May 1, 2006— Comet 73P/Schassmann-Waachmann 3 is going, going — but not quite gone. Astronomers have now added an unprecedented series of highly detailed Hubble Space Telescope images to the thousands of ground-based telescope images documenting the disintegration of the comet as it passes within several million miles of Earth.
The images capture the three-dimensional details of fragments flying free of one of the larger chunks, which then can be seen breaking into smaller and smaller pieces. Although many other comets have been seen breaking up, this is the first to fly apart so conveniently close to Earth telescopes.
"I've never seen anything like this," said Lars Lindberg Christensen, a Hubble astronomer with the European Space Agency, speaking to Discovery News from Germany. "What's clear from the Hubble images is that the fragments are fragmenting. The harder you look, the more fragments you find."
"It's simply spectacular," said astronomer Hal Weaver of the Johns Hopkins University Applied Physics Research Lab.
It's a textbook example of that's called "hierarchical break-up," where big pieces give rise to smaller pieces which give rise to smaller and smaller pieces. "This is a multidimensional laboratory."
Because comet 73P/Schassmann-Waachmann 3 is so close, Hubble images can resolve fragments down to about 3 miles (5 km) wide, said Weaver. And by putting several images together, they can for the first time track individual small fragments and chart their speeds and directions as they fly off.
The images back up the "dirty snowball" theory about comets, Weaver said. The idea is that they just break down into smaller and smaller pieces until they become harmless interplanetary sand grains and dust that become shooting stars when Earth plows through a cloud of them and they burn up in our atmosphere.
A competing theory describes comets as built more like peaches, with hard rocky cores at their centers that remain as small dark asteroids after the gas and dust has been blown away by the solar wind after many passes near the sun.
The current comet disintegration is only the latest chapter in a break-up that's been under way for years. Almost 11 years ago, astronomers watched as the comet broke into four separate fragments, labeled A, B, C, and D.
When the comet swung into the inner solar system again in 2000-2001, it was not at all close to Earth, so astronomers only caught sight of the B and C fragments. The comet orbits the sun every 5.4 years.
The close proximity of 73P/Schassmann-Waachmann 3 this year has made it possible to see many more fragments. It's also possible that the disintegration has accelerated as the comet nears its end. It remains to be seen whether any of the fragments survive for another orbit of the sun.
"It's possible that the C fragment may still come back again," said Weaver.
That might become clearer in the next few weeks. The fragmented comet will make its closet approach to Earth on May 12, when it will be 7.26 million miles (11.7 million kilometers) away, about 30 times further than the moon. Already fragments B and C can be seen with binoculars in the evening sky, to the northeast, between the constellations Hercules and Corona Borealis
(View an April 28 sky chart.)
Hubble will not be able to follow the comet for much longer, said Christensen, because the fragments are approaching the sun. Hubble and its instruments would be damaged if aimed too far in that direction.
As for how the comet got its rather lengthy name, it's after the German astronomers Arnold Schwassmann and Arno Arthur Wachmann who discovered it while asteroid hunting in 1930. At that time the comet was within 6 million miles (9.3 million km) of Earth.
