5.3 Solar-System Targets

Objects within the solar system move with respect to the fixed stars. HST has the capability to point at and track moving targets, including planets, their satellites and surface features on them, with sub-arcsecond accuracy. However, there are a variety of practical limitations on the use of these capabilities that must be considered before addressing the feasibility of any particular investigation.

HST is capable of tracking moving targets with the same precision achieved for fixed targets. This is accomplished by maintaining FGS Fine Lock on guide stars, and driving the FGS star sensors in the appropriate path, thus moving the telescope so as to track the target. Tracking under FGS control is technically possible for apparent target motions up to 5 arcsec per second. In practice, however, this technique becomes infeasible for targets moving more than a few tenths of an arcsec per second. An observation can begin under FGS control and then switch over to gyros when the guide stars have moved out of the FGS field of view. If sufficient guide stars are available, it is possible to "hand off" from one pair to another, but this will typically incur an additional pointing error of about 0.3 arcsec.

Targets moving too fast for FGS control, but slower than 7.8 arcsec per second, can be observed under gyro control, with a loss in precision that depends on the length of the observation.

The track for a moving target is derived from its orbital elements. Orbital elements for all of the planets and their satellites are available at STScI. For other objects, the PI must provide orbital elements for the target in Phase II. The Reuse target offset capability (see above) can be used to insert an offset within 3 days of the observation to eliminate "zero-point" errors due to an inaccurate ephemeris.