Following strong recommendations from the Science Working Group and Division of Planetary Sciences as well a detailed feasibility study NASA has approved changes to the Webb observatory to enable the study of targets within our solar system. A team representing the pointing control system (Northrop Grumman), the fine guidance sensor (FGS, ComDeV), and the STScI have agreed on an approach that will provide Hubble-like tracking and imaging capabilities for planets (Mars and beyond), moons, Kuiper Belt Objects, and distant comets.
Scientists will be able to specify the target name or tracking data in their proposal and the STScI will plan the observation so that guide stars will be available in the FGS during the desired observation. By commanding the pointing control system to force the guide star to move opposite the motion of the solar system target, the Webb will hold the target steady in its IR cameras or spectrographs (1-28 microns). These observations must finish before the guide star moves out of the FGS field of view: imposing a limit on the product of the duration and the apparent angular speed of the target. For targets in the outer solar system, the angular speed is less than 1.4 milliarcsecond per second (Uranus), allowing exposures of almost a day or more before the guide star transits the 1 arcminute field of an FGS sensor. Observations of Io, however, would be limited to approximately an hour. While astronomers must wait for the release of the proposal and planning system (~ 2012) to find the details for proposing a Webb solar system observation, they can familiarize themselves with the recent implementation study JWST-RPT-009982 from the Moving Target Working Group.