GREENBELT, Md., May 12 /PRNewswire/ -- Scientists using NASA's Swift
satellite have detected X-rays from a comet that is now passing the Earth
and rapidly disintegrating on what could be its final orbit around the sun.
Swift's observations provide a rare opportunity to investigate several
ongoing mysteries about comets and our solar system, and hundreds of
scientists have tuned in to the event.
The comet, called 73P/Schwassmann-Wachmann 3, is visible with even a
small, backyard telescope. Peak brightness is expected next week, when it
comes within 7.3 million miles of Earth, or about 30 times the distance to
the Moon. There is no threat to Earth, however.
This is the brightest comet ever detected in X-rays. The comet is so
close that astronomers are hoping to determine not only the composition of
the comet but also of the solar wind. Scientists think that atomic
particles that comprise the solar wind interact with comet material to
produce X-rays, a theory that Swift might prove true.
Three world-class X-ray observatories now in orbit -- NASA's Chandra
X-ray Observatory, the European-led XMM-Newton, and the Japanese-led Suzaku
-- will observe the comet in the coming weeks. Like a scout, Swift has
provided information to these larger facilities about what to look for.
This type of observation can only take place in the X-ray waveband.
"The Schwassmann-Wachmann comet is a comet like no other," said Scott
Porter of NASA's Goddard Space Flight Center in Greenbelt, Md., part of the
Swift observation team. "During its 1996 passage it broke apart. Now we are
tracking about three dozen fragments. The X-rays being produced provide
information never before revealed."
The situation is reminiscent of the Deep Impact probe, which penetrated
comet Tempel 1 about a year ago. This time, nature itself has broken the
comet. Because Schwassmann-Wachmann 3 is much closer to both the Earth and
the sun than Tempel 1 was, it currently appears about 20 times brighter in
X-rays. Schwassmann-Wachmann 3 passes Earth about every five years.
Scientists could not anticipate how bright it would become in X-rays this
time around.
"The Swift observations are amazing," said Greg Brown of Lawrence
Livermore National Laboratory in Livermore, Calif., who led the proposal
for Swift observation time. "Because we are viewing the comet in X-rays, we
can see many unique features. The combined results of data from several
premier orbiting observatories will be spectacular."
Swift is primarily a gamma-ray burst detector. The satellite also has
X- ray and ultraviolet/optical telescopes. Because of its burst-hunting
ability to turn rapidly, Swift has been able to track the progress of the
fast-moving Schwassmann-Wachmann 3 comet. Swift is the first observatory to
simultaneously observe the comet in both ultraviolet light and X-rays. This
cross comparison is crucial for testing theories about comets.
Swift and the other three X-ray observatories plan to combine forces to
observe Schwassmann-Wachmann 3 closely. Through a technique called
spectroscopy, scientists hope to determine the chemical structure of the
comet. Already Swift has detected oxygen and hints of carbon. These
elements are from the solar wind, not the comet.
Scientists think that X-rays are produced through a process called
charge exchange, in which highly (and positively) charged particles from
the sun that lack electrons steal electrons from chemicals in the comet.
Typical comet material includes water, methane and carbon dioxide. Charge
exchange is analogous to the tiny spark seen in static electricity, only at
a far greater energy.
By comparing the ratio of X-ray energies emitted, scientists can
determine the content of the solar wind and infer the content of the comet
material. Swift, Chandra, XMM-Newton and Suzaku each provide complementary
capabilities to nail down this tricky measurement. The combination of these
observations will provide a time evolution of the X-ray emission of the
comet as it navigates through our solar system
Porter and his colleagues at Goddard and Lawrence Livermore tested the
charge exchange theory in an earthbound laboratory in 2003. That
experiment, at Livermore's EBIT-I electron beam ion trap, produced a
complex spectrograph of intensity versus X-ray energy for a variety of
expected elements in the solar wind and comet. "We are anxious to compare
nature's laboratory to the one we created," Porter said.
The German-led ROSAT mission, now decommissioned, was the first to
detect X-rays from a comet, from Hyakutake in 1996. This was a great
surprise. It took about five years before scientists had a suitable
explanation for X-ray emission. Now, ten years after Hyakutake, scientists
could settle the mystery.
For Swift images of comet 73P/Schwassmann-Wachmann 3, visit:
http://www.nasa.gov/swift
SOURCE NASA
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Related links:
http://www.nasa.gov
CONTACT:
Susan Hendrix of NASA Goddard Space Flight
Center, Greenbelt, Md., +1-301-286-7745
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