A New Paradigm for Lunar Orbits

Orbiting the Moon is tricky. There's a big planet nearby (Earth) that tugs on satellites and destabilizes their orbits. NASA researchers have an idea for a new class of orbits that may solve the problem.

 

 

Credit: Pat Rawling

An artist's concept of a manned lunar base.

It's 2015. You're NASA's chief engineer designing a moonbase for Shackleton Crater at the Moon's south pole. You're also designing a com-system that will allow astronauts constant radio contact with Earth.

But you know that direct transmissions won't work--not always. As seen from Shackleton Crater, Earth is below the horizon for two to three weeks each month (depending on the base's location). This blocks all radio signals, which travel line of sight.

The solution seems obvious. Simply place a satellite in a high, circular orbit going almost over the Moon's poles. Better yet, place three satellites into the same orbit 120 degrees apart. Two would always be above the lunar horizon to relay messages to and from Earth.

There's just one problem.

"High-altitude circular orbits around the Moon are unstable," says Todd A. Ely, senior engineer for guidance, navigation, and control at NASA's Jet Propulsion Laboratory. "Put a satellite into a circular lunar orbit above an altitude of about 750 miles (1200 km) and it'll either crash into the lunar surface or it'll be flung away from the Moon altogether in a hyperbolic orbit." Depending on the specific orbit, this can happen fast: within tens of days.