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October
1, 2007: Comet Encke never knew what hit it.
On
April 20, 2007, the comet had just dipped inside the orbit
of Mercury, perilously close to the sun, when a solar eruption
struck and literally tore the comet's tail off. This surely
has happened to comets before, but for the first time in history
a spacecraft was watching. NASA's STEREO-A probe recorded
a fantastic movie of the collision. To play it, click on the
image:
Above: A CME strikes Comet Encke. Click to
play: 7 MB avi, 4
MB gif
"We
were speechless when we saw this," says Angelos Vourlidas
of the Naval Research Lab in Washington, D.C. "I kept
playing the movie over and over."
Vourlidas
is part of a team of NRC researchers who built the Heliospheric
Imager telescope onboard STEREO-A that recorded the event.
He's also the lead author of a paper reporting the collision
in the Oct. 10 issue of the Astrophysical Journal Letters.
The
eruption that hit Encke was a CME or "coronal mass ejection."
Sky watchers on Earth are familiar with CMEs because of the
auroras they create when they occasionally hit our planet.
CMEs are fast-moving and massive, packing billions of tons
of solar gas and magnetism into billowing clouds traveling
a million-plus miles per hour.
Actually,
it is a little surprising that a CME succeeded in ripping
off a comet's tail. For all their mass and power, CMEs are
spread over a large volume of space. The impact of a gossamer
CME exerts little more than a few nanoPascals of mechanical
pressure—softer than a baby's breath.
The
ripping action must have been something else.
"We
believe the explanation is 'magnetic reconnection,'"
says Vourlidas. Magnetic fields around the comet bumped into
oppositely directed magnetic fields in the CME. Suddenly,
these fields linked together--they "reconnected"--releasing
a burst of energy that tore off the comet's tail. A similar
process takes place in Earth's magnetosphere during geomagnetic
storms powering, among other things, the aurora borealis.
"In
a sense, the comet experienced a geomagnetic storm,"
says Vourlidas. "It is the first time we've ever witnessed
such an event on another cosmic body."
"Although
STEREO is primarily designed to study CMEs and their impact
on Earth, we hope this CME strike on Encke will provide insights
to scientists studying comets as well," adds Mike Kaiser,
STEREO project Scientist at NASA's Goddard Space Flight Center.
 Right:
STEREO-A monitors a solar eruption, an artist's concept. [More]
[Larger
image]
Among
astronomers, Encke is one of the most famous and important
comets. It was only the second comet named, after Halley itself,
and it is the shortest-period comet known. Encke loops around
the sun once every 3.3 years compared to, say, once every
75 years for the more typical Halley.
Encke's
short period combined with its trajectory inside the orbit
of Mercury means it may have been blasted by CMEs more often
than any other comet routinely observed by astronomers. Could
this have some effect on the comet's nature and evolution?
"We don't yet know."
Vourlidas'
team is only beginning to study all the data gathered by STEREO.
The spacecraft carries a suite of five different telescopes
(collectively known as SECCHI) each optimized for a different
kind of observation ranging from wide-angle views of CMEs
at visible wavelengths to pinpoint maps of sunspots through
high-resolution ultraviolet filters. The Heliospheric Imager
movie represents just a fraction of the total dataset. In
the months ahead, "we're going to be looking at the CME-comet
interaction in much greater detail to understand the processes
at work," says Vourlidas.
Encke
may not know what hit it, but NASA scientists soon will.
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Author: Dr.
Tony Phillips | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
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STEREO, short for
Solar Terrestrial Relations Observatory, is actually
a pair of spacecraft, A and B, widely separated to provide
stereo views of the sun. STEREO-A was in position to
see Comet Encke when the CME struck.
STEREO
is the third mission in NASA's Solar Terrestrial Probes
program. The HI instrument was built in the U.K. by
Rutherford Appleton Laboratory and the University of
Birmingham with major contributions from NRL and the
Centre Spatial de Liege, Belgium. The Naval Research
Laboratory is the Department of the Navy's corporate
laboratory. NRL conducts a broad program of scientific
research, technology, and advanced development. The
Laboratory, with a total complement of nearly 2,500
personnel, is located in southwest Washington, with
other major sites at the Stennis Space Center, MS; and
Monterey, CA. STEREO is sponsored by NASA's Science
Mission Directorate and is managed by the Goddard Science
and Exploration Directorate.
Web
links:
Solar
Terrestrial Relations Observatory (STEREO) -- home
page
NASA's
Future: The
Vision for Space Exploration
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