Mercury Probe Swings by Venus

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June 5, 2007 — A NASA spacecraft en route to Mercury will lend its cameras, laser mappers and other science instruments for a short, but detailed study of its neighbor planet, Venus.

The spacecraft, called Messenger, is scheduled to fly by Venus Tuesday so it can use the planet's gravity to redirect itself toward the innermost planet in the solar system, Mercury. Though relatively close to Earth, Mercury has had just one terrestrial visitor: the Mariner 10 probe flew past the planet three times in the mid-1970s.

"It just isn't easy getting to Mercury," said NASA program scientist Marilyn Lindstrom.

Mission scientists want to use the flyby of Venus for a test-run of the spacecraft's optical navigation system, which will be key to upcoming visits to Mercury.

"This will be a dress rehearsal," said Messenger lead scientist Sean Solomon, with Carnegie Institution.

The spacecraft will not be alone at Venus. Europe's Venus Express has been orbiting the planet for more than a year. A series of collaborative experiments is planned to study Venus' thick clouds and deadly carbon dioxide atmosphere. Scientists also want to learn more about how the solar wind affects Venus' ionosphere.

The planet's atmosphere is so dense that heat cannot escape, creating an inferno hot enough to melt lead. With surface temperatures of about 900 degrees Fahrenheit, the planet is about twice as hot as it would be if it had no atmosphere.

Scientists are curious to unravel the planet's history, which may hold lessons for understanding global climatic changes affecting Earth as well.

"We have a dramatic greenhouse effect on Venus," said Hakan Svedhem, the Venus Express project scientist. "By studying how that works we can learn better how to get robust models to be applied on Earth when our knowledge [of climate change] is more solid."

Messenger's closest approach to Venus will come at 7:09 p.m. EDT when it skims just 210 miles above the planet's surface. During the hours before and after the flyby, Messenger will photograph both the sun-lit and dark sides of the planet and use its infrared and ultraviolet spectrometers to study Venus' surface feature and cloud structures.

The findings will be coordinated not only with Venus Express' observations, but also with dozens of studies to be made by Earth-based telescopes.

After the flyby at Venus, Messenger will be on track for the first of three passes around Mercury which are to take place in 2008 and 2009. By the time the probe settles into orbit around Mercury in 2011, it will have traveled 4.9 billion miles and made 15 passes around the sun.

The goal of the mission will be to take the first images of the entire planet — only 45 percent of Mercury was imaged during the Mariner 10 mission — and conduct studies of the planet's composition, structure, thin atmosphere and magnetic field.