Messenger spacecraft to orbit Mercury Thursday

Mission is run by Hopkins' Applied Physics Lab

  • View of Mercury from one of Messenger's flybys.
View of Mercury from one of Messenger's flybys. (Courtesy of Johns Hopkins…)
March 16, 2011|By Frank D. Roylance, The Baltimore Sun

Fifteen years of planning and 61/2 years of maneuvering in space will all come down to the crunch Thursday evening as mission managers in Maryland try to slip NASA's Messenger spacecraft into orbit around Mercury.

The braking maneuver, playing out 96 million miles from Earth, will have to slow the desk-size planetary probe by 1,929 mph and ease it into a polar orbit around the planet closest to the sun.

Failure will leave Messenger's managers at the Johns Hopkins Applied Physics Lab near Laurel with less than 10 percent of the fuel the craft left Earth with, and limited options for recovery. And that would put the primary goal of the $446 million mission at risk.

But success will open the doors to at least a year of scientific discovery, yielding close-up, high-definition images, maps and data from a planet that until now has only been observed from Earth or during high-speed flybys. Those earlier visits were by Mariner 10 in 1974 and 1975, and by Messenger in 2008 and 2009.

Being this close to beginning an unbroken year of study from distances as close as 120 miles has planetary scientists on the Messenger team on the edge of their seats.

"The flybys have been wonderful, but they're really dress rehearsals to what this mission was designed to do," said Sean Solomon, of the Carnegie Institution in Washington, the mission's principal investigator. "We're on the threshold of finally getting into orbit, and we are very excited to begin.

"These will be the first observations made by any spacecraft from orbit around a planet that is one of our nearest neighbors," he said. "Mercury has been comparatively unexplored, considering its proximity in our solar system. It's also a special place by virtue of being the planet closest to the sun."

Mercury's orbit, one-third of Earth's distance from the sun, also poses severe challenges for Messenger, which carries a broad shade to protect its scientific instruments from solar heat.

Engineers will have to keep that shade between Messenger and the sun, and they must protect Messenger's instruments from "looking" at the planet's "hot pole" — the side facing the sun and reflecting its heat back into space.

At the same time, they will have to be sure the spacecraft's power-generating solar panels are facing the sun, and that its radio antenna is facing the Earth for data downloads and communications with APL.

Orbital "insertion" begins at 5 p.m. Thursday when NASA turns its Deep Space Network radio antennas on Messenger to help track the spacecraft and monitor its progress.

At 8:42 p.m., the spacecraft will be turned to firing position, and 30 minutes later, its largest rocket engine will begin a 15-minute burn — twice as long as any in the mission since launch.

If all goes well, Messenger will move into a lopsided elliptical orbit that takes it as close as 120 miles from the planet's surface, and as far out as 9,300 miles.

Engineers and scientists at APL's mission control center will watch tracking data in real time, and hope to hear from Messenger by 9:30 p.m.

"Once we get that data on the ground, we'll have a much better idea what the orbit looks like," said Andy Calloway, the Messenger mission operations manager at APL. They hope to announce by 10 p.m. whether they've achieved orbit.

"There are some scenarios we've gone over in the reviews for this activity, months ago, that highlight the different criteria of a missed orbital insertion, and what it would take to get back to the planet," Solomon said.

An initial orbit that is less than ideal might be improved by using some of the remaining fuel to move to one better suited for the science mission ahead. A more complicated recovery could burn up fuel needed to carry out a full year of on-orbit scientific work.

The first images from the spacecraft are not expected for two weeks, after engineers have checked out and turned on all seven of Messenger's cameras and other instruments. Regular scientific data downloads should begin by April 4.

Messenger was launched in 2004. It has since flown a dozen orbits of the sun, gradually tightening the circle, flying past the Earth once and Venus twice en route to Mercury.

Once in orbit, Messenger will do high-resolution imaging of the planet's surface in visible, ultraviolet and infrared wavelengths. It will conduct detailed laser ranging studies, study Mercury's gravity field, magnetic field and the charged-particle space environment the planet is moving through.

Scientists are eager to fly over Mercury's poles, especially the north pole. Previous radar observations from Earth have suggested the presence of water ice in deep polar craters where the sun doesn't shine.

If Messenger confirms it, Solomon said, "We may see that Mercury has a greater volume of polar ices than the moon does, despite being as close to the sun as it is."

Maryland weather blog: Frank Roylance on meteorology

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