Meteors and comets have struck Earth repeatedly. One may have wiped out the dinosaurs 65 million years ago. The big question, of course, is when will the next one hit? Peter Jenniskens, an astronomer at the NASA Ames Research Center near Mountain View, California, hasn’t found the next killer comet, but he has discovered something almost as ominous: the orbit of an as yet unseen comet that could one day slam into Earth.
Jenniskens pieced together the orbit by studying meteor showers, which are essentially comet flotsam vaporizing in our atmosphere. As a comet travels through the solar system on its circuit around the sun, it leaves a trail of debris, like trash spilled from a speeding garbage truck. This debris continues to follow the parent comet’s orbit long after the comet itself has returned to the outer reaches of the solar system. If the trail happens to cross Earth’s orbit, we see it as a meteor shower whenever Earth passes through it.
This means that meteor showers recur regularly. Every November, for example, the Leonid shower lights up night skies (all meteor showers are named after the constellation from which they seem to radiate; the Leonids stream in from the direction of Leo). During particularly intense showers called meteor outbursts, the hourly tally of shooting stars can run into the thousands. Astronomers generally chalked up these outbursts to a clumpy, unevenly spread debris trail.
But after a 1994 outburst of the Aurigid shower, Jenniskens found another explanation for such unusual displays. He noticed that in previous Aurigid outbursts Jupiter and Saturn had been in roughly the same orbital positions. This led him to devise a model in which outbursts are caused not by changes in the amount of debris in a trail but by the trail’s position relative to Earth’s orbit.
In his model the gravitational pull of Jupiter and Saturn makes meteor streams weave in and out of Earth’s orbit. Thus a narrow meteor stream like the Aurigid might miss Earth entirely except for those years when Saturn and Jupiter pull it into position. After some calculations, Jenniskens realized that the lineup of Jupiter and Saturn might also be responsible for a display called the alpha-Monocerotid outburst that had put in puzzling appearances in 1925, 1935, and 1985. He predicted that the early morning of November 22, 1995, would see a spectacular outburst.
Jenniskens turned out to be right. He had the help of some amateur astronomers who were traveling from Holland to Spain to view the Leonids, which were to appear five days before the predicted outburst. The promise of a spectacular display induced the amateurs to stay on to collect data for Jenniskens. It was worth the wait. For about 50 minutes the sky loosed a torrent of meteors. The volunteers used video cameras to record the meteors streaking through the sky. Jenniskens used these data to reconstruct the trajectories of the meteors and determined that the orbit of this debris stream extends out ten times farther than Pluto. If the meteors hadn’t hit our atmosphere, they would have orbited the sun and returned in 10,000 years.
What’s more, Jenniskens reasons that somewhere in this long orbit there must be a comet responsible for all these meteors. Judging from the number of meteors tracked and their estimated total mass, he calculates that the comet is about a half-mile across. Since comets typically zip through the inner solar system at more than 100,000 miles per hour, on some far-off November day this half-mile-wide comet could hit us with a wallop equal to 200,000 megatons of tnt, enough to cause a global catastrophe.
The good news is the risk of a hit on any given pass is minute. The bad news is no one knows exactly where the comet is. Worse still, Jenniskens has identified 13 similar meteor streams that probably come from long-period comets, and he estimates that there may be as many as 70 such bodies that cross Earth’s path at some point in their orbit. Jenniskens thinks these streams should be closely watched for signs of threatening comets. It might not be the sort of thing that excites most professional astronomers, he says, but amateurs have the time and the means to do this work.