From Observational Geometry to Practical Satellite Design: AsteroidFinder/SSB

Abstract

DLR has selected AsteroidFinder as the first payload to be flown on its SSB satellite platform, in the frame of the German national Compact Satellite Program. The scientific goal is to better observe and characterize Near-Earth Objects (NEO), particularly the Aten asteroids and the Inner Earth Objects orbiting completely Interior to Earth’s Orbit (IEO). Only ten mostly Aten-like IEOs have been found so far, of which two are Potentially Hazardous Asteroids (PHA). Ground-based observations of Atens and IEOs are severely constrained by the Earth’s body and its atmosphere. An Earth-orbiting survey telescope can in principle evade these constraints with ease, to become an efficient and cost-effective tool to facilitate the discovery and follow-up of these objects. It may however be constrained by other factors specific to its orbital environment. Analysis of the observational geometry and present technological capabilities has shown that stray light from the Sun and Earth is the most critical performance limiter. The thermal influences of both sunlight and earthlight become important if the capabilities of state-of-the-art detectors are to be fully exploited at low temperatures. Thus, the optical and thermal behaviour of the satellite as a system beyond the scientific instrument itself is strongly coupled, through the shape and layout of the satellite and the parameters of the satellite’s orbit, to the observational geometry of the target asteroids, the Earth, and the Sun. Objects within the Earth’s orbit are to be observed in a region of interest continuing sunward from that covered by ground-based surveys to 30° solar elongation. Their identification is accomplished through apparent motion and parallax, requiring repeated observations of the same field which the satellite has to provide at certain intervals. The strong coupling of optical and thermal influences forces system-level optimization of the geometrical layout of the satellite in accordance with survey pointing patterns. This affects the layout of the telescope and the components visible to its aperture, the positioning of several radiators for different temperature levels, the accommodation of antennae for communication, the placement of deployable baffles, sunshields, and solar panels, etc. AsteroidFinder will also test space-based detection of space debris and artificial satellites at different observational attitudes. All this has to be fitted to the limited envelope of a compact class satellite; within the common envelope and power rating of a small household fridge, and no moving parts but one-time deployables. As of December 2008, AsteroidFinder/SSB is in preparation for phase B. Its launch is planned for 2012, for a one-year baseline mission.