[meteorite-list] The Two Faces of Tempel 1

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 14 Feb 2011 12:40:51 -0800 (PST)
Message-ID: <201102142040.p1EKepoo021787_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.cfm?release=2011-051

The Two Faces of Tempel 1
Jet Propulsion Laboratory
February 14, 2011

Just one year before its Feb. 14 encounter with comet Tempel 1, NASA's
Stardust spacecraft performed the largest rocket burn of its extended
life. With the spacecraft on the opposite side of the solar system and
beyond the orbit of Mars, the comet hunter's rockets fired for 22
minutes and 53 seconds, changing the spacecraft's speed by 24 meters per
second (54 mph). The burn was a result of an international effort to
determine something that could very well be indeterminate -- which face
of Tempel 1 will be facing the sun when Stardust hurtles by tonight,
Feb. 14, the evening of Valentine's Day in the United States.

"Our goal is to re-visit a comet to look for changes that occurred since
NASA's Deep Impact mission took a look five-and-a-half years ago," said
Tim Larson, Stardust-NExT project manager from NASA's Jet Propulsion
Laboratory in Pasadena, Calif. "We may also see the crater that Deep
Impact created in 2005, but because of Tempel 1's rotation, there is no
guarantee. At the end of the day, whatever we see there should provide
some great new science."

While comets have been observed and postulated on for centuries,
cometary science acquired literally "on the fly" is a relatively new
field. Since 1984, there have been spacecraft flybys of six comets. Of
these, none involved the ability to look for changes that may have
occurred as a result of the comet's orbit around the sun. That is, until
Stardust-NExT and Tempel 1 meet tonight.

"You could argue that comet Tempel 1 is the most unique icy dirtball in
our solar system," said Joe Veverka, Stardust-NExT principal
investigator from Cornell University, Ithaca, N.Y. "Not only does it
have many intriguing physical characteristics that fascinate the
scientific community, it also has been analyzed and scrutinized time and
again from the ground and space."

In January 2007, NASA chose Veverka's plan to revisit comet Tempel 1
with NASA's already in-flight Stardust spacecraft. Stardust had just
completed the mission it was designed for - flying to comet Wild 2,
collecting samples of the coma as it hurtled by, and then flying back to
Earth to drop off a sample return capsule so scientists could study
pieces of comet in their labs.

Ask any spacecraft project manager -- re-tasking a spacecraft designed
for a completely different mission is a challenge. To be in the right
place at the right time to see changes in surface features on a small
celestial body that seemingly changes its rotation rate on a whim and is
out of view from observers for most of its five-and-a-half-year orbit
about the sun -- that's something else entirely. But that was the
assignment given to Stardust-NExT team members Mike Belton, Steve
Chesley and Karen Meech.

"As comets sweep though the inner solar system, they come alive," said
Belton, a Stardust-NExT co-investigator from Belton Space Initiatives in
Tucson, Ariz. "They belch gas and dust, and this outgassing can not only
change their orbits, it can also change their rotation rate."

Determining the comet's rotation rate and which side will be illuminated
when is tricky, because the comet had only been seen up close for a
short time in July 2005 during the Deep Impact encounter. From then on,
the comet nucleus, about 6 kilometers (3.7 miles) wide, appeared to
observers to be little more than a point of light in the sky for even
the best telescopes -- including NASA's Hubble Space Telescope. (Tempel
1's orbit takes it as far out as Jupiter's orbit and almost as close as
Mars??? orbit.) But even points of light can bear scientific fruit for
astronomers and space scientists. The flattened, oblong Tempel 1 nucleus
was no exception.

"Its shape is central to what we could learn about its rotation," said
Belton. "A comet reflects the sun's light. When one of its two broad
regions is facing us, it gives off more light. When one of its skinnier
sides is pointed toward the telescope, it gives off less light. So we
felt we could develop an accurate model for the comet's rotation."

The plan was for Belton and Chesley to generate comet rotation models
independently. What both needed was data (and a lot of it) on the amount
of sunlight Tempel reflected and when. Both knew the source for that
information: fellow Stardust-NExT co-investigator Karen Meech. Meech, an
astronomer from the University of Hawaii, reached out to her network of
fellow astronomers around the world.

"They came through (in spades)," said Meech. "In total, 25 telescopes
at 14 observatories around the world allocated about 450 whole or
partial nights to this project. With telescope time at a premium, it was
a massive effort on their parts."

With the Tempel 1 light curve data acquired by Meech in hand, Belton and
Chesley independently worked on determining the rotation rate for Tempel
1. As it turned out, the data revealed it was not so easy.

"The comet doesn't just rotate at a specific rate -- it speeds up and
slows down its rotation depending on what part of its surface is heated
by the sun," said Steve Chesley, Stardust-NExT co-investigator from JPL.
"Overall, the comet's spin is speeding up over time. We expect its
average rotation rate to go up progressively as it continues its orbits
around the sun, but it is hard to define just how much."

In January 2010, after almost a year of analysis, Belton and Chesley
compared notes. Their two independently generated rotational models for
Tempel 1 were remarkably similar. But were they right?

"NASA looked at the data and decided that they were actionable," said
Tim Larson. "Our Feb. 17 burn last year set us up for a flyby when the
comet rotation model suggests the face of Tempel 1 that contains the
Deep Impact crater is facing the sun."

If that is the case, Stardust's camera should be able to see the crater.

"When Stardust completed its prime mission in 2006, it was in an orbit
that could possibly reach only two comet targets in the future," said
Veverka. "One of those two was Tempel 1. I chose it because it is a
fascinating place. If we see the Deep Impact crater, that's great. If we
see the other face of the comet, we will provide science with the most
complete picture of any comet surface to date. Either way, we win."

The wait to find out which face Tempel 1 decides to put forward is
almost over. Tonight, Feb. 14, at 8:40 p.m. PST (11:40 p.m. EST), the
Stardust spacecraft is expected to fly within 200 kilometers (124 miles)
of comet Tempel 1. During the encounter, the spacecraft's navigation
camera will take 72 images. The first one should be down on the ground
soon after midnight at JPL.

"Some people have asked me where I will be when those first images come
down," said Chesley. "I know exactly where I will be. I'll be on the
edge of my seat."

Stardust-NExT is a low-cost mission that will expand the investigation
of comet Tempel 1 initiated by NASA's Deep Impact spacecraft. JPL, a
division of the California Institute of Technology in Pasadena, manages
Stardust-NExT for the NASA Science Mission Directorate, Washington, D.C.
Joe Veverka of Cornell University is the mission's principal
investigator. Lockheed Martin Space Systems, Denver, built the
spacecraft and manages day-to-day mission operations.

For a press kit and other information about Stardust-NExT, please visit:
http://stardustnext.jpl.nasa.gov .

For NASA TV streaming video, scheduling and downlink information, visit:
http://www.nasa.gov/ntv .

The live coverage and news conference will also be carried on one of
JPL's Ustream channels. During events, viewers can take part in a
real-time chat and submit questions to the Stardust-NExT team at:
http://www.ustream.tv/user/NASAJPL2 .

DC Agle 818-393-9011
Jet Propulsion Laboratory, Pasadena, Calif.
agle at jpl.nasa.gov

2011-051
Received on Mon 14 Feb 2011 03:40:51 PM PST