Lunar explorers will need GPS system

A low cost network
Although such a halo orbit would be a boon for communication with the lunar far side, it can be a delicate affair and any spacecraft in such an orbit would be teetering precariously atop a gravitational high spot.

"These unstable equilibrium points are kind of like positioning a marble at the top of a hill.  With just a slight push, you can send it rolling down in many different directions," Hill said.

With only a small nudge from its thrusters a com-sat could keep itself on the correct halo orbit, or be sent back to Earth or toward the moon. Spacecraft can also enter such halo orbits just as easily as leaving them.

In fact, research by Jeff Parker at the University of Colorado has found that it takes less fuel for a satellite to enter a L2 halo orbit than it takes to get a satellite from Earth into geostationary orbit a mere 22,000 miles above our planet.

Further cost savings would come from a technique peculiar to spacecraft at the L1 and L2 Lagrange points known as Liaison Navigation; whereby a spacecraft in a halo orbit times how long it takes for a pulse sent to another craft to be returned. From this timing the range between the satellites can be deduced and both craft can be located in space.

"That means that a constellation of spacecraft at the moon can navigate autonomously as long as one of them is in a halo orbit. This would reduce the cost of operating the constellation," Hill said.

NASA Future astronauts returning to the moon will likely need a network of satellites in lunar orbit to aid daily operations.

Lunar Sat-Nav
As well as providing contact with Earth, a constellation of halo orbiting satellites would give future moon walkers a degree of independence and they need not rely on mission control to tell them their exact whereabouts on the lunar surface.

"The lunar com-sats could provide something similar to GPS for lunar explorers," Hill explains. "Receivers for any orbital Lunar Positioning System would probably be more complicated than the GPS receivers we use on Earth."

Our Earthly GPS system relies on four or more satellites in geostationary orbit to confirm our location, something that will likely not be cost effective out at the moon.

"What we are more likely to see is that lunar explorers will have only one or two lunar com-sats visible at a time, so the explorers will probably need to have their own atomic clocks and wait for a while before they get a good position fix," Hill said.

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