with Simon Mansfield
Surveyor Enters Aerobraking Mode Successfully
JPL - November 10, 1997 - NASA's Mars Global Surveyor spacecraft has successfully resumed aerobraking through the upper atmosphere of Mars, heading toward a new science mapping orbit that is the mirror image of its original target orbit, project officials announced today. Aerobraking resumed Nov. 7 with a brief propulsive maneuver that changed Global Surveyor's flight path slightly. A second maneuver was performed successfully on Nov. 9, with a third maneuver planned for Nov. 12. These small adjustments at the farthest part of the spacecraft's orbit around Mars -- known as the apoapsis -- begin the process of lowering Global Surveyor's orbit into the Martian atmosphere more gradually than originally planned.
The more gradual aerobraking strategy will lead to a new mapping orbit that preserves all of the original scientific objectives of the mission. Selected by the mission's science team, the new orbit is essentially just the reverse of the original orbit: data will be taken from the south to the north along the spacecraft's orbital track, rather than north to south, and mapping will begin one year later than originally planned.
In this new mapping orbit, beginning one-half Mars year (equivalent to one Earth year) later than planned, Mars will be at a point in its orbit that is directly opposite where it would have been in the original mission. From the spacecraft's point of view, the side of Mars that would have been dark will be sunlit and vice versa.
"From the perspective of the science instruments, the orbit will look just like the original orbit, except that instead of taking data from north to south on the sunny side of Mars, Global Surveyor will be making its observations in a south to north direction in the sunlight," said Glenn E. Cunningham, Mars Global Surveyor project manager at NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA. "The new mapping orbit will take the spacecraft down over the Martian equator at 2 a.m. local Mars time during each orbit, rather than the originally planned orbit that would have crossed the Martian equator at 2 p.m."
The new mapping orbit will take an additional year to achieve, due to both the more gradual pace of aerobraking and a six-month hiatus in the spring of 1998, while Mars moves into the correct alignment with the Sun for global mapping. Rather than reaching its final mapping orbit in mid-January 1998, and beginning the science mission in mid-March 1998, Mars Global Surveyor will achieve its final orbital position in mid-January 1999, and mapping will begin in mid-March 1999.
"Essentially, we will begin mapping the surface of Mars in mid-March 1999, during summer in the northern hemisphere," Cunningham said. "Originally we had planned to begin mapping on March 15, 1998, during summer in the southern hemisphere."
During next year's hiatus, Mars Global Surveyor will remain in a fixed, elliptical orbit in which it will pass much closer to the surface of Mars during each periapsis -- or closest part of its orbit around Mars -- than it will in the final mapping orbit. These close-range passes are essentially an opportunity for bonus science and will provide superb opportunities for data acquisition. The spacecraft's full suite of instruments, including the laser altimeter, will be turned on during this time to study the planet up close.
"We expect to gain some spectacular new data during this time," Cunningham said. "The spacecraft's orbit will still be elliptical during this period, with a duration of between eight to 12 hours, but at periapsis, the surface resolution will be much greater, and the lighting angles will be excellent."
Mars Global Surveyor's first two aerobraking maneuvers have gone well, giving the operations team confidence that the spacecraft's unlatched panel will be able to withstand an increased amount of pressure as it begins to dip lower into the Martian atmosphere. The new pressure level (an average of 0.2 newtons per square meter) is about one-third the level of pressure originally planned for aerobraking. Aerobraking is a technique that allows a spacecraft to lower its orbit without relying on propellant, by using the drag produced by a planet's atmosphere. The technique was first demonstrated in the summer of 1993 during the final months of NASA's Magellan mission to Venus.
Although these initial maneuvers have not changed Global Surveyor's orbital period significantly, they will soon begin to reduce the time it takes the spacecraft to complete one revolution around Mars.
"The spacecraft's unlatched solar panel performed as expected during the two drag passes we've conducted so far," Cunningham said. "Although we observed some slight movement during the passes, the panel returned to its initial position and its stiffness did not change. That performance gives us confidence that the mission can proceed without further delay."
A third, five-second burn with spacecraft's small thrusters to be performed at 2:30 a.m. EST on Nov. 12 will lower the spacecraft's periapsis by an additional 2.5 miles (four kilometers). With completion of that maneuver, Global Surveyor will begin the main phase of aerobraking well inside the upper atmosphere of Mars. During this two-month main phase, the spacecraft will be circling Mars every 34.5 hours to start at a periapsis altitude of about 77 miles (125 kilometers), with the apoapsis starting at 27,500 miles (44,400 kilometers) and decreasing with each pass. The final goal is a 248-mile (400 kilometer) circular mapping orbit.
If additional problems arise with the aerobraking process, the new mission plan will offer the Surveyor team other opportunities to reach an elliptical orbit that will satisfy many of the mission's science objectives. These so-called "off-ramps" from the aerobraking process, will be detailed in a new mission plan to be reviewed by NASA officials in February 1998.
During a press briefing today at JPL, scientists also showed stunning new images of layered rock and sediment in the canyon walls of Valles Marineris on Mars. Other images of an ancient valley hint at the presence of active sand dunes and dried-up ponds. The new images are available here.
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