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Understanding > Knowing more > Astéroids and comets III
ASTEROIDS, COMETS AND METEORITES:
A DANGER FOR THE EARTH?
We have studied three types of celestial objects, asteroids, comets and meteorites which have a common characteristic: their orbits can cross the one of the Earth. In particular, asteroids whose orbit crosses that of the Earth have been identified in a so-called near-Earth asteroids category. However, the list of these bodies is growing day after day. Increasingly, most often, the media announce that we have just escape a cataclysm, an asteroid having just brushing us. What is it? Is there a danger for the inhabitants of the Earth? We propose to answer these questions.
The Near-Earth Objects family
NEOs are mostly asteroids, but it must also include the short-period comets which also have orbits crossing the Earth. The comets, smaller and less dense, are, moreover, generating meteor of small sizes able to cross Earth's orbit.
The near-Earth objects are divided into 6 categories described in the table below. In this table, it was recorded q the distance to perihelion in astronomical units, P the orbital periode in years, a the semi-major axis of the orbit and Q the distance to aphelion in astronomical units (AU).
| Group | Description | Definition |
|---|---|---|
| NEC | Comets close to Earth (Near-Earth Comets) | q<1.3 AU, P<200 years |
| NEA | Asteroids close to Earth (Near-Earth Asteroids) | q<1.3 AU |
| Aten | NEA (Near-Earth Asteroids) the semi-major axis of which being smaller than the one of the Earth (from the name of the asteroid 2062 Aten). | a<1.0 AU, Q>0.983 AU |
| Apollo | NEA (Near-Earth Asteroids) the semi-major axis of which being larger than the one of the Earth (from the name of the asteroid 1862 Apollo). | a>1.0 AU, q<1.017 AU |
| Amor | NEA (Near-Earth Asteroids) the orbit of which being outside the orbit of the Earth and inside the orbit of Mars (from the name of the asteroid 1221 Amor). | a>1.0 AU, 1.017<q<1.3 AU |
| PHA | Potentially Hazardous Asteriods: NEA the intersection distance with the Earth (DMIT) being smaller than 0.05 AU and having an absolute magnitude less or equal to 22. | DMIT<=0.05 AU, H<=22.0 |
The profusion of NEOs or objetcs close to the Earth is a recent discovery due to the increase of the power of telescopes.
The table below shows the number of known objects at different dates until today and thus the progress of discovery.
| Date | NEC | NEA | Aten | Apollo | Amor | PHA | PHA<18 | NEC+NEA |
|---|---|---|---|---|---|---|---|---|
| 2022-12-03 | 117 | 2437 | 14218 | 12646 | 2319 | 28074 | ||
| 2022-01-15 | 117 | 27957 | 2198 | 14200 | 11558 | 2251 | 160 | 28074 |
| 2019-12-10 | 1393 | 10835 | 9093 | 2026 | ||||
| 2018-12-05 | 1415 | 9589 | 8132 | 1949 | 19169 | |||
| 2017-11-16 | 1051 | 7251 | 6195 | 1859 | 17146 | |||
| 2014-01-10 | 817 | 5245 | 4445 | 1446 | ||||
| 2013-02-25 | 766 | 4816 | 4062 | 1379 | 179 | 9618 | ||
| 2012-01-01 | 683 | 4190 | 3538 | 1272 | ||||
| 2011-01-01 | 633 | 3834 | 3183 | 1189 | 7650 | |||
| 2009-01-01 | 167 | 5867 | 491 | 2891 | 2475 | 988 | 165 | 6034 |
| 2008-01-01 | 5055 | 434 | 2469 | 2152 | 890 | 161 | ||
| 2007-01-01 | 4392 | 366 | 2203 | 1823 | 806 | 157 | ||
| 2006-01-01 | 3753 | 317 | 1888 | 1548 | 721 | 154 | ||
| 2005-01-01 | 3135 | 261 | 1574 | 1288 | 623 | 143 | ||
| 2004-01-01 | 2603 | 205 | 1286 | 1102 | 534 | 129 | ||
| 2003-01-01 | 2164 | 169 | 1071 | 919 | 450 | 119 | ||
| 2002-01-01 | 44 | 1679 | 126 | 838 | 715 | 360 | 105 | 1723 |
| 2001-01-01 | 40 | 1239 | 96 | 620 | 523 | 283 | 84 | 1279 |
| 2000-01-01 | 39 | 878 | 58 | 452 | 368 | 210 | 70 | 917 |
| 1999-01-01 | 38 | 651 | 46 | 335 | 270 | 161 | 56 | 689 |
| 1998-01-01 | 38 | 447 | 27 | 233 | 187 | 107 | 48 | 485 |
| 1997-01-01 | 38 | 393 | 23 | 203 | 167 | 96 | 43 | 431 |
| 1996-01-01 | 38 | 347 | 21 | 175 | 151 | 85 | 41 | 385 |
| 1995-01-01 | 38 | 315 | 20 | 161 | 134 | 82 | 40 | 353 |
| 1994-01-01 | 36 | 271 | 16 | 140 | 115 | 69 | 35 | 307 |
| 1993-01-01 | 36 | 231 | 14 | 117 | 100 | 62 | 32 | 267 |
| 1992-01-01 | 36 | 199 | 12 | 103 | 84 | 60 | 30 | 235 |
| 1991-01-01 | 34 | 159 | 10 | 76 | 73 | 50 | 26 | 193 |
| 1990-01-01 | 34 | 134 | 9 | 63 | 62 | 43 | 22 | 168 |
| 1980-01-01 | 31 | 53 | 4 | 27 | 22 | 18 | 13 | 84 |
| 1970-01-01 | 29 | 28 | 1 | 13 | 14 | 11 | 8 | 57 |
| 1960-01-01 | 28 | 21 | 1 | 10 | 10 | 9 | 7 | 49 |
| 1950-01-01 | 28 | 13 | 0 | 7 | 6 | 5 | 4 | 41 |
| 1940-01-01 | 24 | 9 | 0 | 3 | 6 | 3 | 2 | 33 |
| 1930-01-01 | 23 | 5 | 0 | 0 | 5 | 0 | 0 | 28 |
| 1920-01-01 | 21 | 3 | 0 | 0 | 3 | 0 | 0 | 24 |
| 1910-01-01 | 19 | 1 | 0 | 0 | 1 | 0 | 0 | 20 |
| 1900-01-01 | 17 | 1 | 0 | 0 | 1 | 0 | 0 | 18 |
Note: "PHA<18" correspond to PHA the absolute magnitude of which being less than 18, i.e. their size is at leat one kilometer.
Comets and meteorites
Comets have highly eccentric orbits and are very sensitive to perturbations by the planets. They often encounter them (see for example the fall of the comet SL9 on Jupiter or the fall of comets on the Sun observed by the SOHO satellite. Smaller comets have a gravitational cohesion very low and often break into pieces . They also eject many particles of all sizes some of which falling on the Earth as falling stars, meteors and meteorites. Every day, a hundred tons of these particles fall on Earth, but mostly the atmosphere protects us. We will see that this is not the case for large-sized meteorites.
Is life coming from space?
Life on Earth seems to have appeared due to the presence
water, organic molecules built from atoms of
a carbon and energy source. The presence of water and of such
molecules on the NEOs
may suggest that life could be of an extraterrestrial origin.
At the time of its formation, the Earth was bombarded by
a multitude of NEOs, asteroids and comets,
which have participated in its increase in size. This phenomenon
took place about 4 billion years ago on an
Earth too hot for life to appear. When the Earth had cooled
and the bombing decreased, the conditions for the development of
life appeared. One hypothesis is that the first
molecules at the origin of this development could
then arrive very quickly on Earth through the
fall of a comet that already contains these molecules,
thus avoiding the long process of apparition of such molecules
at the surface of the Earth autonomously .
Potentially hazardous NEO: which ones and how many?
Potentially hazardous NEO (asteroids or comets) are indeed those whose orbit crosses that of the Earth. They are however very dangerous only if they collide the Earth's surface, that is to say, if they are not stopped by our atmosphere and turned into shooting stars. Up to 50 meters in diameter, there are many chances for them being destroyed by their passage through the atmosphere of the Earth -especially for comets, weaker- producing an energy release up to the equivalent of a bomb of 5 megaton that explodes in the upper atmosphere. Beyond this size, the object will surely reach the Earth causing damage. This damage will remain limited for an object of the kilometer size. For a larger size, the consequences would affect the entire planet. A body of 2 km in diameter will cause an explosion equivalent to a bomb of one million megatons and a body 15 kilometers a bomb of 100 million megatons. In the latter case, most living species will disappear. It is better not to imagine a collision with an even bigger body...
The above table gives the number of such objects found until today. Astronomers believe that there are more than 1000 objects larger than one kilometer (until 25 km) and perhaps a million NEOs whose diameter exceeds 50 meters.
See the tables providing the dates of close approaches either in the future or in the past for the asteroids and the comets. A table providing the correspondance between the absolute magnitude and the size of the objec is available here.
Previous collisions: the case of the "Meteor Crater" in Arizona
The risk of collisions of NEOs with Earth has led scientists to wonder if such collisions had already taken place. We knew the meteorites, but until the early twentieth century, no catastrophic collisions were thought to be possible. The site of the "Meteor Crater" in Arizona was not then interpreted as a crater impact, like lunar craters. In 1891, the controversy was in full swing: was the "Meteor Crater" volcanic or had it been caused by the fall of a giant meteorite? Other meteorites of several tons had been discovered, but slowed by Earth's atmosphere, they had not formed craters.
It was not before 1901 that the presence of small pieces of meteoritic iron around the crater was interpreted as evidence of the meteoritic nature of the crater. Furthermore, the presence of powdered material in the crater was the proof of an enormous pressure which had prevailed at that place, the absence of any volcanic material in this site and the nature of the quartz found there can come only from a strong rise in temperature, proved definitely the meteoritic nature of the crater.
The Meteor Crater is actually the the best preserved impact crater on the surface of the Earth. It measures 1,500 meters in diameter, 200 meters deep and is surrounded by cliffs 50 meters high. Over an area of more than 10 kilometers from the crater we found pieces of meteoritic iron. The power generated by the impact should be of the order of 3 to 4 megatons. The age of the crater is about 50 000 years.
Since, it was discovered other impact craters of all sizes, mainly in deserts because the rain and vegetation are rapidly removing small craters.
Previous collisions: the case of the impact of the Tunguska
On June 30, 1908 at 7 hours 17 minutes, a huge ball of fire visible in Western China to Central Russia crossed the night sky by a deafening noise. An explosion of power of a bomb from 20 to 40 megatons (thousand times the power of the Hiroshima bomb) destroyed more than one thousand square kilometers of forest and killed herds of reindeers in a place called "Tunguska" in Siberia. An orange glow lit up the sky and was seen in Western Europe. Shortly after the impact, a black rain of debris fell on the region. The place was so isolated, no human being has been killed. The nearest humans were deafened by the power of the explosion. 70 kilometers away, in the town of Vanavara, houses were badly damaged and the population projected in the air. The noise was heard up to 800 km. At Kansk, a Railway station of the Trans-Siberian located at 600 kilometers from the impact, passengers were thrown from a train out of their seats and the sound of the explosion made them believe the end of the world ... No scientist then went to see what happened, and the evidence of this phenomenon, in addition to direct witnesses, is a shock similar to that of an earthquake, recorded by the seismograph in Irkutsk more than 1,000 miles away. Many seismographs worldwide also resented an earthquake.
It was not until 1921 that a scientific expedition went there to try to locate a crater. Neither crater nor meteorite were found, but after 6 years of research, a vast area of nearly a thousand square kilometers where all the trees had been lying and curiously stripped of their branches by the breath was located. The witnesses were interviewed and many explanations were born since the fall of an asteroid, from the antimatter, until the explosion of an alien spaceship. The hypothesis of an alien nuclear bomb was also advanced in 1946, but the absence of radiation overturned this hypothesis also very fanciful. Indeed, it is now widely believed -and this is confirmed by computer simulations- that this is an asteroid of 60 meters in diameter and weighing 100,000 tons, which was fragmented in the Earth's atmosphere and exploded 7 kilometers in altitude because of its angle of entry in the atmosphere. This should be a stony asteroid with some ice (maybe a comet?) because a metallic asteroid would reach the ground.
The disappearance of the dinosaurs
The existence of the impact crater in Arizona, as well as the disaster in Tunguska has encouraged researchers to locate other impact craters. Geological and meteorological conditions does not allow these craters to remain apparent for a long time. Erosion, rainfall, wind, crustal motion combined to make disappear quickly -in the astronomically sense- traces left by these collisions. Despite this, we have identified approximately 150 sites today of impacts on Earth, suggesting that there are thousands of impact that have occurred since life began.
Thus, high levels of iridium, a rare metal on Earth but abundant in meteorites, identified in geological layers dating from the late Cretaceous, 65 million years ago, suggest that the fall of a giant meteorite took place. The impact crater was located in the Yucatan peninsula (Central America) and was caused by an object of more than 10 kilometers in diameter. Its fall would have resulted in the disappearance of more than 75 % of living species, especially dinosaurs, because of the dust which has then overrun the atmosphere, interrupting the development of plants and breaking and the food chain for most species. The mammals, small creatures then undeveloped and needing little, have survived, freed competing species which had impeded their development in the past. If the consequences of the fall of such a body are only assumptions, the fall itself is a proved fact.
The effects of impacts should we be afraid?
We have seen two facts. First, impacts have occurred in the past and daily, shooting stars remind us that the Earth does not move in a total vacuum. Second, the observations show that there is a large number of NEOs able to strike the Earth. Even before serious conclusions have been proposed, all kinds of deductions were advanced, and with each new observation of NEOs approaching Earth, the media like to scare people. Better, disaster movies took the subject and depict catastrophic impacts, as in "Armageddon" or "Deep Impact", filmed in 1998. Note, however, that the film "Deep Impact" restores pretty much what would be the fall of a NEO on Earth. Of course, in these films, heroes fly to the threatening object and attempt to deflect its route, with more or less success. We will go back to the opportunities available to us for that purpose.
In fact, regardless of the number of NEOs that we discover, and whatever their orbits, the probability of an impact with the Earth will not increase and will remain as before our observations. What is new is that we are now able to identify dangerous objects and we want to defend against them. Thus, the probability of impact is : - very small for very large objects (the last date 65 millions years), lower for smaller objects (a few tens of meters in diameter) causing only local damage but not negligible (Meteor Crater, Tunguska), high - for pebbles touching the ground, but causing only negligible damage (and no human deaths so far). It is therefore against objects of "Tunguska" type that we must find a parade. One can also say that the probability of accidental death of a human is very high for all types of risks of everyday life (traffic accidents, domestic accidents, ...), great for risks from terrestrial natural disasters (storms, hurricanes, earthquakes, floods, volcanic eruptions) and low for the risk of impact by NEOs. Of course, the death of 500 million human beings, even for one in 1 million years is more impressive than 10 people in a storm ...
The Turin scale
Regarding the risk of collision of Near Earth asteroids with the Earth, collisions which are fortunately very rarely dangerous for humanity but often evoked by the media when passing close to a NEO, the International Astronomical Union created a working group to coordinate observations and to determine the probabilities and consequences of such collisions. To simply inform the public, a scale of risk has been established at a meeting of the working group in Turin in 1999 under the leadership of R. Binzel of MIT in Boston. This scale is similar to the Richter earthquake scale: for each NEO discovered, we can associate a number on the Turin scale describing the risks for humanity during the XXIth century.
| Event without real consequences | 0 | The object is too small to reach the surface of the Earth or the probability of collision is near zero. |
|---|---|---|
| Event for survey | 1 | The probability of collision is very low, same as that of a collision with an object not detected. |
| 2 | The probability of collision is very low but not zero with very limited damage in case of collision. | |
| Event needing precautions | 3 | The probability of collision is of the order of 1% with damage that remain highly localized. |
| 4 | The probability of collision is of the order of 1% but with significant localized damage. | |
| 5 | High probability of localized destruction. | |
| Very dangerous and likely impact | 6 | The probability of overall disaster is large. |
| 7 | The probability of overall disaster is very large. | |
| 8 | Collision with localized destruction. Probability: every 50 to 1000 years. | |
| Sure and very important disaster | 9 | Collision able of destroying a part of the Earth's surface. Probability: every 1000 to 100000 years. |
| 10 | Collision able of causing a climate catastrophe for the whole Earth. Probability: every 100 000 years. |
The Turin scale ranges from 0 to 10: 0 indicates an object having
very little chance of reaching the Earth or being too
small to reach the ground; 10 inénemedicates that the collision is certain and
the object is large enough to cause a worldwide disaster. The five colors, from white to red, have the following meanings:
- white: "Event without real consequences", correspond to number 0;
- green: "Event for survey", corresponds to
objects whose trajectory has a small risk of encountering the Earth.
Monitoring of the orbit must be done to be able
to make a precise calculation;nbsp;
- yellow: "Event needing precautions",
corresponds to the approach of an object having a high chance of reaching the
Earth within a few decades. A
very detailed study of the orbit is necessary;
- orange: "Very dangerous and likely impact", for which
probability is almost certain in the next century.
The precise orbit computation is a top priority;
- red: "Sure and very major disaster" corresponds to
an object whose trajectory will surely encounter the Earth
and is large enough to cause damage very
important on the ground, up to the global catastrophe
in the case of number 10.
The number on the Torino Scale is calculated from the probability of collision and the kinetic energy of of the object (equivalent to a one-megaton explosion corresponding to a kinetic energy of 4.3 × 1015 Joules) as shown in the figure below. An object placed on this scale can see its number decreasing after further studies but not increasing.
The Future: predictions, observations, protection
Since we now have a new understanding of the risk involved by humanity, we must limit it at best.
First thing to do : observe the NEOs, know them at the most exhautive manner and determine their orbits with more precision. Currently, several observational programs are underway. These observational programs mobilize less than 100 people worldwide. The observational program the more productive is the LINEAR program of MIT Lincoln Laboratory, using two small 1m-telescopes, in New Mexico (USA) with the support from NASA and the U.S. Air Force. Also include the NEAT research program in Hawaii and the Spacewatch program at the University of Arizona. Studies of NEO also exist in Japan, France, Italy and China. In addition to the discovery of new objects, physical study of these objects, in particular through radar echo must also be conducted to know them better and one day, to protect us from them.
At the present time, we know more than one half of the objects that are larger than one kilometer. None of these objects will hit the Earth in the near future and we need to keep an inventory of all potentially dangerous NEO. The Spaceguard project of the NASA was to identify 90% of the objects whose size exceeds a kilometer before 2008. The European Spaceguard foundation and the International Astronomical Union coordinate the observational projects at international level. If NEO more than one kilometer large could cause a global catastrophe on Earth, the smaller objects are dangerous too. An impact like the Tunguska in 1908, occurring over an inhabited area, would be disastrous for one or more countries. Knowing all the objects to a minimum size of a few tens meters in diameter will take time. Several decades are still needed, but the probability of impact is so low that few tens years is a very short time compared to these probabilities. The prediction of earthquakes on Earth is much more urgent for humanity.
Second problem to solve : if the trajectory calculations predict soon an impact of an NEO more than 100 meters in diameter, what can we do? Several options are available to us depending on the size of the object. In the case of local damage, populations could be moved, provided that it was not for several million people. The probability of a drop in the ocean is obviously strong with the result a tidal wave. How then evacuate all coasts which are, in general, the most populated places? Alternatively, only for a large body generating a global catastrophe, is to try to destroy or deflect the object before the impact. The solutions advocated in the science-fiction movies (nuclear missiles for destroying the object) are illusory. The fragmentation of a large object could pose as much danger during the fall on Earth. Only the deflection of the orbit is conceivable: it is then imperative to know the date of the impact several years in advance (it is possible to do these calculations of predictions for all known objects, and for several decades in advance, celestial mechanics permitting). We should then deposit on the surface of the object a small reactor (ionic motor, for example) who would provide to the NEO a force even very small, but with time, changing the orbital trajectory that the object will misse the Earth. It could also be possible to mount a solar sail on the NEO, on which the solar radiation pressure would also be sufficient. It is no more science fiction, but very real possibilities. In 2022, NASA attempted to make a deflexion to double asteroid orbit through the DART mission.
As a conclusion
In conclusion, the danger today is not coming from known objects. The past experience shows that the risk is very low, although lower than any other risk of terrestrial origin. There therefore is no need to worry. We can quietly inventory all potentially hazardous objects to leave our descendants the information necessary for their protection.