsion for rockets. Just strap some sails onto a spacecraft, let them catch a few rays and the ingenious vessel will slowly, gradually, pick up speed as incoming photons transfer their momentum to the sail. Could something similar work on an asteroid? A couple of scientists think so. Assuming you had some time -- we're talking decades here -- you could fasten some solar sails on an asteroid, do a little tacking and steer the rock away from Earth.

Of course, even Bruce Willis might not be extreme enough to land on a hunk of rock and try to convert it into a cosmic sailboat. Another option would be to wrap the asteroid in foil or coat it with highly reflective paint. Either solution would have the same effect as a solar sail, harnessing the energy of incoming photons. Then again, who's going to try to wrap foil around a giant potato traveling, say, at 16 miles (25 kilometers) per second [source: Jessa]? Or carry a few million gallons of paint into space?

Luckily, there's another sun-centered solution that might not seem so wacky.

7: Turn the Rock Into a Puffball

You're familiar with puffballs, right? They're the little round mushrooms we often see in fields and forests that reproduce by releasing spores through a topside exit hole. Poke a fresh puffball, and you'll see black smoke shoot out in a jet.

Strangely enough, astronomers think they can get an asteroid to do the same thing, though not by poking it. Instead, they envision parking an unmanned probe in orbit around an offending rock, then aiming a laser at the object's surface. As the laser heats up the rocky substrate, steam and other gases will erupt in fast-moving jets. According to Newton's laws of motion, each burst of gas applies a tiny force in the opposite direction. Heat the asteroid long enough, and you'll have it hissing like a teakettle and moving, centimeter by centimeter, off its original course.

Some see the laser as the limiting factor in this scenario. What if it can't draw enough power to sustain long-term heating? You could arm the probe with an array of mirrors. Once you get the spacecraft in orbit around the asteroid, you simply unfurl the mirrors and orient them so that they direct a beam of concentrated sunlight toward the object's surface. This provides the necessary heating without the need for a high-powered laser.

Then again, why not use the orbiting spacecraft without all of the tricks and gimmicks? Doesn't it have mass and, as a result, gravity? And doesn't gravity pull on nearby objects? Why, yes, Sir Isaac, it does.

6: Invite the Asteroid to a Tractor Pull

Every object in the universe, even something as small as a pebble, has gravity. You can't feel a pebble's gravity because its mass is so small, but it's still there, tugging away on anything that comes close. The close part is important because gravity is also related to the distance separating two objects. The closer they are, the greater the gravitational attraction.

A spacecraft zipping through the solar system obeys the same principles, exerting a gravitational pull directly proportional to its mass and inversely proportional to the distance between it and another object. Now, compared to an asteroid, which might have the mass of Mount Everest, a spacecraft is pretty puny, but its gravity can still make things happen. In fact, if you place an unmanned probe in a close orbit around an asteroid, it will pull ever so slightly on the rock. Over a period of 15 years or more, this almost infinitesimal tug could deflect the asteroid's orbit just enough to protect Earth from a nasty blow [source: BBC News].

Astronomers refer to this as a gravitational tractor and think it's a viable solution -- as long as they know about a potential collision years in advance. Early detection is just as critical to the next idea on the list.

5: Get Pushy With the Planetoid

If the gravitational tractor concept seems too delicate and prissy, you're in luck. A few scientists are proposing another way to make use of a spacecraft that doesn't require slamming it into an asteroid or entering a passive orbit. They studied busy harbors here on Earth and observed how tugboats nudge large ships up to the wharf. Then they developed an asteroid-deflection scenario using a similar technique.

Here's how it works: First, you build a special ship with powerful plasma engines and an array of radiator panels to dissipate heat from the onboard nuclear reactors. After you're alerted of a threat, you launch the vessel and fly it to the offending asteroid. Then you ease the space tug close to the rocky surface and attach the vessel using several segmented arms. Finally, you go easy on the throttle and start a slow, gentle push. If all goes well, 15 to 20 years of pushing in the direction of the asteroid's orbital motion will deflect it just enough to avoid a catastrophe [source: Schweickart].

Still not convinced? Then grab your mitt and keep moving to the next page.

Remember those baseball pitching machines you faced when you were a kid? They had a feeder tube and a wheel assembly to shoot the balls out at 50 to 60 miles (80 to 97 kilometers) an hour. Wouldn't it be great if you could set up a pitching machine on an asteroid? Not to take batting practice, but to save the world?

As crazy as it sounds, astronomers have an idea to do just that. They call their machine a mass driver, but it works the same way. It scoops up rocks from the surface of an asteroid and hurls them out into space. With each throw, the machine applies a force to the rock, but the rock, thanks to Newton's action-reaction law, applies a force back to the machine -- and to the asteroid. Throw a few hundred thousand rocks, and you'll actually shift the asteroid's orbit.

Of course, the concept has invited some criticism. How do you get the mass driver on the asteroid? And how do you keep it powered? A pitching machine plugs into an electrical supply, but extension cords are tough to manage out in space. And what if the darn thing breaks down? A relief pitcher may not be available to finish the game.

Maybe baseball is the wrong sport. Maybe another backyard favorite offers a better solution.

No, REM, we don't feel fine at all, but we might as well get some books and flicks in while we wait. Here are some (non-escapism) picks:

  • "Lucifer's Hammer" by Larry Niven and Jerry Pournelle
  • "Death From the Skies!" by Phil Plait
  • "The Road" by Cormac McCarthy
  • "The Walking Dead" (either the graphic novels or the TV series)
  • "The Day of the Triffids"
  • "Melancholia"
  • South Park's "Deeply Impacted" episode

3: Play Tetherball With the Asteroid

In 2009, a doctoral candidate at North Carolina State University proposed a novel asteroid-deflection technique in his dissertation. This was the idea: Attach one end of a tether to an asteroid and the other end to a massive weight known as a ballast. The ballast acts like an anchor, changing the asteroid's center of gravity and diverting its trajectory over the course of 20 to 50 years, depending on the size of the rock being moved and the weight of the ballast.

The student didn't work out every detail, but he estimated that the tether would need to be somewhere between 621 miles and 62,137 miles (1,000 and 100,000 kilometers) long. He also suggested a crescent-shaped attachment bar similar to those found on globes. This would allow the asteroid to rotate without tangling the tether (no one likes a tangled tether).

Now, if you think this sounds just too wacky to work, you should know that astronomers have embraced space tethers for years. In fact, NASA has used them successfully on several missions to move payloads in Earth's orbit. Future missions call for delivering material to the moon by handing off payloads across a series of tethers.

Still, a tether and ballast system, like most solutions in our countdown, requires time. And time requires early detection. As we'll see next, asteroid detection may be far more important than deflection.

2: Increase Your Reaction Time

When it comes to asteroids, you want to be like the Rolling Stones and put time on your side (yes, you do). Luckily, steps are being taken to survey and detect near-Earth objects, or NEOs.

NASA addresses NEO detection through two surveys mandated by U.S. Congress. The first, known as the Spaceguard Survey, seeks to detect 90 percent of NEOs 1 kilometer (0.621 miles) in diameter. Congress had set the original deadline as 2008, but the work continues as astronomers keep discovering and learning more about these enigmatic rocks. The second survey, the George E. Brown Jr., Near-Earth Object Survey, seeks to detect 90 percent of near-Earth objects 459 feet (140 meters) in diameter or greater by 2020. Both surveys rely on powerful telescopes to repeatedly scan large areas of the sky.

As of March 2012, those telescopes had discovered 8,818 near-Earth objects. Almost 850 of those NEOs were asteroids with a diameter of approximately 1 kilometer or larger. Nearly 1,300 were labeled as potentially hazardous asteroids, or PHAs. PHAs must be at least 492 feet (150 meters) wide and must come within 4.65 million miles (7.48 million kilometers) of Earth [source: NASA]

Now, if you're prone to panic, remember that the key word is "potentially." Not every space rock that makes a close approach to Earth will make an impact. Still, it's a sobering number, especially when you realize that the solar system likely contains hundreds of thousands, or even millions, of asteroids. How many have we just not seen? And how many will go unnoticed until it's too late?

As we grapple with that final question, we must face a harsh reality: Despite our best efforts, a catastrophic impact could be in Earth's future. Next, we'll consider a few civil defense strategies that might be necessary if an asteroid comes knocking.

So, the tether on your tether-and-ballast system got tangled. The gravity tractor wasn't built Ford-tough. What do you do now about that killer asteroid barreling toward Earth? Well, if you tried one of the mitigation strategies just mentioned, the asteroid is most likely (a) big and (b) far away. That gives you some time to prepare for impact, although you won't have any historical precedent to provide best practices.

In fact, many astronomers point to fictional accounts -- "On the Beach" by Nevil Shute, for example -- as the best source material about what we might do and how we might fare in a true global cataclysm. Clearly, astronomers would try to pinpoint where the asteroid would hit so ground-zero areas could be evacuated, and governments would try to build underground bunkers, store food and water, collect animal and plant species, and shore up the global financial, electronic, social and law-enforcement infrastructures. The impact of a smaller asteroid -- say, one about 984 feet (300 meters) wide -- could devastate a region the size of small nation.But a rock bigger than 0.621 miles (1 kilometer) wide would affect the whole world. A rock larger than 1.86 miles (3 kilometers) would end civilization [source: Chapman].

Tsunamis, firestorms and earthquakes might cause additional damage. Either way -- impact in the ocean or land -- public officials might only have days or hours to evacuate heavily populated areas. Millions of lives would likely be lost.

Given these scenarios, you can see why governments around the world are so interested in keeping asteroids far from our biosphere. You can also see why dollars don't always drive decisions -- because the cost of failure far exceeds the cost of even the most elaborate deflection concept.

Even a small, 300-meter asteroid means trouble. If it struck the ocean, an epic tsunami at least 32 feet (10 meters) high would wash over coastal areas, with follow-up waves adding to the misery. The December 2004 tsunami in Southeast Asia might serve as an example, although an asteroid-induced tidal wave might behave quite unexpectedly.

If the rock struck land, it would dig out a crater 1.86 to 2.49 miles (3 to 4 kilometers) across and deeper than the Grand Canyon. Everything within a 31-mile (50-kilometer) radius of the blast would be destroyed [source: Chapman].

Could an Earth sized object orbit Jupiter?

Some moons of Jupiter are quite large. Is there a limit to how big of a moon could naturally form around a planet? Could Jupiter have an Earth sized moon in a stable orbit?

I know our moon is quite high ratio wise compared to us, leading to the preferred theory it was the result of an already existing large body colliding with a pre-Earth body.

Regarding orbital mechanics, nothing prevents a large moon from orbiting a planet. It could even be equal in mass and size to the planet itself and the orbit could still be stable. Double planets have been theorized but are yet to be discovered.

Regarding co-accretion, it happens with double stars, so nothing prevents it from being possible with planets.

The giant impact hypothesis about the origin of the Moon is not only due to the size, but also to the similarity in composition and to the lack of an iron core. That suggests the Moon was formed from material ejected from the proto-Earth's surface during the impact.

Isn't Pluto and it's moon a double planet?

Would it be considered a double planet though? I thought a planet needed to have cleared so much of it's orbit?

Could Jupiter have an Earth sized moon in a stable orbit?

Truth be told, it's not like a moon orbits the planet. It's more like both the moon and the planet orbit their common center of mass.

When you look at it that way, the relative size of these two objects can be anything. They could even be equal in mass. If they are both beyond their Roche limits, then the situation should be long-term stable.

tldr: There's no reason I can think of why Jupiter couldn't have an Earth-sized moon.

In principle, I don't see a reason why a Jupiter-sized planet couldn't have an Earth-sized moon. The only limitation I can think of that would prevent an Earth-sized moon to orbit around Jupiter is the Roche Limit. The Roche Limit describes the distance at which an object would be ripped apart from the gravitational tidal forces exerted on it by the larger object. It's essentially the same principle as how our moon exerts tides on the ocean, but on a much larger scale.

If we assume that the Earth is a sphere with constant density (it isn't, but let's do this for easy calculations), we can use the equation you can find on the Wikipedia page to calculate the limit. Plugging in for the mass of jupiter (1.9x10 27 kg), the mass of the Earth (6.0x10 24 kg), and the radius of Earth (6378 km), we get that the Roche limit for the Jupiter/Earth system is about 5500 55000 km away from Jupiter's center of mass.

Since Jupiter's radius (69900 km) is much larger than the Roche limit, then there should be no problem with an Earth-sized satellite orbiting Jupiter at any arbitrary distance, as long as it wasn't actually touching Jupiter.