July Fourth: Crashing the Party

Deep Impact trajectory to intercept and probe the interior of comet Tempel 1.
Credit: NASA/JPL

On July 4th, before fireworks burst across the United States to celebrate Independence Day, there will be a different kind of explosion in the sky.

A NASA spacecraft will smash head-on into the comet Tempel 1. The impact should carve out a crater on the surface, allowing scientists to see the interior of a comet for the first time.

The Deep Impact spacecraft is now fast approaching Tempel 1, and 24 hours before the scheduled collision it will release a smaller spacecraft that has been designed to hit the comet.

Tempel 1 is orbiting the sun at 37,100 kilometers (23,000 miles) per hour – 10 times faster than a bullet – so the impacting spacecraft is expected to be vaporized when it hits.

"It’s an utterly simple experiment in concept," says Deep Impact Principal Investigator Michael A’Hearn of the University of Maryland. "You have something that you put in front of the comet and let the comet run over it, like putting a penny on a train track. However, it’s a little more challenging that that."

Deep Impact intercepts ballistically the Tempel 1 comet on July 4, 2005.
Credit: NASA/JPL

Tempel 1 will be about 133 million kilometers (83 million miles) from Earth on July 4, so signals sent from mission control will take several minutes to reach the spacecraft. The impacting spacecraft therefore is designed to think for itself during the final phase of the mission, and will perform several navigational adjustments to ensure it hits the comet.

The Deep Impact 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.

"We get one chance, lasting 800 seconds, to take all of the key data – from impact until we’ve flown past." says A’Hearn. "Because we can’t carry 800 different kinds of instruments on Deep Impact, we’ve also got a really extensive Earth-based program of observations."

The Chandra space telescope will collect X-ray data about the comet, Hubble will be looking in the ultraviolet, and the Spitzer space telescope will watch in the infrared. The European Space Agency’s Rosetta space telescope also will be observing, as will 60 Earth-based observatories in 20 countries.

Look near Jupiter for the Tempel 1 comet on July 4, 2005. Click image for full view.
Credit: NASA/JPL

The best places to observe will be from the Western United States and across the Pacific to New Zealand. Although Tempel 1 normally is not bright enough to observe with the naked eye, the impact could cause the comet to brighten considerably. Sky watchers should look for the comet near the bright star Spica and the planet Jupiter early in the morning on July 4, at about 1:52 a.m. Eastern time (10:52 p.m. PDT on July 3).

Scientists expect to release images within a few minutes after impact. It will take 7 or 8 minutes for Deep Impact’s signals to reach Earth, and a few minutes more will be needed to process the images. Better quality images will take at least a day to process.

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.

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.

"Not only are comets thought to be the left over bits and pieces from the outer solar system formation process, but they’re thought to have brought to the Earth much of the water and the carbon-based molecules that allowed life to form," says Don Yeomans, Deep Impact mission scientist at NASA’s Jet Propulsion Laboratory. "So they hold the keys to the birth of the solar system, and they hold the keys, perhaps, to the birth of life itself."

Most comets are not visible without a telescope. Hale Bopp Comet above World Trade Center in foreground, light dot above the Twin Towers is Hale-Bopp, which was visible from urban centers.

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 the comet on a path toward Earth.

As a comet travels near the sun, it becomes heated and its surface chemistry changes. Scientists presume that the material beneath the surface is more pristine, and can therefore provide information about what the solar system was like in its earliest years.

Yeomans says that someday comets could provide resources for colonizing the inner solar system, providing water for explorers to drink, or to use as hydrogen and oxygen rocket fuel. "So in some sense," he says, "comets may be the future watering holes and fueling stations for interplanetary exploration."

Scientists got their first close-up look of a comet’s nucleus with the Stardust mission. The comet Wild 2 turned out to be more complex than anticipated, with multiple spires, craters, and gas jets. Stardust collected material streaming off the comet, and will return that sample to Earth on January 15, 2006.

Related Web Pages

Deep Impact
Comet Blaster Blasts Off
Cometary Big Dig

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