Deep Impact Sees Heart of Target Comet

Comet Tempel 1’s nucleus appears as the red blotch at the center of these false-color views based on Deep Impact observations from May 30, 2005. The actual image is on the upper left with a mathematical model Tempel 1’s atmosphere to right. The lower left image shows the difference between the two upper views, yielding Tempel 1’w nucleus. At bottom right, a trace through the center of the comet shows the brightness of the nucleus. CREDIT: University of Maryland/NASA/Ball Aerospace.

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.

• Deep Impact: Special Report