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Question: Tempel 1 is the comet that best satisfied a variety of requirements when the mission team proposed. Can you guess some of the reasons this is so? Clues are: Size, rotation, orbital path, brightness, and schedule.


  • Size - The larger the comet, the easier it is to target and hit it.

  • Rotation - The comet must rotate slowly enough so the crater doesn't move out of view during our 14 minutes of observation.

  • Orbital path - The comet must travel close enough to the Earth to allow for earth-based observation by telescopes. We must also be able to intercept the comet releasing the impactor in front of the comet with a direct line of sight to Earth.

  • Brightness - Comet Tempel 1 has less coma (fluorescing gas and dust) around its nucleus than some other comets. You might think of it as a flash light that is getting dim because its batteries are beginning to die. Less coma means a safer entry to the nucleus for the impactor as it moves through the coma and a clearer view of the impact on the nucleus once the crater is excavated. It has to be bright enough for us to find it both in space and through telescopes on Earth. It is all a balancing act.

  • Schedule - The project members needed to choose a comet that would be in the right place at the right time according to the time range NASA was funding. Tempel 1 is at its brightest and closest point to Earth when impact occurs. If NASA funded Deep Impact for a different time period, it would probably have meant choosing a different comet.

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