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Deep Impact: The First Look inside a Comet

by Laurie Clark and Karen Meech

On July 3 HST, NASA's Deep Impact (impactor, right) will collide with Comet Tempel 1, giving us a first look inside a comet.

Comets are time capsules that hold clues to the formation and evolution of our solar system. They are made of ice, gas, and dust—the primitive debris from the solar system's earliest and coldest period 4.5 billion years ago. NASA's Deep Impact mission will be the first to explore a comet's interior by creating a crater that will let us look deep inside.

In January, NASA launched Deep Impact from Cape Canaveral, and the spacecraft began its six-month journey to Comet Tempel 1, discovered in 1867 by German astronomer Wilhelm Tempel. Deep Impact consists of two parts, a larger "flyby" spacecraft and a smaller "impactor" spacecraft. The two will remain connected until 24 hours before impact, when the impactor will be released into the orbital path of the comet. The flyby spacecraft will then divert to a new path to safely collect images of the comet's approach, impact, and aftermath, and send them back to Earth.

On July 3, at approximately 8:10 p.m. Hawaii Standard Time (HST), Comet Tempel 1 will hit the 794-pound (360 kg) impactor at 23,000 miles per hour (37,000 km/hour), producing a crater as large as a football stadium and as deep as a seven-story building. The impact will eject ice and dust from the surface of the crater to reveal fresh material below. Scientists will analyze the images transmitted by the flyby spacecraft to learn more about the comet's interior. This knowledge will help us better understand the formation of the solar system and what happens when a comet collides with Earth.

An artist's concept of Deep Impact's encounter with Comet Tempel 1.
Top: As the comet approaches the Sun, it acquires a tail.
Middle: The "impactor" of Deep Impact (center) heads toward Comet Tempel 1 (right), while the "flyby" portion of the spacecraft (left) is ready to observe the collision and send data back to Earth.
Bottom: The impact crater on Comet Tempel 1 is expected to be larger than a football stadium.
NASA art by Pat Rawlings.

As a member of the Deep Impact Science Team, IfA astronomer Karen Meech has played two key roles for this mission: She has studied the comet intensively since 1997 to learn more about its orbit, size, and shape, information needed to target the spacecraft correctly. She has also been responsible for coordinating the worldwide Earth-based observations (ground-based and Earth-orbiting satellites). Meech has traveled to Chile, Germany, Australia, Arizona, Taiwan, and Iceland to conduct research and workshops informing the professional community about the Deep Impact mission and how they can best use the ground-based observations to complement the results from the spacecraft.

Mauna Kea will play an important part in this mission. All its telescopes will be focused on the event before, during, and after impact. This type of worldwide coordination and observing resource allocation is unprecedented since the impact of Comet Shoemaker-Levy 9 with Jupiter in 1994. However, Deep Impact differs in that it is a controlled experiment because the Deep Impact mission team has complete knowledge of the impactor, which is almost half copper to minimize corruption of spectral emission lines that are used to analyze the nucleus.

Hawaii is one of the few parts of the world where it will be possible to view this extraordinary event in real-time with the naked eye. The comet will be well placed for viewing in the southwest sky, very close to the bright star Spica and the planet Jupiter. Scientists expect that at first, because the dust will not have spread out yet, the comet will appear to be a new bright star, but in the hours and days following the impact, the dust will spread out and add to the comet's tail. For the best viewing, go to a dark site away from city lights. Visit the IfA Web site to learn more about Deep Impact, and as information is received, organized activities taking place around Hawaii for the Deep Impact event.

Impact results will be available on the Internet so that everyone can be part of this pioneering space mission.