The January flyby resolved some scientific questions that had lingered since Mariner 10's visit. Messenger confirmed that volcanism and lava flows have shaped the floors of giant impact craters, and that the planet shrank as it cooled, leaving its surface laced with fractures and high cliffs.
Scientists also measured Mercury's magnetic field, deduced the nature of the planet's iron core and liquid iron mantle, discovered water in the planet's thin atmosphere and analyzed the molecules of sodium blasted into space by the solar wind.
Monday's close pass will be followed by a final one Sept. 29, 2009. And if all goes according to plan, on March 18, 2011, Messenger will become the first spacecraft ever to orbit Mercury. It will spend at least a year there snapping pictures and gathering scientific data to send back to Earth.
Today, Messenger is traveling about 32 miles per second. It has put about 2.8 billion miles on its odometer during 1,521 days in space. There are about 898 days to go before it enters orbit around Mercury.
January's flyby produced 1,200 photos, providing scientists with views of about half of the terrain that Mariner 10 was unable to see.
Next week's flyby should yield 1,200 more pictures during 30 hours of observations, providing first-ever views of another 30 percent of the planet's surface, an area the size of South America.
"We're hoping the new images will allow us to extend all that insight [gained from the January flyby] almost to the rest of the planet," said Sean C. Solomon of the Carnegie Institution of Washington, the mission's principal investigator.
Of all the traditional nine planets, only Pluto has not been visited by a spacecraft from Earth. Another APL mission, named New Horizons, was launched in January 2006. It has passed the orbit of Saturn and is expected to fly past Pluto, which is no longer classified as a planet, on July 14, 2015.
