This chance finding suggests that a spacecraft designed to search for the ions marking a comet's trail could help find answers to questions such as: Are there any unseen comets hurtling through space? Are any of these invisible comets on a collision course with the Earth? What can these comets tell us about how our solar system was formed?

In the April 6 issue of the British journal Nature, two Ulysses science teams report that on May 1, 1996, the spacecraft --which studies solar winds at the poles of the Sun-- passed through the tail of comet Hyakutake.

At that time the comet was more than 300 million miles away from Ulysses, a distance greater than three times that of the Earth from the Sun. This means Hyakutake's tail is far longer than scientists had previously thought a comet's tail could be.

Moreover, it suggests that the tails of other comets also stretch far out in space, marking the paths of these giant balls of dirty ice as they travel past the sun.

"There are several amazing things about this discovery, including the huge amount of luck involved and the fact that it was made independently and almost simultaneously by our team and by the magnetometer team," said University of Maryland physicist George Gloeckler, who is the chief inventor of the Ulysses solar wind ion composition spectrometer.

Gloeckler and Johannes Geiss of the International Space Science Institute in Switzerland are the principal investigators for the instrument. Gloeckler said that the spacecraft's crossing of a comet's tail wasn't realized until 1999, three years after the event occurred, because no one thought there was any possibility a comet tail could be present in that location.

"Our detection of ions characteristic of a comet's tail was like someone finding a needle in a haystack when they didn't even know a needle was there," said Geiss.

"However, the most amazing and important thing about this discovery is that it points to a new way of detecting and studying cometary ions, and, in the process opens up a whole new area of science," Gloeckler said.

According to Gloeckler, the recognition that comets probably have tails that stretch far out across regions of the solar system means that the process of sampling a comet's ions can be much easier and cheaper than previously thought possible.

Until this discovery, scientists had thought that comet tails and their ions dissipated rather quickly, and thus could be detected only by expensive missions designed to rendezvous with and make a close flyby of a known comet.

"My colleagues and I now believe that with a much more sensitive version of the ion composition spectrometer found on Ulysses, 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," Gloeckler said.

A spacecraft with a very sensitive ion composition spectrometer might be able to detect many small, as yet unknown, comets and provide orbital data that would help scientists determine if any pose a collision risk to the Earth.

In addition, new insights might be gained into the composition of comets, which scientists believe contain material unchanged since the formation of the solar system.

"Such a spacecraft could also pick up and analyze ions from materials, such as interstellar dust, that have entered the solar system from distant regions of space." Gloeckler said. "This could provide a new way of learning about far regions of the universe that now can only be studied through telescopes."

Ulysses, launched in 1990, is a joint venture of NASA and the European Space Agency. The spacecraft -- which is managed by the Jet Propulsion Laboratory in Pasadena, California -- studies the Sun from a high- latitude orbit, mostly at right angles to the plane of orbiting planets.

Ulysses studies the Sun's magnetic fields, solar winds and cosmic rays near the Sun's North and South Poles, away from the equator, where Earth orbits. Ulysses has no camera, but its ten sophisticated instruments can observe phenomena not detectable by visible observations.

Gloeckler is lead author of the Nature paper on the ion findings, along with F.M. Ipavich, also of the University of Maryland; N.A. Schwadron, L.A. Fisk and T. H. Zurbuchen, all of the University of Michigan; J. Geiss and R. von Steiger of the International Space Science Institute in Switzerland; H. Balsiger of the University of Bern in Switzerland; and B. Wilken of the Max-Planck Institute in Germany. The other Nature article, on the Ulysses magnetometer findings, was authored by J. Jones and A. Balogh of the Imperial College in Britain and T. Horbury of Queen Mary and Westfield College in Britain.