Countdown Begins for Deep Impact With Comet

Stefan Lovgren
for National Geographic News
July 1, 2005
Watch a National Geographic News video news report on Deep Impact (requires Windows Media Player).

NASA's Deep Impact spacecraft will be launching a projectile into the surface of comet Tempel 1 on Monday, possibly causing the most spectacular fireworks on U.S. Independence Day.

"The time of comet encounter is near, and the major mission milestones are getting closer and closer together," said Rick Grammier, Deep Impact project manager at NASA's Jet Propulsion Laboratory (JPL), Pasadena, California. "After all the years of design, training and simulations, we are where we want to be. The flight and science teams are working the mission plan, and we are good to go for encounter," he added.

The flyby spacecraft will use medium- and high-resolution imagers and an infrared spectrometer to collect and send to Earth pictures and spectra of the event. Also watching Deep Impact will be NASA's Hubble and Spitzer space telescopes, the Chandra X-ray Observatory, the Swift and Submillimeter Wave Astronomy satellites, and the European Space Agency's XMM-Newton X-ray observatory and Rosetta spacecraft.

Observatories on Earth will view the impact and its aftermath.

"Those in the U.S. living west of the Mississippi [and in] Hawaii, and [those living in] New Zealand will have ringside seating for the impact event in a dark sky," said Don Yeomans, a Deep Impact mission scientist at JPL.

Scientists, who know very little about the interior of comets, hope the trash can-size projectile will smash a hole deep enough in the comet's icy exterior to reveal what lies inside it.

Learning more about what comets are made of will help explain the role they played in forming our 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 mission will help us understand the earliest phases of the solar system by learning what's really inside a comet in its pristine form," said Jay Melosh, a professor of planetary science at the University of Arizona in Tucson. Melosh is also a member of the Deep Impact science team.

The experiment could also help scientists devise ways to deflect rogue comets if they threaten to collide with Earth in the future.


During the early morning hours of July 3, a flyby spacecraft will deploy a 39-inch-wide (1-meter-wide), copper-fortified probe into the path of Tempel 1. The comet is about half the size of Manhattan and is hurtling through space at 23,000 miles an hour (37,000 kilometers an hour).

If all goes according to plan, over the next 22 hours both the spacecraft and the probe will move toward the comet. When the probe, which has autonomous navigation, hits the comet, the flyby craft will pass 310 miles (500 kilometers) below.

"We are really threading the needle with this one," said Rick Grammier, the Deep Impact project manager at JPL. "We are attempting something never done before at speeds and distances that are truly out of this world."

But scientists don't know what will happen when the projectile hits the comet, because no one knows how hard its outer layers are. The impact could make a small dent or a crater the size of a football stadium. According to one long-shot theory, the comet may even disintegrate.

"We expect to be surprised," Melosh said.

The flyby spacecraft, which carries the most powerful telescope ever sent into deep space, will observe and record data about the impact. A camera on the impactor itself will snap pictures as it hurtles toward the comet.

The comet may become nearly a hundred times brighter as a result of the impact, scientists speculate, and the smash-up could create a fireworks display visible in a dark sky by the naked eye.

Deep Freeze

Comets are composed of ice, gas, and dust—primitive debris from the time of the early solar system—and they could answer basic questions about how the solar system was created.

"[The comets] have spent most of their lives in the deep freeze of the outer solar system, [so] they likely retain the original ices and materials that were present during the outer solar system formation process that began 4.6 billion years ago," Yeomans said.

Tempel 1, discovered in 1867 by German astronomer Ernst Tempel, 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.

Tempel 1 is probably three times as long as it is wide. It has been in its present orbit a long time and has made many passages through the inner solar system.

But scientists know little of the comet's nucleus.

"We still need to determine its brightness, orientation, and shape," Grammier said.

Scientists hope the mission will help them understand the composition and interior structure of the heart of Tempel 1.

"Like a field geologist using a hammer to probe a rock's strength and density, the Deep Impact spacecraft will probe the comet's strength and density by observing the reaction of the nucleus to a controlled impact," Yeomans. said

Nuking Comets

Deep Impact is expected to barely alter the comet's orbital path around the sun, and will not put either the comet or a chunk of it on a collision course with Earth.

However, the mission may help scientists devise ways to deflect any other comet that threatens to collide with Earth in the future.

"The kind of information we could get out of this—how porous or how strong the comet is—is just the sort of thing we need to evaluate any potential deflection schemes," Melosh said. "We're completely ignorant about that now."

Possible deflection options range from landing a spacecraft on the surface of a comet and trying to push it in a different direction to using nuclear weapons to blow it up.

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