A major milestone in space exploration occurred today as Surveyor began the aerobraking phase of its mission. This event began at 9:37 a.m. on Wednesday as the spacecraft flew through the upper fringes of the Martian atmosphere at the low point and start of its fourth orbit around the red planet. At that time, Surveyor was slightly to the northwest of the tallest mountain in the solar system, a 89,000-foot (27 km) tall volcano named Olympus Mons.
For the next four months, the spacecraft will skim through the upper Martian atmosphere as it passes through the low point of every orbit. During these atmospheric passes, the spacecraft will slow slightly due to air resistance. This loss of momentum will cause Surveyor to lose altitude on its next pass through the orbit's high point. Surveyor will use this innovative aerobraking technique to lower the high point of its orbit from its current value of 33,555 miles (54,000 km) to near 250 miles (400 km).
Today's atmospheric pass occurred at an altitude of 93 miles (150 km). The spacecraft experienced almost no loss of momentum from air resistance because the Martian atmosphere is extremely thin at this height. Tomorrow, the flight team will fire Surveyor's tiny rocket thrusters to lower the altitude of the next atmospheric pass to 81 miles (130 km). This next pass will occur Friday morning at the low point and start of the fifth orbit. At this lower altitude, the atmosphere will be slightly thicker.
The flight team will continue to lower the altitude of the spacecraft's atmospheric pass until Surveyor encounters enough atmosphere to slow by an appreciable amount on every orbit. According to navigator Dan Johnston, this situation will probably occur at an altitude of about 68 miles (110 km). This gradual "dipping" into the Martian atmosphere is necessary because the atmosphere has not yet been fully characterized by Surveyor's atmospheric science team.
In other news, data returned from the Magnetometer science instrument has indicated the presence of a magnetic field around Mars. Until now, scientists were uncertain as to the absence or presence of a Martian magnetic field. Please visit the following web sites to view a press release containing more details about this important discovery. The second of the two sites listed here is the home page for the Magnetometer.
Status report prepared by:
Office of the Flight Operations Manager
Mars Surveyor Operations Project
NASA Jet Propulsion Laboratory
California Institute of Technology
Pasadena, CA 91109