The final "transfer to mapping orbit" burn lowered Global Surveyor's closest approach over Mars from 253 miles (405 kilometers) to approximately 229 miles (367 kilometers). Later this week, the flight team will turn on, focus and calibrate the spacecraft's camera and power up several other science instruments, including the thermal emission spectrometer and laser altimeter.

"Reaching our mapping orbit has been a long time coming for all involved. We are delighted to finally be able to do this mission as it was designed, in the proper mapping orbit with all the instruments working at their full potential," said Dr. Arden Albee, the Mars Global Surveyor project scientist at the California Institute of Technology, Pasadena, CA.

The mapping orbit was designed so that Surveyor passes over a given part of Mars at the same local time each orbit. At about 2 p.m. local Mars time, the spacecraft will cross the equator flying northward on the daytime side and about 2 a.m., it will cross the equator flying southward on the nighttime side. This timing is essential for effective interpretation of atmospheric and surface measurements, because it allows scientists to separate local daily variations from longer-term seasonal and annual trends.

"We still have a few minor adjustments to fine-tune the orbit during the next few weeks. Our plan at this point is to conduct the first three one-week mapping cycles with Surveyor's high-gain communication antenna in the stowed position. After we have these first mapping cycles completed, we plan to deploy the antenna and continue mapping in that configuration," said Glenn E. Cunningham, deputy director of the Mars Exploration Program at NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA.

Launched in November 1996 and in Mars orbit since September 1997, Mars Global Surveyor carries a dish-shaped high-gain antenna that will be deployed on a 6.6-foot-long boom. The antenna was stowed during launch and the early orbital phase at Mars to reduce the chances of its being contaminated by the exhaust plume from the spacecraft's main engine.

During deployment, the boom is pushed outward by a powerful spring. A damper mechanism cushions the force of the spring and limits the speed of the deployment, somewhat like an automobile shock absorber or the piston-like automatic closer on a screen door. Last year, engineers became aware of problems with similar damper devices on deployable structures such as solar panels on other spacecraft.

"Until we deploy the antenna, we must turn the entire spacecraft periodically to transmit data to Earth," Cunningham explained. "This means that we have to stop acquiring science data. The advantage of deploying the high-gain antenna is that we can then use its gimbals to point the antenna at Earth and send science data back at the same time the instruments are pointed at Mars."

The first phase of the primary mapping mission is scheduled to begin on March 8. The deployment of high gain antenna is currently scheduled for March 29, pending approval by NASA Headquarters officials in mid-March.