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NASA spacecraft speeding toward Thursday comet flyby

Posted: November 3, 2010

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A recycled NASA spacecraft is hurtling toward an encounter Thursday with comet Hartley 2, an icy ball of primordial rock, dust and gas that could hold the key to better understanding the birth of the solar system.

Artist's concept of the Deep Impact spacecraft approaching Hartley 2. Credit: NASA/JPL-Caltech/UMD
About the size of a compact car, the robotic spacecraft will shoot past the comet at a distance of about 700 kilometers, or 435 miles. Closest approach is expected around 1400 GMT (10 a.m. EDT) Thursday.

The probe's two cameras will track the comet as it flies by at a relative speed of more than 27,500 mph, snapping more than 118,000 images during the approach, encounter and departure sequences.

Scientists say the mission is collecting about 1.5 gigabytes of data during the flyby, yielding unprecedented views of Hartley 2, which will become just the fifth comet ever imaged by a nearby spacecraft.

"This is going to give us the most extensive observation of a comet to date," said Tim Larson, the EPOXI project manager at the Jet Propulsion Laboratory in Pasadena, Calif.

EPOXI is the extended mission for NASA's Deep Impact spacecraft, which dropped a high-speed impactor to comet Tempel 1 in July 2005, blowing a hole in the nucleus and spraying icy debris into space. The Deep Impact mothership continued flying, and NASA offered the craft for scientists to propose new missions.

"After delivering the impactor to comet Tempel 1 in July of 2005, the spacecraft was still in good shape (and) willing to do more work," Larson said. "It just needed a new reason for living, and NASA, in its effort to go green by reusing spacecraft and recycling as much as possible, approved a new mission for the project."

NASA selected EPOXI, a Discovery mission of opportunity to use the probe's high-resolution telescope for extrasolar planet observations and visit another comet in the inner solar system.

Engineers guided the spacecraft toward Hartley 2 using nearly a dozen engine burns and three flybys of Earth to use the planet's gravity to slingshot the probe closer to its target. The maneuevers were accomplished as the craft executed its planetary observations.

According to NASA, the EPOXI mission costs $42 million for operations from 2007 through 2011. By comparison, the development and flight of Deep Impact cost $267 million, not including the price of a Delta 2 launch vehicle.

Michael A'Hearn, the Deep Impact principal investigator, is leading the EPOXI science team.

"The flyby spacecraft will be going past comet Hartley 2, which is a rather different kind of comet than was Tempel 1," A'Hearn said. "In particular, the nucleus is much smaller, but it's also much more active in releasing gas and dust, so we expect to find differences on the nucleus that will help us explain how the comet works."

The approaching Deep Impact flyby probe took this picture of Hartley 2 on Nov. 2. Credit: NASA/JPL-Caltech/UMD
The probe started observing Hartley 2 on Sept. 5, taking daily images with its on-board instruments. The spacecraft started downlinking about 2,000 frames per day beginning Oct. 1, according to Larson.

The Deep Impact spacecraft was expected to begin the encounter phase of the flyby late Wednesday, when controllers planned to pivot the probe to point its science deck toward the comet. The maneuver would also move the craft's high-bandwidth communications antenna away from Earth, curtailing the stream of data coming from the instruments.

A low-gain antenna will keep engineers 13 million miles away on Earth informed on the craft's health, but the probe won't beam back any up-close imagery until 30 minutes after the flyby.

Beginning 50 minutes before closest approach, the spacecraft will switch control to an automatic navigation algorithm to keep its two cameras centered on Hartley 2.

The probe's most sensitive camera, called the high-resolution instrument, will collect color images of Hartley 2 and study the comet's composition with an infrared spectrometer. The 11.8-inch telescope will provide the most detailed pictures of a comet ever taken.

The medium-resolution imager will gather context views of the comet's nucleus and take pictures of Hartley 2's coma, or the cloud of ice, dust and gas surrounding the object's core. From a distance of 700 kilometers, or 435 miles, the camera will have a resolution of about 23 feet.

"We've chosen 700 kilometers because we're balancing several desires and several threats," Larson said. "The closer we get to the comet, the better our image resolution, so we want to get as close as we can. But we're also limited by the turn rate of the spacecraft. As the comet zooms by us, we have to be able to turn fast enough to track it all the way by."

Officials fear particles of ice or rock could damage the spacecraft if it flies closer to the comet.

Scientists expect to find Hartley 2 an active comet.

The approaching spacecraft has already captured jets of poisonous cyanide gas coming from the comet. Scientists also spotted two more erupting plumes from Hartley 2 on Oct. 26, evidence the comet is reaching a peak in activity near the sun.

Hartley 2 was selected as a backup target for EPOXI after its original destination, comet Boethin, went missing. Researchers believe Boethin broke apart into pieces too small to be tracked from Earth.

Despite progressively better images from the Deep Impact probe and the best efforts from other telescopes, researchers know little about Hartley 2.

The Hubble Space Telescope captured this view of Hartley 2 in September. Credit: NASA/ESA/Hal Weaver
Its nucleus is estimated around 1.4 miles across and rotates once every 18 hours. Those dimensions make Hartley 2 the smallest comet every imaged by a flyby probe.

Three other comets, Tempel 1, Wild 2 and Borrelly, averaged about 4 miles in diameter. The mean diameter of Halley's Comet is about 7 miles.

Radar data from the Arecibo observatory in Puerto Rico show the comet's rocky center is elongated in shape.

"It's a much smaller nucleus," A'Hearn said. "Despite that, it puts out more gas every minute than does Tempel 1. This says it's a really active nucleus for its size."

The EPOXI mission to Hartley 2 will add another comet to the growing list of icy bodies visited by spacecraft. Scientists say they are eager to compare and contrast Hartley 2 with the other comets.

But Hartley 2's wild behavior, including its unexpectedly active jets and eruptions, will give scientists rich data to analyze over the coming months. According to A'Hearn, the objective is to determine how Hartley 2 has evolved since its formation at the genesis of the solar system.

"This is what sets it apart and what enables us to try to separate the features on the nucleus that are driven by the outgassing process and the evolution of the comet from the aspects of the nucleus that are primordial and can tell us how the planets formed 4.5 billion years ago," A'Hearn said.