Deep Impact detects comet nucleus
Wed Jun 22, 2005 at 08:50 UTC
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!"
Image Credit: NASA/JPL-Caltech/UMD
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.
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."
University of Maryland News Release
