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July 10, 2003

Catherine E. Watson
Johnson Space Center, Houston, Texas
Phone: 281/483-5111

Release: #J03-80

CATCHING A COMET'S TAIL IN THE EARTH'S UPPER ATMOSPHERE

For more than 20 years, NASA has flown high-altitude research
aircraft to collect cosmic dust - debris of comets and
asteroids that fills the inner solar system. In late April
though, they made the first attempt to collect dust particles
from a very specific target - comet Grigg-Skjellerup.

Until that flight, scientists had no way of knowing the cosmic
origin of the dust particles they collected. Using a computer
model developed by Dr. Scott Messenger, a researcher in the
Office of Astromaterials Research and Exploration Science at
NASA's Johnson Space Center in Houston, they were able to
determine exactly when to fly in order to catch a piece of the
comet they wanted to examine.

"In effect, NASA is exploring the solar system with
airplanes," Messenger said.

Messenger predicted that the week of April 22 was the best
opportunity for collecting Grigg-Skjellerup particles, as the
Earth passed through the dust stream created by the comet as
it flew around the Sun.

Dust streams from comets are similar to those that produce
meteor showers, but are different in several important ways.
First, the particles are much smaller than meteor particles,
which are about the size of a grain of sand. Second, the dust
streams hit the Earth's atmosphere at much lower speeds,
enabling the dust to survive entry into the atmosphere without
melting. Third, these streams are very young, produced as
recently as 30 years ago, while many meteor streams are
hundreds of years old.

This last aspect is what makes the comet dust particles
possible to identify in the dust collections, even among a
very abundant background of interplanetary dust. The fresher
cometary dust particles can be identified by their lack of
solar flare damage tracks and implanted gas from the solar
wind. Dust particles from comet Grigg-Sjkellerup will be
identified by a detailed examination of the collected samples,
a process that could take years.

"The key measurements will be performed by transmission
electron microscopy, a technique that gives compositional
information on an atomic scale," Messenger said.

Comet samples are a good place to look for the ingredients of
the early solar system, which themselves came from the
remnants of early stars in the universe.

"Identifying cometary dust particles will allow us to study
the most distant parts of the solar system on a microscopic
scale," Messenger said.

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