— A rare meteor shower predicted to hit Earth on 1 September should give astronomers only their second chance to study an ancient comet's crust. It could also help them develop a warning system against an otherwise insidious threat a comet aimed at Earth from the dark fringes of the solar system.
September's shower, called the alpha Aurigids, has only been seen three times before, in 1935, 1986 and 1994. The reason for this elusiveness is the shower's unusual origin.
Most meteor showers are caused by short-period comets, dirty iceballs that loop around the inner solar system on orbits lasting less than 200 years, shedding debris each time they approach the Sun's heat. This debris builds up into a broad band along the comet's orbit. Every year, when we pass through, it burns up in the atmosphere and appears as shooting stars.
The Aurigids come from a comet that takes 2000 years to orbit the Sun. With such infrequent visits, Comet Kiess can't build up a broad dust band; it only generates a narrow trail of debris each time.
The showers happen when Earth passes through one of these dust trails in particular, which was thrown off by the comet in 83 BC. "It is only a very narrow trail, and it is only once in a while that it crosses Earth's path," says Peter Jenniskens of NASA's Ames Research Center in Moffett Field, California, US.
He thinks the gravity of Jupiter and Saturn controls the path of the dust trail, waving it around like a garden hose, occasionally aiming it at Earth. Along with his colleague Jérémie Vaubaillon at Caltech, US, Jenniskens has calculated that the hose should be pointed at us again this year.
Several teams of astronomers will be watching the shower, both from the ground and from two aircraft following the Earth's shadow.
They are hoping to see fragments of the ancient crust of Comet Kiess. For 4.5 billion years before some gravitational accident nudged it towards the inner solar system, Kiess was drifting among a vast swarm of icy bodies called the Oort cloud lying far beyond the planets.
All that time, high-energy particles called cosmic rays bombarded the comet, and astronomers suspect that created a hard crust by blasting out some of its more volatile substances.
Only once before have astronomers knowingly seen a shower from a long-period comet, when Jenniskens predicted an appearance of the alpha Monocerotids in 1995. They penetrated unusually far into the atmosphere, suggesting that they were made of relatively tough material, perhaps from such a cosmic-ray-produced crust.
This time, astronomers will be looking at the spectral signature of evaporating meteors to test this theory. "Now we are better prepared, we can do more in-depth studies to understand the properties of the material," Jenniskens told New Scientist.
He also wants to know whether meteor showers such as this could warn of planetary peril. At present, astronomers can only spot a long-period comet a few years before it arrives in the inner solar system, leaving little time to deflect it if it were pointed right at Earth.
But if it had visited the inner solar system before, the resulting meteor shower might be used to trace the comet's orbit and get a much earlier warning. The size and number of Aurigid meteors will tell the researchers how debris has spread along the orbit and how these showers evolve.
They are keen for amateurs to contribute their observations. "We're interested to know what is the brightest, biggest Aurigid," says Jenniskens. "Somebody is going to capture that, and it's probably not going to be us."
The best view of the meteors will be from the west coast of North America, before dawn on 1 September. Based on past showers, there should be up to 200 bright meteors visible per hour, and they may have an unusual blue-green colour.
The shower probably won't return for at least 50 years, according to Jenniskens' calculations. "It's a once in a lifetime event."
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Journal reference: EOS (7 August 2007, vol 88, no 32)