Scientists will have a rare opportunity to get a unique look at Venus Tuesday, when a NASA probe bound for Mercury will slingshot around Earth's sister planet and conduct dual science observations with a European craft in orbit.

An artist's concept shows MESSENGER flying by Venus. Credit: NASA/JHUAPL

The main purpose of Tuesday's flyby is to use Venus's gravity to slow the MESSENGER spacecraft's velocity relative to the Sun by more than five miles per second, guiding the probe's orbit further into the inner solar system for a series of swings past Mercury beginning next January.

But planetary scientists around the world are savoring a chance to have two different spacecraft in the vicinity of Venus. MESSENGER's observations this week will complement those of Venus Express, a European Space Agency mission orbiting the planet.

"This is the first time that we are able to take observations from two different vantage points (at Venus)," said Sean Solomon, MESSENGER principal investigator at the Carnegie Institute of Washington.

MESSENGER, which stands for Mercury Surface, Space Environment, Geochemistry, and Ranging, is gradually decreasing its velocity to rendezvous with the solar system's innermost planet in March 2011. The $427 million mission was launched in 2004 and subsequently flew past Earth and Venus in 2005 and 2006, respectively.

Three flybys of Mercury are planned next year and in 2009 before MESSENGER slides into orbit around the enigmatic planet in 2011.

MESSENGER will close to a distance of about 209 miles from Venus Tuesday. The craft will make its closest approach at 2308:19 GMT (7:08:19 p.m. EDT) as it zips over the planet's cloud tops at about 30,000 miles per hour.

The probe will approach Venus from the planet's daytime side before entering darkness as it passes behind the planet. Closest approach will be in darkness but in full view of Earth, Solomon said.

The flyby will induce the largest change in speed for the spacecraft during its mission, according to Eric Finnegan, MESSENGER mission systems engineer.

Controllers at the Johns Hopkins Applied Physics Laboratory in Maryland sent commands to begin a regimen of science observations Monday. The investigations will continue until Thursday, spanning about 73 hours, according to Finnegan.

MESSENGER will store the data before beginning to downlink the information back to Earth on Thursday.

"By the end of the weekend, we should have a good majority of the data on the ground to start looking at," Finnegan said.

Venus Express will send all of its data to the ground by June 15, according to Hakan Svedhem, the mission's project scientist.

MESSENGER's seven science instruments should collect about six gigabits of data this week, including about 630 images, Finnegan said.

"There will certainly be images of our sister planet, which I think anybody can appreciate and marvel at," Solomon said.

Finnegan said Tuesday's flyby will mark the first time MESSENGER's entire science package will be operational.

In addition to gathering imagery, MESSENGER's surface and atmospheric spectrometer will create profiles of the Venusian atmosphere and observe the surface and cloud decks, according to project officials.

MESSENGER will also look for evidence of lightning on Venus's dark side.

MESSENGER and Venus Express will make concerted simultaneous of the surface and clouds.

"The clouds move fairly quickly across the surface of Venus and we can't exactly observe it at the same time," Svedhem said.

MESSENGER's laser altimeter will also attempt to determine the heights of cloud decks in the planet's atmosphere.

Venus's atmosphere is primarily composed of carbon dioxide, and its clouds are made of sulfuric acid. Strong winds of more than 200 miles per hour thrive in the upper atmosphere, and the planet's surface pressure is more than 90 times higher than on Earth.

The atmosphere creates a runaway greenhouse effect on Venus, resulting in sweltering surface temperatures of nearly 900 degrees Fahrenheit.

"One of the science topics of Venus Express is to learn, more in general, how climate change works on planets," Svedhem said.

MESSENGER's magnetometer and plasma and neutron spectrometers will collect information on the interaction between charged particles in the solar wind and the planet's upper atmosphere.

Venus Express carries comparable instruments to provide another vantage point for magnetic field and solar wind observations.

Svedhem said he believes the most interesting science coming from the tandem studies will be associated with the synchronized observations of the planet's weak magnetic field and probing the planet's cloud decks using MESSENGER's altimeter.

More than 20 teams of astronomers on Earth will also be observing Venus near the time of closest approach. Their observations will supplement the results from MESSENGER and Venus Express, Svedhem said.

The MESSENGER team is using Tuesday's flyby as a rehearsal for the spacecraft's encounters with Mercury in the next few years.

"This is a warm-up for us," Solomon said.

MESSENGER was unable to gather science data during the probe's previous flyby of Venus last October because the planet was located on the opposite side of the solar system, hindering consistent communications with the spacecraft.

"Because we knew we were going into radio blackout and because we knew we had the second flyby coming up in June with a much closer approach and with a full view of the Earth, we elected not to turn on any of the MESSENGER instruments at the time of that flyby," Solomon said.

MESSENGER will next set its sights on Mercury. A deep space maneuver is scheduled for this fall using the spacecraft's main engine to precisely aim the craft toward a flyby of the planet on Jan. 14.

Three flybys in the next two years will further decrease the probe's velocity and provide opportunities to map the entire planet in color before the craft enters orbit for a more detailed observation program in 2011.

Detailed imagery of Mercury is only available for about 45 percent of the planet's surface. NASA's Mariner 10 probe took those pictures as it flew by Mercury three times in 1974 and 1975.