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A few hundred such near-Earth asteroids are known, ranging in size up to four kilometres. Tens of thousands probably exist, with estimates placing the number of NEAs larger than one kilometre 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:
Notice that all Atens and Apollos have eccentric orbits that cross the orbit of the Earth, making them potential threats to our planet while Amors do not cross Earth's orbit but some may come very close.
Also sometimes used is the Arjuna asteroidThe Arjuna asteroids are a class of near-Earth asteroids whose orbits are very Earth-like in character, having low inclination, orbital periods close to one Earth year, and low eccentricity. Arjunas are very uncommon, but of significance owing to the extr classification for asteroids with extremely Earth-like orbits. Near-Earth asteroid is a more restrictive term than near-Earth objectNear-Earth objects (NEO are asteroids, comets and large meteoroids whose orbit intersects Earth's orbit and which may therefore pose a collision danger. Due to their size and proximity, NEOs are also more easily accessible for spacecraft from Earth and ar.
The general acceptance of the Alvarez hypothesis, explaining the Cretaceous-Tertiary extinction eventThe Cretaceous-Tertiary (KT) extinction event also known as the KT boundary (from German: , was a period of extremely frequent extinction of species, about 65. 5 million years ago. It corresponds to the end of the Cretaceous period and the beginning of th as the result of a large asteroid or comet impact eventImpact events are caused by the collision of large meteoroids, asteroids or comets (generically- Bolides) with Earth and may sometimes be followed by mass extinctions of life. For discussion of impacts in general, not just on Earth, see crater. The geolog, 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 16July 16 is the 197th day (198th in leap years) of the year in the Gregorian Calendar, with 168 days remaining. Events 622 Beginning of the Islamic calendar. 1769 First performance of Mozart's opera The Abduction from the Seraglio. 1769 Father Junipero Ser, 19941994 is a common year starting on Saturday, and was designated the International year of the Family''. Events January events January 1 North American Free Trade Agreement (NAFTA) goes into effect January 6 Nancy Kerrigan is clubbed on the right leg by an, 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 4581 Asclepius (1989 FC) missed the Earth by 400,000 miles passing through the exact position where the earth was only 6 hours before. If the asteroid had impacted it would have created the largest explosion in recorded history.
Asteroids with a 1 kilometre diameter hit the Earth a few times in each million year interval. Large collisions with 5 kilometre 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 (29075) 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. (29075) 1950 DA has a diameter of a kilometre.
On March 18, 2004, LINEAR announced a 30 metre asteroid 2004 FH which would pass the Earth that day at only 42,600 km (26,500 miles), about one-tenth the distance to the moon, and the closest miss ever noticed. They estimated that similar sized asteroids come as close about every two years.
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.