Countdown to Mystery Moon Begins
By Amit Asaravala
Wired.Com
02:00 AM Dec. 24, 2004 PT
After hitching a ride on the back of NASA's Cassini orbiter for
seven years through more than 2 billion miles of space, the
European Space Agency's Huygens space probe is finally stepping out
on its own.
Mission controllers at the Jet Propulsion Laboratory in Pasadena
plan to cut the 700-pound probe loose from Cassini Friday evening,
initiating a 21-day free-fall that will end with Huygens crashing
into the surface of Saturn's mysterious moon Titan. The journey is
expected to answer a question that astronomers and scientists have
been asking ever since the Voyager spacecraft spotted the moon's
thick atmosphere in 1980: Just what exactly is hidden underneath
all that haze and smog?
Though mission scientists don't think Huygens will stumble upon
anything like the green forests and blue oceans here on Earth --
nor any forms of life, for that matter -- they do think it will
encounter conditions very similar to those that existed soon after
the Earth formed.
"(Titan) is the only moon in the solar system with a thick,
nitrogen-rich atmosphere" -- relatively similar to that on Earth --
said Huygens project scientist Jean-Pierre Lebreton. "In this
atmosphere, we may have chemical reactions similar to ones
occurring before life appeared on Earth. Going in situ will allow
us to better understand what causes those chemical reactions to
take place."
Scientists are especially interested in figuring out where the
small percentage of methane in Titan's atmosphere is coming from.
Normally, light from the sun would break it down into other
chemicals that would then fall on the moon's surface like rain. But
the fact that some methane still exists in the atmosphere suggests
that the surface might harbor pools of liquid methane which
regularly evaporate back into the atmosphere.
The Cassini spacecraft, launched on Oct. 15, 1997, has tried to
peer through Titan's haze to find these oceans several times. But
despite using radar and cameras that can see "around" the haze by
analyzing different wavelengths of light, the orbiter has revealed
little more about the moon's surface than the fact that it contains
rough and smooth areas.
However, these observations were all made from a distance of at
least 590 miles from the moon. Huygens, on the other hand, will
enter Titan's atmosphere and drift down to its surface, taking
readings and sending them back to Cassini as it goes. The plunge
into Titan's atmosphere is expected to last about two and a half
hours and will take place on Jan. 14.
As it drifts to the surface, held aloft by three successive
parachutes, Huygens will use six instruments to gather as much data
as possible about the composition, density and volatility of
Titan's atmosphere. The probe will also take photographs in
multiple directions -- and listen for sounds.
"We are going to hear what Titan sounds like," said Lebretton.
"There is a simple microphone on board, but it should be able to
collect sound -- for example, if there is lightning, we will hear
it. We may also detect sounds due to winds."
If the mission scientists are lucky, they may even receive data
directly from the surface of Titan for as long as 30 minutes after
Huygens touches down, provided the instruments survive the impact.
Likewise, they may continue to get readings if the probe lands in a
lake or ocean, though the existence of such is now in doubt.
"If we believe the observations made by Cassini on Oct. 26 and Dec.
12, we are not seeing oceans," said Lebretton. "It's still a
possibility, but we're not sure. We'll see what we find when we get
to the surface."
Cassini is scheduled to eject the Huygens probe at approximately
7:08 p.m. Pacific Standard Time on Dec. 24. The maneuver must go
exactly as planned, or else Huygens could slip away from the
plotted trajectory, possibly jeopardizing the mission. The probe
has no thrusters onboard to correct itself if it goes off
course.
Lebretton said the project team is not all that worried, especially
given the impressive track record of the Cassini mission so
far.
"There is a certain tension, but things are going so well," he
said. "We are fairly confident this will happen."