Scientists and engineers who launched NASA's Messenger spacecraft in 2004 to study the planet Mercury are hoping to learn more about another planet - Venus - when their spacecraft soars by that cloud-shrouded world tonight.
Among other things, they would like to know more about global warming on Venus and why the "greenhouse" effect has made that planet's atmosphere hot enough to melt lead, while Earth's climate has so far remained habitable.
The $426 million, Maryland-built Messenger spacecraft will fly within about 210 miles of Venus' surface just after 7 p.m. It will use Venus' gravity to bend its course toward a first encounter with Mercury in January, according to mission managers at the Johns Hopkins University's Applied Physics Laboratory near Laurel.
If all goes well after Mercury flybys in 2008 and 2009, Messenger will begin orbiting the planet in March 2011. It will be the first spacecraft to visit Mercury since Mariner 10 flew by in 1975.
"The reason it's taken so long is that it just isn't easy to get to Mercury," said Marilyn Lindstrom, Messenger program scientist for NASA. That requires flying into the "well" formed by the sun's gravity without speeding up so much that no engines could slow it down to orbital speed.
Messenger first flew by Venus in October. But Venus was behind the sun then as seen from Earth, out of radio contact, so no scientific data or photos were obtained. This time, scientists will train all seven Messenger instruments on Venus and its atmosphere.
Work will be coordinated with observations by the European Space Agency's Venus Express mission, in orbit around Venus since April 2006, and by 20 observatories around the world.
A key target will be Venus' clouds and atmospheric dynamics, which may hold clues to the greenhouse process that's heating the planet's dense, carbon dioxide atmosphere.
The same phenomenon is what keeps Earth's warmth from radiating back into space, leaving the surface cold and uninhabitable. But too much warming can be dangerous. Most scientists believe rising carbon dioxide levels, due in part to human activities, are responsible for Earth's gradually warming climate.
Earth and Venus are about the same size, and some scientists believe the two were nearly identical when they formed 4.5 billion years ago.
"By studying how that works on a planet with a runaway greenhouse effect, we can learn better how to get robust models to be applied on Earth when our knowledge [of climate change] is more solid," said Hakan Svedhem, project scientist for the Europeans' Venus Express mission.
Pictures, movies assembled from successive still images and scientific data from this evening's Venus flyby will be recorded and transmitted during the next several days, said Eric Finnegan, mission systems engineer for Messenger at APL. "Certainly by the end of the weekend we should have the majority of the data on the ground to start looking at," he said.
If skies are clear, anyone can spot Venus soon after the flyby. Venus is the very bright "evening star" that's hung high in the western sky after sunset all spring. The spacecraft has been approaching the planet in recent days, from the right of Venus as the planet is seen from Earth, said Messenger team member Clark R. Chapman.
At 7:08 p.m. Messenger will skim just 210 miles above the planet's surface, traveling at a relative speed of 30,000 mph. Venus will be too dim to be seen in the daylight. After sunset, by about 9 p.m., Venus will appear in the dusk, the brightest object in the western sky.
By then, Messenger will be "an imperceptible distance to the left of Venus," Chapman said, "with its motion slightly changed so that it is accurately on course for its first flyby of Mercury on Jan. 14, 2008."
Mercury, too, is currently visible in the evening sky, below Venus and to the right. But it is closer to the horizon, much dimmer and harder to find at dusk without very clear skies.
Messenger is expected to make 630 photographs and other observations during a 73-hour period before and after the closest approach to Venus.
Its cameras will photograph the nightside and the day-lit side of the planet, in color and in black and white. Mapping spectrometers will study cloud composition and surface features in infrared and ultraviolet wavelengths.
Laser-ranging instruments will attempt the first measurement of the distance to the planet's cloud tops, and sensors will measure the interaction of the solar wind and magnetic field with the planet's atmosphere.
The observations will be done simultaneously with others by the Venus Express spacecraft, orbiting 41,000 miles above the planet, and by astronomers on Earth. The multiple views will allow scientists to combine and compare their results for a better understanding of the planet.
Messenger was launched Aug. 3, 2004, from Cape Canaveral, Fla., on a seven-year journey projected to carry it 4.9 billion miles, on 15 loops around the sun before reaching its target.
The voyage entails what Finnegan described as a "roller coaster" of course adjustments. It includes two gravity assists each from Venus and Mercury, three major deep-space rocket firings and 13 smaller maneuvers designed to tailor Messenger's path.
Today's Venus flyby will be the first chance for scientists and engineers to switch on all seven of the spacecraft's scientific instruments. "This will be a dress rehearsal for the flybys of Mercury in 2008 and 2009," said Sean Solomon of the Carnegie Institution of Washington, who is the principal investigator for the mission.
For more information, go to http:--messenger.jhuapl.edu