Advances in SPICE Support of Planetary Science

Introduction: SPICE is the de facto international standard for determining the geometric conditions– parameters such as altitude, lighting angles, and LAT/LON coverage of an instrument footprint– pertaining to scientific observations acquired by instruments on board robotic spacecraft. This system, comprised of data and allied software, is used for planning science observations and for analyzing the data returned from those observations. Use of SPICE is not a NASA requirement but is recommended by NASA's Planetary Data System and by the International Planetary Data Alliance. Owing in part to its reliability, stability, portability and user support, the use of SPICE has spread to many national space agencies, including those of the U.S., Europe (ESA), Japan, Russia and India. SPICE has been in use since the Magellan mission to Venus and so has many well-known capabilities. But the NAIF Team responsible for implementing SPICE continues to add new features; this presentation describes a number of these. Geometry Finder: Until rather recently the kind of computations possible with SPICE were of the style "compute quantity X at time T." Not long ago the NAIF Team began adding somewhat of the inverse capability: "within these start and stop times, find the time intervals when such-and-such a geometry condition is true" (example: body X is occulting body Y), or "within these start and stop times, find the time(s) when a particular observation geometry parameter satisfies numeric condition X" (examples: altitude of my spacecraft is between Y and X km; phase angle has reached a relative or absolute minimum). This rather extensive new subsystem is called the "geometry finder" (GF) subsystem. While it can take non-trivial effort to learn how best to use the GF capabilities, doing so places a great deal of capability in a SPICE user's hands. Shape Models: Target bodies–planets, satellites, comets and asteroids–have traditionally been modeled as tri-axial ellipsoids within SPICE. This low fidelity approach is clearly not sufficient for modern missions. NAIF is in the process of adding to SPICE a pair of new shape models: a tessellated plate model and a digital elevation model. Both of these are instantiated within a new SPICE data product named Digital Shape