Approach a world with weak gravity

Phobos has a heavily cratered, irregular surface and very weak gravity. The image here is a later observation from NASA’s Mars Reconnaissance Orbiter, acquired in 2008. It provides context for the destination; it is not a picture returned by either Soviet spacecraft. NASA identifies the prominent Stickney crater and explains how the displayed color combines several HiRISE channels.

Weak gravity changes what happens after an impulse, but it does not remove the need to navigate. A spacecraft approaching a moon is moving in a system that includes Mars, the moon and the spacecraft itself. Shape, terrain, relative velocity and uncertain surface properties all matter. A familiar Earth landing intuition can be misleading in this environment.

The final Phobos 2 plan included release of a stationary platform and a mobile hopper near the moon. The National Research Council’s mobility report describes the proposed hopper using mechanical rods to orient its instruments after touchdown and spring-loaded legs to move to another site. This was a planned way to compare local measurements across the surface, not a rover journey that actually occurred.

The report describes a design intended to make hops of up to about 20 meters and investigate multiple sites during a short battery-powered lifetime. Those figures are design goals. Contact with the parent spacecraft was lost before deployment, so no sequence of hops on Phobos demonstrated that performance. Ground engineering work and an achieved surface traverse require different evidence.

In an original flat-ground exercise, a test object leaves a surface vertically at 0.2 meters per second under a constant downward acceleration of 0.01 meters per second squared. Ignoring all other forces, it reaches its highest point after 20 seconds and rises 2 meters. These are invented teaching values, not a flight prediction for the irregular moon.

The calculation uses time to the top equal to upward speed divided by acceleration, and height equal to speed squared divided by twice the acceleration. For the same initial upward speed, weaker constant gravity produces a longer ascent and greater height. A real landing design must add local gravity, terrain, rotation and the object’s orientation; the simple exercise does not prove that a proposed hop is safe.