Scientists and engineers in Maryland are preparing for their second close look at the planet Mercury as NASA's Messenger spacecraft soars toward its second flyby.
If all goes well, Messenger will fly within 125 miles of Mercury's surface at 4:41 a.m. Monday. A day later, it will send back 1,200 pictures and a wealth of other data on the planet nearest the sun. Planetary scientists say they're hoping to photograph another 30 percent of the planet that has never been seen.
Mission managers said yesterday that the Maryland-built spacecraft is on course after becoming the first to use the subtle pressure of solar radiation for precision steering toward its target.
By adjusting the orientation of the spacecraft and its solar panels over several months, "solar sailors" at Messenger's control center at the Johns Hopkins University's Applied Physics Laboratory were able to change its trajectory and shrink a 105-mile course error to less than a mile - all without burning any onboard fuel.
"The push from the sun allowed Messenger to fly closer to the target," said Scott Murchie, an APL scientist. That made Messenger "the first to use solar sailing as a means to control its trajectory."
Their success allowed mission controllers to avoid three thruster firings that could have cost them more than half of Messenger's reserve propellant. Now the fuel will be available for future maneuvers and for orbit adjustments after it begins circling Mercury in March 2011.
Solar sails, or "light sails," were first envisioned in the 17th century. A few spacecraft have used solar radiation pressure to make minor attitude or orbital adjustments. A European communications satellite uses solar sail panels to control its orientation in space. There are also plans to use solar sails for spacecraft propulsion. But APL believes Messenger is the first to use it for precision steering to its target.
The $446 million Messenger mission was designed and built at APL. The project was conceived in 1996, approved by NASA in 1999 and launched in 2004 - only the second spacecraft to visit the planet. The first was Mariner 10, which flew by in 1975.
Messenger's arduous journey requires two Venus flybys and three Mercury flybys, using the two planets' gravity to slow itself down enough to enter orbit around Mercury in 2011.
The Venus flybys took place in 2006 and 2007. The first Mercury flyby was Jan. 18.
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.