Over The Horizon

A Very Near-Earth Asteroid

The smallest asteroid ever seen whizzed by in mid-March, missing the Earth by 170,000 kilometres -- just half the Moon's distance. It came within 140,000 kilometres of the Moon. This was a record close approach by a solar system object to either body.

The tiny asteroid was just a few metres across and was visible only in big telescopes. It was discovered by the Spacewatch Telescope, a 90-cm aperture telescope on Kitt Peak in Arizona, dedicated to searching for near-earth asteroids. When discovered on March 14th, it was 1.8 million kilometres from Earth and coming straight at us. It was extremely faint, with a magnitude of 20.5.

By the following night the object, designated SS0171, was 380 000 kilometres away and crossing the sky at 1.5 degrees (three full moon diameters) per hour. Although it was as close as the moon, it was just 17th magnitude in brightness, visible only in very big telescopes. The Minor Planet Center calculated an accurate track and advised observatories in the Japan, the US, Australia and New Zealand.

Sadly, the weather was cloudy at Mt John Observatory near Tekapo, so no observations could be attempted. It would have been a very difficult target anyway -- by the time it was visible here, it would have been 16th magnitude moving at 10 degrees per hour. No-one else has reported observations, either.

What would have happened if SS0171 had hit the Earth? In part it depends on what it was made of. If it was metallic, like a nickel-iron meteorite, it would reach the ground in one piece and make a crater tens of metres in diameter. A hard, rocky object would be likely to break up on its way through the air, landing to produce a group of smaller craters. An asteroid of soft rock would fragment in the lower atmosphere, producing an airburst like a nuclear bomb.

In an even closer contact, a very large meteor fireball exploded about 20 kilometres above the western Pacific northwest of Nauru, in early February. Information on the fireball sighting is slowly emerging from US military sources.

The energy released by the explosion was equivalent to between 10 kilotons and one megaton of TNT. The meteor was observed by several satellites and was seen from the ground by fishermen who reported it as very bright but making no sound. The absence of sound shows that the meteor was a long way above ground, where atmospheric temperature gradients deflect sound upward.

Calculations by NASA's Ames Research Center show that the original meteor must have weighed 1,000-40,000 tons and been 9-30 metres in diameter, depending on the object's speed and the actual energy released. Such an event occurs once in 5 to 50 years, depending on the exact size of the object.

Alan Gilmore, Pam Kilmartin, Mt John Observatory