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It Takes a Cosmic Village to View a Comet


Like people gazing skyward to watch Independence Day fireworks, an international array of telescopes will train expert eyes on a dramatic encounter between NASA's Deep Impact spacecraft and a passing comet. The explosive event will happen 133.6 million kilometers (83 million miles) from Earth in the early hours of July 4 Eastern Daylight Time (late July 3 Pacific Daylight Time). Telescopes on the ground and others orbiting in space will document the mission's crucial moments using different wavelengths of light.

Artist's concept showing Deep Impact just before impact with comet Tempel 1.
Artist's concept showing Deep Impact just before impact with comet Tempel 1.
CREDIT: Maas Digital.

Comets are dirty balls of ice that hold clues to our own solar system's formation and evolution. Deep Impact is the first space mission to attempt to break the surface of a comet and reveal the secrets inside. The mission will send a 360-kilogram (816-pound) impactor into the path of comet Tempel 1. After releasing the impactor, the main spacecraft, called the flyby craft, will move safely aside and collect information. During this phase, every moment counts. The flyby spacecraft will have just over 13 minutes to gather its precious data.

"The flyby craft is constrained to an 800-second interval from the time of impact to the time it will no longer be able to observe the impact site," said Dr. Karen Meech of the University of Hawaii, Honolulu, co-investigator of the Deep Impact Earth-based campaign. "So ground and Earth-orbital observations will be key to realizing the full scientific potential of the mission."

The time chosen for the impact was partially determined by the location of Earth-bound telescopes and their ability to observe the event. On encounter night, telescopes will jump into action like volunteers in a fire brigade, performing specific duties based on their unique abilities.

Artist's concept of Spitzer Space Telescope.
Artist's concept of Spitzer Space Telescope.

A trio of NASA's Great Observatories will be watching from orbit: the Hubble Space Telescope with its keen optical vision; the Chandra X-ray Observatory using its powerful X-ray eyes; and the Spitzer Space Telescope with its infrared detectors to measure the heat emitted from the comet at impact. NASA's Galaxy Evolution Explorer and the Canadian and French space agencies' Far Ultraviolet Spectroscopic Explorer will look for specific molecules in the comet. The European Space Agency's Rosetta spacecraft will watch in multiple wavelengths from a different side of the Sun than the other space telescopes. Rosetta is en route to comet Churymov-Gerasimenko and will be the closest telescope to Tempel 1.

On the ground, at least 60 observatories in 20 countries will focus their attention on the bold encounter. Telescopes on Mauna Kea in Hawaii - in particular some of the world's largest telescopes, the W.M. Keck, Subaru and Gemini North telescopes - have teamed up to maximize the unique capabilities of each facility. Radio telescopes, including the Institut de Radio Astronomie Millimétrique telescope in Spain, the James Clerk Maxwell Telescope on Mauna Kea, and Kitt Peak telescope in Arizona, will observe molecules in the comet that vibrate at radio frequencies, in particular the breakdown products of water. Water is probably the most abundant molecule in comets. Prior to Deep Impact's launch, Spitzer and Hubble came up with the best estimates of the comet's size, shape, reflectivity and rotation rate, to help the impactor zero in on its target.

Gemini North Observatory atop Mauna Kea in Hawaii.
Gemini North Observatory atop Mauna Kea in Hawaii.
CREDIT: Gemini North Observatory.

''The combination of Hubble and Spitzer observations provided scientists with knowledge of the nucleus' shape, size, and shininess, information that we could not get from many years of observing from the ground," said Dr. Carey Lisse of Johns Hopkins University's Applied Physics Laboratory, Laurel, Md. Lisse is team leader for the Spitzer and Chandra studies of the comet event.

Meech and Lisse will be stationed at a NASA facility on Mauna Kea on the night of the impact to coordinate the massive observing effort. They will ensure that mission-critical observations are made and communicated to the Deep Impact science team at NASA's Jet Propulsion Laboratory in Pasadena, Calif. A video teleconference will link all the major facilities around the world.

While only part of the world will have a view of the comet through telescopes at the time of impact, astronomers all over the globe will be able to see the resulting effects over the next several hours, days, and possibly weeks. Some of the observations from professional observers will be posted on the Deep Impact mission site at, and amateur observers are also asked to submit observations at

"The level of cooperation among planetary colleagues as well as from colleagues outside the field is unprecedented," said Meech. "The experiment has not only engaged people's curiosity, but also a desire to help the Deep Impact mission make the most of its one-shot opportunity."

This fourth of July, a cosmic village of telescopes will make history as they watch the spectacular feat, while the rest of the world waits eagerly for information and pictures.

Gay Hill (818) 354-0344
Jet Propulsion Laboratory, Pasadena, Calif.


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