The reason? A recently implemented navigational technique that makes use of solar-radiation pressure (SRP) to guide the probe has been extremely successful at maintaining MESSENGER on a trajectory that will carry it over the cratered surface of Mercury for a second time on October 6.

SRP is small and decreases by the square of the distance away from the Sun. But, unlike rockets, so-called solar sailing requires no fuel. And although SRP's thrust is small, it will continue as long as the Sun is shining and the "sail" is deployed, providing a continuous acceleration source for the probe.

MESSENGER's mission designers and its guidance and control team at the Applied Physics Laboratory in Laurel, Md., along with the navigation team, at KinetX, Inc., in Simi Valley, Calif., once viewed SRP as something of a challenge to overcome, particularly for the critical gravity-assist flybys -0 one of Earth, two of Venus, and three of Mercury -0 that the spacecraft would be executing to position it for Mercury orbit insertion in 2011.

"Because of the changing proximity to the Sun during MESSENGER's cruise phase, the SRP varies from one to 11 times the value experienced at Earth," explains APL's Daniel J. O'Shaughnessy, MESSENGER's Guidance and Control Lead Engineer.

This variation in magnitude, as well as the attitude-dependent direction of the resulting disturbance force and torque, presents a significant challenge to mission designers and the guidance and control team, he says.

"The Mercury flybys are designed to take the probe within approximately 200 kilometers of the planet, so precision targeting is absolutely critical," O'Shaughnessy says. Fly too low and the probe could crash into the planet. Fly too far away and MESSENGER might have to use its reserve fuel to correct for the acceleration loss. Either way, getting off target could jeopardize the mission.

SRP was seen as an impediment to precise targeting, until the first Mercury flyby in January 2008. About 26 days before that historic event, MESSENGER fired its thrusters to fine-tune its trajectory and aim for the 200-kilometer-altitude flyby point.

Prior to the maneuver, the probe was on a course to miss the flyby aim point by more than 2,000 kilometers.

After the maneuver, the probe was still about 9.5 kilometers off from its target.

"We still had one more opportunity for another trajectory-correction maneuver four days before the flyby, but we were able to skip it by solar sailing the spacecraft closer to the intended aim point," explains APL's Jim McAdams, who designed MESSENGER's trajectory.

Three days earlier than originally planned, the team tilted MESSENGER's solar panels an extra 20 degrees away from the Sun. The resulting change in solar-array orientation moved the flyby altitude very close to the target aim point.

Ultimately, MESSENGER missed its target altitude by only 1.4 kilometers. This targeting was "spectacular," McAdams says.

The MESSENGER team has planned a more extensive use of this technique for the second Mercury flyby. "We've developed a process to use the SRP force as a control for the trajectory," explains O'Shaughnessy.

Using the knowledge developed from the first flyby, the team has developed a carefully planned sequence of probe-body attitude and solar-array orientations that, if all goes according to plan, should reduce the number of trajectory correction maneuvers needed in the future.

According to NASA, the only other visitor to Mercury used solar sailing. In 1974, when the Mariner 10 spacecraft ran low on attitude-control gas, its engineers angled the spacecraft's solar arrays into the Sun and used solar radiation pressure for attitude control, and it worked.

But MESSENGER's use of the technique represents the first time that a spacecraft has successfully used solar sailing as a propulsion-free trajectory control method for the targeting of planetary flybys.

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