The outgassing activity of periodic comets, and hence their observability, depends upon how close these icy bodies approach the sun. The following table shows the evolution of the comet's perihelion distance and orbital period as a result of various passages by Jupiter for a few hundred years in the past and future. From the 17th century until the comet's discovery, repeated passages by Jupiter reduced the comet's perihelion passage distance to a point where the comet's activity allowed it to be discovered in April 1867.
Comet 9P/Tempel 1 was the first discovery of a periodic comet by Ernst Wilhelm Leberecht Tempel and the 9th periodic comet to be recognized as such. After its discovery in 1867, the comet was observed during its next two returns to perihelion in 1873 and 1879. However a close Jupiter approach in 1881 to within 0.55 AU pushed the comet's perihelion passage distance out to 2.1 AU so that despite attempts to recover the comet, it was not seen again for nearly a century. As a result of additional Jupiter approaches in 1941 and 1953, the perihelion distance evolved back in toward the Earth's orbit and the comet was again observed during its 1967, 1972, 1978, 1983, 1989, 1994 and 2000 returns to perihelion. Between 2000 and 3000, the comet's perihelion passage distance will reach a minimum of 1.48 AU in 2161 and a maximum of 2.37 AU in 2833. Because the orbital period of comet 9P/Tempel 1 is roughly one half that of Jupiter, the comet's orbit is in a so-called 2:1 resonance with Jupiter and hence its motion is rather stable, without wide variations in its orbital evolution. In the following Table, the comet's perihelion distance and orbital period are given for times on either side of each Jupiter close approach. This Table is meant to show the evolution of Tempel 1's perihelion distances and how they can change as a result of Jupiter close approaches. While there are no Jupiter close approaches within the interval, information is also provided for the returns to perihelion between 1967 and 2022.
|Time Epoch||Perihelion Distance (AU)||Orbital Period (years)||Jupiter Close Approach Distance (AU)|
|1867 May||1.56||5.65||Comet discovery in 1867|
The Minimal Effect of the Deep Impact Impactor on the Orbital Motion of Comet 9P/Tempel 1
The collision with comet 9P/Tempel 1 takes place near the comet's perihelion point and at a relative velocity of 10.2 km/s. The 370 kg impactor will impart a very modest 0.0001 mm/s velocity change in the comet's orbital motion and by so doing decrease the comet's perihelion distance by 10 meters and decrease its orbital period by far less than a second of time. This is to be compared to a change of some 378 billion meters in the comet's perihelion distance due to the passage by Jupiter in 2024. Because the comet is so much larger and more massive than the impactor, there will be practically no change in the comet's orbital motion as a result of the Deep Impact collision. As is evident from the above Table, the changes imparted in the motion of comet Tempel 1 by Deep Impact are completely negligible when compared to the orbital changes on the comet due to periodic passages near the giant planet Jupiter.
It has been suggested that one effective technique for deflecting a small comet or asteroid that is on an Earth threatening trajectory would be to run into it with a massive spacecraft at high velocity several years prior to its predicted Earth encounter. The optimal technique for this type of kinetic energy impact would involve a head on crash of a massive spacecraft with the comet at the comet's perihelion to cause it to lose a bit of its orbital energy and hence change its orbital velocity by a few millimeters per second. Over ten years' time, a 7 mm/s change in the comet's velocity would modify its orbital position by one Earth radius thus allowing the comet to miss the Earth entirely. Although, the impulse given to comet 9P/Tempel 1 in 2005 will not materially affect this comet's orbit, this same impact magnitude could substantially affect the trajectory of a much smaller comet. For example, it is interesting to note that the impulse delivered to comet Tempel 1 in 2005 would be sufficient to move, in ten years time, a comet of diameter 125 meters by one Earth radius.