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The Closest Object Ever

The closest natural object ever was discovered on 1991 January 18 by D Rabinowitz with the Spacewatch, Kitt Peak. Its discovery brightness was 17.5 magnitude. It was followed by Rabinowitz and J V Scotti for 5 hours, during which it moved 7.1 degrees to the east and south.

From these measures an Apollo type orbit was computed with the following approximate elements: T="1991" Mar 2.06, Peri="70.58," Node="118.34," i="1.96" (equinox 1950.0), q="0.713" AU, e="0.682," a="2.24" AU. All the observations are satisfied within 1 arcsec.

With H="28.5" the object is presumably some 10 times smaller than 1990 UN, until now the smallest known celestial object. 1991 BA is estimated to be 5-10 metres across.

During the period it was observed its distance decreased from 0.0052 to 0.0033 AU and as it passed the Earth on Jan 18.72 it was only 0.0011 AU (170,000 km) away.

On 1991 October 7 another, similar object (1991 TU) was observed passing about 750,000 km from the Earth.

A very recent close approach to the Earth occurred on 1996 May 19. With a diameter of about 500 metres, this is probably the largest asteroidal object ever to come close to the Earth. The object is called 1996 JA1 and it passed the Earth at a distance of about 450 000 kilometres, roughly the distance of the Moon.

Other minor planets which have come close to the Earth are;

Adonis, discovered in 1936, passed 0.015 A U from Earth, not observed again until rediscovered in 1977.

Hermes, discovered in 1937 when it came within 0.006 A U of the Earth, since lost.

Icarus, discovered in 1949, Passed within 0.04 A U of the Earth in 1968.

Apollo, discovered in 1932 when it came within 0.07 A U of the Earth, it was then lost to be rediscovered in 1973.

The closest comet to approach the Earth was Comet P/Lexell. It was discovered in 1770 by the famous comet searcher Messier but takes its name from the calculator of its orbit who showed that the reason for its non-reappearance was that it had passed close to Jupiter and its orbit had been perturbed. Comet Lexell has not been seen since.

For comparison purposes the following are approximate distance in the Solar System.

Diameter of the Earth 13,000 km.

Height of a geostationary satellite 36,000 km.

Distance of the Moon 384,000 km.

Distance of the Sun 150,000,000 km. (1 Astronomical Unit (AU)).

Possible collisions with the Earth
Astronomers have become increasingly worried by the discovery of these asteroids that come so close to the Earth. Predictions of their frequency are hard to make and so the likelihood of a possible collision of one of these objects with the Earth can not be estimated with any confidence.

Some estimates put the chances of a collision as high as once every 100 years. The only recorded collision of an object with the Earth in recent history was the Tunguska event in Siberia in 1908.

Although the chance of such a collision may be small the effects would be disastrous. For instance, if 1991 BA had hit the Earth travelling at a relative speed of 20 km/s it would have caused an explosion equivalent to about 40 kilotons of TNT (about three times the Hiroshima bomb). There are asteroids in similar orbits to 1991 BA which are far bigger; the effects of a collision with one of these could be catastrophic for the continuation of civilization or even of man-kind.

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