[meteorite-list] NASA's Swift Looks to Comets for a Cool View

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 15 Dec 2008 12:11:23 -0800 (PST)
Message-ID: <200812152011.MAA02945_at_zagami.jpl.nasa.gov>

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

NASA's Swift Looks to Comets for a Cool View
12.03.08
 
NASA's Swift Gamma-ray Explorer satellite rocketed into space in 2004
on a mission to study some of the highest-energy events in the universe.
The spacecraft has detected more than 380 gamma-ray bursts, fleeting
flares that likely signal the birth of a black hole in the distant
universe. In that time, Swift also has observed 80 exploding stars
and studied six comets.

Comets? ... Comets are "dirty snowballs" made of frozen gases mixed with
dust. X-rays come from superhot plasmas. What do cold comets have in
common with exploding stars or the birth of black holes?

"It was a big surprise in 1996 when the NASA-European ROSAT mission
showed that comet Hyakutake was emitting X-rays," says Dennis Bodewits,
a NASA Postdoctural Fellow at the Goddard Space Flight Center in
Greenbelt, Md. "After that discovery, astronomers searched through ROSAT
archives. It turns out that most comets emit X-rays when they come
within about three times Earth's distance from the sun."

Bodewits is working with the Swift team at NASA's Goddard Space Flight
Center in Greenbelt, Md., to study comets using data from the
spacecraft's Ultraviolet/Optical Telescope (UVOT) and X-Ray Telescope
(XRT). "Swift is an excellent platform for studying dynamic processes in
comets," he says.

Ultraviolet wavelengths let astronomers identify the chemical
composition of the comet's atmosphere, observe the structure of dust
emission, and identify the rotation of the comet's icy nucleus. X-rays
reveal the structure of the comet's gas and the state of the solar wind,
a stream of charged particles that flows from the sun at speeds upwards
of 900,000 mph.

The piece, known as fragment C, is believed to be the comet's main body,
which began splintering in 1995. In 2006, astronomers counted 66
fragments. Telescopes -- including NASA's Hubble and Spitzer -- revealed
dust and condensations trailing several pieces. But fragment C showed no
unusual changes -- except to Swift's ultraviolet eye. "It's subtle, but
Swift caught clouds of dust and perhaps small pieces that no one else
was able to," Immler says.

The UVOT also includes an ultraviolet grism, which combines a grating
with a prism to separate incoming light by wavelength. "Swift's grism
spans the wavelengths where carbon-bearing molecules and the hydroxyl
molecule are most active. This gives us a unique view into the types and
quantities of gas a comet produces, and that gives us clues about the
origin of comets and the solar system," Bodewits explains. In fact, with
the failure of the Hubble Space Telescope's ultraviolet spectrograph in
2004, Swift is currently the only space observatory covering this
wavelength range.

As a comet's surface warms near the sun, the ices turn to gas and form a
tenuous atmosphere, or coma, measuring hundreds of thousands of miles
across. The solar wind pushes this gas back to form a comet's glowing
gas tail. X-ray emission is a side effect of this interaction.

The X-rays arise through a process called charge exchange. Fast-moving
ions in the solar wind snatch electrons from uncharged atoms in the
comet's atmosphere. The solar-wind ions give off X-rays as the relocated
electrons settle into their new home. Because the interaction occurs
over such a broad region, the total power output of these emissions can
reach one billion watts.

Charge exchange may play important roles in any objects where hot,
expanding gas collides with cooler gas. One example: Young stars
interacting with the gas and planets that might surround them. Comets
provide excellent laboratories to explore these interactions.

When Comet 17P/Holmes underwent a surprising outburst in October 2007,
Bodewits tasked both Swift and NASA's Chandra X-ray Observatory to
observe it. "The comet was too bright to observe with the UVOT. We were
afraid we'd damage the instrument," Bodewits says. "Despite this, we're
still not sure whether we detected Holmes with the XRT or Chandra."

At the time of the outburst, Holmes was about 19 degrees above the
ecliptic, the plane in which the planets orbit the sun. At that
elevation, the comet was probably experiencing a cooler, steadier flow
from the solar wind. "The source of this cooler flow wasn't hot enough
to produce the ions Holmes needed to make X-rays," Bodewits notes.

Four years ago today, Swift captured its first x-rays. The radiation
came from Cygnus X-1, one of sky's strongest sources at these energies.
The system, located within our galaxy, contains a hot, blue-giant star
orbited by a black hole.

"Swift has operated two years longer than we had hoped," says Neil
Gehrels, the mission's lead scientist at NASA's Goddard Space Flight
Center. "And while gamma-ray bursts and stellar explosions are the
satellite's bread and butter, it's clear that Swift has a lot to
contribute to other areas of astronomy."
 
Francis Reddy
NASA's Goddard Space Flight Center
 
Received on Mon 15 Dec 2008 03:11:23 PM PST