The Pause before the Crash

The cometary crater left behind as simulated in digital rendering prior to the July 4th encounter.
Credit: NASA, Maas Production

The Deep Impact spacecraft is getting ready for its date with comet Tempel 1. Mission scientists and engineers are on the edge of their seats, waiting for the action to begin.

"If I could bottle all of the adrenaline that I’ve seen flowing in the last day, I probably wouldn’t have to sleep for a year," says Rick Grammier, Deep Impact project manager at NASA’s Jet Propulsion Laboratory in Pasadena, California.

A specially designed impactor spacecraft is scheduled to hit the comet at 1:52 a.m. Eastern time on July 4 (10:52 p.m. PDT on July 3).

Exactly 24 hours beforehand, the smaller impactor spacecraft will detach from the mothership. During its final two hours, the impactor will have three chances to self-adjust its position to make sure it is in the path of the comet. Scientists are hoping the resulting impact will blast a crater in the surface of Tempel 1, allowing them to see the interior of a comet for the first time.

The mothership spacecraft will take pictures before, during and after impact, scooting underneath the comet and then turning back around to watch the fireworks. The impacting spacecraft has a camera as well, and will take pictures right up to the moment of collision.

Tempel 1 is orbiting the sun at 37,100 kilometers (23,000 miles) per hour – 10 times faster than a bullet – so the spacecraft will perform their maneuvers on their own. The encounter will be so quick, human reflexes would not be able to direct everything in time.

Image of Borrelly taken by Deep Space 1.
Ground based image of comet 9P/Tempel 1 set for July 4th encounter and impactor.
Credit: NASA/JPL

"These two spacecraft have been designed to accomplish something that the best fighter pilots in our country would not be able to do," says Monte Henderson, Deep Impact project manager at Ball Aerospace in Boulder, Colorado. "We had to design a navigation and control system that would allow the flyby and the impactor spacecraft to perform this very intricate ballet."

However, if the impactor spacecraft fails to detach from the mothership as planned on Sunday, the mission scientists will direct the mothership to fly into the comet.

Tempel 1 will be about 133 million kilometers (83 million miles) from Earth on July 4. Space telescopes observing the impact include the 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. About 60 professional ground-based telescopes will be watching as well.

The comet will not be visible to the naked eye, but a person viewing with a medium resolution telescope should be able to witness the quick brightening that is expected to occur at the moment of impact. The comet will have already set in the sky for everyone except those in the Southwest and West Coast of the US, and across the Pacific to New Zealand. Still, if the effects of the impact persist for 24 hours, they may be viewable in other parts of the world the next evening.

Deep Impact trajectory to intercept and probe the interior of comet Temple1.
Credit: NASA/JPL

While the scientists are not sure how big a crater will result from the collision, they estimate it will be the size of a small house to the size of a football stadium, and several stories deep. Like all comets, the nucleus of Tempel 1 is made of rock and ice, but it is not known how firmly that material is bound together. The nucleus could be weak and fluffy, or hard as concrete.

The Deep Impact spacecraft recently saw Tempel 1 experience two outbursts, where the coma of gas and dust surrounding the nucleus briefly brightened. During the outbursts, the amount of water in the coma doubled. The exact cause of the outbursts is unknown, although it may be the result of heating as the comet approaches the sun.

Most of the comets in the solar system formed 4.5 billion years ago, at the same time the planets were forming. Tempel 1 formed in the Kuiper Belt, a gas and dust-filled region in the solar system located beyond Neptune’s orbit.

Tempel 1 now orbits the sun every 5.5 years, tracing a path between the orbits of Mars and Jupiter. The impact is expected to slightly alter the course of the comet’s orbit, but mission scientists say this won’t push it on a path toward Earth.

Comets have hit the Earth many times in the past, delivering water and organic compounds to our planet. While such materials are thought to be necessary for life, comets can also be a threat to life — a large cometary impact could cause an extinction event like the one that killed off the dinosaurs 65 million years ago. Michael A’Hearn, Deep Impact principal investigator, says that learning about the architecture of a comet could help us avoid a similar catastrophe.

"If you are worried about defending the Earth from possible impactors, it’s sure a lot easier to change the course of something if you know what it is you are changing the course of," says A’Hearn. "You cannot design a sensible defense system without knowing what the impactors are."

Related Web Pages

Deep Impact
Griffith Observatory Observing Guide to Comet Tempel 1
Blasting Cap On A Comet
Bombing the Comet