[meteorite-list] NASA's Space Eyes Focus on Deep Impact Target

[Ron Baalke]

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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