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.

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 Mercury Laser Altimeter, MLA. The central laser is surrounded by four receiver telescopes. [More]

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.

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.

Data from Venus Express reveal oxygen ions escaping from Venus' atmosphere--a result of solar wind interaction. [More]

"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."

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."

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.