NASA's Messenger probe will make its second pass by Venus on Tuesday, which will help guide the spacecraft on its circuitous journey to Mercury and give scientists a close-up look at Earth's cloud-shrouded neighbor.
The fly-by also will give researchers the rare opportunity to pair with a European spacecraft studying Venus.
"This is the first time that we are able to take observations from two different vantage points," said Sean Solomon of the Carnegie Institution of Washington, the principal investigator for the Messenger program.
During Messenger's first pass by Venus in October, the planet was on the opposite side of the sun, putting the probe out of radio contact for two weeks.
The spacecraft also did not pass as close to the planet, so researchers decided against turning on its instruments, Solomon said.
If successful, Messenger, launched in August 2004, would be the first spacecraft to orbit Mercury, arriving in 2011.
The spacecraft cannot fly straight to Mercury because it isn't carrying enough fuel, so it is flying once past Earth, twice past Venus and three times past Mercury for gravity assists before slowing enough to slip into orbit around the small, hot planet.
While planets are often used to give probes gravity boosts to speed their travel, Messenger is using Venus to slow down and help guide it into orbit around Mercury, said Andy Calloway, Messenger mission operations manager at the Johns Hopkins University Applied Physics Laboratory in Laurel.
Messenger is the seventh in NASA's Discovery program of lower-cost, scientifically focused space missions. The Applied Physics Laboratory, which built and operates the probe, manages the mission for NASA.
Eric Finnegan, Messenger mission systems engineer at the Applied Physics Laboratory, said the Venus flyby begins a "veritable inner-planetary roller coaster" ride over the next 18 months as it homes in on its final destination.
Messenger will pass 209 miles above the surface of Venus, using the planet to decelerate from 22.7 to 17.3 miles per second, putting the probe on target to pass Mercury for the first time in January. Messenger will be the first probe to orbit Mercury and the first to visit the planet since Mariner 10 zoomed by three times in the mid-1970s.
Researchers also will team with the European Space Agency's Venus Express mission to observe the planet second-closest to the sun.
Messenger and the European spacecraft will make joint measurements, for example, of how solar wind affects the planet's atmosphere.
Hakan Svedhem, a Venus Express project scientist with the European Space Agency, said that astronomers currently have an excellent view of Venus and that many will be watching the planet as the two probes take measurements.
"So, we will get lots of data," Svedhem said.
The flyby "provides an excellent opportunity, really, to study Venus from two points close to Venus and also from many points on Earth."