Lunar explorers will need GPS system

By David Powell

updated 2:38 p.m. ET April 30, 2008

Communicating effectively with astronauts on the moon was an essential part of the Apollo missions. Without reliable radio contact there would have been no live feed of Armstrong's first steps and in all likelihood no first steps at all.

According to new research, the next footsteps on the lunar surface could be beamed back to Earth via the moon's very own network of communication satellites. The setup could double as a GPS for moonwalkers.

"What we did in the Apollo missions will not be enough," said Keric Hill of Texas A&M University. "We will need some kind of communication relay system to ensure 24/7 coverage of the moon."

Without it, interesting areas for exploration such as the lunar poles, which may harbor water ice in permanently shadowed regions, will remain out of reach.

The moon's far side is also currently inaccessible without a relay satellite, for it is the ultimate radio dead spot; the only place in our solar system that never faces Earth.

Points of contact
Getting radio signals to these hard-to-reach places is going to require a go-between that can cope with the constant gravitational nudges from the Earth, moon and sun.

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One potential path a lunar communication satellite (com-sat) could take is by following a "frozen orbit" around the moon. In such an orbit the satellite's orbital characteristics remain constant despite prods from the moon's lumpy gravity field.

This uneven gravity field is due to mascons, large concentrations of mass in the lunar crust.

"You can think of it (a frozen orbit) as a roller coaster ride over the lunar mascons. If you pick the path just right, the tugs and pulls of the mascons will end up cancelling each other out. At the end, the spacecraft will be right back where it started in the orbit," Hill told SPACE.com.

An alternative to the bumpy ride of the frozen orbit is to place a com-sat in a halo orbit around a region known as a Lagrange point.

There are five Lagrange points in the Earth-moon system marking the region where a spacecraft can remain stationary relative to the Earth and moon.

"A halo orbit at L2 (the Lagrange point above the lunar far side) would be an ideal location for a lunar communication relay, since a spacecraft there could always see the far side of the moon. Some halo orbits are large enough that a spacecraft would always be out from behind the moon and able to see the Earth," Hill said.

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