Descent to an airless body

There are two fundamental arrival strategies - from a closed orbit (circular or otherwise) around the target body, and from a hyperbolic or near-linear trajectory directly to the surface.

Landing places some significant requirements on the thrust capability of the landing propulsion. Obviously the thrust-to-weight ratio (in that gravity field) must exceed unity if the vehicle is to be slowed down. The AV requirements will depend significantly on the trajectory and thrust level chosen, and can in the case of a hover, be infinite; a lower bound is given by the impulsive approximation analogous to the Hohmann transfer between coplanar orbits - first an impulse is provided to put the vehicle on a trajectory that intersects the surface, on the opposite side in the case of a descent from orbit. A second impulse can then be applied to null the velocity at the impact site.

In practice the trajectory of the vehicle, the performance of the propulsion system and the topography of the target body are inadequately known for such a strategy to be performed open-loop, except in the case of landing on very small bodies where the orbital and impact velocities are low enough that the second, arrival AV can be safely provided by impact forces rather than propulsively. Thus some sort of closed-loop control is needed.

Compensation for varying propulsive performance (both due to engine performance variations, especially if feed pressure may vary in blowdown mode, and due to the progressively reducing mass of the vehicle) can be achieved by monitoring the spacecraft acceleration with onboard accelerometers. The NEAR spacecraft used this technique, with thruster cutoff after a fixed time as a backup against accelerometer failure. In fact because NEAR hit the ground while its expected descent profile indicated it should still have been falling, it tried to compensate for the upward force from the ground by firing thrusters downward, pushing the spacecraft into the regolith until the burn timed out.

For closed-loop control of the trajectory relative to the ground, some sort of navigation information is needed.

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