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> Hubble Witnesses Comet Collision

JULY 2005

Hubble Witnesses Fireworks
From Comet Collision


Hubble's view: From
80 million miles away

On July Fourth, not all the fireworks were on Earth. NASA's Deep Impact spacecraft helped create some spectacular celestial fireworks by crashing into comet 9P/Tempel 1, a potato-shaped object about 3 miles (5 kilometers) wide and 7 miles (11 kilometers) long.

NASA's Hubble Space Telescope had a front-row seat for the big event, providing the best visible-light view from Earth of the collision. The telescope watched as an 820-pound probe, called an impactor, punched through the comet's crusty surface. The impactor hurtled into the comet's core at 23,000 miles per hour, releasing lots of energy. The impactor, made up mostly of copper, formed a crater big enough to swallow a large stadium.

By breaking through the outer baked crust, the collision unleashed ancient material that had been trapped inside the comet since its birth billions of years ago. It will take astronomers a long time, however, to analyze all the information collected about the comet and share the results.

The mission


Deep Impact

Scientists launched the Deep Impact spacecraft six months ago on its one-way trip to the comet. The spacecraft was as large as a mid-size sport utility vehicle and had two parts: a flyby spacecraft and the impactor. Both parts were fitted with instruments designed to record the event.

The flyby spacecraft launched the impactor and then moved out of Tempel 1's path to avoid being hit by the comet. As the impactor traveled to its planned meeting with the comet, its camera snapped images of the nucleus.


View from
the flyby

Tempel 1's nucleus is only the fourth ever seen. A comet is surrounded by dust, which makes its nucleus difficult to view. The flyby spacecraft's camera spotted the nucleus because it was very close to the comet, about 300 miles away. The instruments onboard the flyby craft took detailed images of the impact and recorded other data needed to identify the ejected material.

A key to the past

Why were astronomers daring enough to take on a comet? They want to study the material locked beneath a comet's icy surface. For billions of years, comets have been "baked" by sunlight as they pass through the inner solar system. Their warmed surfaces develop a black, carbon-rich crust. The baked crust seals material inside the comet. By cracking through the surface crust, astronomers can learn more about a comet’s makeup and structure.

Looking inside a comet also may help explain how our solar system formed about 4.5 billion years ago. Comets are an important link to our early solar system. These celestial snowballs are the remains of material created in the coldest outer regions of our solar system. They may still contain the original building blocks of the Sun and the planets.

What are the parts of a comet?

of a comet


The heart of a comet is its nucleus, an object made up of chunks of ice and rock that is surrounded by a cloud of gas and dust, called a coma. The icy comet releases the gas and dust when it is warmed by sunlight. The solar wind pushes the coma away from the comet, forming one or more tails of gas and dust. The gas and dust surrounding a comet prevent scientists from learning more about the makeup of the nucleus. Astronomers are eager to learn whether comets exhaust their supply of gas and ice over time or keep some of it in their interiors. By breaking through Tempel 1's surface, scientists may find an answer to this question. They also may discover how Tempel 1's interior is different from its surface.

Tempel's tale

Amateur astronomer Ernst Wilhelm Leberecht Tempel of Marseilles, France, discovered comet 9P/Tempel 1 in 1867 while using a telescope to search for comets. It is named "Tempel 1" because it was the first comet spotted by Tempel. The "9P" part of the name means that Tempel 1 was the ninth periodic comet discovered by sky watchers. Periodic comets have stable orbits and can be regularly observed from Earth.

Tempel 1 has completed more than 100 orbits around our star. The frequent trips have helped it develop a crusty layer of carbon-rich material. Little else, however, is known about the comet. Astronomers are eager to learn whether Tempel 1, which has depleted much of its outer layers of gas and ice, has kept some gas and ice in its interior.

The encounter with the Deep Impact probe did not change Tempel 1's course. So, the comet will continue to orbit the Sun every 5.5 years.

A visible impact

Besides the Hubble telescope, an array of other space-based observatories viewed this once-in-a-lifetime event, including the ultraviolet Galaxy Evolution Explorer, the infrared Spitzer Space Telescope, and the Chandra X-ray Observatory. Many ground-based observatories also witnessed the smashup.

The views obtained by all of the observatories provided a treasure trove of information about the comet. The cameras aboard the Deep Impact flyby craft, for example, provided a close-up view of the comet, from about 300 miles away. From its distance of 80 million miles away, Hubble captured a wider view of the encounter, before, during, and after the impact. Hubble's view measured changes in the comet's brightness. By analyzing those changes, astronomers determined how much material was released from the impact. This information will help astronomers learn about Tempel 1's makeup.


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