Efforts to reach the
Martian moon Phobos have long been outshined by missions to the red planet
itself. Now, scientists in Russia, Canada and the U.S. are preparing their own
missions to the largest moon of Mars.
Mars actually
has two moons: Phobos and Deimos. They might more properly be called
satellites, however, because they are extremely small, only a few kilometers in
diameter.
In fact,
some scientists think Phobos and Deimos could be asteroids that somehow ended
up orbiting Mars instead of crashing into the planet, or they could be
leftovers from the time of planetary formation. Another option is that the
moons are fragments of Mars, blasted off the planet's surface by a large
asteroid or comet impact.
Whatever
their origins, in
terms of space missions the moons have been overshadowed by the planet they
orbit. If a spacecraft is sent on a 6-month journey of many millions of miles,
it seems anti-climatic to have it plop down on a tiny asteroid-like body when
Mars itself is so close, beckoning with its many different regions to explore
and a complex history to unravel. Russian scientists, however, have been trying
to send
a spacecraft to Phobos, the larger of the two Mars moons, for many years.
Phobos 1
and Phobos 2 were launched within 5 days of each other in July 1988. Phobos 1
lost communications due to a software glitch that September, but Phobos 2 had
better luck and made it all the way to Mars orbit. Before the spacecraft could
send its two probes to Phobos, however, the spacecraft's signal was lost due to
a computer malfunction.
The Russian
Federal Space Agency proposed sending a new mission to Phobos in 1999. Called
Phobos-Grunt ("grunt" being the Russian word for "soil"), this mission aims to
gather a sample of soil from the moon's surface and send it back to Earth for
analysis. Although an unmanned sample return mission sounds complicated, the
Russian space program has succeeded many times in conducting similar missions
closer to home.
From Our
Moon to Mars
Between
1959 and 1976, the USSR sent a fleet of robotic spacecraft to investigate
Earth's Moon. Luna 16, Luna 20, and Luna 24 each dug up lunar soil and then
sent a sample back to Earth via an ascent vehicle located on top of the lander.
These lunar soil samples are now housed at the Vernadsky Institute of
Geochemistry and Analytical Chemistry in Moscow, Russia.
The
director of the Institute, Erik Galimov, thinks samples from Phobos could help
answer many mysteries about the Martian moon's origin and composition. He says
that we have been able to learn some details about Phobos by gazing at the moon
with telescopes, or by quick flybys of Mars orbiters such as ESA's Mars
Express. These glimpses have shown that Phobos is made of dark material that does
not reflect much light. This makes scientists suspect it has a chemical
composition similar to that of carbonaceous
chondrite asteroids.
But other
aspects of the moon can't be learned simply by staring. For one thing, Galimov
says that remote instruments can't tell us the absolute age of Phobos, while a
sample would allow us to measure that precisely. A sample also would allow
scientists to measure the chemical isotopes, which could then be compared to
the isotopic
signatures of Martian meteorites that have landed on Earth to see how
similar Phobos is to the planet it orbits. Similar studies were conducted on
lunar samples, and they indicate that the Earth and our Moon formed from the
same kind of source material.
A closer
examination of Phobos might reveal if it is one large hunk of rock, or a
gravitationally-bound rubble pile as many scientists now suspect. The question
of whether Phobos contains water ice, a potential resource for future Mars explorers,
also could be answered.
"I think it
will be a very positive contribution, because we would have in our hand samples
from the Mars area," says Galimov.
One
attraction Phobos holds over Mars is that it costs much less money to go there.
Phobos has no atmosphere to contend with, and much lower gravity. Even so, the
latest estimated cost for the entire Phobos-Grunt mission is 2.4 billion rubles
(about 72 million US dollars). Adding to the complications are the number of
instruments that have been added the design plan over the years (the current
count is 11 instruments – most NASA spacecraft have 3 or 4 instruments
onboard).
One of the
planned Phobos-Grunt experiments is the Living Interplanetary Flight
Experiment, or LIFE, provided by the U.S.-based Planetary Society. This
experiment would carry various microorganisms, as well as a soil colony from
the Arctic permafrost, to see how well they endure space travel. The experiment
is controversial because of planetary protection issues – scientists hope to
find evidence for life on Mars someday, and are careful to sterilize spacecraft
so we don't inadvertently carry life there. If Phobos-Grunt should accidentally
crash on Phobos or Mars, the life contained in the spacecraft probably would
not survive for long in the harsh environment, but most scientists don't want
to risk contamination regardless.
The launch of
Phobos-Grunt was scheduled to take place this October, but it now seems likely the
launch date will be pushed back to 2011 at the earliest. Whenever the mission
does launch, it will take 3 years for the lander to reach the small moon, touch
down on its surface, and then return its sample to Earth.
More
Missions
The
Russians are not the only ones who dream of Phobos. Another mission headed by
Optech Inc. and the Mars Institute, and funded as a concept study by the
Canadian Space Agency, is called PRIME, for "Phobos Reconnaissance and International
Mars Exploration." The PRIME mission would be composed of an orbiter and lander,
and each would carry 4 instruments designed to study various aspects of
Phobos's geology. Yet another mission concept, the Phobos-Deimos Sample Return
Mission (SRM), is being studied by NASA's Glenn Research Center. This mission
would rely on low-cost electric propulsion to land on Phobos and Deimos and
bring back samples from the two moons.
These
mission studies are proceeding in part on the philosophy that Phobos could be
key to the future human exploration of Mars. Not only does the ease of landing
and taking off from Phobos make it a good pit stop between Earth and Mars, but
because Phobos is tidally-locked, always showing the same face to Mars, it
could be a stable location for a Mars communications relay or some sort of
monitoring station.
At present,
neither PRIME nor the Phobos-Deimos SRM has a projected launch date.
In the meantime,
spacecraft currently orbiting Mars can study Phobos and Deimos whenever they
happen to pass by. The MER rovers Spirit and Opportunity also have caught
glimpses of the moons, most notably when either Phobos or Deimos has passed in
front of the sun. Neither moon is large enough or positioned so as to block out
the sun entirely like our Moon does during a solar eclipse, but perhaps they
cast a brief shadow over the rovers as they flit by overhead.
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