A comet passing close to earth this week has for some unknown reason broken up to form mini-comets, writes John Moore.
A comet passes the earth tomorrow, a relatively near miss given it will come within 10 million kilometres of us as it rushes by in its orbit around the sun. Yet scientific interest has less to do with its proximity than with the fact that for some unknown reason it is breaking up.
In mid-May, periodic comet 73P/Schwassmann-Wachmann 3 will be the closest comet to pass by Earth for more than 20 years. This may be the last time anyone will ever see it again however, given it is disintegrating into tiny mini-comets.
First discovered nearly 76 years ago by astronomers Arnold Schwassmann and Arno Wachmann at the Hamburg Observatory in Germany, the comet has since undergone an unexpected thousand-fold brightening over the intervening years. On close inspection by telescopes on Earth in 1995, the brightening was attributed to the comet having split into three distinct fragments. New observations last March showed the three chunks had fragmented even further.
The latest images taken by the Hubble Space Telescope revealed more than 40 fragments and it looks as though the breakup isn't going to stop.
"It seems as though some of the fragments are themselves forming their own sub-fragments," says Don Yeomans, head of NASA's Near Earth Object Program in Pasadena. "This is a rare opportunity to watch a comet in its death throes."
The most likely explanation for the break-up is thermal stress on the comet's nucleus, cracking it open as it nears the sun in its 5.3 yearly orbit. However, the break-up could also be due to outbursts of trapped volatile gases, or as has been recently suggested, a small object like a stray interplanetary boulder may have struck it causing it to fragment.
If the latter proves to be the case, then Earth could be in for a new meteor shower in 2006. Any collision would likely produce debris that would burn up in spectacular fashion in our atmosphere.
"It wouldn't be the first time a dying comet produced a meteor shower," says astronomer, Paul Wiegert, of the University of Western Ontario. "However, if thermal stresses are the cause, a debris dust cloud slowly expanding off the comet should reach Earth in about 2022, producing nothing spectacular."
None of the comet's fragments will come closer to Earth than nine or 10 million kilometres during its closest approaches between May 12th and 28th. That is more than 20 times the distance from the Earth to the moon. The main fragment will pass close to Earth on May 12th at a distance of approximately 12 million kilometres (7.5 million miles), while the next large fragment will pass by on May 14th at about 10 million kilometres away.
If both fragments remain as they are right now, they should easily be visible from Ireland using binoculars or small telescopes. Look eastwards and the comet should have passed the brilliant blue-white star, Vega, in the constellation of Lyra around May 8th, and around May 12th it should have passed through the Milky Way in Cygnus.
The Hubble Space Telescope will be taking high-resolution pictures of the event as it passes close to Earth, and the giant radio telescope in Arecibo, Chile, will "ping" the fragments to determine their shape and spin.
The observations should provide invaluable information about the pristine material buried below the crust of the comet, giving astronomers a unique opportunity to look at dust and gases that have been around since the sun and planets formed some 4.5 billion years ago.