Catching A Comet By The Tail

The finding, announced in the April 6 edition of the British journal Nature, came from some strange readings radioed to Earth in 1996 by the spacecraft Ulysses, which is supposed to be studying solar winds. On May 1, 1996, the data from Ulysses suddenly went haywire for a few hours, explained Nathan Schwadron, an assistant research scientist in atmospheric, oceanic and space sciences at the University of Michigan's College of Engineering.

The solar wind, which usually blows past the spacecraft at about 700 kilometers per second, became strangely hot and calm, and the number of charged particles encountered by the spacecraft soared at precisely the same instant. This disturbance lasted for only a few hours and was missed until very recently.

"We said, 'All right, we've got something. Now, what is it?'" said Prof. George Gloeckler of the U-M Department of Atmospheric, Oceanic and Space Sciences. "It turns out, after you do all the numbers, that we sailed right through the wake of Comet Hyakutake." The comet, named for a Japanese amateur astronomer who discovered it, blazed across our night skies in spectacular fashion in 1996 and made an unusually close pass by the sun.

"This tail extends more than a half a billion kilometers," Schwadron said. "That's more than three times the distance from the Earth to the sun. It's just unbelievable."

Gloeckler, who also holds a professorship at the University of Maryland's Institute for Physical Science and Technology, is the lead author of the Nature paper along with Schwadron, Lennard Fisk and Thomas Zurbuchen at the University of Michigan and Johannes Geiss of the International Space Science Institute in Switzerland.

"Comets are the most primordial things in our solar system," Geiss said. "If we can better understand their chemical makeup, we can get a handle on what was going on in the past, and where we've been." Some theorists even propose that comets "seeded" Earth and other planets with the building blocks of life.

Not only did Ulysses show that comet tails are longer than anyone realized, this data is significant just because it's only the fourth time anyone has directly sampled the contents of a comet like this. Ulysses is equipped with spectrometers, instruments that can identify chemicals. Hyakutake's tail was found to be mostly carbon and oxygen with some nitrogen, and water.

Coincidentally, in the same edition of Nature, another team operating a magnetometer aboard Ulysses reports that magnetic field lines were altered for precisely the same period that the Michigan team detected the change in solar wind and a dramatic, 1,000-fold rise in particles. "If you were not looking at these data closely, you would have missed it," Schwadron said.

"It brings up a whole new way to study comets, and I think opens up a whole new area of science," Schwadron said. With better data from instruments designed to intercept these long tails of ionized comet dust, "we would know exactly how stars have processed material over time," he said. And that, in turn, leads astrophysicists and cosmologists to pondering some really cosmic questions: "Where did we come from? How old is the universe? What were the initial conditions for creating our solar system?"

Ulysses is a joint venture of NASA's Jet Propulsion Lab and the European Space Agency which studies the sun from a "high latitude" orbit, one which is mostly at right-angles to the plane of the planetary orbits in our solar system. It was launched on Oct. 6, 1990, by the shuttle Discovery and then went on a sling-shot maneuver around Jupiter to attain the high speed needed for its elliptical orbit. Its mission is to explore what the sun's magnetic field, solar winds and cosmic rays are like closer to the sun's North and South Poles, rather than around its equator, where our planet orbits.

Relevant links:

GIF and QuickTime animations of the data: http://www-personal.engin.umich.edu/~nathanas/Distrib/index.html

Solar-Heliospheric Lab at U-M: http://www.sprl.umich.edu/

Ulysses: http://ulysses.jpl.nasa.gov/

Nature: http://www.nature.com