[meteorite-list] X-Rays Fly As Cracking Comet Streaks Across the Sky (Schwassmann-Wachmann 3)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon May 15 11:54:21 2006
Message-ID: <200605151551.IAA27414_at_zagami.jpl.nasa.gov>

http://www.nasa.gov/mission_pages/swift/bursts/swift_comet.html

X-Rays Fly As Cracking Comet Streaks Across the Sky
Christopher Wanjek
Goddard Space Flight Center
May 12, 2006

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.
Received on Mon 15 May 2006 11:51:26 AM PDT