# Is there any known asteroid or comet whose path goes through the Earth?

Is there any known celestial body of at least 500 ft (150 m) diameter whose Minimum Orbit Intersection Distance (MOID) is within the Earth, meaning it's sure that it can impact the Earth? I know Comet 109P/(Kegler-)Swift-Tuttle has a very close MOID at 130,000 km (81,000 mi) and the Earth probably would be struck by particles from its tail if it came that close. And Apophis can come as close as 47,200 km (29,300 mi) but that's an asteroid (no tail). Are there any large bodies whose MOID is closer?

JPL and ESA provide tools to search for small bodies meeting user-defined criteria. I tried:

• H <= 24
• MOID <= 1e-4 au

with each and got these results in common:

• (89959) 2002 NT7, an H=16.5 (~1.4 km) asteroid with a 2.29 year period. MOID≈4000 km but closest approach between 1900 and 2200 is 0.37 au; orbit rather highly inclined (i=42.3°).

• (292220) 2006 SU49, an H=19.4 (390-780 m) asteroid with a 1.68 year period. MOID≈10,000 km; 3.2 lunar distance (LD) approach expected in 2029.

• 2014 DA, an H=22.7 (85-170 m) asteroid with a 1.85 year period and MOID≈4000 km. Discovered after a 9.3 LD approach; other approaches uncertain (JPL condition code 7) due to limited observational data.

• 2008 BO16, H=22.9 (80-160 m), 3.78 year period, MOID≈15,000 km, condition code 7.

• 2010 VB1, H=23.2 (70-140 m), 1.21 year period, MOID≈8000 km, 1.5 LD approach expected in 2068.

Each tool also listed a few bodies which the other didn't. MOID computation methods vary, and I wouldn't assume that it's known very precisely even for numbered asteroids. The ESA database appears to store MOID at a resolution of 1e-5 AU (1500 km).

JPL Sentry uses different criteria and shows different results including Apophis and Bennu.

## Asteroid 2018 VP₁ may be heading for Earth. But there’s no need to worry

Jonti Horner does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

### Partners

University of Southern Queensland provides funding as a member of The Conversation AU.

The Conversation UK receives funding from these organisations

Social media around the world lit up over the weekend, discussing the possibility that an asteroid (known as 2018 VP₁) could crash into Earth on November 2.

It seemed only fitting. What better way to round off a year that has seen catastrophic floods, explosions, fires, and storms – and, of course, a global pandemic?

A massive planetesimal, smashing into Earth. Exactly what won’t happen on November 2 with 2018 VP₁… NASA/Don Davis

But you can rest easy. The asteroid does not pose a threat to life on Earth. Most likely, it will sail harmlessly past our planet. At worst, it will burn up harmlessly in our atmosphere and create a firework show for some lucky Earthlings.

## Is there any known asteroid or comet whose path goes through the Earth? - Astronomy

Asteroids are material left over from the formation of the solar system. One theory suggests that they are the remains of a planet that was destroyed in a massive collision long ago. Asteroids may also be material that never coalesced into a planet. If the estimated total mass of all known asteroids was gathered into a single
object, the object would be less than 1,500 kilometers (932 miles) across -- less than half the diameter of our Moon. However, there may have been more material originally.

Much of our understanding about asteroids comes from examining pieces of space debris that fall to the surface of Earth. Asteroids that are on a collision course with Earth are called meteoroids. When a meteoroid strikes our atmosphere at high velocity, friction causes this chunk of space matter to incinerate in a streak of light known as a meteor. If the meteoroid does not burn up completely, what's left strikes Earth's surface and is called a meteorite.

The upper air burst into life!
And a hundred fire-flags sheen,
To and fro they were hurried about!
And to and fro, and in and out,
The wan stars danced between

Inspiring poets (Cooleridge might have the spectacular Leonid meteor shower of 1797) and spectacular to all, meteor showers and meteors can be very impressive.

Because asteroids are material left over from the very early solar system, scientists are interested in their composition. Spacecraft that have flown through the asteroid belt have found that the belt is really quite empty and that asteroids are separated by very large distances. Before 1991 the only information obtained on asteroids was though Earth based observations. Then on October 1991 asteroid 951 Gaspra was visited by the Galileo spacecraft and became the first asteroid to have hi-resolution images taken of it. Again on August 1993 Galileo made a close encounter with asteroid 243 Ida. This was the second asteroid to be visited by spacecraft. Both Gaspra and Ida are classified as S-type asteroids composed of metal-rich silicates. Of all the meteorites examined, 92.8 percent are composed of silicate (stone), and 5.7 percent are composed of iron and nickel the rest are a mixture of the three materials. Stony meteorites are the hardest to identify since they look very much like terrestrial rocks.

 A photo of comet Kohoutek with a well developed tail

The main distinction between asteroids and comets seems to be that comets have more volatiles and more elliptical orbits. But there are interesting ambiguous cases such as 2060 Chiron (aka 95 P/Chiron) and 3200 Phaethon and the Kuiper Belt objects which seem to share some aspects of both categories.

Unlike the other small bodies in the solar system, comets have been known since antiquity. There are Chinese records of Comet Halley going back to at least 240 BC. The famous Bayeux Tapestry, which commemorates the Norman Conquest of England in 1066, depicts an apparition of Comet Halley.
As of 1995, 878 comets have been cataloged and their orbits at least roughly calculated. Of these 184 are periodic comets (orbital periods less than 200 years) some of the remainder are no doubt periodic as well, but their orbits have not been determined with sufficient accuracy to tell for sure.

Comets are sometimes called dirty snowballs or "icy mudballs". They are a mixture of ices (both water and frozen gases) and dust that for some reason didn't get incorporated into planets when the solar system was formed. This makes them very interesting as samples of the early history of the solar system.

When they are near the Sun and active, comets have several distinct parts:

nucleus: relatively solid and stable, mostly ice and gas with a small amount of dust and other solids
coma: dense cloud of water, carbon dioxide and other neutral gases sublimed off of the nucleus
hydrogen cloud: huge (millions of km in diameter) but very sparse envelope of neutral hydrogen
dust tail: up to 10 million km long composed of smoke-sized dust particles driven off the nucleus by
escaping gases this is the most prominent part of a comet to the naked eye
ion tail: up to 100 million km long composed of plasma and laced with rays and streamers caused by
interactions with the solar wind.

Comets are invisible except when they are near the Sun. Most comets have highly eccentric orbits which take them far beyond the orbit of Pluto these are seen once and then disappear for millennia. Only the short- and intermediate-period comets (like Comet Halley), stay within the orbit of Pluto for a significant fraction of their orbits.

After 500 or so passes near the Sun off most of a comet's ice and gas is lost leaving a rocky object very much like an asteroid in appearance. (Perhaps half of the near-Earth asteroids may be "dead" comets.) A comet whose orbit takes it near the Sun is also likely to either impact one of the planets or the Sun or to be ejected out of the solar system by a close encounter (esp. with Jupiter).

By far the most famous comet is Comet Halley but SL 9 was a "big hit" for a week in the summer of 1994.

Meteor shower sometimes occur when the Earth passes thru the orbit of a comet. Some occur with great regularity: the Perseid meteor shower occurs every year between August 9 and 13 when the Earth passes thru the orbit of Comet Swift-Tuttle. Comet Halley is the source of the Orionid shower in October.

Asteroids are sometimes also refered to as minor planets or planetoids (not to be confused with "lesser planets" which refers to Mercury and Pluto). Very small rocks orbiting the Sun are sometimes called meteoroids to distinguish them from the larger asteroids. When such a body enters the Earth's atmosphere it is heated to incandescence and the visible streak in the sky is known as a meteor. If a piece of it survives to reach the Earth's surface it is known as a meteorite.

Millions of meteors bright enough to see strike the Earth every day (amounting to hundreds of tons of material). All but a tiny fraction burn up in the atmosphere before reaching the ground. The few that don't are our major source of physical information about the rest of the solar system (some new information about cometary flux to earth).

Harry ɿurious' after being 'shunted aside' with Meghan for final Royal event

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## NASA: Massive asteroid close call due Saturday, but won’t be hitting Earth

An asteroid large enough to cause planet-wide devastation will hurtle unsettlingly close to Earth early Saturday morning — but a near-miss means we’re safe for now, astronomers say.

The kilometer-wide asteroid NASA officially calls 2002 PZ39 will get closest to earth at 6:05 a.m. Saturday, when it will be 3.6 million miles away.

This asteroid is cruising at around 34,000 mph, and will at its closest be about 15 times the distance of the moon.

“There’s no hazard or danger,” Paul Chodas, director of NASA’s Center for Near Earth Objects Studies, insisted to the Herald on Thursday. “We’ve been watching this asteroid for years, and we know its orbit very well.”

But the asteroid will be back, as its orbit essentially intersects with Earth’s orbit. It will come within just 251,000 miles of where we were Tuesday night.

“This is certainly an object that we need to keep an eye on,” astrophysicist Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics told the Herald. “Maybe it wouldn’t be enough to send us extinct, but it would make for a very different world.”

Of asteroids this size, McDowell said, “This is an existential threat in the long term for our species.”

If this asteroid’s path were just slightly different and were to impact Earth, McDowell said, “It plows into the Earth, it makes a huge crater, it throws enormous amounts of earth material into the atmosphere. It could set fire to a large fraction of a continent. Then you’d have a lot of dust in the atmosphere for years and years afterward. It would wipe out all life for a thousand miles around, and then knock-on consequences for the environment for decades.”

Astronomers, NASA and other foreign space agencies have their eyes on the skies to monitor what McDowell said is likely hundreds of sizable space rocks whose paths come close to earth. The most notorious asteroid hit on Earth was the Chicxulub impactor in the Yucatan Peninsula 66 million years ago, which is blamed for the mass extinction of the dinosaurs.

Humans wouldn’t be able to do anything if an object is only spotted within days of impact. The key, McDowell said, is to figure out years in advance, so astronauts could fly up and do something about it. But they wouldn’t blow it up, a la Bruce Willis in “Armageddon” — they’d likely just attach rocket boosters to it to speed it up or slow it down just slightly, so it gets to the intersection point with Earth’s orbit just a couple hours sooner or later than we’ll be there.

“If you can just slow it down by an hour, you just avoid that hit,” McDowell said.

Chodas said, “There is no known asteroid that has any chance of hitting the Earth over the next hundred years.”

The next truly close call is on April 13, 2029 — Friday the 13th — when the asteroid Apophas will come even closer to Earth than the satellites we use for GPS. That 300-meter-long rock will fly so close we’ll be able to see it, Chodas said — though he’s said NASA is now sure it’s not going to hit us then.

## Contents

The average distance to the Moon (or lunar distance (LD)) is about 384,400 km (238,900 mi), which is around 30 times the diameter of the Earth. [3] Below are lists of close approaches less than one LD for a given year. (See also near-Earth asteroids published by the International Astronomical Union [note 1] and NEO Earth Close Approaches.)

• Discovered > 1 year in advance
• Discovered > 7 weeks in advance
• Discovered > 1 week in advance
• Discovered up to 1 week in advance
• < 24 hours' warning
• No warning

### Closest known per year Edit

From the list in the first section, these are the closest-known asteroids per year that approach Earth within one lunar distance. More than one asteroid per year may be listed if its geocentric distance [note 2] is within a tenth of the lunar distance, or 0.10 LD. For comparison, since a satellite in a geostationary orbit has an altitude of about 36,000 km (22,000 mi), then its geocentric distance is 0.11 LD (approximately three times the width of the Earth).

The table shows that the years 2016 and 2017 had a total of 13 such close encounters that are known. Of these, eight were undetected until after they'd happened and only one was detected with more than 24 hours' notice. 2018 has fared better so far, with six out of the eight known close encounters being detected beforehand, albeit with less than 24 hours' notice in most cases.

This list does not include any of the hundreds of objects that collided with Earth, which were not discovered in advance, but were recorded by sensors designed to detect detonation of nuclear devices. Of the objects so detected, 78 had an impact energy greater than that of a 1-kiloton device (equivalent to 1000 tons of TNT), including 11 which had an impact energy greater than that of a 10-kiloton device i.e. comparable to the atomic bombs used in the Second World War. [4]

Rows highlighted red indicate objects which were not discovered until after closest approach

Year Date of
closest approach
Date
discovered
Object Nominal
geocentric
distance
(in 000's km)
Nominal
geocentric
distance
(in LD)
Size of object
(in meters)
(H) Ref
2095 2095-09-06 2010-09-05 2010 RF 12 39.1 0.10 7 28.4 JPL · CAD
2034 2034-05-06 2014-04-29 2014 HB177 88.9 0.23 6–14 28.1 JPL · CAD
2032 2032-08-14 2008-02-18 2008 DB 125.0 0.32 19–43 25.7 JPL · CAD
2029 2029-04-13 2004-06-19 99942 Apophis 38.0 0.10 310–340 19.7 JPL · CAD
2028 2028-06-26 2001-11-20 (153814) 2001 WN 5 248.7 0.65 610–1400 18.2 JPL · CAD
2021 2021-02-09 2021-02-09 2021 CZ3 22.7 0.06 1.7–3.9 30.9 JPL · CAD
2020
2020-11-13 2020-11-14 2020 VT 4 9.3 0.02 4.8–11 28.7 JPL · CAD
2020-09-24 2020-09-18 2020 SW 28 0.07 4.3–9.7 28.9 JPL · CAD
2020-08-16 2020-08-16 2020 QG 9.3 0.02 2.9–6.4 29.8 JPL · CAD
2020-05-04 2020-05-04 2020 JJ 13.5 0.03 2.7–6 30 JPL · CAD
2020-02-01 2020-02-01 2020 CW 15.7 0.04 0.83–1.9 32.6 JPL · CAD
2019 2019-10-31 2019-10-31 2019 UN 13 12.6 0.033 1.0–2.2 32.1 JPL · CAD
2019-04-04 2020-02-15 2020 CD 3 13.1 0.034 1.9–3.5 31.7 JPL · CAD
2019-01-08 2018-01-08 2019 AS 5 15.1 0.039 0.7–2.5 32.3 JPL · CAD
2019-03-01 2019-03-01 2019 EH1 23.5 0.061 2–6 30.1 JPL · CAD
2019-03-04 2019-03-05 C09Q4H2 26.6 0.069 [note 3] 1–3 31.9 Pseudo-MPEC
CNEOS Distance (T)
2019-01-17 2019-01-16 P10LGkb 33.7 0.088 [note 4] 1–3 31.6 Pseudo-MPEC
CNEOS Distance (H) (T)
2019-09-05 2019-09-06 2019 RP1 37 0.10 7.3–16 27.8 JPL · CAD
2018 2018-06-02 2018-06-02 2018 LA 5.0
(Impact)
0.0130
(Impact)
2018-10-19 2018-10-19 2018 UA 13.7 0.036 2–6 30.2 JPL · CAD
2018-01-22 2018-01-22
(Unconfirmed)
A106fgF 20? 0.03? 2–5 30.6
2018-06-17 2018-06-17
(Unconfirmed)
A107j4p 30.8 0.080 4–11 28.9
2018-11-16 2018-11-17 2018 WG 30.9 0.080 3–10 29.3 JPL · CAD
2018-12-02 2018-11-29 2018 WV 1 33.0 0.086 4 30.2 JPL · CAD
2018-08-10 2018-08-11 2018 PD 20 33.5 0.087 7–22 27.4 JPL · CAD
2018-01-18 2018-01-18 2018 BD 39.2 0.10 2–6 30.2 JPL · CAD
2017 2017-04-04 2017-04-03 2017 GM 16.3 0.042 2–7 29.9 JPL · CAD
2017-10-20 2017-10-21 2017 UJ2 18.0 0.047 1–5 30.8 JPL · CAD
2017-10-22 2017-10-30
(Unconfirmed)
YU95BEF 19.4 0.051 5–15 28.2
2017-03-02 2017-03-02 2017 EA 20.9 0.054 1–5 30.7 JPL · CAD
2017-11-26 2017-11-26 2017 WE30 30.1 0.078 1–3 31.8 JPL · CAD
2017-11-14 2017-11-20
(Unconfirmed)
P10ELNY 31.7 0.083 4–12 28.8
2017-11-08 2017-11-16
(Unconfirmed)
A104Vqx 33.2 0.086 4–14 28.4
2016 2016-02-25 2016-02-26 2016 DY30 14.3 0.04 2–5 30.5 JPL · CAD
2016-09-11 2016-09-11 2016 RN41 23.7 0.06 1–5 31.0 JPL · CAD
2016-01-12 2016-01-13 2016 AH164 26.7 0.07 3–7 29.6 JPL · CAD
2016-03-11 2016-03-15 2016 EF195 [5] 31.7 0.08 16–31 25.6 JPL · CAD
2016-11-05 2016-11-14
(Unconfirmed)
XV88D4F 36.7 0.09 2–7 30.0 [6]
2016-01-14 2016-01-14 2016 AN164 37.0 0.10 2–5 30.5 JPL · CAD
2015 2015-09-22 2015-09-24 2015 SK7 26.6 0.07 3–14 28.9 JPL · CAD
2015-11-15 2015-11-14 2015 VY105 34.6 0.09 4–9 29.0 JPL · CAD
2015-02-17 2015-02-18 2015 DD1 39.3 0.10 1–3 30.4 JPL · CAD
2014 2014-01-02 2014-01-01 2014 AA 0.45 [7] [8]
(Impact)
0.001
(Impact)
2014-06-03 2014-06-02 2014 LY 21 16.7 0.04 4–8 29.1 JPL · CAD
2014-09-07 2014-09-01 2014 RC 39.9 0.10 12–25 26.8 JPL · CAD
2013 2013-12-23 2013-12-23 2013 YB 27.3 0.07 3 31.4 JPL · CAD
2013-02-15 2012-02-23 367943 Duende 34.1 0.09 30 24.0 JPL · CAD
2012 2012-05-29 2012-05-28 2012 KT 42 20.8 0.05 9 29.0 JPL · CAD
2011 2011-02-04 2011-02-04 2011 CQ 1 [9] 11.9 0.03 1 32.1 JPL · CAD
2011-06-27 2011-06-22 2011 MD 18.7 0.05 14 28.0 JPL · CAD
2011-02-06 2011-02-07 2011 CF22 40.2 0.10 2.4 30.9 JPL · CAD
2010 2010-11-17 2010-11-16 2010 WA 38.9 0.10 2–6 30.0 JPL · CAD
2009 2009-11-06 2009-11-06 2009 VA 20.4 0.05 4–13 28.6 JPL · CAD
2008 2008-10-07 2008-10-06 2008 TC 3 5.9
(Impact)
0.0152
(Impact)
2008-10-09 2008-10-09 2008 TS26 12.6 0.03 0.5-1.6 33.2 JPL · CAD
2008-10-20 2008-10-21 2008 US 32.9 0.09 1–4 31.4 JPL · CAD
2007 2007-10-17 2007-10-21 2007 UN12 69.7 0.18 4–11 28.7 JPL · CAD
2006 2006-02-23 2006-02-22 2006 DD1 117.5 0.31 12–30 26.5 JPL · CAD
2005 2005-11-26 2005-11-25 2005 WN3 83.8 0.22 3–6 29.9 JPL · CAD
2004 2004-03-31 2004-03-31 2004 FU 162 12.9 0.03 4–12 28.7 JPL · CAD
2003 2003-09-27 2003-09-28 2003 SQ222 84.2 0.22 2–6 30.1 JPL · CAD
2002 2002-12-11 2002-12-13 2002 XV90 117.7 0.31 19–47 25.5 JPL · CAD
2001 2001-01-15 2001-01-19 2001 BA16 79.0 0.21 15–38 26.0 JPL · CAD
1999 1999-03-12 2013 2013 EC20 315.4 0.82 3–12 29.0 JPL · CAD
1994 1994-12-09 1994-12-09 1994 XM1 105.5 0.27 5–16 28.2 JPL · CAD
1993 1993-05-20 1993-05-21 1993 KA2 149.2 0.39 3–11 29.0 JPL · CAD
1991 1991-01-18 1991-01-18 1991 BA 168.2 0.44 4–13 28.6 JPL · CAD
1990 1990-09-19 2003 2003 SW130 213.9 0.56 3–10 29.1 JPL · CAD
1984 1984-01-10 2016 2016 TB57 294.8 0.77 13–43 26.0 JPL · CAD
1982 1982-11-04 2012 2012 TY52 314.4 0.82 111–358 21.2 JPL · CAD
1979 1979-09-02 2014 2014 WX202 334.3 0.87 3–8 29.6 JPL · CAD
1976 1976-10-17 2013 2013 UG1 328.1 0.85 70–226 22.4 JPL · CAD
1971 1971-04-11 2002 2002 JE 9 237.0 0.62 122–393 21.2 JPL · CAD
1965 1965-10-27 2005 2005 VL1 289.2 0.75 10–33 26.6 JPL · CAD
1959 1959-01-27 2012 2012 BX 34 203.4 0.53 6–21 27.6 JPL · CAD
1957 1957-12-10 2010 2010 XW58 60.8 0.16 22–71 24.9 JPL · CAD
1955 1955-06-19 2015 2015 LR21 225.6 0.59 11–34 26.5 JPL · CAD
1954 1954-03-13 2013 2013 RZ53 102.7 0.27 1–4 31.1 JPL · CAD
1949 1949-01-01 2003 2003 YS70 259.6 0.68 3–10 29.1 JPL · CAD
1938 1938-11-02 2018 2018 RW 105.0 0.27 2-5 30.3 JPL · CAD
1936 1936-01-06 2010 2010 VB1 212.6 0.55 61–140 23.2 JPL · CAD
1935 1935-03-08 2015 2015 DD54 182.8 0.48 18–57 25.4 JPL · CAD
1925 1925-03-29 2012 2012 FT35 39.3 0.10 4-9 29.2 JPL · CAD
1922 1922-06-07 2017 2017 LD 18.2 0.047 11 27.5 JPL · CAD
1918 1918-09-17 2011 (458732) 2011 MD 5 350.1 0.91 556–1795 17.9 JPL · CAD
1914 1914-12-31 1998 (152680) 1998 KJ 9 232.9 0.61 279–900 19.4 JPL · CAD
1910 1910-05-09 2007 2007 JB21 288.7 0.75 18–57 25.4 JPL · CAD

A notable case is the relatively large asteroid Duende, which was predicted nearly a year in advance, coincidentally approaching just a few hours after the unrelated Chelyabinsk meteor, which was unpredicted, but injured thousands of people when it impacted.

### Largest per year Edit

From the lists in the first section, these are the largest-known asteroids per year that approach Earth within one LD. (More than one asteroid per year may be listed if its size is 100 m [330 ft] or more.) For comparison, the 1908 Tunguska event was caused by an object about 60–190 m (200–620 ft) in size, while the 2013 Chelyabinsk meteor which injured thousands of people and buildings when it generated a large airburst over Russia was estimated to be just 20 m (66 ft) across.

The table shows about 14 events in the 12 decades of 1900–2020 involving a body with an upper size estimate of 100 m (330 ft) or more making a close approach to Earth within one LD, with one (the Tunguska object) making impact.

Year Date of
closest approach
Object Nominal
geocentric
distance
(in 000s km)
Nominal
geocentric
distance
(in LD)
Est. size
(in m)
(H) Ref
2029 2029-04-13 99942 Apophis 38.0 0.10 310–340 19.7 JPL · CAD
2028 2028-06-26 (153814) 2001 WN 5 248.7 0.65 921–942 18.3 JPL · CAD
2019 2019-07-25 2019 OK 78 0.2 57–130 23.3 JPL · CAD
2018 2018-01-03 2018 AH 298 0.77 65–226 22.5 JPL · CAD
2018-05-15 2010 WC 9 203 0.53 42-136 23.5 JPL · CAD
2018-04-15 2018 GE 3 193 0.50 48–110 23.6 JPL · CAD
2017 2017-07-21 2017 QP 1 63 0.16 31–91 24.3 JPL · CAD
2016 2016-03-21 2016 FN56 384 1.00 35–86 24.2 JPL · CAD
2015 2015-01-18 2015 BP 513 240 0.62 12–27 26.7 JPL · CAD
2014 2014-03-05 2014 DX 110 349 0.91 20–40 25.7 JPL · CAD
2013 2013-08-04 2013 PJ 10 371 0.97 60 24.6 JPL · CAD
2012 2012-04-01 2012 EG 5 230 0.60 60 24.3 JPL · CAD
2011 2011-11-08 (308635) 2005 YU 55 324 0.84 360 21.9 JPL · CAD
2011-12-03 2011 XC 2 347 0.90 100 23.1 JPL · CAD
2010 2010-11-02 2010 UJ7 286 0.74 20–49 25.4 JPL · CAD
2009 2009-03-02 2009 DD 45 72 0.19 15–47 25.8 JPL · CAD
2008 2008-02-15 2008 CK 70 371 0.97 22–71 24.9 JPL · CAD
2007 2007-01-18 2007 BD 324 0.84 18–57 25.4 JPL · CAD
2006 2006-02-23 2006 DD1 117 0.31 11–34 26.5 JPL · CAD
2005 2005-12-05 2005 XA8 217 0.57 15–49 25.7 JPL · CAD
2004 2004-03-18 2004 FH 49 0.13 15–49 25.7 JPL · CAD
2003 2003-12-06 2003 XJ7 148 0.39 11–36 26.4 JPL · CAD
2002 2002-06-14 2002 MN 120 0.31 40–130 23.6 JPL · CAD
2001 2001-01-15 2001 BA16 306 0.80 13–43 26.0 JPL · CAD
1999 1999-08-12 2016 CD137 179 0.47 13–43 26.0 JPL · CAD
1994 1994-12-09 1994 XM1 105 0.27 5–16 28.2 JPL · CAD
1993 1993-05-20 1993 KA2 149 0.39 3–11 29.0 JPL · CAD
1991 1991-04-08 2012 UE 34 322 0.84 46–149 23.3 JPL · CAD
1990 1990-09-19 2003 SW130 186 0.48 3–10 29.1 JPL · CAD
1988 1988-10-16 2010 UK 322 0.84 9–30 26.8 JPL · CAD
1982 1982-11-04 2012 TY52 314 0.82 111-358 21.4 JPL · CAD
1980 1980-05-18 2009 WW7 74 0.19 4-11 28.9 JPL · CAD
1976 1976-10-17 2013 UG1 328.1 0.85 70–226 22.4 JPL · CAD
1971 1971-04-11 2002 JE 9 237.0 0.62 122–393 21.2 JPL · CAD
1936 1936-01-06 2010 VB1 212.6 0.55 48–156 23.2 JPL · CAD
1925 1925-08-30 (163132) 2002 CU 11 347.0 0.90 443–467 18.5 JPL · CAD
1918 1918-09-17 (458732) 2011 MD 5 350.1 0.91 556–1795 17.9 JPL · CAD
1914 1914-12-31 (152680) 1998 KJ 9 232.9 0.61 279–900 19.4 JPL · CAD

The year 2011 was notable as two asteroids with size 100 m (330 ft) or more approached within one lunar distance.

Objects with distances greater than 100 km (62 mi) are listed here, although there is no discrete beginning of space.

### Objects < 50 meters Edit

Asteroids smaller than about 50 m (160 ft). [10]

2020 QG—Closest asteroid flyby to not hit Earth, at 2,900 km (1,800 mi), closest approach on 16 August 2020. [11] [12]

Nominal
geocentric
distance (AU)
Nominal
geocentric
distance (km)
Size (m)
(approximate)
Date of
closest approach
Object Ref
0.000079 11,900 1 February 4, 2011 2011 CQ 1 JPL · CAD
0.000084 12,500 1 October 9, 2008 2008 TS26 JPL · CAD
0.000086 12,900 6 March 31, 2004 2004 FU 162 JPL · CAD
0.000088 13,100 1.9–3.5 April 4, 2019 2020 CD 3 JPL · CAD
0.000125 18,700 10 June 27, 2011 2011 MD [13] JPL · CAD
0.000137 20,400 7 November 6, 2009 2009 VA JPL · CAD
0.000139 20,800 4–10 May 29, 2012 2012 KT 42 JPL · CAD
0.000177 26,500 3–14 September 22, 2015 2015 SK7 JPL · CAD
0.00018 27,000 3 December 23, 2013 2013 YB JPL · CAD
0.000221 33,000 4 December 2, 2018 2018 WV 1 JPL · CAD
0.000227 33,900 5 December 19, 2004 2004 YD5 JPL · CAD
0.000228 34,100 40×20 [14] February 15, 2013 367943 Duende [15] JPL · CAD
0.000260 38,900 3 November 17, 2010 2010 WA JPL · CAD
0.000262 39,300 1–3 February 17, 2015 2015 DD1 JPL · CAD
0.000267 39,900 12–25 September 7, 2014 2014 RC JPL · CAD
0.000269 40,200 2.4 February 6, 2011 2011 CF22 JPL · CAD
0.000328 49,100 30 March 18, 2004 2004 FH JPL · CAD
0.000346 51,800 5–10 October 12, 2010 2010 TD 54 JPL · CAD
0.000383 57,300 25 May 28, 2012 2012 KP 24 [16] JPL · CAD
0.000437 65,400 8 January 27, 2012 2012 BX 34 JPL · CAD
0.000482 72,100 9 September 8, 2010 2010 RK53 JPL · CAD
0.000483 72,200 19 March 2, 2009 2009 DD 45 JPL · CAD
0.000484 72,400 2–7 December 11, 2013 2013 XS21 JPL · CAD
0.000531 79,400 7 September 8, 2010 2010 RF 12 JPL · CAD
0.000564 84,300 5 September 27, 2003 2003 SQ222 JPL · CAD
0.000568 85,000 15 March 18, 2009 2009 FH JPL · CAD
0.000635 95,000 17 October 12, 2012 2012 TC 4 JPL · CAD
0.000704 105,400 10 December 9, 1994 1994 XM1 [17] JPL · CAD
0.000856 128,000 2 October 13, 2015 2015 TC 25 JPL · CAD
0.000862 129,000 15–30 January 13, 2010 2010 AL 30 JPL · CAD
0.000998 149,200 7 May 20, 1993 1993 KA2 [17] JPL · CAD
0.001124 168,200 6–10 January 18, 1991 1991 BA JPL · CAD
0.001539 230,200 47 April 1, 2012 2012 EG 5 JPL · CAD
0.001655 247,600 12 September 8, 2010 2010 RX 30 JPL · CAD
0.002454 367,100 10–17 March 4, 2013 2013 EC JPL · CAD
0.00257 384,400 average distance to the Moon [3]
0.002899 433,600 22 April 9, 2010 2010 GA 6 JPL · CAD

### Objects > 50 meters Edit

Asteroids larger than about 50 m (160 ft). [10] [18]

Nominal
geocentric
distance (AU)
Nominal
geocentric
distance (km)
Size (m)
(approximate)
Date of
closest approach
Object Ref
0.000521 78,000 57–130 July 25, 2019 2019 OK JPL · CAD
0.000802 120,000 73 June 14, 2002 2002 MN JPL · CAD
0.00155 * 233,000 500 December 31, 1914 (152680) 1998 KJ 9 JPL · CAD
0.00159 * 239,000 200 April 11, 1971 2002 JE 9 JPL · CAD
0.00210 * 314,000 200 November 4, 1982 2012 TY52 JPL · CAD
0.002172 324,900 360 November 8, 2011 (308635) 2005 YU 55 JPL · CAD
0.00219 * 328,000 150 October 17, 1976 2013 UG1 JPL · CAD
0.0022 ** 329,000 100 April 8, 1991 2012 UE 34 JPL · CAD
0.0023 * 340,000 730 August 30, 1925 (163132) 2002 CU 11 JPL · CAD
0.0023 340,000 100 December 3, 2011 2011 XC 2 JPL · CAD
0.00257 384,400 average distance to the Moon [3]
<0.00266 * <398,000 100 January 6, 1936 2010 VB1 JPL · CAD
0.002891 432,400 500 July 3, 2006 2004 XP 14 JPL · CAD
0.003704 554,200 250 January 29, 2008 2007 TU 24 JPL · CAD
0.004241 * 634,500 300 April 26, 1942 69230 Hermes JPL · CAD
0.004572 684,000 300 March 22, 1989 4581 Asclepius JPL · CAD
0.004950 740,500 300 October 30, 1937 69230 Hermes JPL · CAD
0.0062 * 930,000 200 December 27, 1976 2010 XC 15 JPL · CAD
0.00836 1,251,000 325 June 8, 2014 2014 HQ 124 JPL · CAD
0.0093 * 1,390,000 5000 August 27, 1969 (192642) 1999 RD 32 JPL · CAD
0.0124855 1,867,800 400 December 16, 2001 (33342) 1998 WT 24 JPL · CAD
0.036415 5,447,600 1000 June 14, 2012 2012 LZ 1 JPL · CAD
0.043294 6,476,600 1600 November 5, 2012 (214869) 2007 PA 8 [19] JPL · CAD
0.046332 6,900,000 5400 December 12, 2012 4179 Toutatis [20] JPL · CAD

Asteroids with large uncertainty regions are not included.

* Asteroid approach did not occur during an observed apparition. Passage is calculated by integrating the equations of motion.

** Only the nominal (best-fit) orbit shows a passage this close. The uncertainty region is still somewhat large due to a short observation arc.

Incomplete list of asteroids larger than about 50 m (160 ft) predicted to pass close to Earth (see also asteroid impact prediction and Sentry (monitoring system)): [18] [21]

Nominal
geocentric
distance (AU)
Nominal
geocentric
distance (km)
Size (m)
(estimated)
Date of
closest approach
Object JPL-Ref
0.000256 38,300 325 April 13, 2029 99942 Apophis JPL · CAD
0.000670 100,200 75–170 October 19, 2129 2007 UW 1 JPL · CAD
0.000721 107,800 50–120 April 8, 2041 2012 UE 34 JPL · CAD
0.001572 235,200 170–370 January 2, 2101 (456938) 2007 YV 56 JPL · CAD
0.001585 237,000 360±40 November 8, 2075 (308635) 2005 YU 55 JPL · CAD
0.001629 243,700 370–840 December 1, 2140 (153201) 2000 WO 107 JPL · CAD
0.001635 ** 244,600 190–420 October 26, 2087 2011 WL 2 JPL · CAD
0.001663 248,800 700–1500 June 26, 2028 (153814) 2001 WN 5 JPL · CAD
0.001980 296,200 170–370 January 22, 2148 (85640) 1998 OX 4 JPL · CAD
0.002222 332,500 190–250 May 28, 2065 2005 WY 55 JPL · CAD
0.002241 335,200 75–170 March 23, 2146 2009 DO111 JPL · CAD
0.00257 384,400 for comparison, this is the average distance to the Moon [3]

A list of predicted NEO approaches at larger distances is maintained as a database by the NASA Near Earth Object Program. [22]

** Only the nominal (best-fit) orbit shows a passage this close. The uncertainty region is still somewhat large due to a short observation arc.

Objects which enter and then leave Earth's atmosphere, the so-called 'Earth-grazers', are a distinct phenomenon, in as much as entering the lower atmosphere can constitute an impact event rather than a close pass. Earth grazer can also be short for a body that "grazes" the orbit of the Earth, in a different context.

## 'Oumuamua Was Neither Comet nor Asteroid. So What Was It?

Jason Arias/Quanta Magazine

Like a hit-and-run driver who races from the scene of a crash, the interstellar guest known as ’Oumuamua has bolted out of the solar system, leaving confusion in its wake. Early measurements seemed to indicate that it was an asteroid—a dry rock much like those found orbiting between Mars and Jupiter. Then by this past summer, astronomers largely came around to the conclusion that it was instead a comet—an icy body knocked out of the distant reaches of a far-off planetary system.

Now a new analysis has found inconsistencies in this conclusion, suggesting that ’Oumuamua may not be a comet after all. Whether it’s actually a comet or an asteroid, one thing is clear: ’Oumuamua is not quite like anything seen before.

The object was first spotted a year ago by scientists with the Pan-STARRS telescope in Hawaii. ’Oumuamua (a Hawaiian word meaning “scout”) appeared to be a rocky, elongated asteroid at first, a stubby cosmic cigar.

Other astronomers quickly joined in the hunt, measuring everything they could. (One team even trained radio telescopes on it to check whether it might be transmitting extraterrestrial broadcasts. It was not.) By last December, a team of astronomers published ’Oumuamua’s electromagnetic spectra, which can be used to probe what an object is made of. The researchers found that ices with organic material similar to those seen in comets in our solar system lurked just below ’Oumuamua’s surface that ice could have survived a long interstellar journey.

They also looked at ’Oumuamua’s rotation. Many asteroids tend to spin around their long axis like an expertly thrown football. ’Oumuamua, by contrast, tumbled slightly like an errant pass by Charlie Brown.

An animation of ’Oumuamua’s journey through the solar system. ESO

A few months later, another collaboration found that ’Oumuamua wasn’t just being pulled by the sun’s gravity. Instead, it was being slightly accelerated by an unseen force, which they argued could only be attributed to comet “outgassing” acting like a thruster. With this additional information, the case appeared to be closed. “Interstellar asteroid is really a comet,” read the headline of a press release put out by the European Space Agency.

The explanation seemed to fit with what we know about our own solar system. In the distant reaches beyond Neptune, countless comets orbit our sun. Anytime one of these comets gets too close to a planet, it could be ejected out into the galaxy. In contrast, there are far fewer asteroids in the asteroid belt, and they orbit closer to the sun, where they’re harder to knock into interstellar space. “There are more comets, and it’s easier to fling them away from a planetary system,” said Ann-Marie Madigan, an astrophysicist at the University of Colorado, Boulder. “For the first interstellar traveler that we see in our solar system, for that to be an asteroid, would be shocking.”

Yet comets have tails. And ’Oumuamua, if it was indeed made out of icy rock and propelled by jets of gas as it passed by the sun, should have displayed a tail that would settle the question of its origin. Yet no tail was ever found.

Now in a new study that is currently under peer review, Roman Rafikov, an astrophysicist at the University of Cambridge, argues that the same forces that appeared to have accelerated ’Oumuamua — the same forces that should have also produced a tail — would have also affected its spin. In particular, the acceleration would have torqued ’Oumuamua to such a degree that it would have spun apart, breaking up into smaller pieces. If ’Oumuamua were a comet, he argues, it would not have survived.

“There’s very strong and unequivocal evidence on both sides,” said Rafikov. “If it’s an asteroid, then it’s really unusual, with exotic scenarios for its formation.” He proposed such a scenario earlier this year, whereby an ordinary star dies, forming a white dwarf, and in the process rips apart a planet and launches the shards clear across the galaxy. ’Oumuamua is one of those shards. “Basically, it’s a messenger from a dead star,” he said.

In part to help resolve the impasse, researchers have tried to identify the star system where ’Oumuamua originated by combing through the newly released data troves of the Gaia space telescope. Perhaps it came from a binary star system, or a system with a giant planet, either of which could have launched the object into interstellar space.

’Oumuamua’s path through the solar system. Distances of the closest passage between ’Oumuamua and other objects are measured in astronomical units (AU) 1 AU is the distance between Earth and the sun. Tom Ruen

But of all the possible candidate star systems, none provided a match. ’Oumuamua’s trajectory was at least two light-years away from all the candidates anyway — too far for them to be its source. And if ’Oumuamua got launched hundreds of millions of years ago, all the local stars will have shifted quite a bit since then. “It’s unlikely you’d ever be able to track it back to a single individual parent system, which is a shame, but it’s just the way things are,” said Alan Jackson, an astronomer at the University of Toronto.

Ultimately the transient nature of the observations has frustrated astronomers’ ability to solve the mystery of our first interstellar guest. “We had only a few weeks, with almost no planning, to make the observations,” said Matthew Knight, an astronomer at the University of Maryland. “Everybody’s trying to wring out every last bit of information they can from what data we were able to collect as a community.” Had ’Oumuamua been spotted earlier, or had Hurricane Maria not taken Puerto Rico’s Arecibo Observatory out of action, astronomers would have more to go on.

## Asteroids? Comets are more Dangerous!

Asteroids have always frightened us. They are rocky, they are near, in the asteroid belt between Mars and Jupiter, and they are numerous. However, Comets may be even more dangerous, more numerous, faster, more unstable, and substantially larger.

Today we live in a great world interconnected by vast networks of electromagnetic communications which order our civilization into a vast complex of multi-level existence. We might say that as a world species, we are an organism, even if at times, we seem to attack our own. Today we have achieved the highest level of not only human existence, but of any living existence that we are aware of. Never before has a living organism reached the level of evolution that we have. In all ways, no matter the ultimate impact on the rest of the Earth, the human organism as a whole is quite exquisite, quite beautiful.

Yet, despite our advanced technology, despite our advanced weapons, despite our mastery of today’s technology, our civilization is not beyond the possibility of a sudden, and irrevocable termination. There are a number of possible scenarios which can result in our end. A few are simply unavoidable, as for example a volcanic episode of some kind, or perhaps a very severe climate change, or perhaps an astronomical event of some kind. Yet this last category does have one possible remedy if it is of a specific kind. If the destructive event had to do with an asteroid, or comet, there might indeed be a chance to avoid absolute destruction.

We have all heard of the Dinosaur extinction 65 million years ago. We have all heard that a very large asteroid, possibly as large as six or seven miles across hit the Earth and caused enormous and widespread destruction all over the planet and putting an end to the reign of the Dinosaur. The destroyer was supposed to be an asteroid possibly dislodged from what is known as the Asteroid belt, which lies between Mars and Jupiter, by some collision or gravitational perturbation in space. Once dislodged the Asteroid is supposed to have hit the Earth and created a huge impact crater at Chicxulub, in what is now Mexico and the Gulf of Mexico, being only a few miles from famous Cancun. But was the culprit an asteroid? Or was it quite possibly something else? Was it a comet instead?

Comet Siding is now heading towards Mars and is scheduled for impact or near miss in October 2014

### Asteroids tend away from Earth, Comets tend toward it

Although asteroids are quite numerous, and quite near by comparison, tend to be much smaller than the largest comets and they also tend also to be slower than comets. Due to their nearness they simply do not have as much time, or gravitational fetch, to accelerate as fast as would comets which originate from much further away. Beyond this, Asteroids tend in fact to be more stationary once in orbit than are Comets because they are usually made of rocky, or metallic material, where Comets are usually made of Ice and Dust. This ice will often melt once it comes into close orbit around the Sun, and this often tends to result in explosive propulsion which gives the Comet the ability to actually change direction and orbit and thus in theory at least able to cover more target area than an Asteroid.

Moreover, the Asteroid belt is stewarded by the giant planet Jupiter, whose powerful gravitational field will tend to pull the asteroids away from the Sun and Earth and out closer to itself. Comets however tend to encounter the very opposite effect from Jupiter in that if ever they should pass close to Jupiter in their history, they will tend to be drawn towards the Sun and Earth. This same would apply with encounters they may have with the other giant planet, Saturn, as well. Thus any comet coming close enough to Jupiter or Saturn will actually tend to come nearer to the Earth and present an even greater danger. Even Neptune and Uranus are in some ways large enough to have this effect, though to a lesser degree than the giant planets Jupiter and Saturn, being that they are substantially smaller, though still four times as large as Earth.

#### Kuiper belt objects

There are two gigantic breeding grounds for Comets in the Solar System. The first, known as the Kuiper belt, is a large tube like region around the Sun, beyond the orbit of the planet Neptune. This region is quite extensive and may reach as far or more than 50 astronomical units (au) which are each equal to the distance between the Earth and Sun. That is a very great distance away but there are possibly many hundreds of billions of these objects out there and some are quite large, with unstable orbits.

A few of these objects are nearly as large as the former planet Pluto which by the way is also a kuiper belt object itself. Years ago Pluto was designated as a planet, or the ninth planet of the Solar System, but it has since been demoted to a so called Trans-Neptunian Object because it’s orbit extends beyond Neptune. However, whatever these designations are worth, the reality is that there are many other Trans-Neptunian objects in the Kuiper belt, and though none so far have been found to be as large as Pluto, many are nearly as large. In the past twenty years, hundreds of very large objects have been found to exist in the kuiper belt orbiting around the Sun, and by some estimates there are at least 70,000 objects with a diameter larger than 100 kilometers in the Kuiper belt.

This is somewhat disconcerting. The truth is that many of these objects do not really have stable orbits at all no matter what anyone might say. Any of these small planetoids can easily be swayed by the Giant planets in our solar system, like Neptune, or Uranus, or Saturn, or Jupiter, or can even collide with one another and take a very different path than the one they are on now. The Kuiper belt in not as gravitationally stable as are the lower orbits around the Sun, and thus it does not take all that much to move an object in the direction of the Sun, even a very large object from orbit. Any small collision, or any gravitational nudge can conceivably send a very large object into the gravitational field of the Sun and thus cause it to accelerate towards Earth.

The possibility always exists that one day we may wake up and find that a number of very large objects are heading towards Earth orbit. It is quite possible that a single disturbance can result in the breakup of a large object into smaller but deadly comets and head towards the inner Solar System as they make a b-line towards the Sun. Many would say that this is nothing more than an another foolish end of the world scenario that never happened, or never will. But when we look very carefully at the facts we can see that these things not only can happen, but almost without question have happened before!

Where the Comets lie in wait-in the deep darkness of the Solar System

#### Oort cloud objects

There is a region beyond the Kuiper belt, that is even larger and more extensive, and even more gravitationally unstable. Though much further away, the gigantic Oort cloud , named after Jan Oort, the Dutch Astronomer who is credited with its discovery, is the probable well source of all objects in the Kuiper belt. This region is spherical and exists at the very edge of our solar system. It is huge, and contains trillions of objects and is where most long period comets originate from. The theory is that it is a leftover from the formation of the Solar System billions of years ago. Objects in the Oort cloud are so far away from the Sun, that they are barely subject to the Sun’s gravitational pull, and thus are at all times more prone to extreme deviations even after small changes in the overall field. For example even a nearby encounter with a star can perhaps cause objects in the Oort cloud to change their orbital direction, sometimes towards the Sun and Earth.

There are two types of comet orbit designations. One is a short period comet which comes from the Kuiper belt. These tend to show up periodically, like Halley’s comet for example. They go around the Sun, then head back out to the Solar System’s outer limits, only to return in a few decades.

Long period comets are comets that tend to have exceedingly long period orbits and originate in the Oort cloud. These can have an orbital period as long as 50,000 years! That is to say we might be due to see a comet appear for the first time in human history that originally started its journey some 25,000 years ago! Indeed, this object might not only appear on the horizon on any given day, it could be heading for Earth, and it could be gigantic. In fact it could be much larger than the asteroid commonly supposed to have killed off the Dinosaurs that is often quoted as six miles across, but some Trans-Neptunian Objects can easily be hundreds of miles of across.

#### Earth strikes happen often

The incidence of comet strikes on Earth are actually quite numerous. Most are tiny and we hardly notice. Others, larger tend to make themselves known. However, though relatively rare, they happen, and they may well happen in streaks. This last statement should not be taken lightly. The reason may be that one general perturbation may be responsible for dislodging a number of comets from their orbit at once. We may not simply see one comet get close to Earth, there may be a number of these all at once. The same goes for asteroids, as they too can happen in streaks. Indeed the last few sightings have seemingly come together in multiple occurrences.

The most recent of these, the meteor that struck Russia’s Chelyabinsk region, is said to have disintegrated in the atmosphere, just like one such comet or asteroid did in 1909, the famous Tunguska event in Siberia. Yet, though they disintegrated in air, their blasts were formidable causing quite a bit of damage. If not for a few accidentals, like for example the fact that these regions are quite isolated, we might well have been looking at a horrendous tragedy of unheralded proportions.

More often than not objects that strike the Earth are most often said to be asteroids, but many actually turn out to be comets. The fact is that comets are far more likely to blow up in the atmosphere than asteroids. It actually turns out that the last event at Chelyabinsk really was an asteroid because particles were subsequently found which as far as we know designated it as an actual asteroid. But the Tunguska event, which was much more powerful and destructive, left no crater and no particles. And many now believe it was indeed a comet that disintegrated in air before actually striking the Earth. Yet the shock wave was large enough to flatten trees for miles around.

S ome pretty big objects out there. Not much holding them where they are.

Comets can get awfully large and cause an awful amount of destruction. Potentially, due to both size and speed, they can actually cause far superior destruction than the greatest of asteroids. They are also probably far more numerous, and worst of all, they are far less easy to detect. As we have said before they tend to be made of ice and so will be subject to melting as they approach the Sun. This can cause the explosive release of gas within the comet and change its orbit. Thus they are not only hard to predict because they come from so far away, they are also inherently unpredictable due to the material they are made from. Yet they are both numerous, and potentially extremely destructive and are well known to appear from time to time.

We can of course say that comet or asteroid strikes are only a matter of luck. True, these are a matter of luck, but mostly bad luck. The Earth has gone four billion years without a life ending comet blast. A sufficiently large comet like the ones we are referring to here, nearing 100 kilometers in diameter are not survivable. It is quite unlikely that any life at all is going to survive, not even sea life, should an object that large get past the Moon and strike Earth at full speed. And yet there are billions of these objects out there! Any one of those objects might have begun its trip towards our planet thousands of years ago, and we would not detect it if it is under planetoid size especially since these things tend to have what are called eccentric orbits that do not adhere to what is called the ecliptic, or the usual orbit of planets around the Sun. Therefore, these comets, especially those long period comets originating in the Oort cloud might actually come from the direction of the poles where there is very little monitoring since there are so few people watching. We may not discover such a body or group of bodies till they are only a few months away. Should that happen now, we would probably have to endure an agonizing few months of utter social degeneration before the final, ghastly cataclysm, a nightmare greater than any we can imagine, or authentically describe.

So what should we do about this admittedly unlikely event? Well first let us consider if it is really all that unlikely. We have not seen evidence that a very large comet has struck Earth because if it did, we probably wouldn’t be here at all. It would have wiped out all life in all probability. But if in four billion years it has not happened, and the possibility has existed that long, could we say possibly that such a terrible event might be well overdue? Do we know? Truth is we really honestly do not know what the true odds are here because we don’t as yet know how many such objects there really are out there, and just how often do they tend to drop in for a visit. Remember we have only been monitoring the skies for a few thousand years, these objects may take hundreds of thousands of years to appear, or millions even. Thus they may in the distant past have come close quite often, we were simply not here to see them. That is an awfully scary notion when you think about it. Yes, it is obviously somewhat annoying to consider such an unlikely event and try to make it a possibility, as we are sure many of you might be thinking by now. Why stir up such unlikely scenarios anyway with so much more pressing matters to think of? Yet the reality is that in this particular case we really don’t know how unlikely it is, or is not. We have had a spate of near misses in the past few years. Are they getting more numerous?

Consider the event recently where the Chelyabinsk meteor strike actually happened while another asteroid, larger that it, was scheduled to just miss Earth, and thankfully it did! But it was a terrible shock to hear that on that very same day when one asteroid missed us, another hit us! On the day we were struck, two asteroids were gunning for our planet at the same time! There is at least some small reason to consider that these things are not as farfetched as we might believe.

We are learning that in the past the Earth has been struck by meteors of varying size and destructive power many times. But apparently not by a terminal body of very large dimensions, though we know now that they do exist, and can potentially head our way. Yet after four billion years, perhaps we should consider it as a possibility.

### Comet Shoemaker-Levy left no doubt, comets can strike planets

A few years ago we witnessed the comet Shoemaker-Levy strike Jupiter. It was a large comet, approximately six miles in diameter, and the damage done was tremendous. Had it struck the Earth, you would probably not be reading this, and I would not be writing. Astronomers were dazzled by its force and power, but they were also surprised by the fact that it had broken up into smaller pieces by Jupiter’s enormously powerful gravitational field, yet still managed to do a great deal of damage. Had it hit Earth, it probably would not have broken up at all and would have hit us in one piece and some things very ugly and sad would have been left behind. But we needed no more proof that these things really do happen. Those comets are out there. They are real. They can become our most horrific nightmare any day or night. They can destroy us, and everything we have ever known of, or ever loved on Earth.

But again, what should we do? Is there a good enough reason to begin to develop counter measures? Considering costs, and so many other problems should we even bother with such a small, purely random event? Are there any sound reasons to proceed to some sort of genuine planetary defense against such rare, if albeit destructive occurrences?

We think there are. In the end, we do have the technology. Yes it will be expensive, admittedly, but the technology is there. We can build a specific kind of rocket, or possibly even lasers that would be able to significantly deflect or destroy a reasonably sized body. No it could not probably protect us from a planetoid, but we could probably protect our world from what might be smaller bodies which are after all far more numerous and so perhaps more dangerous when taken altogether. There is a general consensus that such a system could be built in earnest if we really wanted to build it. Even if it meant nuclear tipping a rocket, consider what we are talking about. Consider carefully what it would mean if suddenly we discovered a very large comet was heading towards us, or a group of such comets. Consider what this might be like to live the last few months in sheer agony and regret for not having taken steps to prevent it. Having some option would certainly be desirable. But without actually building such a system, and testing it years in advance, it could never become operational in the few months to a year that we might have warning. This is why it is perhaps time to consider building a defense system against either asteroids, or comets. To save all life on Earth is no small feat, nor is it mean, or ignoble when you really think about it. After so many billions of years, our time may be running out. The planet Earth has given us everything we have. She has nurtured, protected and sustained life for four billion years, perhaps now is the time for life to do the same for her, our one common mother. Perhaps it is in some way a duty to consider.

### A Post Script : The End of the World Hypothesis

A post script is in order. Recently a theory published by an astronomer Michael Rampino, warned of the potential for a mass extinction as the galactic planes lines up with the Kuiper belt during the age of Aquarius, and possibly begins to knock some of these Trans Neptunian objects towards Earth being that their orbits tend to be unstable and easily changeable through the action of the galactic core gravitational force. He named this the “Shiva” hypothesis, Shiva being the Hindu Goddess of destruction. Though we do not honestly know that this hypothesis is all that likely, who’s to know in the end? To be sure, the Maya predicted this age would be the end of ages, and this is a new age as the Sun enters a new astrological configuration in the heavens. And it is true that indeed the Solar System is actually lining up with the galactic core plane during this coming age. We don’t know in the end what truth, or lack of there may be in these events as causes for a new wave of extinctions, as the Shiva hypothesis proposes. However, what we do know is that these icy , distant, dark objects really are out there, they visit us often, and have caused massive destruction in the past, and but only a few years ago on Jupiter. Thus far our lovely planet has escaped the worst of it, but its time may be running out. We may be overdue for some such occurrence. Perhaps it is time to consider in earnest taking an insurance policy against the very worst that can happen. It would be a shame to lose it all now, wouldn’t it?

## List of Earth-crossing minor planets

An Earth-crosser is a near-Earth asteroid whose orbit crosses that of Earth as observed from the ecliptic pole of Earth's orbit. [1] The known numbered Earth-crossers are listed here. Those Earth-crossers whose semi-major axes are smaller than Earth's are Aten asteroids the remaining ones are Apollo asteroids. (See also the Amor asteroids.)

An asteroid with an Earth-crossing orbit is not necessarily in danger of colliding with Earth. The orbit of an Earth-crossing asteroid may not even intersect with that of Earth. This apparent contradiction arises because many asteroids have highly inclined orbits, so although they may have a perihelion less than that of Earth, their paths can never cross. An asteroid for which there is some possibility of a collision with Earth at a future date and which is above a certain size is classified as a potentially hazardous asteroid (PHA). Specifically, an asteroid is a PHA if its Earth minimum orbital intersection distance (MOID) is <0.05 AU and its absolute magnitude is 22 or brighter. [2] The concept of PHA is intended to replace the now abandoned strict definition of ECA (Earth-crossing asteroid) which existed for a few years. Determining if an asteroid was an ECA required calculation of its orbits millennia into the future, including planetary gravitational perturbations, to assess whether a collision with Earth was possible and this has proved to be impractical. [3]

Having a small MOID is not a guarantee of a collision. On the other hand, small gravitational perturbations of the asteroid around its orbit from planets that it passes can significantly alter its path. For instance, 99942 Apophis will approach Earth so closely in 2029 that it will get under the orbit of the Earth's geostationary satellites. [4]

Of the Earth-crossing asteroids, 3753 Cruithne is notable for having an orbit that has the same period as Earth's.

## Halley's in the Space Age

When Halley's Comet came by Earth in 1986, it was the first time we could send spacecraft up to look at it.

That was a fortunate occurrence, as the comet ended up being underwhelming in observations from Earth. When the comet made its closest approach to the sun, it was on the opposite side of that star from the Earth &mdash making it a faint and distant object, some 39 million miles away from Earth.

Several spacecraft successfully made the journey to the comet. This fleet of spaceships is sometimes dubbed the "Halley Armada." Two joint Soviet/French probes (Vega 1 and 2) flew nearby, with one of them capturing pictures of the heart or nucleus of the comet for the first time.

The European Space Agency's Giotto got even closer to the nucleus, beaming back spectacular images to Earth. Japan sent two probes of its own (Sakigake and Suisei) that also obtained information on Halley.

Additionally, NASA's International Cometary Explorer (already in orbit since 1978) captured pictures of Halley from 17.3 million miles (28 million km) away.

"It was inevitable that this most famous of all comets would receive unprecedented attention, but the actual magnitude of the effort has surprised even most of those involved in it," NASA noted in an account of the event.

Sadly, the astronauts aboard Challenger's STS-51L mission were also scheduled to look at the comet, but they never got the chance. The shuttle exploded about two minutes after launch on Jan. 28, 1986, due to a rocket malfunction, killing all seven astronauts.

It will be many decades until Halley's gets close to Earth again, but in the meantime you can see its remnants every year. The Orionid meteor shower, which is spawned by Halley's fragments, occurs annually in October. Halley's also produced a shower in May, called the Eta Aquarids.

When Halley's sweeps by Earth in 2061, the comet will be on the same side of the sun as Earth and will be much brighter than in 1986. At least one study has pointed out that it is difficult to predict Halley's orbit on a scale of more than 100 years, and that the comet could collide with another object (or be ejected from the solar system) in as little as 10,000 years, although not all scientists agree with the hypothesis.

When Halley next returns to Earth's vicinity, one astronomer predicted it could be as bright as apparent magnitude -0.3. This is relatively bright, but well below that of the brightest star in Earth's sky: Sirius, at magnitude -1.4 as seen from Earth.

There is a group of comets called "Halley family comets" (HFC) because they appear to share the same orbital characteristics of Halley, including being highly inclined to the orbits of Earth and other planets in the solar system. However, this family has a range of inclinations, which prompts other astronomers to suggest they may have a different origin than Halley. Some suggest these comets could have evolved from members of the Oort Cloud, or from Centaurs (objects that generally have a closest approach between Jupiter and the Kuiper Belt.) Alternatively, HFCs could have come from somewhere just beyond Neptune.

While it will be decades before we can send another spacecraft to Halley's Comet, there have several other missions that have studied comets from up close. Between 2014 and 2016, for example, the Rosetta probe examined Comet 67P/Churyumov&ndashGerasimenko up close and made comparisons to other comets. One of its key findings was uncovering that Comet 67P had a different kind of water (specifically, a different deuterium-to-hydrogen ratio) than what is seen on Earth. Back in the 1980s, similar examinations of Halley by the Giotto probe also showed that Halley has a different D-to-H ratio in its water than on Earth.

Other notable cometary missions include NASA's Stardust (which captured samples of comet 81P/Wild and returned them to Earth), NASA's Deep Impact (which deliberately sent an impactor into 9P/Tempel on July 4, 2005), and the European Space Agency's Philae (which landed on Comet 67P in 2014.)