Scientists Plan "Deep Impact" Crash With Comet

Stefan Lovgren
for National Geographic News

January 29, 2004

As principal investigator of NASA's Deep Impact mission, Michael A'Hearn will be in charge of shooting a projectile into the surface of a frozen ball of ice and rock, comet Tempel 1, creating a crater the size of a football stadium.

So does he feel like a kid in a sandbox?

"Yeah, there's something to that," A'Hearn chuckled.

But A'Hearn and his team are not shooting up celestial objects just for fun. Instead, by impacting a copper projectile about the size of a trash can into Tempel 1, scientists hope to find out what the comet is made of.

That's important because comets were instrumental in the formation of the solar system. Many scientists consider comets to be the source of most of the water and organic material that was once delivered to all terrestrial planets.

"The goal is to learn more about how the solar system was formed," said A'Hearn, an astronomy professor at University of Maryland in College Park.

It is the first space mission to probe beneath a comet's surface to reveal secrets of its interior. The experiment could also help scientists devise ways to deflect rogue comets if they threaten to collide with Earth in the future.

To build public support, NASA is inviting space enthusiasts to its Web site (Deep Impact Home Page) to add their names to a CD that will be carried by the projectile onto the comet. The submission deadline is January 31, 2004.

One Shot

Planning for the Deep Impact mission began in 1999. It culminates on July 4, 2005, when a "fly-by" spacecraft will release a smaller "impactor" spacecraft, which will smash into comet Tempel 1 at 37,000 kilometers (22,000 miles) per hour.

Since the fly-by spacecraft is traveling at almost the same high speed after slowing down slightly, scientists have only an 800-second window to make their observations before launching the projectile, which is equipped with a camera and has autonomous navigation.

"It's a one shot-deal," said A'Hearn.

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

After releasing the impactor, the fly-by spacecraft will observe and record data about the impact, ejected material 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.

"The impact should be easy to see from Earth with binoculars, if you know where to look," said A'Hearn.

Origin of Life

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 the comet's interior remain relatively unchanged from the time they were formed.

[NASA recently embarked on a multimillion-dollar research program, involving an interdisciplinary team of scientists from around the world, to study 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 about four billion years ago.]

The target comet, Tempel 1, was discovered in 1867 by astronomer Ernst Tempel. It was probably formed beyond the planet Neptune in the Kuiper Belt, a disk-shaped region that is the source of most of the so-called short-period comets.

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

"It's a pretty average periodic comet," said A'Hearn. "There's nothing unusual about it, which is what we want."

Along for the Ride

The sign-up campaign, meanwhile, is part of NASA's effort to publicize the Deep Impact mission. A CD containing the names of those who signed on board for the one-way trip will be obliterated along with the copper-tipped impactor.

"This is an opportunity to become part of an extraordinary space mission," said Don Yeomans, an astronomer at the Jet Propulsion Laboratory in Pasadena, California, and a member of the Deep Impact science team. "[People] can hitch along for the ride and be part of what may be the best space fireworks show in history."

While the experiment will not throw Tempel 1 off its course, it will help scientists devise ways of how to deflect a comet in the unlikely event that one threatens Earth, a doomsday scenario depicted in movies such as Armageddon and Deep Impact.

"If you want to deflect a comet it's very important to understand how it will react to what you do to it," said A'Hearn "This mission will tell us directly what would be the best methods to use."

A'Hearn says the Deep Impact mission is unique because it's an active experiment. "This mission takes us from the passive, look-and-see-what's-there, which is traditional of all astronomy and planetary science, into the realm of real experimentation."