JPL: Ulysses meets a comet

From: Brian D Williams (
Date: Thu Apr 06 2000 - 07:59:52 MDT

PASADENA, CALIF. 91109 TELEPHONE (818) 354-5011

Contact: Jane Platt (818) 354-0880



     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

     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


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