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Posted by baalke on October 24, 2007, 5:22 pm
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http://www.jpl.nasa.gov/news/features.cfm?feature=1502
Ulysses Catches Record for Catching Comets by Their Tails
Jet Propulsion Laboratory
October 19, 2007
When it was launched 17 years ago, scientists and mission engineers
for
the Ulysses project knew they should expect, well, the unexpected.
After
all, the joint NASA/European Space Agency-managed spacecraft was going
where no spacecraft had gone before - above and below the sun's poles.
But the surprises the team expected were wholly in the area of solar
research - which would make sense, as the primary mission of the
Ulysses
spacecraft is to characterize the sun and its influence on the space
environment. That was before the spacecraft met up with some of the
solar system's most mysterious and beautiful deep-space nomads.
"Ulysses has flown through and acquired data from the tails of comets
on
three separate occasions," said Edward J. Smith of NASA's Jet
Propulsion
Laboratory in Pasadena, Calif. Smith serves as the U.S. project
scientist for the Ulysses mission. "No other spacecraft in history has
done that."
Ulysses' first cometary tail encounter occurred in 1996. Back then,
comet Hyakutake was dazzling scientists and the public alike with its
noteworthy appearances in the nighttime spring sky. On May 1, 1996,
while Ulysses was cruising through space studying the solar wind, its
data suddenly went wild for a few hours.
"As we were not looking for comets, we did not realize the
significance
of the data right away," said Smith. "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."
At the time of the unexpected encounter, Ulysses was hundreds of
millions of miles from comet Hyakutake and far beyond the visible
tail.
As their analysis began ruling out other possibilities, the science
team
came to a startling conclusion - Hyakutake's tail extended more than
480
million kilometers (300 million miles, or three times the distance
from
Earth to the sun), making it the longest comet tail ever recorded.
The once-in-a-lifetime chance encounter with a comet tail happened
again
in 2004 when Ulysses flew through the ion tailings of comet
McNaught-Hartley. Unlike Hyakutake, comet McNaught-Hartley seemed to
be
at the wrong location for Ulysses to intercept its tail. By chance, an
eruption of particles from the surface of the sun, called a coronal
mass
ejection, carried cometary material to Ulysses. Such a collision has
recently been observed for the first time by NASA's Stereo spacecraft
(http://www.nasa.gov/mission_pages/stereo/news/encke.html). A movie
(http://www.nasa.gov/mpg/191284main_encke_scienceatnasa.mpg) shows the
disruption and reformation of periodic comet Encke's tail.
Ulysses racked up its third, and perhaps most scientifically
revealing,
comet tail encounter this past February when it again flew through the
ion tailings of a comet named McNaught (a different comet than the one
encountered in 2004, but discovered by and named after the same
astronomer). The nucleus of this comet McNaught was some 257 million
kilometers (160 million miles) from the spacecraft during encounter.
Ulysses' solar wind ion composition spectrometer instrument, developed
by University of Michigan heliophysicist George Gloeckler, found that
even at such a great distance, the tail had filled the solar outflow
with unusual gases and molecules. In response, the solar wind that
usually measures about 700 kilometers per second (435 miles per
second)
at that distance from the sun, was less than 400 kilometers per second
(249 miles per second) inside the comet's tail, as measured by one of
Ulysses' instruments called "Solar Wind Observations Over the Poles of
the Sun."
The interaction between comets' tails and the solar wind has been
studied for decades. A comet's ion tail always points away from the
sun,
whether the body is traveling toward or away from the sun along the
comet's elliptical orbit. It was this finding that eventually led in
1958 to the discovery of solar wind. The magnetism and velocity of the
solar wind are so strong, the effect pushes the comet's tail forward.
A
paper on Ulysses' latest crossing of a comet tail was published in the
Oct. 1 issue of Astrophysical Journal.
"I recall saying a few years back that the odds that Ulysses' flight
path would intersect that of a comet tail were probably less likely
than
finding a needle in a haystack," said Smith. "Now that we have
encountered three, I cannot help wondering when nature will have
another
one in store for us."
Smith is part of an international science team that has been working
Ulysses data since its 1990 launch from the payload bay of Space
Shuttle
Discovery. Ulysses scans the sun's magnetic field, solar plasmas,
solar
radio noise, energetic particles, galactic cosmic rays and cosmic dust
between the poles and the equator - imparting a more complete
perspective of the sun's atmosphere. Understanding Earth's nearest
star
and its processes is of paramount importance, as the space weather
created by the sun has a huge effect on the third rock from it and its
inhabitants. The sun's gaseous outer atmosphere can create huge space
storms. This violent space weather, in turn, can affect Earth's
electrical grid, cell phone communications, satellite functioning, and
the operation of astronauts in orbit.
"Such unique science is a tribute to the durability of the mission and
the intellectual curiosity of our science team," said Ed Massey of
JPL,
who serves as Ulysses' NASA project manager.
The Ulysses spacecraft was built by Dornier Systems of Germany for the
European Space Agency. NASA provided the launch via space shuttle and
the upper stage boosters. The U.S. Department of Energy supplied a
generator that powers the spacecraft; science instruments were
provided
by both U.S. and European investigators. The spacecraft is operated
from
JPL by a joint team from the European Space Agency and NASA.
More information about NASA's Ulysses mission is available at
http://ulysses.jpl.nasa.gov .
------------------------------------------------------------------------
Media contact: DC Agle/JPL
818-393-9011
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