All objects in the solar system rotate as they orbit the sun and Comet 9P/Tempel 1, the target of Deep Impact, is no exception. It has been found to spin with a period of about 1.71 days as a result of an intensive observing campaign that measures periodic fluctuations in comet's brightness with time.
Knowledge of the rotational properties of the comet is significant to the Deep Impact project for two reasons. The first has to do with planning the mission and the second has to do with the interpretation of the data that the Deep Impact mission will acquire at the comet.
For planning purposes the spin state must be known to great precision by the time of the last Earth flyby en route to the encounter with the comet if it is to be useful. If this precision cannot be achieved then knowledge of the spin matters little. We would like to use the rotation period to adjust the arrival time at the comet so that the largest possible lighted area of the comet's nucleus presents itself to the impactor at the time of encounter. This will provide the highest assurance of an impact in a lighted area. To do this an accuracy of a few seconds in the 1.71 day rotation period is required. To achieve this kind of accuracy an international observing program using ground-based telescopes has been underway since early 1999. At present the accuracy is at the level of a few minutes and it is too early to tell if the necessary accuracy will be achieved. Should the rotation period not be known sufficiently well the arrival time will be determined by other considerations, such as the ability of the spacecraft to communicate to two stations simultaneously during the encounter and the aspect of the target at impact will be left to chance.
For scientific interpretation accuracy of the rotation period is not the primary consideration. What is needed here is knowledge of the physical nature of the spin state. Here an analogy with the flight of a football is useful. The shape of a football is not too different from that expected for the shape of 9P/Tempel 1's nucleus. It will therefore have similar rotational properties. For a kicked football the ball spins end-over-end as it flies through the goal posts. This end-over-end spin is the most likely spin state of the comet nucleus. Physicists call it the lowest rotational energy state. However, a perfect pass has the football spinning rapidly around its long axis. Physicists call this the highest rotational energy state and it is unlikely for the Deep Impact comet. However, some wobbling spin state in between these two extremes is possible and attempts will be made to determine if this is the case. If this indeed turns out to be true, then together with knowledge of the precise shape obtained from Deep Impact images, we can use knowledge of the rotation to infer how material is distributed within the interior of the comet's nucleus.