11 September 1997
Mars Orbit Insertion
This date marked the arrival of Mars Global Surveyor at the red planet. A 22-minute firing of Surveyor's main rocket engine placed the spacecraft into a highly elliptical orbit taking 44 hours, 59 minutes, and 34 seconds to complete. The initial orbit had a low point of 262 km above the northern hemisphere, and a high point of 54,026 km above the southern hemisphere.
On September 17th, the flight team started performing a series of orbit changes to lower the low point of its orbit into the upper fringes of the Martian atmosphere. On every pass through the atmosphere, the spacecraft slowed down by a slight amount because of air resistance, and the high point of the orbit to begin to drop. The original plan was to use this "aerobraking" scheme to lower the high point of the orbit from 56,026 km down to 450 km by repeatedly flying through the atmosphere for four months.
The flight team performed a maneuver to raise the low point of the orbit out of the atmosphere. This suspension of aerobraking was performed because air pressure from the atmosphere caused one of Surveyor's two solar panels to bend backward by a slight amount. The panel in question was slightly damaged shortly after launch in November 1996.
The decision to lower the low point of the orbit back into the atmosphere and resume aerobraking came after several weeks of analysis. Flight team members concluded that aerobraking is safe, provided that it occurs at a more gentle pace than proposed by the original mission plan.
Under the new mission plan, aerobraking will occur with the low point of the orbit at an average altitude of 120 km, as opposed to the original altitude of 110 km. This slightly higher altitude results in a decrease of 66% in terms of air resistance pressure experienced by the spacecraft. During these six months, aerobraking will reduce the orbit period down to between 12 to 6 hours.
Sometime slightly before May, aerobraking will be temporarily suspended to allow the orbit to drift into the proper position with respect to the Sun. Without this hiatus, Surveyor would complete aerobraking with its orbit in the wrong solar orientation.
In order to maximize the efficiency of the mission, these six months will be devoted to collecting as much science data as possible. Data will be collected between two to four times per day, at the low point of each orbit.
During these six months, aerobraking will continue and will shrink the high point of the orbit down to 450 km. At this altitude, Surveyor will circle Mars once every two hours.
Aerobraking is scheduled to terminate at the same time the orbit drifts into its proper position with respect to the Sun. In the desired orientation for mapping operations, the spacecraft will always cross the day-side equator at 2:00 p.m.(local Mars time) moving from south to north. This geometry was selected to enhance the total quality of the science return.