Unlike most maneuvers, this one was conducted in a so-called "open loop," explained MESSENGER Project Manager Peter Bedini.

"In closed-loop maneuvers, the accelerometers are used to trim the burn, and thrusters are employed as needed to adjust the direction as perceived by the accelerometers," said Bedini, of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md.

"To test the unlikely scenario that we will not have accelerometer data for the huge Mercury orbit insertion, we did this second part without using those data as feedback."

The first part of the maneuver, which occurred on December 4, increased the probe's speed relative to the Sun by 219 meters per second (489 miles per hour) to about 30.994 kilometers per second (69,333 miles per hour).

Today's maneuver, which began at 3:30 p.m. EST, increased MESSENGER's speed by an additional 24.7 per second, for a total velocity change of 247 meters per second.

Mission controllers at APL verified the start of the maneuver about 13 minutes, 18 seconds later, when the first signals indicating spacecraft thruster activity reached NASA's Deep Space Network tracking station in Canberra, Australia.

"We are now on target for the third Mercury flyby next September," Bedini said. "The fifth deep-space maneuver two months later will position MESSENGER for Mercury orbit insertion in March 2011," enabling it to become the first spacecraft to orbit the innermost planet.

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