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June
5, 2007: Picture this: A spaceship swoops in from
the void, plunging toward a cloudy planet about the size of
Earth. A laser beam lances out from the ship; it probes the
planet's clouds, striving to reach the hidden surface below.
Meanwhile, back on the craft's home world, scientists perch
on the edge of their seats waiting to see what happens.
Sounds
like science fiction? This is real, and it's happening today.
The
spacecraft is MESSENGER, and the planet is Venus. On June
5, 2007, MESSENGER will fly past Venus just 338 km above the
planet's surface--and it will shoot a laser into the clouds.
Above:
Venus, photographed by the Galileo spacecraft in 1990. [More]
MESSENGER
is on a mission to Mercury, not Venus. But the spacecraft
must pass by Venus for a gravity assist en route. In passing,
researchers hope to learn a few things about Earth's "evil
twin," an Earth-sized world with sulfuric acid clouds,
a choking carbon dioxide atmosphere, and a surface hot enough
to melt lead.
"We
are treating the Venus flyby as a full dress rehearsal for
the first flyby of Mercury in January 2008," says Sean
Solomon, the mission's principal investigator at the Carnegie
Institution of Washington. "All of the spacecraft's science
instruments will be turned on during the flyby."
Of
particular interest is the laser experiment, which aims to
measure the location of Venus' cloud decks. "It could
either fizzle or be a major result," says Ralph McNutt,
MESSENGER's project scientist at the Johns Hopkins University
Applied Physics Lab. "We've never sent a laser to Venus
before. This could give us some unique information about the
planet's clouds."
 The
name of the laser is MLA--short for Mercury Laser Altimeter.
It was designed to map the rocky topography of Mercury, but
MLA turns out to have some nice properties for the study of
Venus. Solomon explains: "Venus' atmosphere and clouds
are nearly transparent at several infra-red wavelengths."
The wavelength of the laser (1064 nm) is close to one of these
spectral "windows," so it may be able to penetrate
deep into the atmosphere. "It's a long shot, but we may
even see returns from the surface of Venus," he speculates.
Right:
The Mercury Laser Altimeter, MLA. The central laser is surrounded
by four receiver telescopes. [More]
In
planetary exploration, "you must give yourself the chance
to be lucky," says McNutt. "Having the MLA on MESSENGER
and turning it on for the Venus flyby is exactly that. If
it turns out that there are significant results, then a Venus
orbiter in the future with such instrumentation could help
us understand why our 'sister planet' is so different from
Earth."
In
addition to the laser, MESSENGER will scrutinize Venus using
high-resolution cameras, a suite of spectrometers ranging
in wavelength from infra-red to gamma-rays, an energetic particle
counter and a magnetometer. Data from these instruments may
shed new light on the chemistry of Venus' atmosphere and how
it interacts with the solar wind.
Unlike
Earth, Venus has no global magnetic field to protect it from
solar wind. A gale of charged particles traveling 300 km/s
(almost a million mph) hits Venus with full force, and to
some degree this erodes the planet's atmosphere.
 "There
is still much to understand about how solar wind removes material
from the top of Venus' atmosphere," says Solomon. "We
know that the process over time is very effective for light
elements such as hydrogen, because the Venus atmosphere has
a ratio of heavy hydrogen (deuterium) to hydrogen that is
higher than on Earth by a factor of more than 100. Some of
our measurements may capture this process in action."
Right:
Data from Venus Express reveal oxygen ions escaping from Venus'
atmosphere--a result of solar wind interaction. [More]
The
European Space Agency already has a well-instrumented ship
named Venus Express orbiting Venus, but this does not diminish
the value of the MESSENGER flyby. On the contrary, having
two spacecraft at Venus at the same time, even temporarily,
is a big bonus, says Solomon. "It gives us an unprecedented
opportunity to study Venus atmospheric circulation, cloud
structure, chemistry, and solar wind interaction from the
perspective of two platforms observing simultaneously with
a complementary suite of instruments."
"Venus
has been visited by more spacecraft than any other planet.
Nonetheless, every time we visit a planet with a new set of
instruments, we make discoveries--as New Horizons demonstrated
when it flew by Jupiter earlier this year." (See Science@NASA's
Fantastic
Flyby for details.)
What
will come of this flyby? Solomon is certain of only one thing:
"The history of planetary exploration has taught us to
expect surprises."
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The
MESSENGER project is the seventh in NASA's Discovery Program
of
lower-cost, scientifically focused space missions. The Applied
Physics Laboratory built and operates the MESSENGER spacecraft
and
manages the mission for NASA's Science Mission Directorate,
Washington. The Discovery and New Frontiers Program Office
at NASA's Marshall Space Flight Center manages the Discovery
Program for NASA Headquarters.
Author: Dr.
Tony Phillips | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
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