Solar system objects, with the exception of the outermost planets, are moving sufficiently fast that their motion has to be compensated for by the spacecraft's attitude control system. This involves trimming the gyros to the drift rate of the target, so that the telescope tracks at the same rate as the target motion. During the exposure, the telescope pointing is held either by gyros or tracking on a nearby planetary satellite.
The Earth, Moon, Jupiter, Saturn and Venus are so bright that the FES is saturated when viewing them. A technique has been developed for using the FES measurement of scattered light to put an object as bright as Venus into an aperture, but both the Moon and the Earth must be observed by blind offset techniques under gyro control. The coordinates of solar system objects with a geocentric distance less than 1 A.U. (the Moon and some comets and asteroids) should be corrected for the parallax shift due to IUE's orbit. The Galilean satellites of Jupiter can be tracked to within 2 arc minutes of the planet.
Moving-target observers must prepare an ephemeris of target position in right ascension and declination (epoch 1950), including spacecraft parallax correction to geocentric coordinates, before arrival at GSFC. An ephemeris should also be prepared for any satellite to be used for offsetting. The ephemeris should also give the target drift rates in right ascension and declination (arc seconds/hour). If a planetary satellite is to be used as a "guide star," a table giving the distance and change of distance between target and guide object as a function of time is essential. The time interval between entries in this ephemeris depends on the rates involved, the accelerations, and the degree of pointing accuracy your observing program requires. Plan to contact the Resident Astronomers well in advance of your visit to discuss your observing run's requirements.