Feature

Catch A Falling Star

It is said that comets blaze forth the death of kings. A study at the University of Canterbury may show that meteors blaze forth the death of comets.

By Cathryn Crane

Sighting a shooting star is commonly thought to be a rare thing, but Andrew Taylor sees thousands each day. He uses radar to do it, tracking the trail of ionized gas that tiny dust particles leave behind them as they burn up in Earth's atmosphere.

There are a lot of dust particles to watch -- some 1,000 tonnes falls on the Earth every day. A significant proportion of this meteoric dust is associated with comets. As comets swing in around the sun, volatile gases heat up and bubble off, pushing dust and debris away. The comet leaves a stream of dust in its wake and when the Earth passes through this stream of dust meteor showers occur.

One of the best known showers occurs from October 16th to 29th, with the greatest number of meteors around October 21st. This is the Orionid shower, named after the constellation from which it appears to emanate. The shower is associated with Halley's comet, as are a number of other meteor showers that occur at different times of the year.

Three years ago, when Halley's comet came close to Earth, Taylor began monitoring the meteor streams in the comet's orbit as part of his PhD studies. A low-tech radar system was combined with a personal computer and special signal processing software to keep track of the dust particles. At about 100 kilometres above the Earth, the atmosphere gets thick enough for meteors to produce vapour trails as they burn up. Taylor's radar detects the trails at the 90-kilometre level; much lower than this and the tiny particles vaporise.

The results have been rather startling, according to Taylor. Halley's comet has been found to move backwards and forwards across the Earth's orbit. Over the last 600 years, Halley's comet has returned every 76 years. Ancient records and modern calculations show that this orbital period has changed over time, ranging from 69 to 79 years. As the comet's orbit changes over long time spans, it lays down different "strata" of meteoric dust representing different "epochs" in the comet's life.

The data shows five distinct bands with different characteristic orbits that confirm the wanderings of the comet. Band 3, for example, is associated with the comet's spectacular return in 1066 and with the three or four orbits before that.

Taylor plans to use the same technique on the Apollo asteroids, which are pieces of hurtling rock that dip in close to the sun. In doing so, they have orbits which are similar to those of comets. There are around 20 Apollo asteroids and, as the name suggests, it has been thought that the fragments are escapees from the asteroid belt between Mars and Jupiter. However, there are indications that they may be extinct comets which have lost all their gases and volatile material.

"If these were once comets, you'd expect to see a meteor stream associated with them, so we're going out looking for that," says Taylor.

Cathryn Crane is a freelance journalist with an interest in environmental issues.