NASA space eyes focus on Deep Impact target

Dolores Beasley/Marta Metelko
Headquarters, Washington June 2, 2005
(Phone: 202/358-1753/1642)

Whitney Clavin
Jet Propulsion Laboratory, Pasadena, Calif.
(Phone: 818/354-4673)

RELEASE: 05-139

NASA'S SPACE EYES FOCUS ON DEEP IMPACT TARGET

On July 4, NASA's Deep Impact spacecraft will attempt an
extraordinarily daring encounter with the far-flung comet Tempel 1 hurtling
through space at tens of thousands of miles per hour. As if that is not
challenging enough, the comet's size, shape and other characteristics are
not entirely known.

Two of NASA's eyes in the sky, the Spitzer and Hubble Space Telescopes,
helped scientists prepare for the comet encounter. From their orbits high
above Earth, the telescopes watched Tempel 1 in early 2004. Together they
came up with the best estimates of the comet's size, shape, reflectivity and
rotation rate. The data may help Deep Impact snap pictures of the dramatic
rendezvous and increase the probability of making contact with the comet.

"Even tiny adjustments to our model of Tempel 1 are crucial to hitting the
target and setting camera exposure times," said Dr. Carey Lisse, Johns
Hopkins University Applied Physics Laboratory, Laurel, Md. Lisse is team
leader for the Tempel 1 Spitzer studies.

Previous observations of Tempel 1 taken with ground-based telescopes
indicated the comet is dark and oblong, with a width of a few miles. Spitzer
and Hubble refined these measurements, revealing a matte black comet
approximately 8.7 by 2.5 miles, or roughly one-half the size of Manhattan
Island, N.Y.

"Spitzer was crucial in pinning down the comet's size," said Dr. Michael
A'Hearn, University of Maryland, College Park. He is principal investigator
for Deep Impact and the Hubble observations. "We'll know exactly what it
looks like when we get there," he said.

The Deep Impact spacecraft was launched on January 12, 2005. Its mission is
to study the primordial soup of our solar system, which is sealed away
inside comets.

On July 3, as it approaches Tempel 1, the spacecraft will separate into two
parts. The impactor will attempt the tricky task of placing itself in the
path of the speeding snowball, while the second part, the flyby spacecraft,
swings around for a ringside view.

After the impactor is released, its specialized software will steer it
toward the sunlit portion of Tempel 1's nucleus. To program the software,
mission planners at NASA's Jet Propulsion Laboratory (JPL) needed to know
the size and reflectivity of Tempel 1's surface. Since its surface can't be
observed directly from Earth, scientists turned to Spitzer's infrared eyes
to measure its size.

When viewing a comet in visible light from very far away, only reflected
sunlight can be seen, so a big, dark comet can look the same as a highly
reflective, small comet. In infrared light, a comet's radiated heat is
measured, providing a direct look at its size.

Once the size of Tempel 1 was known, scientists could calculate surface
reflectivity. They calculated the amount of reflected, visible light
observed by Hubble and found Tempel 1 reflects only four percent of the
sunlight that falls on it.

"Knowing the reflectivity also tells us how to set up our cameras," Lisse
said. "Like photographers, it's important for us to know our subject before
the shoot."

Tempel 1's shape and two-day rotation rate were derived from long-term
observations made by various telescopes, including Hubble, Spitzer and the
University of Hawaii's 2.2-meter telescope at Mauna Kea.

In addition to the flyby spacecraft images, at least 30 telescopes around
the world, including Spitzer, Hubble and the Chandra X-ray Observatory, will
be watching the dramatic impact. By analyzing the material blown out of the
interior of the comet, this global network of telescopes will assemble a
list of the raw ingredients that went into making the planets in our solar
system.

JPL manages the Deep Impact mission for NASA. For information about NASA and
the Deep Impact mission on the Web, visit:

http://www.nasa.gov/home/index.html

http://www.spitzer.caltech.edu/spitzer

http://hubblesite.org/news/

http://deepimpact.jpl.nasa.gov/


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Jacques :-)

www.spacepatches.info