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Royal Greenwich Observatory
Information Leaflet No. 3: 'The Closest Object Ever'.
The Closest Object Ever:
1991 BA, 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:
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.
H is an asteroid or comet's absolute magnitude. That is, the magnitude it would have if placed 1AU from both Sun & Earth. Given reasonable assumptions about the albedo and colour, this can be translated into an equivalent diameter.
(Definition of H by Tony Beresford)
On December 9, 1994, the object 1994 XM1 was discovered by J. V. Scotti with the 0.9m
Spacewatch telescope (at the Steward Observatory in Kitt Peak),
it came to within 112,000 Km of the Earth.
See I.A.U. Minor Planet Electronic Circular 1994-X05.
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;
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 distances in the Solar System:
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.
Produced by the Information Services Department of the Royal Greenwich Observatory.
PJA May 23 15:26:51 GMT 1996
The Apollo Group is a set of asteroids with perihelia inside the orbit of the Earth. Included in the group are Icarus and Oljato. The group is named after the asteroid Apollo which was discovered in 1932.
Icarus-1566 has one of the smallest perihelion distances, at 0.205 Astronomical Units, which brings it within the orbit of Mercury.
Oljato-2201 is an asteroid measuring some 1.4 km in diameter. Oljato's orbit major axis varies greatly due to its close approaches to Earth and Venus.
The Amor Group is a set of asteroids with Perihelion inside the orbit of Mars. They have perihelion distances of between 1.018 and 1.3 Astronomical Units. The group is named after the asteroid Amor which was discovered in 1932.
Perihelion (opposite of aphelion) is the point in its orbit where a planet
is closest to the Sun.
When referring to objects orbiting the Earth the term Perigee is used; the term Periapsis is used for orbits around other bodies.
For up-to-date information on the closest approaches to the Earth by Minor Planets, see Closest Approaches to the Earth by Minor Planets at the IAU - Minor Planet Center.
Updated: December 12 '98, June 26 '14
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