NASA Readies "Deep Impact" Craft for Comet Smash

Stefan Lovgren
for National Geographic News
December 16, 2004
The launch of NASA's Deep Impact mission to smash a spacecraft into a comet on July 4, 2005, has been delayed until at least January 12. That means the mission team will have fewer potential launch dates to choose from, because their target comet will only be in range for a short time.

But scientists said this week that they are still confident the mission will go off as planned.

"We expect to provide some great fireworks for all our observatories … next Fourth of July," engineer Rick Grammier said at a news conference this week. Grammier is the Deep Impact project manager at NASA's Jet Propulsion Laboratory in Pasadena, California.

If all goes as planned, a flyby spacecraft will shoot a projectile about the size of a trash can into the surface of a frozen ball of ice and rock, comet Tempel 1, creating a crater the size of a football stadium.

The mission is the first time a spacecraft will touch the nucleus of a comet. Scientists hope to find out more about what the comet is made of.

Comets are seen as the building blocks of the solar system. Many scientists believe comets are the source of most of the water and organic material that was long ago delivered to terrestrial planets.

Software Update

Planning for the Deep Impact mission began in 1999. It should culminate on July 4, 2005, when the "impactor" spacecraft is expected to smash into comet Tempel 1 at 22,000 miles an hour (37,000 kilometers an hour).

Since the flyby spacecraft is traveling at almost the same high speed, scientists have only a 13-minute, 20-second window to make their observations before launching the projectile, which is equipped with a camera and is built to reach the comet largely on autopilot.

The launch was initially scheduled to take place at the end of this month, but has been delayed at least twice. The latest delay came after scientists determined that further software testing was needed.

Boeing, the spacecraft manufacturer, also decided to replace a part on the Delta 2 rocket that will carry the craft.

The craft must launch from Florida's Cape Canaveral space station by January 28 in order to make a rendezvous with Tempel 1 on July 4, 2005. Otherwise, the scientists will have to choose another, less enticing target.

Tempel 1 was discovered in 1867 by astronomer Ernst Tempel. The comet probably formed beyond the planet Neptune in the Kuiper belt, a disk-shaped region that is the source of most short-period comets, comets that orbit within our solar system.

Tempel 1 is probably three times as long as it is wide—smaller than comet Halley. It's been in its present orbit a long time, and has made many passages through the inner solar system.

Mystery Interior

Comets are composed of ice, gas, and dust—primitive debris from the formation of the solar system about 4.5 billion years ago. The Deep Impact mission could answer basic questions about how the solar system was created, because scientists believe the material in comet interiors remains relatively unchanged from the time they were formed.

(One NASA research program, involving an interdisciplinary team of scientists from around the world, studies whether comets supplied the raw material to form life on Earth. Scientists believe that Earth suffered a prolonged series of cometary impacts at its formation billions of years ago.)

"There are no data on the interior [of a comet], and that's what we hope to solve with Deep Impact," the mission's principal investigator, Mike A'Hearn, said at the news conference. A'Hearn is an astronomy professor at the University of Maryland in College Park.

Each time the comet passes close to the sun, it heats up, changing the surface layer of the comet. Thus, only the interior of the comet preserves what scientists anticipate are clues to the formation of the solar system.

Big Bang

The impact is expected to create a crater 100 meters (330 feet) in diameter and up to 30 meters (100 feet) deep. But scientists know so little about comets that cratering experts can't even agree on what physics are relevant to the impact, and so can't agree on what exactly will happen.

"Some people think the nuclei are strong [and will stay in place] … others think we will fracture the nucleus into several pieces. Other people think we may just compress material downward," A'Hearn said.

After releasing the impactor, the flyby spacecraft will observe and record data about the impact, the material ejected from the crater, and the structure and composition of the crater's interior. Professional and amateur astronomers and telescopes on Earth will also observe the impact and its aftermath, and the results will be broadcast on the Internet.

"This is a unique mission, in that much of the scientific return is going to come from an Earth-based observation campaign, both from Earth orbit and from the ground," said Karen Meech, an astronomer at the University of Hawaii and a Deep Impact co-investigator.

For people who know where to look, the impact should even be easy to see from Earth with binoculars, scientists say.

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