Comets, meteors and asteroids

Radar images of the near-Earth asteroid Toutatis obtained with NASA's 70 m radio telescope at Goldstone. These were made during the close encounter between Toutatis and the Earth in 1992. Image: NASA The closest recorded encounter between a natural object and the Earth took place on 31 March 2004. Meteoroid 2004 FU₁₆₂ passed just 6500 km above the ground. 2004 FU₁₆₂ is only 10 m across so if it had collided with the Earth it would almost certainly have exploded harmlessly in the upper atmosphere.

Another close approach to the Earth occurred on 19 May 1996, when 1996 JA1 passed the Earth at a distance of 450,000 km or about as far away as the Moon. With a diameter of about 500 metres, this is probably the largest asteroidal object recorded so close to the Earth.

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

• Adonis, discovered in 1936, passed 2.25 million km from Earth, not observed again until rediscovered in 1977.
• Hermes, discovered in 1937 when it came within 900,000 km of the Earth, since lost.
• Icarus, discovered in 1949, passed within 6 million km of the Earth in 1968.
• Apollo, discovered in 1932 when it came within 10.5 million km of the Earth, it was then lost to be rediscovered in 1973.
• Toutatis, discovered in 1989, passed within 1.56 million km of the Earth on 29 September 2004.

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. In the last century, one of the most dramatic collisions of an object with the Earth was the Tunguska event in Siberia in 1908.

Even a collision between a small object and a city could cause great damage. If an asteroid like 1996 JA1 hit the Earth at a typical velocity of 20 km per second, the energy released would be about the same as a 0.4 Megatonne nuclear weapon or 30 times the force of the bomb dropped on Hiroshima.