Small bodies

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

Figure 5.2. NEAR descent profile to scale from initial orbit (as viewed from the Sun), after Dunham et al. (2002).

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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