A few hundred such near-Earth asteroids are known, ranging in size up to four kilometers. Tens of thousands probably exist, with estimates placing the number of NEAs larger than one kilometer in diameter at up to 2,000.
Astronomers believe that NEAs only survive in their orbits for 10 million to 100 million years. They are eventually eliminated either by collisions with the inner planets, or by being ejected from the solar system by near misses with the planets. Such processes should have eliminated them all long ago, but it appears they are resupplied on a regular basis.
Some of the NEAs with highly eccentric orbits appear to actually be extinct "short period" comets that have lost all their volatiles, and in fact a few NEAs still show faint comet-like tails. These NEAs were likely derived from the Kuiper belt, a repository of comets residing beyond the orbit of Neptune. The rest of the NEAs appear to be true asteroids, driven out of the asteroid belt by gravitational interactions with Jupiter.
There are three families of NEAs:
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2 An example of a recent asteroid impact 3 External Link |
The general acceptance of the Alvarez hypothesis, explaining the Cretaceous-Tertiary extinction event as the result of a large asteroid or comet impact event, has raised the awareness of the possibility of future Earth impacts with asteroids that cross the Earth's orbit.
The threat of an Earth impact was emphasized by the collision of the comet Shoemaker-Levy 9 with Jupiter on July 16 1994, resulting in explosive impacts that would have been catastrophic on Earth. To be sure, Jupiter is far larger and more massive than the Earth and so undergoes far more impacts, but the event still provided an illustration that such things do happen and can be unimaginably destructive.
In fact on March 23, 1989 the 1,000-foot diameter Apollo asteroid 1989FC missed the Earth by 400,000 miles passing through the exact position where the earth was only 6 hours before. If the asteroid did impact it would have created the largest explosion in recorded history.
Asteroids with a 1 kilometer diameter hit the Earth a few times in each million year interval. Large collisions with 5 kilometer objects happen every ten million years. Small collisions (but still potentially dangerous ones) occur a couple times each month.
Although there have been a few false alarms, a number of asteroids are definitely known to be threats to the Earth. Asteroid 1950 DA was lost after its discovery in 1950 since not enough observations were made to allow plotting its orbit, and then rediscovered on December 31 2000. Proper calculation of its orbit then demonstrated that it has a 1 in 300 chance of hitting the Earth on March 16 2880. This probability is a thousand times greater than any other known asteroid threat, and 50% greater than all other known asteroid threats combined. 1950 DA has a diameter of a kilometer.
It is difficult to determine the chances of its impact better than that. The uncertainty is due to minor irregularities in the Sun's shape, and so its gravitational field; weakening of the Sun's gravity through mass loss from the solar wind of particles that streams out from its atmosphere; uncertainties in the masses and so the gravitational pull of the planets; variations in the tidal pull of the surrounding galaxy; the subtle pressure of sunlight; and, in particular, a phenomenon known as the "Yarkovsky effect".
This effect was discovered by a Russian engineer named I.O. Yarkovsky a century ago. It is a subtle process: the heating of the asteroid's surface causes it to emit thermal radiation, which creates a slight amount of thrust. It is somewhat unpredictable, since an asteroid's ability to soak up heat from the Sun depends on its terrain, and the effect is also influenced by the asteroid's spin orientation and rotation rate.
Astronomers have been conducting surveys to locate the NEAs. One of the best-known is the Spacewatch project, which uses an old 90 centimeter telescope sited at the Kitt Peak Observatory in Arizona, updated with automatic pointing, imaging, and analysis gear to search the skies for intruders. The project was set up in 1980 by Tom Gehrels and Dr. Robert S. McMillan of the Lunar and Planetary Laboratory of the University of Arizona in Tucson, and is now being operated by Dr. McMillan.
The Spacewatch project has acquired a 1.8 meter telescope, also at Kitt Peak, to hunt for NEAs, and has provided the old 90 centimeter telescope with an improved electronic imaging system with much greater resolution, improving its search capability. These new resources promise to increase the rate of NEA discoveries by Spacewatch from 20 to 30 a year to 200 or more.
NASA has considered a more extensive network of six 2.5-meter automated telescopes named "Spaceguard" to watch the skies for such intruders, but at present astronomers are still assessing the actual level of the threat posed by NEOs, and moving to such a "defensive" network is not generally regarded as justified at present.
Nonetheless, the fact that an impact of an NEA a kilometer or more in size would be a catastrophe unparalleled in human history has kept the idea of a defensive network alive, as well as led to speculations on how to divert objects that might be a threat. Detonating a nuclear weapon above the surface of an NEA would be one option, with the blast vaporizing part of the surface of the object and nudging it off course with the reaction. This is a form of nuclear pulse propulsion.
However, it is becoming increasingly obvious that many asteroids are "flying rubble piles" that are loosely glued together, and a nuclear detonation might just break up the object without adjusting its course. In some ways, being struck with a loose cloud of smaller asteroids is worse than being struck with just one big one. This has led to a variety of other ideas for dealing with the threat:
The NEA threat
Thinking on the matter continues - see Asteroid deflection strategies - and if there is no prospect of immediate action, the issue isn't going away, either.
On June 6 2002 an object with an estimated diameter of 10 meters collided with Earth. The collision occurred over the Mediterranean Sea, at approximately 34°N 21°E. The energy released was estimated (from infrasound measurements) to be 26 kilotons to TNT, comparable to a medium-size nuclear weapon [1]. At that time India and Pakistan were at a heightened state of alert, ready to initiate a nuclear weapon war with each other. If this asteroid impact had hit in this area the results might have been catastrophic.
An example of a recent asteroid impact
External Link
There is an excellent article in the current (October 2003) issue of Scientific American regarding NEA's and long term strategies for protecting Earth from them.\n