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Mars Global Surveyor Solar Panel Will Not Hinder Mission Goals

Douglas Isbell               November 27, 1996
Headquarters, Washington, DC
(Phone:  202/358-1753)

Diane Ainsworth
Jet Propulsion Laboratory, Pasadena, CA
(Phone:  818/354-5011)

RELEASE:  96-250

MARS GLOBAL SURVEYOR SOLAR PANEL WILL NOT HINDER MISSION GOALS

     Mission engineers studying a solar array on NASA's Mars 
Global Surveyor that did not fully deploy during the 
spacecraft's first day in space have concluded that the 
situation will not significantly impair Surveyor's ability to 
aerobrake into its mapping orbit, or affect its performance 
during the cruise and science portions of the mission. 

     The solar panel under analysis is one of two 11-foot 
(3.5-meter) wings that were unfolded shortly after the Nov. 7 
launch and are used to power Global Surveyor. Currently, the 
so-called -Y direction array is tilted 20.5 degrees away from 
its fully deployed and latched position.

     "After extensive investigation with our industry 
partner, Lockheed Martin Astronautics, using a variety of 
computer-simulated models and engineering tests, we believe 
the tilted array poses no extreme threat to the mission," 
said Glenn Cunningham, Mars Global Surveyor project manager 
at NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA.  "We 
plan to carry out some activities in the next couple of 
months using the spacecraft's electrically driven solar array 
positioning actuators to try to gently manipulate the array 
so that it drops into place.  Even if we are not able to 
fully deploy the array, we can orient it during aerobraking 
so that the panel will not be a significant problem." 

     Diagnosis of the solar array position emerged from two 
weeks of spacecraft telemetry and Global Surveyor's picture-
perfect performance during the first trajectory maneuver, 
which was conducted on Nov. 21.  The 43-second burn achieved 
a change in spacecraft velocity of about 60 miles per hour 
(27 meters per second), just as expected. The burn was 
performed to move the spacecraft on a track more directly 
aimed toward Mars, since it was launched at a slight angle to 
prevent its Delta third-stage booster from following a 
trajectory that would collide with the planet. 

     Both the telemetry data and ground-based computer models 
indicate that a piece of metal called the "damper arm," which 
is part of the solar array deployment mechanism at the joint 
where the entire panel is attached to the spacecraft, 
probably broke during the panel's initial rotation and was 
trapped in the two inch space between the shoulder joint and 
the edge of the solar panel, Cunningham said. 

     Engineers at JPL and Lockheed Martin Astronautics, 
Denver, CO, are working to develop a process to clear the 
obstruction by gently moving the solar panel.  The damper arm 
connects the panel to a device called the "rate damper," 
which functions in much the same way as the hydraulic closer 
on a screen door acts to limit the speed at which the door 
closes.  In Global Surveyor's case, the rate damper was used 
to slow the motion of the solar panel as it unfolded from its 
stowed position.

     Engineers have been re-evaluating the aerobraking phase 
of the Global Surveyor mission, which begins in September 
1997 after the spacecraft is captured into an elongated orbit 
around the planet using its on-board rocket engine.  The 
solar arrays are essential to the aerobraking technique and 
will be used to drag the spacecraft into its final, circular 
mapping orbit.  First tested on the Magellan spacecraft at 
Venus, aerobraking allows the spacecraft to carry less fuel 
to a planet and take advantage of its atmospheric drag to 
gradually lower itself into the correct orbit.

     "Since we launched early in our window of opportunity, 
we will not have to aerobrake as fast to reach the mapping 
orbit, and this reduces the amount of heating that the solar 
panels are exposed to," Cunningham said.  "In the event that 
our efforts to latch the solar array properly in place are 
not successful, this reduced heating should allow us to tilt 
the array in such a way to prevent it from folding up and yet 
still provide enough useful aerobraking force."  Additional 
analysis and testing will be performed over the next several 
months to verify this hypothesis. 

     Meanwhile, Mars Global Surveyor continues to perform 
very well as it completes its first two weeks in space, with 
on-going science instrument calibrations being performed this 
week.  At the same time, the Mars Relay radio transmitter has 
been turned on for a post-launch checkout.  Radio amateurs 
around the world are gearing up to participate in a radio 
tracking experiment in which they will become receiving 
stations for the low-power beacon signal transmitted by the 
Mars Relay radio system. 

     Mars Global Surveyor is approximately 3.4 million miles 
(5.5 million kilometers) from Earth today, traveling at a 
speed of about 74,000 miles per hour (119,000 kilometers per 
hour) with respect to the Sun.

     Mars Global Surveyor is the first mission in a sustained 
program of robotic exploration of Mars, managed by JPL for 
NASA's Office of Space Science, Washington, DC. 

                           -end-

Note to Editors:  A line-drawing of Mars Global Surveyor 
showing the current position of the solar panel in its fully 
deployed position, including a blow-up which shows the area 
in which the broken deployment mechanism is located, can be 
found under "News Flashes" on JPL's World Wide Web home page 
using the following URL: http://www.jpl.nasa.gov