|Ground-based radar observations provide information on the trajectories, shapes, spin states, and surface structure of near-Earth asteroids in advance of spacecraft missions. This is crucial for mission planning and for reducing mission risks. I will illustrate this with two examples: 4179 Toutatis, target of a flyby by the Chang’e 2 spacecraft; and 2008 EV5 (hereafter EV5), which has been proposed variously as a target for ESA’s MarcoPolo R mission, one version of the Asteroid Retrieval Mission, and a deep-space human asteroid mission.
Toutatis is near to a 4:1 orbital resonance with Earth, and makes sets of close Earth approaches separated by several decades. It was observed briefly during the previous set of close approaches in the 1930s, discovered after the first flyby in the most recent set in 1988, and was observed with radar using the Goldstone and Arecibo facilities at 4 year intervals between 1992 and 2012. The radar observations show that Toutatis is elongated, bifurcated, and has a complicated non-principal-axis spin state dramatically altered by tidal torques from the Earth and Sun. The radar astrometry and spin-state model allowed Chang’e 2 to make an exceptionally close flyby of the asteroid in 2012, passing within 1 km of Toutatis’ surface.
EV5 was observed with optical, infrared, and radar techniques during its most recent close Earth approach in 2008. The radar images showed that EV5 is a ~400 m spheroid, with an equator-aligned ridge broken by at ~150 m concavity. It has several ~10-m boulders, or at least exposed high-standing and well-defined blocks, on its surface. EV5 will be readily accessible by spacecraft in the early-to-mid 2020s, and so has been proposed as the target for several different missions. ESA’s MarcoPolo R mission proposed to go to EV5, retrieve samples from its surface, and return them to Earth; ESA has elected to not pursue MarcoPolo R at this time. One version of NASA’s proposed Asteroid Retrieval Mission would go to EV5, remove a boulder from its surface, and return that boulder to a distant lunar orbit for use by astronauts. And EV5 has also been considered as a target for deep-space human missions, with opportunities for a yearlong mission spending two months at the asteroid.
Radar reconnaissance of other near-Earth asteroids provides a broad selection of well-characterized targets to choose from for future exploration.