"We are very well acquainted with the trajectory of Comet 73P Schwassmann-Wachmann 3," said Don Yeomans, manager of the agency's Near-Earth Object Program, at Jet Propulsion Laboratory. "There is absolutely no danger to people on the ground or the inhabitants of the International Space Station, as the main body of the object and any pieces from the breakup will pass many millions of miles beyond the Earth."

Yeomans' pointed statement about 73P is apparently in response to vague reports on the World Wide Web that a piece of the comet will hit Earth during the flyby. Although the number of fragments has risen to nearly four dozen, he said none of them poses an impact hazard.

None of the comet's fragments will come closer than 5.5 million miles to Earth during its closest approaches between May 12 and 28 - or more than 20 times the distance between Earth and the Moon.

The main fragment, C, will pass closest to Earth on May 12 at a distance of approximately 7.3 million miles. It will be visible to small telescopes during the morning hours in the constellation Vulpecula, or the Coathanger.

73P is no stranger to astronomers, who have been observing the comet for more than 75 years. They have computed its trajectory repeatedly and have refined the level of precision over time - so 73P's orbit is well known. The real news is that Hubble's images show many more fragments than had been reported by ground-based observers.

The comet is currently comprised of a chain of more than 40 fragments, named alphabetically and stretching across several degrees on the sky. Observers have noted some dramatic brightenings among some of the fragments, which suggests they are continuing to disintegrate - some could disappear altogether.

Hubble caught two of the fragments, B and G, shortly after large outbursts in activity. Hubble also photographed fragment C, which at the time was less active. The resulting images reveal that some sort of destructive process is taking place, in which fragments are continuing to split into smaller chunks.

Hubble has found several dozen house-sized fragments trailing each main fragment. Sequential images of the B fragment, for example, taken a few days apart, suggest the chunks are pushed down the tail by outgassing from the icy, sunward-facing surfaces of the chunks, much like space-walking astronauts are propelled by their jetpacks.

The smaller chunks have the lowest mass, so they are being accelerated away from the parent nucleus faster than the larger chunks. Some of the chunks seem to dissipate completely over the course of several days.

Ancient relics of the early solar system, comet nuclei are porous and fragile mixes of dust and ice. They can break apart from tidal forces when they pass near large bodies, which is what happened to Comet Shoemaker-Levy 9 when Jupiter's gravity tore it to pieces in 1992.

Shoemaker-Levy's fragments all plunged directly into Jupiter's atmosphere two years later, creating gaping holes in gas giant's cloudtops and leaving ugly blackened scars that lasted a year.

Comet nuclei also can fly apart from rapid rotation. Or, they can break up from the thermal stresses as they pass near the Sun, or explosively pop apart like corks from champagne bottles due to the outburst of trapped volatile gases.

"Catastrophic breakups may be the ultimate fate of most comets," said Hal Weaver, a planetary astronomer with the Johns Hopkins University Applied Physics Laboratory, who led the team that made the recent Hubble observations.

The comet is named for German astronomers Arnold Schwassmann and Arno Arthur Wachmann, who discovered it during a photographic search for asteroids in 1930. At the time, 73P passed within 5.8 million miles of Earth, or 24 times the Earth-Moon distance. The comet orbits the Sun every 5.4 years, but it was not seen again until 1979. The comet was missed again in 1985 but has been observed during every return since.

During the fall of 1995, astronomers witnessed a huge outburst from 73P, and shortly afterwards they identified and labeled four separate nuclei as fragments A, B, C and D, with C being the largest and the presumed principal remnant of the original nucleus.

Only C and B were definitively observed during the next return in 2000-2001, possibly because of poor geometry involving Earth's location versus the cometary fragments. This year's better observing circumstances may have aided the detection of all of the new fragments, but it also is possible the disintegration of the comet is now accelerating.

Related Links
NASA Near Earth Object Program
73P Tracking

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