NASA
scientists want to learn all they can about comet Tempel 1 before they drive a
stake into its heart. Today they improved their view with the first image of
the comet's nucleus.
When Deep Impact releases a probe to
slam into Tempel 1 on July 3-4, scientists will photograph the event in an
effort to pin down the comet's structure and composition. The more they can
learn in advance, the more they'll glean from the explosive event, which may be
visible
to the naked eye from Earth.
The heart
of a comet is made of ice and rock. Around that, gas and dust that boil off
create a dense halo that obscures the solid surface, called the nucleus.
Images
taken in May by Deep Impact have been processed to reveal the nucleus,
according to a NASA statement released today. The atmosphere has been
subtracted from the image to reveal the core.
The nucleus
is not a sphere. Rather, its about 9 miles (14 kilometers) long and 3 miles (4.8
kilometers) wide. The latest image was taken when the spacecraft was about 20
million miles (32 million kilometers) from Tempel 1.
"Being able
to distinguish the nucleus in these images helps us to better understand the
rotational axis of the comet's nucleus, which is helpful for targeting this
elongated body," said University of Maryland astronomer Michael A'Hearn, who
leads the mission.
"We
detected the nucleus a lot sooner than expected, but now we'll be watching the
nucleus all the way to impact," said Carey Lisse, a member of the Deep Impact
team and leader of the effort to extract views, of the nucleus from the
spacecraft images.
The
brightness of the nucleus as determined from these images was close to that
predicted from earlier
observations with the Hubble and Spitzer space telescopes and observations
from large telescopes on the ground, the scientists said.
"The
early detection of the nucleus in these images helps us to set the final
exposure times for our encounter observations," said Michael Belton,
deputy principal investigator for the Deep Impact Mission. "Next we need
to determine, using additional nucleus detections, how the comet is rotating in
space, so we can figure out what part we will hit on July 4th."
Deep Impact
consists of a flyby mothership the size of a subcompact car and a five-sided
impactor spacecraft about the size of a washing machine.
The
spacecraft will approach the comet and collect images and spectra of it. Then,
some 24 hours before the 2 a.m. (EDT) July 4th impact, the flyby spacecraft
will launch the impactor into the path of the onrushing comet. Like a copper
penny pitched up into the air just in front of a speeding tractor-trailer
truck, the 820-pound impactor will be run down by the comet, colliding with the
nucleus at an impact speed of some 23,000 miles per hour.
Scientists
expect the image to create a crater several hundred feet in size, ejecting ice,
dust and gas from the crater and revealing pristine material beneath. The event
will be imaged and analyzed by the mothership and several other ground- and
space-based observatories.
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