On September 4, the MESSENGER team announced that it would not need to implement a scheduled maneuver to adjust the probe's trajectory. This is the fourth time this year that such a maneuver has been called off. 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 - one of Earth, two of Venus, and three of Mercury - 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.
MESSENGER Team Member Highlight
While the scientists on the MESSENGER team decided what features to image, and the guidance and control team calculated the pointing of the instrument, Nori Laslo - at 29 one of the youngest members on the team - pieced together the commands to tell the camera precisely what to do. Read more about Laslo in the latest MESSENGER Team Highlight, available online at http://messenger.jhuapl.edu/who_we_are/member_focus.html.
Mercury Does a Sunset Tango with Mars and Venus in September
Sky watchers using binoculars and scanning the horizon about 15 to 30 minutes after sunset on September 7 will see a Venus-Mercury-Mars grouping that looks like an isosceles triangle, with the Mars-Mercury and Mars-Venus sides measuring about 2.5 degrees in length and the Mercury-Venus side about 4 degrees. About 10 degrees to the upper left of the triangle will be Spica, the brightest star of the constellation Virgo.
On September 18, Venus, Mercury, and Mars will form an equilateral triangle, whose sides are 4 degrees in length. And Spica will become a part of this arrangement, sitting a few degrees to the left of the triangle. Mercury will remain in the evening sky until October 6, and then reappear on November 25.
One Month to Go until Second Flyby of Mercury!
With just 30 days until MESSENGER's second encounter with Mercury, the spacecraft remains safe and healthy, with all systems operating nominally. All instruments are on except for the Mercury Laser Altimeter (MLA), which will be powered on September 25 and configured for the encounter. "The final command load for the upcoming flyby is now complete, has been reviewed by the team, and is being tested on the hardware simulator," says APL's Peter Bedini, MESSENGER's project manager. "A successful simulation will represent the passing of the last major milestone in the encounter preparations."
MESSENGER is about 55.95 million miles (90.04 million kilometers) from the Sun and 87.55 million miles (140.9 million kilometers) from Earth. At that distance, a signal from Earth reaches the spacecraft in 7.8 minutes. The spacecraft is moving around the Sun at 77.5 million miles (124.7 kilometers) per hour.
MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and after flybys of Earth, Venus, and Mercury will start a yearlong study of its target planet in March 2011. Dr. Sean C. Solomon, of the Carnegie Institution of Washington, leads the mission as Principal Investigator. The Johns Hopkins University Applied Physics Laboratory built and operates the MESSENGER spacecraft and manages this Discovery-class mission for NASA.
