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During an unplanned rendezvous, the Ulysses spacecraft found itself gliding though the immense tail of Comet Hyakutake, revealing that comet tails may be much, much longer than previously believed.

“The odds that Ulysses’ flight path would intersect the comet tail were probably less likely than someone breaking the bank at Monte Carlo,” said Dr. Edward Smith of NASA’ s Jet Propulsion Laboratory, Pasadena, CA, the Ulysses project scientist and a co-investigator for its magnetometer instrument. Before the unexpected encounter, Ulysses was hundreds of millions of kilometers, or miles, away from Comet Hyakutake and far beyond the visible tail.

“This tail extends half a billion kilometers (more than 300 million miles). That’s more than three times the distance from the Earth to the Sun,” said Dr. Nathan Schwadron, of the University of Michigan in Ann Arbor, a member of one of two Ulysses teams that made the discovery independently of one another. Findings from both teams appear in the April 6 issue of the journal Nature.

“This makes it the longest comet tail ever recorded,” said Dr. Geraint Jones from Imperial College, London, of the Ulysses magnetometer team.

Comet Hyakutake, one of the brightest comets of the 20th century, made a dazzling nighttime appearance in the spring of 1996, when it made a close pass by the Sun. While Ulysses was cruising through space studying the solar wind on May 1, 1996, its data suddenly went wild for a few hours. For example, the solar wind seemed to almost disappear and was replaced by gases not normally found in the solar wind, and the magnetic field in the solar wind was distorted. Since Ulysses scientists were not looking for comets, they did not realize the significance of the data right away.

“The discovery was made quite by accident, a bit like finding a needle in a haystack when you weren’t even looking for a needle in the first place,” said Dr. George Gloeckler of the University of Maryland, principal investigator of the Ulysses solar-wind ion-composition spectrometer team. The instrument studies the content and electrical charge of ionized gases. While his team detected ions typically found in comets, the magnetometer team observed magnetic field directional changes like those associated with comet tails.

Comets are of great interest, because they may be the frozen leftovers of the birth of our solar system. They could hold clues to the formation of Earth and life, since one theory holds that comets “seeded” Earth and other planets with the building blocks of life.

Comets are made of dirty ice, and as they approach the Sun and heat up, they emit gas and dust, forming gas and dust tails. The gas slows the solar wind and the portion of the magnetic field near the comet. The parts of the magnetic field farther from the comet continue to travel rapidly past it. Magnetic fields can be stretched like rubber bands. The magnetic field is draped around the comet and stretches out behind it in a hairpin shape.

Gloeckler is lead author of the Nature paper on the ion findings, along with Schwadron, and Drs. Lennard Fisk and Thomas Zurbuchen, also of the University of Michigan, and Dr. Johannes Geiss of the International Space Science Institute in Switzerland. The other Nature article, on the Ulysses magnetometer findings, was authored by Jones and Professor Andre Balogh of Imperial College and Dr. Timothy Horbury of Queen Mary and Westfield College, London.

Jones at Imperial College looked more closely at the magnetic field data because of the publication of the unusual 1996 solar wind event in the Journal of Geophysical Research. It was authored by Dr. Peter Riley, formerly of Los Alamos National Laboratory in New Mexico, and based on data from the Ulysses solar wind instrument. Jones and Horbury saw that the data looked like a cometary tail, and Jones searched until he found the tail’s source — Hyakutake. Gloeckler and his colleagues noticed the event independently and realized it was cometary material.

Ulysses, launched in 1990, is a joint venture of NASA and the European Space Agency (ESA). The spacecraft 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 some phenomena not detectable by visible observations. Scientists now know that sensitive instruments, like those found on Ulysses, can detect comet tail particles that are not normally visible. The Jet Propulsion Laboratory (JPL) manages Ulysses for NASA’s Office of Space Science, Washington, D.C.

More information on the Ulysses mission is available at: and