# Estimating close approach between all asteroids themselves

I am currently writing a C++ program to show asteroids in 3D, and find close approaches or collisions. I got my orbital elements from JPL https://ssd.jpl.nasa.gov/sbdb_query.cgi. So far so good, with over a million asteroids drawn in 5ms with my old Quadro GPU

But the problem is the variation of the elements over time. For example, CERES:

epoch,a,e,i,om,w,ma 2459200.5,2.766089105818,.07816842657453,10.58789954719,80.27235841368,73.72488984426,205.5454154582 2459310.5,2.765760313090,.07831877879848,10.58807660401,80.26860808947,73.73699886586,229.1146825391

After only 4 months, the eccentricity and all other elements have changed as seen on the second line.

How can I compute the change in the parameters without knowing their derivatives?

Or where to get those derivatives?

I searched all JPL small bodies site and their telnet access or email, but could not find a way to download orbital elements with derivatives or a way to compute the change.

I am aware that the difference might be just a pixel on the screen but it represents hundreds of thousands of kilometres. What I want to do is study the close approach between the asteroids themselves and eventually near collision. I already did that for artificial satellites using SGP4 propagator using OpenCL running on GPU video card. I can propagate 20,000 satellites (include debris) in a few milliseconds and get results exact to 1 kilometre. Compare with Celestrak Socrate. A prediction of a 1,000-kilometre approach must be possible.

Does anybody know how?

This is a common problem with ephemeris or TLEs. They change over time and one might want to know the values after the times that are given.

Most folks use an interpolation scheme to estimate values at a certain time. I would suggest starting with linear interpolation. If this doesn't give you good enough fidelity, you could move on to a more complicated interpolation scheme. Industry often uses a cubic spline.

For your example, using linear interpolation for the Ceres data, the epoch values are $$t_1 = 2459200.5$$ and $$t_2 = 2459310.5$$ (in days) with semi-major axes $$a_1 = 2.766089105818$$ and $$a_2 = 2.765760313090$$ (in AU). The linear interpolation estimate of the change in semi-major axis in AUs per day is $$dfrac{da}{dt} approx dfrac{a_2-a_1}{t_2-t_1} approx -2.98902 imes 10^{-6}$$. If you want to know the estimate for the semi-major axis at $$t_2+25$$ days, the answer is $$a_2+25dfrac{da}{dt} = 2.76568558759$$.

If you have the elements for several times, you can compare the estimates for pairs of times. If you get similar results, then a linear interpolation will be fairly accurate.

Thanks folks for helping. I knew about SPK files but from Horizon's telnet inteface there is a limit of 200 bodies per request. Making over 5000 request might be possible but not very productive. I also looked at the DE421 file from ftp://ssd.jpl.nasa.gov/pub/eph/planets/bsp/ but it looks like the corrections for the 8+1 planets

here is the output from BRIEF, a SPICE utility: https://naif.jpl.nasa.gov/naif/utilities.html

BRIEF -- Version 4.0.0, September 8, 2010 -- Toolkit Version N0066 Summary for: de421.bsp Bodies: MERCURY BARYCENTER (1) SATURN BARYCENTER (6) MERCURY (199) VENUS BARYCENTER (2) URANUS BARYCENTER (7) VENUS (299) EARTH BARYCENTER (3) NEPTUNE BARYCENTER (8) MOON (301) MARS BARYCENTER (4) PLUTO BARYCENTER (9) EARTH (399) JUPITER BARYCENTER (5) SUN (10) MARS (499) Start of Interval (ET) End of Interval (ET) ----------------------------- ----------------------------- 1899 JUL 29 00:00:00.000 2053 OCT 09 00:00:00.000

I can always do a statistical estimate, any close approach being false but the overall count being close to reality. Too bad to drop the project, I was getting nices pictures from it. The coloring is done against eccentricity, red=0

## Different Types of Asteroids (C, S, and M) – The Definitive Guide

Many people think of asteroids as being small chunks of rock that drift aimlessly and randomly through space. The reality, of course, is a little different, but it wasn’t until relatively recently that we began to discover more about them. For example, we now have a better idea of their origins and their composition.

Asteroids are the leftovers from the formation of the solar system. The planets came together about 4.6 billion years ago, with the rocky planets forming closest to the Sun. The asteroid belt, between Mars and Jupiter, is the remains of a planet that failed to form, most likely due to the gravitational interference of Jupiter. Most asteroids can be found there, although some can also be found in other locales within the solar system.

The largest asteroid, 1 Ceres, as imaged by NASA’s Dawn spacecraft. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The first asteroid was discovered on January 1st, 1801. Officially known as 1 Ceres, it has a diameter of 974km and is the largest known asteroid. (The smallest known asteroid is only six feet across, but others could be smaller.)

As of August 2020, there are a little over 990,000 known asteroids. In theory, anyone can discover an asteroid, with the discoverer having the honor of being able to name it after almost anyone they desire. Traditionally they were named for gods and goddesses (like the planets themselves) but nowadays they could be named for scientists, politicians, writers, actors or musicians.

We know asteroids are rocky in nature, but not all rock is the same. Bouncing radar signals off asteroids as they pass by the Earth can provide clues to their composition and, therefore, their type.

There are three main groups, based upon the composition of the asteroid:

1. C – Chondrite, clay and silicate rocks
2. S – Stony, silicate materials and nickel-iron
3. M – Metallic, nickel-iron

While other types exist, these three account for over 90% of the known asteroids in our solar system.

## NEO Basics

The comets and asteroids that are potentially the most hazardous because they can closely approach the Earth are also the objects that could be most easily exploited for their raw materials. It is not presently cost effective to mine these minerals and then bring them back to Earth. However, these raw materials could be used in developing the space structures and in generating the rocket fuel that will be required to explore and colonize our solar system in the twenty-first century.

Whereas asteroids are rich in the mineral raw materials required to build structures in space, the comets are rich resources for the water and carbon-based molecules necessary to sustain life. In addition, an abundant supply of cometary water ice could provide copious quantities of liquid hydrogen and oxygen, the two primary ingredients in rocket fuel. It seems likely that in the next century when we begin to colonize the inner solar system, the metals and minerals found on asteroids will provide the raw materials for space structures and comets will become the watering holes and gas stations for interplanetary spacecraft.

Per Wikipedia, the total mass of the asteroid belt is 3-4% that of the moon. That's spread over a ring around the solar system larger than Mars' orbit. Average density, therefore, isn't much different from zero, especially as most of that mass is concentrated in a handful of large asteroids.

Why is it drawn the wrong way? It looks cooler. Why isn't it corrected? It frequently is. Arthur C. Clarke's 2001 mentions it in passing, which is the first time I recall reading it. Even TV Tropes points it out in its discussion of asteroids in fiction. Those pictures still get drawn because of the Rule of Cool.

#### Attachments

Here's another one from Asteroid Day Live 2020. Populated by many (supposedly) prominent astronomers .

Surely the Astronomers themselves should start by setting an example.

#### Attachments

NASA frequently captions pictures as "artist interpretation". Not only would primary source data confuse most readers, human eyes cannot even see many of the wavelengths used for imaging such as infrared (IR) and radio frequencies (RF).

Spacecraft likely use wide radar to detect asteroids and narrow lidar or maser to identify details none of which safely register on human senses. Artists work with engineers to create pictures that untrained humans can comprehend.

Note that non-radiating solar system objects remain invisible to the eye until illuminated by the sun or by a transmitter or from occluding background light sources. The entire field of radio astronomy requires interpretation and data analysis to portray images.

Yes, and a nuance to that (aside from your point about astrophotos) sometimes they are meant to be accurate, sometimes they are pure art, and sometimes a mixture of the two. For the one above, the spacecraft is accurate and the background is art.

What annoys me is corporate press release graphics of (for example) a new plane that looks totally unrealistic/unlikely to even work.

”Just how crowded is the asteroid belt?
Let's do the math!

There are about 3500 - 4000 or so cataloged asteroids with diameters over 1 kilometer. The asteroid belt is a band between 2 - 3.3 Astronomical Units wide, give or take, but the asteroids are bunched up into families so are not uniformly spread out in this vast volume some 100 million miles wide and perhaps another 20 million miles thick.

The volume of this space is about 4 million trillion cubic miles. The average distance between the asteroids would be about 100,000 miles. But there are likely to be lots of smaller objects too. Both the Voyager 1 and 2 spacecraft passed through the asteroid belt. They were not hit by anything except interplanetary dust which is very common in the asteroid belt, and that is your biggest problem to worry about.”

## Preparing for 2029

Making the most of the 2029 flyby will rely on baseline data: what scientists know about Apophis before its dramatic encounter with Earth. That means the observations gathered this year matter. Apophis will be at its closest to Earth this year on March 5 at 8:15 p.m. EST (0115 GMT on March 6).

"Closest" here is a relative term: the asteroid will remain a healthy 0.11 astronomical units (the average distance between the Earth and the sun, or about 93 million miles or 150 million km). That's nearly 44 times the distance between Earth and the moon.

But that's close enough for scientists' most powerful tool for studying asteroids from Earth: planetary radar. Take a powerful radar beam, point it at a mysterious object, then wait. Use a sensitive radio telescope to catch the echo that bounces back, run it through some complicated processing, and the result is a sonogram-like image.

"We like asteroids that come close but, you know, just enough so that we can get a really good signal and we can get really great images," Brozović said.

With good radar images, scientists can tell, for example what shape an asteroid is: potato, peanut, or even a pair of cherries bound only by gravity. Under particularly friendly circumstances, radar can detect boulders on the surface of a space rock. It also hones scientists' ability to track an asteroid's orbit.

Scientists' top priority while preparing for the 2029 Apophis flyby is sharpening their view of the rock's shape and its intricate rotations, Binzel said. "We know Apophis is in a very complicated spin state, it's sort of spinning and tumbling at the same time," he said. "The 2021 encounter gives us an epoch in time."

When scientists look to make predictions about what precisely will happen to Apophis during the 2029 encounter, they'll feed the current best wisdom of the object's shape and twisted rotation into models &mdash but the resulting predictions will only be as robust as the data.

Inconveniently, Earth lost its most powerful planetary radar system in December, when Arecibo Observatory's radio telescope in Puerto Rico collapsed. Each radar system has its strengths and weaknesses, and Arecibo would have shone during this preparatory close approach. Without it, scientists aren't sure how much they'll be able to improve existing radar observations of Apophis.

But they'll try, thanks to the planetary radar system at NASA's Goldstone Deep Space Communications Complex in California, which is due to study Apophis from March 3 to March 14 to cover this flyby. Researchers also hope to use the Green Bank Telescope in West Virginia to catch the echos, rather than having to switch Goldstone's settings back and forth between send and receive if they can use two telescopes, the data will be sharper.

"Arecibo was really a powerhouse, the most powerful radar on the planet, so we just can't make that up," Brozović said. "But we're still going to get good data."

## Asteroid tracker news: SEVEN space rocks to pass Earth this week - latest charts

### European Space Agency share asteroid 2020 OM3 graphic

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Astronomy experts have collected a wide range of all the asteroids about to make what are known as Earth close approaches. And the data shows that this week, a total of seven 'Near Earth Objects' will skim past our planet - the largest of which being up to 88-metres in size. Express.co.uk has compiled all the data for this week.

### 2020 RV2

The first of seven asteroids to enter Earth&rsquos neighbour this week is named 2020 RV2.

NASA has confirmed the space rock will be at its closest tonight, at 10.56pm PT (6.56am October 6).

At this time the asteroid will be 0.03790 Astronomical Units (AU), or 3,523,025 miles.

Astronomical units are the approximate distance from Earth to the Sun and equal to roughly 92,955,807 miles (150 million km) or 8 light-minutes.

Asteroid news: NASA has confirmed seven space rocks will safely pass Earth this week (Image: Getty)

Asteroid tracker news: A surprising number of space rocks will safely blast past the planet this week (Image: Express)

Many may think this is a far too great a distance to be considered a &lsquoclose approach&rsquo.

However, this is a technically correct term, especially when considering the incomprehensible distances of infinite space.

The space rock, which could measure between 19 and 44m-wide, is travelling at an incredible 4.16 km/s (9,305 mph).

Asteroid tracker news: (Image: Express)

### 2020 RR2

This space rock measures between 21 and 47 metres, and is barreling through space at speeds of 4.08km/s.

The asteroid will most its closest approach to Earth at 2.55pm on October 6.

### 2020 RK2

Asteroid 2020 RK2 will make a close approach at 1.12pm on October 7.

Measuring up to 83-metres, this space rock is one of the faster ones to pass Earth this week, as it is currently travelling at speeds of 6.68 km/s.

### 2019 SB6

This asteroid will pass Earth at its closest point at 23.23pm on October 7 - just seven hours after 2020 RK2.

### 2020 SR6

Travelling at speeds of 5.26 km/s asteroid 2020 SR6 measures in at between 20m and 44m.

### 2020 SX3

The fastest of all the asteroids passing Earth this week is travelling more than twice as fast.

Dubbed 2020 SX3 by astronomers, this space rock is barrelling through space at 10.88 km/s (24,337mph).

And coincidently, with a maximum diameter of 88m, this is now known to be the largest space rock to pass us in the coming days.

Asteroid tracker news: (Image: Express)

To put this into perspective, were this building-sized beast to collide with our planet, experts estimate it potentially have an amount of energy equal to the largest nuclear bombs ever made - as much as 50 megatons.

An asteroid on such as scale would flatten reinforced concrete buildings 5 miles from ground zero.

This would consequently totally annihilate every major cities in the world.

Fortunately, neither this one or any other asteroid is known to be on a collision course with this blue planet.

The closest of this week&rsquos near-earth approaches is also being made by 2020 SX3.

This space rock has been confirmed to be coming within 0.01135 AU, or a mere 1,055,048 miles.

### 2020 RO1

The final asteroid of the week is 2020 RO1, which is the slowest of this week's encounters at 3.21 km/s.

The rock is a pretty decent size, however, measuring 22m to 50m.

The asteroid will make its close approach at 4.14pm on October 9.

All the data is publicly available on NASA&rsquos Close Approach Data table.

Some people may be surprised to learn Earth is actually bombarded by more than 100 tons of cosmic dust and sand-sized space particles.

A car-sized asteroid can collide with Earth's atmosphere annually, creating an eye-catching fireball as it safely incinerates before reaching the surface.

Then approximately every 2,000 years, an asteroid the size of a football pitch hits Earth, creating significant damage to a wide area.

Then roughly every few million years, an object large enough to threaten all life on Earth arrives.

## Three Thanksgiving Asteroids to Zoom Past Earth This Week, Ahead of Lunar Eclipse

Three asteroids are set to sail past the Earth around Thanksgiving, ahead of a lunar eclipse early next week.

On November 25, a space rock dubbed 2020 VK6 will make its closest approach to Earth at 4:05 p.m., coming within 3.1 million miles of Earth, according to NASA's Center for Near Earth Studies (CNEOS).

This is equivalent to around 13 times the average distance between the Earth and the moon, CNEOS data shows.

Astronomers estimate that the asteroid, which is travelling at around 25,000 miles per hour, measures up to 157 feet in diameter.

Another two asteroids will make their closest approach to the Earth late on November 25 and on Thanksgiving day itself.

Known as 2018 RQ4 and 2020 VO2, both of these space rocks are similar in size&mdashup to 80-90 feet in width.

The latter will pass the Earth at a similar distance to 2020 VK6. But 2018 RQ4 could come as close as about 265,000 miles&mdashonly slightly greater than the average distance between our planet and the moon.

This is the asteroid's "minimum possible close-approach distance," however, with the expected distance calculated to be 1.9 million miles.

These asteroids will fly past the Earth just a few days ahead of another notable astronomical event: a full moon featuring what's known as a "penumbral lunar eclipse."

These events occur when the moon passes through the more diffused part of the Earth's shadow&mdashcalled the penumbra.

During these eclipses, parts of the moon appear slightly darker than usual&mdashalthough the effect is often very subtle, and sometimes imperceptible, to the untrained eye.

"A penumbral lunar eclipse happens when our moon is 'kissed' only marginally by the most external regions of the structure of the Earth's shadow," astronomer Gianluca Masi, from the Virtual Telescope Project, previously told Newsweek.

The upcoming penumbral eclipse will begin at 02:32 a.m. ET on November 30 and will end four hours and 21 minutes later. Maximum eclipse will be at 09:42 a.m. ET, at which point around 82 percent of the moon will appear darker than usual.

The eclipse will be visible across most of North America, the Pacific Ocean and northeastern Asia, according to timeandate.com.

Lunar eclipses can only happen during full moons. These occur when the Earth is located directly behind the sun and the moon, and all three bodies are lined up in the same plane. In these moments, which occur roughly once every month, the moon appears fully illuminated, like a perfect circle.

## Earth-Approaching Asteroids

Asteroids that stray far outside the main belt are of interest mostly to astronomers. But asteroids that come inward, especially those with orbits that come close to or cross the orbit of Earth, are of interest to political leaders, military planners&mdashindeed, everyone alive on Earth. Some of these asteroids briefly become the closest celestial object to us.

In 1994, a 1-kilometer object was picked up passing closer than the Moon, causing a stir of interest in the news media. Today, it is routine to read of small asteroids coming this close to Earth. (They were always there, but only in recent years have astronomers been able to detect such faint objects.)

In 2013, a small asteroid hit our planet, streaking across the sky over the Russian city of Chelyabinsk and exploding with the energy of a nuclear bomb (Figure (PageIndex<1>)). The impactor was a stony object about 20 meters in diameter, exploding about 30 kilometers high with an energy of 500 kilotons (about 30 times larger than the nuclear bombs dropped on Japan in World War II). No one was hurt by the blast itself, although it briefly became as bright as the Sun, drawing many spectators to the windows in their offices and homes. When the blast wave from the explosion then reached the town, it blew out the windows. About 1500 people had to seek medical attention from injuries from the shattered glass.

A much larger atmospheric explosion took place in Russia in 1908, caused by an asteroid about 40 meters in diameter, releasing an energy of 5 megatons, as large the most powerful nuclear weapons of today. Fortunately, the area directly affected, on the Tunguska River in Siberia, was unpopulated, and no one was killed. However, the area of forest destroyed by the blast was large equal to the size of a major city (Figure (PageIndex<1>)).

Together with any comets that come close to our planet, such asteroids are known collectively as near-Earth objects (NEOs). As we will see (and as the dinosaurs found out 65 million years ago,) the collision of a significant-sized NEO could be a catastrophe for life on our planet.

Figure (PageIndex<1>) Impacts with Earth. (a) As the Chelyabinsk meteor passed through the atmosphere, it left a trail of smoke and briefly became as bright as the Sun. (b) Hundreds of kilometers of forest trees were knocked down and burned at the Tunguska impact site.

Visit the video compilation of the Chelyabinsk meteor streaking through the sky over the city on February 15, 2013, as taken by people who were in the area when it occurred.

View this video of a non-technical talk by David Morrison to watch &ldquoThe Chelyabinsk Meteor: Can We Survive a Bigger Impact?&rdquo Dr. Morrison (SETI Institute and NASA Ames Research Center) discusses the Chelyabinsk impact and how we learn about NEOs and protect ourselves the talk is from the Silicon Valley Astronomy Lectures series.

Astronomers have urged that the first step in protecting Earth from future impacts by NEOs must be to learn what potential impactors are out there. In 1998, NASA began the Spaceguard Survey, with the goal to discover and track 90% of Earth-approaching asteroids greater than 1 kilometer in diameter. The size of 1 kilometer was selected to include all asteroids capable of causing global damage, not merely local or regional effects. At 1 kilometer or larger, the impact could blast so much dust into the atmosphere that the sunlight would be dimmed for months, causing global crop failures&mdashan event that could threaten the survival of our civilization. The Spaceguard goal of 90% was reached in 2012 when nearly a thousand of these 1-kilometer near-Earth asteroids (NEAs) had been found, along with more than 10,000 smaller asteroids. Figure (PageIndex<2>) shows how the pace of NEA discoveries has been increasing over recent years.

Figure (PageIndex<2>) Discovery of Near-Earth Asteroids. The accelerating rate of discovery of NEAs is illustrated in this graph, which shows the total number of known NEAs, the number over 140 meters in diameter, and the number over 1 kilometer in diameter, the size that poses the dominant impact risk on Earth.

How did astronomers know when they had discovered 90% of these asteroids? There are several ways to estimate the total number, even before they were individually located. One way is to look at the numbers of large craters on the dark lunar maria. Remember that these craters were made by impacts just like the ones we are considering. They are preserved on the Moon&rsquos airless surface, whereas Earth soon erases the imprints of past impacts. Thus, the number of large craters on the Moon allows us to estimate how often impacts have occurred on both the Moon and Earth over the past several billion years. The number of impacts is directly related to the number of asteroids and comets on Earth-crossing orbits.

Another approach is to see how often the surveys (which are automated searches for faint points of light that move among the stars) rediscover a previously known asteroid. At the beginning of a survey, all the NEAs it finds will be new. But as the survey becomes more complete, more and more of the moving points the survey cameras record will be rediscoveries. The more rediscoveries each survey experiences, the more complete our inventory of these asteroids must be.

We have been relieved to find that none of the NEAs discovered so far is on a trajectory that will impact Earth within the foreseeable future. However, we can&rsquot speak for the handful of asteroids larger than 1 kilometer that have not yet been found, or for the much more numerous smaller ones. It is estimated that there are a million NEAs capable of hitting Earth that are smaller than 1 kilometer but still large enough to destroy a city, and our surveys have found fewer than 10% of them. Researchers who work with asteroid orbits estimate that for smaller (and therefore fainter) asteroids we are not yet tracking, we will have about a 5-second warning that one is going to hit Earth&mdashin other words, we won&rsquot see it until it enters the atmosphere. Clearly, this estimate gives us a lot of motivation to continue these surveys to track as many asteroids as possible.

Though entirely predictable over times of a few centuries, the orbits of Earth-approaching asteroids are unstable over long time spans as they are tugged by the gravitational attractions of the planets. These objects will eventually meet one of two fates: either they will impact one of the terrestrial planets or the Sun, or they will be ejected gravitationally from the inner solar system due to a near-encounter with a planet. The probabilities of these two outcomes are about the same. The timescale for impact or ejection is only about a hundred million years, very short compared with the 4-billion-year age of the solar system. Calculations show that only approximately one quarter of the current Earth-approaching asteroids will eventually end up colliding with Earth itself.

If most of the current population of Earth-approaching asteroids will be removed by impact or ejection in a hundred million years, there must be a continuing source of new objects to replenish our supply of NEAs. Most of them come from the asteroid belt between Mars and Jupiter, where collisions between asteroids can eject fragments into Earth-crossing orbits (see Figure (PageIndex<3>)). Others may be &ldquodead&rdquo comets that have exhausted their volatile materials (which we&rsquoll discuss in the next section).

Figure (PageIndex<3>) Near-Earth Asteroid. Toutatis is a 5-kilometer long NEA that approached within 3 million kilometers of Earth in 1992. This series of images is a reconstruction its size and shape obtained from bouncing radar waves off the asteroid during its close flyby. Toutatis appears to consist of two irregular, lumpy bodies rotating in contact with each other. (Note that the color has been artificially added.)

One reason scientists are interested in the composition and interior structure of NEAs is that humans will probably need to defend themselves against an asteroid impact someday. If we ever found one of these asteroids on a collision course with us, we would need to deflect it so it would miss Earth. The most straightforward way to deflect it would be to crash a spacecraft into it, either slowing it or speeding it up, slightly changing its orbital period. If this were done several years before the predicted collision, the asteroid would miss the planet entirely&mdashmaking an asteroid impact the only natural hazard that we could eliminate completely by the application of technology. Alternatively, such deflection could be done by exploding a nuclear bomb near the asteroid to nudge it off course.

To achieve a successful deflection by either technique, we need to know more about the density and interior structure of the asteroid. A spacecraft impact or a nearby explosion would have a greater effect on a solid rocky asteroid such as Eros than on a loose rubble pile. Think of climbing a sand dune compared to climbing a rocky hill with the same slope. On the dune, much of our energy is absorbed in the slipping sand, so the climb is much more difficult and takes more energy.

There is increasing international interest in the problem of asteroid impacts. The United Nations has formed two technical committees on planetary defense, recognizing that the entire planet is at risk from asteroid impacts. However, the fundamental problem remains one of finding NEAs in time for defensive measures to be taken. We must be able to find the next impactor before it finds us. And that&rsquos a job for the astronomers.

Asteroids that stray far outside the main belt are of interest mostly to astronomers. But asteroids that come inward, especially those with orbits that come close to or cross the orbit of Earth, are of interest to political leaders, military planners—indeed, everyone alive on Earth. Some of these asteroids briefly become the closest celestial object to us.

In 1994, a 1-kilometer object was picked up passing closer than the Moon, causing a stir of interest in the news media. Today, it is routine to read of small asteroids coming this close to Earth. (They were always there, but only in recent years have astronomers been able to detect such faint objects.)

In 2013, a small asteroid hit our planet, streaking across the sky over the Russian city of Chelyabinsk and exploding with the energy of a nuclear bomb (Figure 1a). The impactor was a stony object about 20 meters in diameter, exploding about 30 kilometers high with an energy of 500 kilotons (about 30 times larger than the nuclear bombs dropped on Japan in World War II). No one was hurt by the blast itself, although it briefly became as bright as the Sun, drawing many spectators to the windows in their offices and homes. When the blast wave from the explosion then reached the town, it blew out the windows. About 1500 people had to seek medical attention from injuries from the shattered glass.

A much larger atmospheric explosion took place in Russia in 1908, caused by an asteroid about 40 meters in diameter, releasing an energy of 5 megatons, as large the most powerful nuclear weapons of today. Fortunately, the area directly affected, on the Tunguska River in Siberia, was unpopulated, and no one was killed. However, the area of forest destroyed by the blast was large equal to the size of a major city (Figure 1b).

Together with any comets that come close to our planet, such asteroids are known collectively as near-Earth objects (NEOs). As we will see (and as the dinosaurs found out 65 million years ago,) the collision of a significant-sized NEO could be a catastrophe for life on our planet.

Figure 1: Impacts with Earth. (a) As the Chelyabinsk meteor passed through the atmosphere, it left a trail of smoke and briefly became as bright as the Sun. (b) Hundreds of kilometers of forest trees were knocked down and burned at the Tunguska impact site. (credit a: modification of work by Alex Alishevskikh)

Watch this video compilation of the Chelyabinsk meteor streaking through the sky over the city on February 15, 2013, as taken by people who were in the area when it occurred.

View this video of a non-technical talk by David Morrison to watch “The Chelyabinsk Meteor: Can We Survive a Bigger Impact?” Dr. Morrison (SETI Institute and NASA Ames Research Center) discusses the Chelyabinsk impact and how we learn about NEOs and protect ourselves the talk is from the Silicon Valley Astronomy Lectures series.

Astronomers have urged that the first step in protecting Earth from future impacts by NEOs must be to learn what potential impactors are out there. In 1998, NASA began the Spaceguard Survey, with the goal to discover and track 90% of Earth-approaching asteroids greater than 1 kilometer in diameter. The size of 1 kilometer was selected to include all asteroids capable of causing global damage, not merely local or regional effects. At 1 kilometer or larger, the impact could blast so much dust into the atmosphere that the sunlight would be dimmed for months, causing global crop failures—an event that could threaten the survival of our civilization. The Spaceguard goal of 90% was reached in 2012 when nearly a thousand of these 1-kilometer near-Earth asteroids (NEAs) had been found, along with more than 10,000 smaller asteroids. Figure 2 shows how the pace of NEA discoveries has been increasing over recent years.

Figure 2: Discovery of Near-Earth Asteroids. The accelerating rate of discovery of NEAs is illustrated in this graph, which shows the total number of known NEAs, the number over 140 kilometers in diameter, and the number over 1 kilometer in diameter, the size that poses the dominant impact risk on Earth.

How did astronomers know when they had discovered 90% of these asteroids? There are several ways to estimate the total number, even before they were individually located. One way is to look at the numbers of large craters on the dark lunar maria. Remember that these craters were made by impacts just like the ones we are considering. They are preserved on the Moon’s airless surface, whereas Earth soon erases the imprints of past impacts. Thus, the number of large craters on the Moon allows us to estimate how often impacts have occurred on both the Moon and Earth over the past several billion years. The number of impacts is directly related to the number of asteroids and comets on Earth-crossing orbits.

Another approach is to see how often the surveys (which are automated searches for faint points of light that move among the stars) rediscover a previously known asteroid. At the beginning of a survey, all the NEAs it finds will be new. But as the survey becomes more complete, more and more of the moving points the survey cameras record will be rediscoveries. The more rediscoveries each survey experiences, the more complete our inventory of these asteroids must be.

We have been relieved to find that none of the NEAs discovered so far is on a trajectory that will impact Earth within the foreseeable future. However, we can’t speak for the handful of asteroids larger than 1 kilometer that have not yet been found, or for the much more numerous smaller ones. It is estimated that there are a million NEAs capable of hitting Earth that are smaller than 1 kilometer but still large enough to destroy a city, and our surveys have found fewer than 10% of them. Researchers who work with asteroid orbits estimate that for smaller (and therefore fainter) asteroids we are not yet tracking, we will have about a 5-second warning that one is going to hit Earth—in other words, we won’t see it until it enters the atmosphere. Clearly, this estimate gives us a lot of motivation to continue these surveys to track as many asteroids as possible.

Though entirely predictable over times of a few centuries, the orbits of Earth-approaching asteroids are unstable over long time spans as they are tugged by the gravitational attractions of the planets. These objects will eventually meet one of two fates: either they will impact one of the terrestrial planets or the Sun, or they will be ejected gravitationally from the inner solar system due to a near-encounter with a planet. The probabilities of these two outcomes are about the same. The timescale for impact or ejection is only about a hundred million years, very short compared with the 4-billion-year age of the solar system. Calculations show that only approximately one quarter of the current Earth-approaching asteroids will eventually end up colliding with Earth itself.

If most of the current population of Earth-approaching asteroids will be removed by impact or ejection in a hundred million years, there must be a continuing source of new objects to replenish our supply of NEAs. Most of them come from the asteroid belt between Mars and Jupiter, where collisions between asteroids can eject fragments into Earth-crossing orbits (see Figure 3). Others may be “dead” comets that have exhausted their volatile materials (which we’ll discuss in the next section).

Figure 3: Near-Earth Asteroid. Toutatis is a 5-kilometer long NEA that approached within 3 million kilometers of Earth in 1992. This series of images is a reconstruction its size and shape obtained from bouncing radar waves off the asteroid during its close flyby. Toutatis appears to consist of two irregular, lumpy bodies rotating in contact with each other. (Note that the color has been artificially added.) (credit: modification of work by NASA)

One reason scientists are interested in the composition and interior structure of NEAs is that humans will probably need to defend themselves against an asteroid impact someday. If we ever found one of these asteroids on a collision course with us, we would need to deflect it so it would miss Earth. The most straightforward way to deflect it would be to crash a spacecraft into it, either slowing it or speeding it up, slightly changing its orbital period. If this were done several years before the predicted collision, the asteroid would miss the planet entirely—making an asteroid impact the only natural hazard that we could eliminate completely by the application of technology. Alternatively, such deflection could be done by exploding a nuclear bomb near the asteroid to nudge it off course.

To achieve a successful deflection by either technique, we need to know more about the density and interior structure of the asteroid. A spacecraft impact or a nearby explosion would have a greater effect on a solid rocky asteroid such as Eros than on a loose rubble pile. Think of climbing a sand dune compared to climbing a rocky hill with the same slope. On the dune, much of our energy is absorbed in the slipping sand, so the climb is much more difficult and takes more energy.

There is increasing international interest in the problem of asteroid impacts. The United Nations has formed two technical committees on planetary defense, recognizing that the entire planet is at risk from asteroid impacts. However, the fundamental problem remains one of finding NEAs in time for defensive measures to be taken. We must be able to find the next impactor before it finds us. And that’s a job for the astronomers.

### Key Concepts and Summary

Near-Earth asteroids (NEAs), and near-Earth objects (NEOs) in general, are of interest in part because of their potential to hit Earth. They are on unstable orbits, and on timescales of 100 million years, they will either impact one of the terrestrial planets or the Sun, or be ejected. Most of them probably come from the asteroid belt, but some may be dead comets. NASA’s Spaceguard Survey has found 90% of the NEAs larger than 1 kilometer, and none of the ones found so far are on a collision course with Earth. Scientists are actively working on possible technologies for planetary defense in case any NEOs are found on a collision course with Earth years in advance. For now, the most important task is to continue our surveys, so we can find the next Earth impactor before it finds us.

## Ep. 29: Asteroids Make Bad Neighbors

This week we’re talking about asteroids. And not just any asteroids, but Near Earth Objects. How do astronomers find these things, why are they buzzing around the Earth, what are the chances we’ll actually get hit, and what would happen if we did get hit? How could we stop them?

### Shownotes

Near Earth Objects

Dealing with the Threat

Dinosaur Killer?

Also Mentioned:

• Phil “the Bad Astronomer” Plait’s blog and video podcast, Q & BA weekly podcast and daily space news feed. Please note, production of this podcast is on hold until Fraser finishes school this semester, but back episodes are still available.

### Transcript: Asteroids Make Bad Neighbours

Fraser Cain: This week’s topic is near-Earth asteroids. Every year, we hear about a new space rock that may or may not get really close to the planet in the next few decades. How do astronomers find these things? Why are they buzzing around the Earth? What are the chances we’ll get hit? What will happen if we do get hit?

That’s a lot of questions, so you can start anywhere Pamela.

Dr. Pamela Gay: Well, according to NASA, right now there’s absolutely nothing that’s going to hit us. So we’re safe right now (right now).

Fraser: For the duration of this recording?

Pamela: For the duration of this recording and as far as they know, none of the known asteroids, comets, anything out there, is going to hit us. Period.

Fraser: All right, that buys us a little time and gives us a chance to talk about it. We’ll check in again maybe at the end of the recording.

Fraser: Okay, what is a near-Earth asteroid, for starters? I guess the name kind of says itself

Pamela: Well the more general term is “near-Earth object” â€“ we have these NEO’s, and they get broken up into a bunch of different groups: There are the near-Earth comets, these are chunks of ice that decided they’re going to come into near where we are and as they orbit the solar system, they get a little too close and they have the potential to hit us.

Fraser: Do they stay close, or do they zip through and then they’re gone again?

Pamela: These are defined as things with periods less than 200 years that go out to the outer parts of the solar system, come in to the inner parts of the solar system, go back and forth, back and forth.

Now, periods less than 200 years means some of them do actually end up living in the inner part of the solar system. They end up living with orbits that never go out further than Mars or Jupiter. They’ve gotten perturbed by close encounters with Mars, close encounters with Earth. All the different planets’ gravities have changed their orbits until their orbits keep them in the inner part of the solar system right in the way of potentially running into us. So far as we know, none of them are going to do that.

Fraser: I think we talked about a bunch of those comets in our meteorite episode.

Fraser: Because they’re the ones feeding the meteorites into the space dust that we crash through.

Okay. Right, so cometsâ€¦ what else?

Pamela: We also have near-Earth asteroids. These are chunks of rock. Some of them are crustaceous, carbonâ€¦ they’re basically dirt. Others have more metallic cores. They come in a variety of different things that resemble chunks of Earth’s crust that have been placed in space. Again, these are things that are in near the Earth’s orbit, by definition they have orbits smaller than 1 and 1/3rd of the Earth’s orbit and they’re hanging out.

Within in that, we also have all these other smaller orbit sizes, more dangerous things, so there are things called Apollos. These are things that have orbits that are greater on average than one Earth orbit, so you’d think that we’re safe, because they’re greater on average than one Earth orbit. Unfortunately, that means they can go in closer than Mercury and go way far out (well, not way far out, but moderately far out) and cross our orbit in the process. So, the Apollos can hit us as they cross our orbit. It keeps life interesting.

There are also these things called amors. These generally stay interior to Mars and outside of Earth’s orbit, but sometimes they cross. Most of the time they’re past the Earth’s orbit and before Mars’ orbit.

There are also (and I may mis-pronounce this) the Atens. These are Earth-crossing, near-Earth asteroids that have an average orbit size that’s smaller than the Earth’s orbit.

So we classify these strictly based on what does their orbit look like, what does the period of their orbit look like.

Fraser: So I guess with the Atens, it’s the same deal they’re mostly inside our orbit, but they can reach out and bonk into us as well.

Pamela: So you can have something that their average distance from the Sun is less than the Earth’s distance, but if they get close enough on the one side, they can cross the Earth’s orbit on the other side.

Fraser: Now, there are some asteroids that are quite close to our orbit, aren’t there? Like Toutatis, right?

Pamela: There’s this special class called PHA’s â€“ Potentially Hazardous Asteroids. These are objects whose minimum orbit intersection distance (MOD) is 0.05 times the distance from the Earth to the Sun away from the Earth. They’re averaging about 5% of the distance from our Sun, is how close they get to us. They’re not necessarily going to hit us, but they get close enough that we can get really good pictures of some of them.

Fraser: How many of these asteroids are there?

Pamela: Of Near-Earth Objects, there’s roughly 11-1200 of the 1km sized ones â€“ the ones that can completely destroy the planet Earth.

Fraser: The big ones. Right. We’ll talk about that in a bit.
[laughter]

Pamela: If you go to NASA’s Near-Earth Asteroid site (it’s conveniently named neo.jpl.nasa.gov), they actually go through and list, at any given moment, all of the asteroids and comets and things like that, that are going to get a little bit too close for comfort. There’s a couple hundred just sitting there, with their statistics showing you how scarily close they’re going to get, and how remarkably low the probability of them hitting us is.

Fraser: I’ve hard that there’s, as you say, 1200 of the really big ones. But there’s hundreds of thousands of little ones.

Fraser: They’re buzzing around us all the time.

Pamela: The fact is, we’re constantly getting hit by small things. Anytime that you see a meteor going across the sky, that’s a really tiny thing. Anytime you see a bullwood, an explosive burst from a meteorite, that’s something hitting our atmosphere. So we’re constantly hitting things that are floating around in our orbit. Most of these just cause pretty fireworks. Occasionally you get stories of people having something coming crashing through their roof that actually turns out to be a meteor. You find things in the desert, you find things in Antarctica. Most of these things are too small for us to have to worry about, and that’s good because most of them are too small for us to see before they hit our atmosphere.

Fraser: How do astronomers find them?

Pamela: There’s a bunch of different programs that go out and take picture after picture after picture of areas specifically along the ecliptic in the sky. This is the area in the sky that the sun travels through and where we see all the planets. More or less, the majority of the asteroids and comets confine themselves to the ecliptic. There are some exceptions, things get thrown around through gravitational interactions and to find asteroids and comets, they look for things that move in their pictures.

So, you take two pictures and subtract them. The thing that appears in both pictures but in two separate locations, that didn’t subtract out, that’s going to be your fast moving object that’s in the inner-part of the solar system.

Fraser: I know they do this with computers now, but they used to do this manually. Didn’t they have some kind of light-box they could flip back and forth?

Pamela: This is actually what Claude Tombaugh did when he was trying to find Pluto. He’d take multiple images, set up the exact same way on the sky, and put them side by side in a blink box that allowed him to flip back and forth between which of the two he was looking at and our vision allows us to see things… it’s the persistency of vision. If you put a flashlight on a string and spin it, you’ll see a wheel of light. The light’s only in one given point in a moment, so our eye is able to perceive, “this should still be here” and then when we see something that has moved, we can actually see what appears to be motion when it goes back and forth between the two images. Our brain does neat tricks that allow us to perceive the motion of asteroids and planets in blinked pictures.

Fraser: We’re not talking about a bunch of astronomers with their telescopes and cameras and looking at blink boxes, it’s an industrial process these days, right?

Pamela: Nowadays there are multiple telescopes set up around the planet that scan each part of the sky along the ecliptic five times in a given night. They’re constantly going through processing, looking for things and discovering new things on a regular basis and calculating the orbits in rapid fire.

A lot of times you have multiple teams working, where you have one telescope that’s doing these five images a night discovery. “Okay, here’s a new object that I havenâ€™t seen before.” Then you’ll have folks going out on different telescopes following up and trying to do precise astrometry to calculate orbits. One of the telescopes that’s used for that is the 30″ out at McDonald Observatory which I worked on for my dissertation.

These folks are going out and calculating orbits, doing all the follow-up work which, in some ways, is the most important part because once you discover them, yay you know there’s a new object, but that object could be going in any direction. It’s the follow-up work, to see where it’s moved several days, several months or several years later that allows us to figure out the long-term motion of these objects.

Fraser: How does that follow up work happen? That’s where more people get brought in to the process, right?

Pamela: Exactly. So in this case, you’ll have individuals going out to the telescope, taking large field images of the object. The large field allows you to get more stars involved in “okay, so, my moving object appears to be three arc-minutes off this objects, five arc-minutes off this object..” with all these different comparisons between multiple objects you can get very precise locations.

You do this multiple times and you can see how the object is moving across the sky. You can then build models of “I know that the Earth was here relative to the solar system on this day and the object appeared here. I know on this later day the Earth was here in the solar system, the object appeared here.” How does everything have to be moving to make sense out of that?

Using, again, complicated computer software, they make very sophisticated orbital calculations that in the long term allow them to take into account “this object’s going to be influenced by Mars, this object’s going to be influenced by Jupiter” and figure out orbit after orbit after orbit. What is the risk of the object hitting the Earth?

Fraser: They’re not looking at “will this rock hit us â€“ is it on a collision course with us today?” They’re able to calculate it out for orbits and orbits and orbits into the future.

Pamela: For hundreds of years. That’s one of the really neat things about this. We can look to the future and figure out “when exactly might we get creamed?”

Fraser: So what range or what size of objects will these automated surveys turn up right now?

Pamela: It depends on the reflectivity of the object. A really reflective object you can see even when its smaller. On average for finding things that are from tens of metres to kilometres in size, majority of objects that we find are around several tens of metres in size. The solar system doesn’t have (luckily) lots and lots and lots of these kilometre asteroids waiting to cream us.

So we’re finding all different sizes and we find them, often, when they’re very close to the Earth. In fact, there was one that was just 30m (100′) in diameter that passed within about 1/10th the distance from the Earth to the Moon, back in 2004, that we discovered just three days before it passed that close to the Earth. So we’re finding lots of smaller objects out there.

Fraser: So we’re able to see the bigger objects at various points along the orbit, but the surveys are also turning up the really small objects as they zip right past us closely.

Fraser: Yeah. What will it take, then, to find more of these objects? I don’t want to find out about these objects three days before they’re going to hit us. I want to know a hundred years before they’re going to hit us.

Pamela: The trick is beating down the noise. These things are moving, which makes it hard to get a lot of photons from them. We don’t know ahead of time exactly how fast they’re moving. To get a good image of something, you want your telescope to track it across the sky. We know the rate that the stars move across the sky, and the asteroids will actually appear to move relative to the stars, so if it’s a very small, faint object, we might need a 5-10 minute exposure to get good signal. But if we don’t know over those five or ten minutes how fast the telescope needs to be moving, we’re not going to get a good image.

If we go up into space, where we get rid of our atmosphere, that’s one less thing that’s going to be blocking the photons and interfering with the light, and it gets a bit easier to find them.
We also have to try different tracking speeds, bigger collecting areas â€“ at the end of the day, you want to survey as much of the sky with as sensitive a detector as you can day after day after day. In space we’re not limited by where the Sun is. Here on Earth we can only observe at night. If we go into space we can observe all the time, we just have to observe the direction near the Sun, but that opens a whole lot more of the sky to look at.

Fraser: Oh I see. It’s like the place where you most want to look, which is toward the Sun, is invisible to us for most of the day.

Pamela: This is actually a real problem.

Pamela: The things that are most likely to hit us are going to be on really elongated orbits and they’re probably going to be coming straight out of the Sun as they come toward us. To see the things on the collision course, we need to be able to see near the Sun.

Fraser: What are the chances that an asteroid is going to hit us?

Pamela: Very very low. But there is this chance. There’s a probability that we’re going to get something like Tungesta, where a comet explodes in the atmosphere roughly every thousand years. As you move to larger and larger and more and more destructive objects, they happen more infrequently, but the probability is still there. The key is, currently we know of nothing that’s going to hit us.

Fraser: We don’t know of anything that’s going to hit us, but we’re fairly certain or almost sure that something’s going to hit us eventually.

Fraser: All right. Let’s say, then, that we’ve got a 50m, one of those smaller objects, that does end up on a collision course with the planet. What would happen if it actually did hit the Earth?

Pamela: With one of these 50m ones, some of it’s going to make it through the atmosphere and hit the Earth with a great deal of force. If it hits on land, it’s going to create a big crater, throw a lot of dust into the air, destroy that area but it’s not going to create global havoc.

Fraser: How much of an area? Are we looking at a small town, a city, a continent?

Pamela: Tucson would be toast if it got hit.

Fraser: That’s just a 50m asteroid.

Pamela: It’s kind of alarming but kind of cool all at once. It’s the “when it bleeds, it leads” section of astronomy.

Fraser: Yeah, that seems to be the directions I seem to take this show, but we’ll try and keep it back on track.

Okay, let’s go a little bigger then.

Pamela: From 50m you destroy a city, but when you get up to 1km, you destroy the planet.

Fraser: You don’t actually blow up the planet?

Pamela: No, no. The Earth will still be there, and there will probably still be critters roaming around on it, but at this point you create essentially the asteroid impact version of the nuclear winter. You throw so much stuff up into the atmosphere that you cool the planet off a bit. You also create (if it hits water) you can have tsunamis that wipe out all the coastlines on one half of the planet. Some rather bad things happen.

In general, though, you always want asteroids to hit the ground. When they hit the ground, they throw up lots of dust, lots of dirt and earth. If they hit in the wrong place they might trigger volcanoes, but when they hit water, the water can cascade around the planet and destroy much, much larger sections.

Fraser: So how big of an object took out the dinosaurs? That was what, 65 million years ago, right?

Pamela: That’s actually a bit controversial. There are people who are saying that might not actually have been the whole story.

Back in 1980, there was the discovery that there’s an iridium layer around the crustaceous-tertiary boundary, where all the dead dinosaurs are located. People got to thinking “so, why is there iridium here?” Iridium is pretty rare on the Earth’s surface, but it’s found really commonly in asteroids. A pair of scientists got to thinking, a father and son (Louis and Walter Alverez) and suggested that perhaps an asteroid struck the Earth, deposited the Iridium, triggered mass extinctions and killed everything.

This was linked with the Yucatan crater, the Chicxulub (and I apologise for what I just did to the pronunciation of that). Since then, there’ve been some highly controversial studies that have said, “well, perhaps that crater isn’t the only crater that was responsible for that particular extinction.” There are several other craters that might also have been linked with it, as well as a huge volcanism event in India. Perhaps all of these things together worked to wipe out the dinosaurs, so we might actually have multiple asteroids to blame for the dinosaurs.

Fraser: Right. So, still it sounds like it was a pretty bad time to be a dinosaur.

Pamela: It was definitely a bad time to be a dinosaur.

Fraser: All right, so now we’ve freaked everybody out.

But I guess the hope of these automated asteroid watching programs is to find these asteroids and maybe give us a chance to do something about it. Once again, let’s rewind, so we haven’t got hit, we’ve got some time, what can we do?

Fraser: And how much time would we need? I mean, when you think about Armageddon or Deep Impact or any of those asteroid disaster movies, it’s always some astronomer, “Oh my God!” looking through their telescope and you’ve got six months and then kaboom.

Pamela: Okay, so first of all, you’re not going to identify that it’s going to wipe the planet out from one observation. My personal greatest annoyance with Deep Impact was that they seemed to be working from one picture and it takes lots and lots of pictures to build an accurate orbit.

That aside, you want to discover these things as far in advance as possible. If something’s far, far away, you can basically walk up to it, thwomp it and change its orbit enough that it won’t hit the Earth. If something’s really close, you have to exert a much stronger force to divert it the same amount because it has less time to divert over.

So we want to find things years in advance if possible. That will give us the time to build what we need to build, to go out to it, tap it and push it in a new direction, or go out and stand in front of it and gravitationally attract it in a new direction. Things that are close, it’s a lot harder to divert.

Fraser: So, what are the best or feasible strategies that we’ve got right now to divert them? In all of those disaster movies it’s always a space shuttle full of nuclear missiles. That can work?

Pamela: That would lead to total destruction of the planet Earth.

With an asteroid, if it’s nice and intact, it hits and it destroys, like, a continent (if it’s a big one) and the rest of the time it has repercussions: you have, basically, nuclear winter. But you only have mass total crater-forming carnage on the one place. If you knock the asteroid into ten thousand little pieces, all of those pieces are going to hit the planet. So we’re now going to have carnage that wraps itself around the globe.

Fraser: Sort of like the Shoemaker-Levy 9 collision with Jupiter, right?

Pamela: Exactly. As the planet rotated, new parts of the comet hit Jupiter and we ended up with this shoestring of pockmarks on the face of Jupiter.

Fraser: So if you flew up with your nuclear weapons, blew up an asteroid into a lot of parts, then it would just be like hitting the Earth with shotgun pellets. Big ones.

Fraser: Once again, you’re not going to completely destroy the Earth, we’re just merely wiping out major life.

Pamela: Exactly. No big deal.

Fraser: Earth’s still going to be fine.

And the bacteria under the surface of the planet will finally have it’s day.

Pamela: Something new will emerge.

Fraser: Yeah. Rise of the slime.

Pamela: Exactly. So don’t blow up asteroids.

Fraser: Okay, but that’s the major idea that everyone’s had for â€“ I mean, that’s the big idea, right?

Pamela: If you’re going to go visit an asteroid, with the space shuttle, attach the space shuttle, fire the engines and just push it in a new direction. Now, our space shuttle is never going to go that far. Our space shuttle struggles to go more than several hundred miles above the surface of the Earth.

Fraser: And our space shuttle wouldn’t have the fuel. I can just imagine the amount of fuel it would take to push. All right. Get real. Let’s hear some realistic strategies here.

Pamela: To get real, you build yourself a fairly reasonable size space probe, put what’s called an ion engine on it, which is where you literally take ions, accelerate them using magnetic fields, and as they leave the spacecraft, they’re going in one direction and conservation of momentum causes the space craft to go in the opposite direction. You can get these little ions which don’t have a lot of mass, going at huge velocities. Since they’re going extremely fast, your much larger mass will still get some velocity that’s noticeable pushing it forward.

Well, if this works to move spacecraft, it will also (if you give enough time) work to move entire asteroids. You can go out and push an asteroid using an ion engine.

Fraser: So you just equip an asteroid with an ion engine.

Fraser: And have it start pushingâ€¦ and use fuel, I guess, from the asteroid, and eventually you’ll change the orbit.

Pamela: You can also go out and just gravitationally pull on it. This is one of those things that it doesn’t seem like it will work, but it actually does. If you take a big chunk of metal (which a space craft can be), stick it near the asteroid, the mutual gravitational gravitation of the two will change the path of the asteroid just enough to prevent it from hitting the Earth.

Fraser: I did a podcast on this, on the Universe Today podcast feed, where there’s actually a really good proposal right now. They take an ion-powered space craft, with a tether down to a chunk of mass like a nuclear reactor, and they put the reactor as close as possible to the asteroid itself and then fire the ion engine continuously. The asteroid would be attracted to the reactor, to the mass on the spacecraft, and obviously the spacecraft would be way more attracted to the asteroid itself, so you fire the engine and you continuously tugging against the pull of gravity. As long as you have enough fuel, you can be slowly pulling the asteroid away from its current orbit. There’s some really neat proposals out there right now that are going around. That seems like the most feasible strategy I’ve heard so far.

Pamela: So, all we have to do is budge them a small amount. There’s also this â€“ it’s not related to protecting the Earth by deflecting an asteroid, but the Planetary Society has this really neat contest going on about how do you tag an asteroid.

One of the problems we have is getting extremely accurate orbits. In the case of asteroids that look like they have some sort of a probability of hitting us, we want to work as quickly as we can to get as accurate an orbit as we can to figure out if we need to make the financial expenditures necessary to go out and divert it.

We recently had a scare with the asteroid Apophus. It’s not going to hit the Earth. It’s not going to hit us any time. So, we know that it’s not a worry, but we thought it was for a while. The Planetary Society put out this call for proposals on how could you go out and put a radio transponder on an asteroid so that we can track it and get accurate positions over time. One of the first steps to diverting an asteroid is actually just radio tagging it, like you might radio tag an endangered species, so we can keep track of it as it migrates through the solar system.

Fraser: And the better you know its position, the better your chances of predicting its future position and knowing whether or not its going to hit the Earth.

Fraser: Good, well I think that gives us a really good overview of the threat. I think asteroids make bad neighbours.

Pamela: Well, they might have future uses as things that we can mine for elements, so bad neighbours, good neighbours, it all depends on how we’re exploding them.

All right, thanks Pamela. We’ll talk to you again next week.

Pamela: It’s been my pleasure, Fraser.

This transcript is not an exact match to the audio file. It has been edited for clarity.