Missions to small bodies differ from those to larger worlds because the low surface gravity means that an orbiter (or rendezvous) spacecraft can approach close enough to perform a surface mission while hovering (with little or no thrust) and the speed of a landing can be very low. This blurs the distinction between orbiters and landers, and may enable orbiter spacecraft to survive landing, as shown by the landing of NEAR on asteroid Eros. Low gravity also means that a landing vehicle may risk being lost entirely on rebound from the surface, or ejected by outgassing in the case of a comet nucleus. Anchoring systems may thus be required. On the positive side, the low gravity also makes it easy to achieve mobility by jumping, and to perform ‘touch and go’ surface-sampling manoeuvres (e.g. Yano et al., 2003; Sears et al., 2004). Most small bodies are highly irregular, and their gravitational fields can be challenging environments in which to navigate. Dust thrown up from the surface (whether from natural cometary activity or the action of a spacecraft) is another hazard. Many small bodies, particularly comets, are in elliptical orbits and so experience wide variations of temperature and solar power production with time and surface location.

Phobos 1F

The Phobos project involved two large Mars orbiters, Phobos 1 and Phobos 2 (Sagdeev et al., 1988; TsUP, 1988).