In some respects the safe descent onto a small body like an asteroid is easier, since the AV requirement and the thrust requirements are low. However, the three-dimensional trajectory may be rather more complicated, since the descent from orbit may take a significant fraction of a rotation period (Figure 5.2), and thus the required thrust direction rotates a significant angle in inertial space.
A successful descent was accomplished by the NEAR spacecraft onto the asteroid Eros in 2001. A significant complication on small bodies is that their gravity fields are likely to be appreciably non-spherical. Light time and the
limited autonomy and landing capability of NEAR (which was only designed for orbital operations) meant that the descent had to be performed open-loop, through a purely pre-programmed sequence. NEAR was in a near-circular 34 X 36 km retrograde orbit and performed a 2.57 m s~ deorbit burn, changing inclination from 180° to 135°. Four separate braking manoeuvres were pre-programmed to execute at fixed times during the 4.5 h descent. Impact velocity was determined to be 1.5-1.8 ms_ 1 (vertical) and 0.1-0.3 ms~1 horizontal, around 500 m from the target point. The target was selected such that the descent trajectory (Figure 5.3) would maximize the number of low-altitude surface images; the longitude was selected such that the spacecraft could maintain continuous Earth pointing during descent while its body-fixed camera saw the surface of Eros throughout.
A soft-landing concept for Eros, employing electric and hydrazine thrusters to reduce the impact velocity to 0.7 m s~ , was proposed as early as 1971 (Meissinger and Greenstadt, 1971). The vehicle would perform a closed-loop controlled descent monitored by a radar altimeter and three-beam Doppler radar.
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