[meteorite-list] Deep Impact Detects Comet Nucleus

[Ron Baalke]

http://www.newsdesk.umd.edu/scitech/release.cfm?ArticleID=1087

For Immediate Release
June 21, 2005
Contacts: Lee Tune, 301-405-4679 or ltune at umd.edu

Maryland-Led Deep Impact Detects Comet Nucleus

COLLEGE PARK, Md. -- For the first time, scientists have processed
images from NASA's Deep Impact spacecraft and clearly
seen the solid body, or nucleus, of the comet through the vast cloud of
dust and gas that surrounds it. The new images provide important
information about the mission's target: the "heart" of comet Tempel 1.

The images were taken at the end of May with the spacecraft's medium
resolution camera, at a distance of some 20 million miles from the
comet. Unprocessed, the images are dominated by the comet's huge cloud
of dust and gas, which scientists call the coma. However, scientists
used a neat photometric trick to isolate the relatively small (3-mile by
9-mile) nucleus from the comet's coma, or atmosphere. The much larger,
but less dense atmosphere was mathematically identified and then
subtracted from the original images leaving images of the nucleus, the
bright point in the center of the coma.

"Its exciting to see the nucleus pop out from the coma," said University
of Maryland astronomer Michael A'Hearn, who leads the Deep Impact
mission. "And 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."

"This is an important milestone for the Deep Impact team," explained
Carey Lisse, a member of the Deep Impact team and leader of the effort 
to extract views of the nucleus from the spacecraft images. "From here 
on in we just watch the nucleus grow and grow and become brighter and 
bigger as the spacecraft closes in on the comet. We detected the 
nucleus a lot sooner than expected, but now we'll be watching the 
nucleus all the way to impact!"

As illustrated in the attached figure, Deep Impact images
taken on May 29-31 contain a well-formed coma with a detectable point
source at the position of the brightest pixel. 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. At present, the
nucleus contributes about 20 percent of the total brightness near the
center of the comet.

"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."

5 - 4 - 3 - 2 - 1 - IMPACT

Deep Impact -- which consists of a sub-compact-car-sized flyby
spacecraft and a five-sided impactor spacecraft about the size of a
washing machine -- carries four instruments. The flyby spacecraft
carries two imaging instruments, the medium resolution imager and the
high resolution imager, plus an infrared spectrometer that uses the same
telescope as the high-resolution imager. The impactor carries a single
imager. Built to science team specifications by Ball Aerospace &
Technologies Corp., the three imaging instruments are essentially
digital cameras connected to telescopes. They record images and data
before, during, and after impact.

At the beginning of July, after a voyage of some 268 million miles, the
joined spacecraft will reach comet Tempel 1. 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. A'Hearn and his fellow mission scientists expect the
impact to create a crater several hundred feet in size; ejecting ice,
dust and gas from the crater and revealing pristine material beneath.
The impact will have no significant affect on the orbit of Tempel 1,
which poses no threat to earth.

Nearby, Deep Impact's 'flyby' spacecraft will use its medium and high
resolution imagers and infrared spectrometer to collect and send back to
Earth pictures and data of the event. In addition, the Hubble and
Spitzer space telescopes, the Chandra X-ray Observatory, and large and
small telescopes on Earth also will observe the impact and its aftermath.

The University of Maryland, College Park, conducts
the overall mission management for Deep Impact, which is a Discovery
class NASA program. NASA's Jet Propulsion Laboratory (JPL) handles 
project management for the Deep Impact mission. The spacecraft was 
built for NASA by Ball Aerospace & Technologies Corporation,
Boulder, Colo.

[Image]
Figure 1 - A false color image of the comet, taken on 30 May 2005, is
shown in the upper left. To its right is a mathematical model of the
comet's atmosphere. The bottom left image is the difference between the
two upper images and shows the nucleus. In the bottom right a trace
through the center of the comet shows the brightness of the nucleus. In
these images North is approximately up and East is to the left. The
direction to the Sun is towards the upper left hand corner. The picture
is about 100,000 miles across.