A Comet's Long Tail Tickles Ulysses.

When a newly discovered comet named Hyakutake passed near Earth about 4 years ago, it made a major spectacle of itself. As well as flaunting two dust tails, it sported a ghostly bluish-white stream of charged particles, extending a third of the way across the sky (SN: 6/1/96, p. 346).

This ion tail, it now turns out, went on for an even longer stretch. A recent review of 1996 data from the Ulysses spacecraft reveals this tail as the longest ever recorded. Spanning at least 571 million kilometers--about 3.5 times the Earth-sun distance--the tail might have extended to the fringes of the solar system, researchers report in the April 6 NATURE.

Astronomers say that they are amazed to find an intact ion tail so far from its parent.

This is the first time that the tail of a known comet has been detected by analyzing the solar wind, the breeze of charged particles that blows out of the sun. Scientists had thought that the only way to sample comet material was to fly costly missions close by the object. They now plan to look for undetected comets simply by searching for their tails.

"A spacecraft could travel through regions of the solar system picking up ions from the many invisible comet tails that probably crisscross our solar system," says George Gloeckler of the University of Maryland, College Park. He coauthored one of the two Ulysses studies in the NATURE issue. A NASA-European Space Agency mission launched in 1990, Ulysses explores the polar regions of the sun.

Gloeckler and his colleagues weren't hunting comets when they sifted through data on the composition of the solar wind. They were looking for changes in the wind that would indicate a solar eruption. However, when they examined the composition of the solar wind hitting Ulysses on May 1, 1996, the researchers found something puzzling.

The population of highly ionized gas particles--the main constituents of the solar wind---abruptly dropped to a fifth of its previous value. At the same time, the density of singly ionized particles, including carbon and oxygen, rose dramatically. Such ions are rare in the solar wind but common in the tails of the few comets, such as Halley's, that astronomers have examined extensively. Whenever the solar wind runs into an ion tail, it carries the tail material along with it.

But if the craft had intercepted a comet's tail, where was the comet? No known comets resided at Ulysses' distance from the sun, nearly four times the Earth-sun separation. Using the velocity of the solar wind, the team calculated that 8 days earlier, Comet Hyakutake had been more than half a billion kilometers distant, yet in the right place--along the line connecting the sun and the spacecraft--to have generated the ions.

A team based in London came to the same conclusion after analyzing data from a magnetometer aboard Ulysses.

That detector revealed that on May 1, 1996, the magnetic field traveling along with the solar wind had an unusual configuration, report Geraint H. Jones and Andre Balogh of Imperial College and Timothy S. Horbury of Queen Mary and Westfield College. The field exhibited a hairpin shape, a sign that the solar wind had run into some dense obstacle and the magnetic field had draped around it. Comets are prime suspects in such behavior: The high density of ions near the core of a comet is highly effective at slowing the solar wind.

The magnetic field disturbance indicated that the tail was curving away from the sun, the researchers say. This geometry is characteristic of a rapidly moving comet, such as Hyakutake, at its closest approach to the sun.

"The two papers together make a very convincing case," says comet researcher John C. Brandt of the University of New Mexico in Albuquerque. He suggests that the ion tail remained intact as it was pushed more than half a billion kilometers because the polar solar wind is remarkably steady. Had the tail been carried by the sun's gusty equatorial wind, it might have been torn apart, Brandt speculates.

If planetary scientists can find small comets by detecting the tails, he notes, they will be better able to test models that describe how these amalgams of dust and ice formed in the early solar system.