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.
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 lunarcrust.
"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.
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.
"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.