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May 7, 2002

Stopping a comet in its tracks
By Jeremy Hsieh

(Special to The Diamondback)

Scientists from the Deep Impact project examine components of the spacecraft, set to launch Jan. 2, 2004 from Cape Canaveral, Fla. COURTESY OF MICHAEL A'HEARN

For university astronomy professor Michael A'Hearn, the proverbial sky is not the limit. A'Hearn heads Deep Impact, a project to study the Tempel 1 comet - by crashing a multi-million dollar spacecraft into it.

Deep Impact, a six-year, $279 million project, has united NASA, Ball Aerospace and Technologies Corp. in Colorado, the Jet Propulsion Laboratory at the California Institute of Technology and hundreds of scientists around the world, including two from the university, in an effort to trace the origins of the solar system.

The collision between the spacecraft and the comet will create a crater and spew the contents of the comet's interior outward. A second spacecraft will observe the comet bits and transmit data back to Earth for analysis.

NASA is footing the bill for the endeavor, which is part of its Discovery Program, and providing oversight and review for the project. JPL is responsible for operations in flight, and BATC for construction of the two-mission spacecraft. A'Hearn will be joined on the project by Lucy McFadden, an associate professor in the university's astronomy department.

Comets are of particular interest to scientists because they believe their nuclei, or solid centers surrounded by gaseous outer layers, hold a cosmic record that will reveal the solar system's beginnings.

"The reason for wanting to understand a comet is that the comets in their interior should preserve a record of the ices that condensed at the beginning of the solar system and therefore, they tell us what their temperature and density conditions were when all the planets formed," A'Hearn said. "The outer layers have been altered by the comet going around the sun, so we're trying to get down deep enough to find out how different the interior is."

Initial plans to study comets called for landing a spacecraft on one. In February 2001, NASA's NEAR spacecraft did land on the asteroid Eros. But landing on an asteroid is very different from landing on a comet.

"Landing on Eros was relatively easy because Eros was much bigger, had significant gravity - there was no outgassing to push you away and there was a pretty good consensus that the material would be like rock and dirt on Earth, so you'd know how to anchor in it," A'Hearn said. "There is no such consensus for cometary nuclei."

Noted astronomer Michael Belton, who headed the project's first incarnation in 1996, and Alan Delamere, a current science team member, designed Deep Impact as an alternative to a landing mission. NASA rejected the first proposal.

"The big objections the first time around were that we were using a 'dumb' impactor," A'Hearn said. "The second major criticism was that the target was an extinct comet."

Extinct comets have exhausted the volatile substances that produce the comet's gaseous outer layer and fluorescent tail.

Several factors went into deciding a new target: The target had to be active, readily reachable from Earth, visible from Earth at the time of impact and approachable from its sunlit side. The mission also had to be launched within the two-year window NASA mandated.

"Given all these constraints, Tempel 1 was the only good candidate," A'Hearn said.

The Spacecraft

The mission involves two spacecrafts, the "impactor" and the "flyby." The two will be flying together as an integrated unit for the vast majority of the mission.

The main spacecraft, the flyby,will be a tall, pentagonal prism weighing about 1,450 pounds with three primary functions. It will carry the impactor to Tempel 1, record the entire event and relay data the two machines collect back to Earth. It will sport two custom-made telescopes, a high-resolution instrument and a medium-resolution instrument, which will allow it to capture detailed images of the comet during impact from a safe distance of at least 300 miles.

These instruments will be on a panel mounted on one side of the flyby. Debris shields will provide protection to the instruments. The largest part of the flyby is its eight-by-eight foot solar panel, to be mounted opposite the instrument panel.

The impactor will be a 770-pound spacecraft similar in shape to the flyby, but about half as tall. It will house a short-range transmitter to relay information to the flyby, a battery, a small propulsion system and the impactor targeting system. The ITS is a telescope that will record images of the comet as the impactor approaches it.

The telescope will also feed data to the propulsion system for small trajectory adjustments, the impactor's "smart" aspect. It is designed to function independently of the flyby only in the final 24 hours of the mission, the amount of time the two will be separated.

Both spacecraft will function autonomously during impact because of the roughly eight-minute communication time-lag between Earth and the spacecraft.

The Mission

Launch is scheduled for January 2, 2004, at the Kennedy Space Flight Center in Cape Canaveral, Fla. An unmanned, dual-stage, $65 million Boeing Delta II rocket will deliver the impactor and flyby into space.

After deployment, the dual spacecraft will enter the sun's orbit. For the next 12 months, it will orbit the sun. In January 2005, it will pass by the moon and the instruments will be tested before a course change commits it to a trajectory that will intersect with Tempel 1.

Problems severe enough to cause the mission to fail at this stage are unlikely, A'Hearn said. Many of the systems on the spacecraft are redundant, meaning that if one fails another can take its place.

The spacecraft will spend the next six months moving into the path of Tempel 1, whose roughly two-mile radius makes it a little less than half the size of Washington, A'Hearn said.

On July 3, 2005, the impactor and flyby will separate. The impactor's battery will power it independently for the next 24 hours until the comet overtakes and destroys it on July 4. Scientists do not know the shape of the comet, so the impactor is programmed to target the comet's center of brightness and will hit within an area of about 300 feet in radius - a difficult task.

"If the nucleus were a perfect, uniform sphere, it would be easy. The problem is we don't know the shape of what we're trying to hit. And we won't know until a day or two before we get there, and that's probably too late to do anything," A'Hearn said.

"It's very different from launching a military rocket ... to a known target. If you want to launch a military rocket on a military target, you know what you're looking for ... It's like, back in the Cold War, trying to target a missile at the Kremlin, and you don't know what the Kremlin looks like, and you don't know where in Moscow it is."

Much of the impactor will be vaporized on impact, A'Hearn said.

Even though the collision will occur about 80 million miles from Earth, it will be visible from small telescopes and possibly the naked eye.

A'Hearn refused to comment on what would happen if the impactor misses the comet, but said the mission was unlikely to fail.

"All our simulations show a much better than 99 percent probability of hitting it," he said.

A'Hearn's main concern was that the impactor would create a crater in a shadowy area, making it difficult for the flyby and ground observers to see the crater and witness its formation.

The flyby will record the impact and, scientists hope, the images of the comet's nucleus. In the 14 minutes following impact, the flyby will slow down and pass underneath the comet. Scientists will study how the crater forms and the makeup of the ejected material.

"During the crater formation, the ejecta flow out as a function of time; the material comes from deeper and deeper in the crater," A'Hearn said. "The first stuff comes from the surface, and the last stuff comes from the bottom. We'll also look at the final crater. Is it layers? Or is it just a steady gradient?"

For a month after the collision, the flyby will transmit all the images and data it collected during impact back to Earth, though certain images will be available almost immediately. There is a significant chance that the flyby will be damaged beyond functionality at some point after impact, but not before sending back the primary data. If it survives, the spacecraft will enter an orbit around the sun and become space junk.

Construction has been underway since last May, when NASA approved the new Deep Impact proposal. The designs passed a critical review in February, giving BATC the green light to proceed with construction.

The three telescopes, the HRI, MRI and the ITS, are currently in construction. The telescopes and two spacecraft are being built separately. They will be finished by the end of the year and will be assembled by next summer, said Harold Montoya, flight systems manager for Deep Impact at BATC.

"Most of us have been around this for awhile, so the problems we have are not unusual. We try to work through them," he said. "We've actually been very pleased with our progress. We're on track to reach our launch date and I don't see any major obstacles at this point."

(This article was reproduced with permission from the author. The original can be found at:

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