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
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.
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 (35,405 km) 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.
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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