PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
Contact: Diane Ainsworth
FOR IMMEDIATE RELEASE
March 14, 1997
MARS PATHFINDER PASSES GLOBAL SURVEYOR ON ITS WAY TO MARS
Like two ships passing in the night, NASA's Mars Pathfinder
spacecraft will begin to overtake Mars Global Surveyor tonight,
moving closer to Mars than its companion orbiter and closing in
for the final four-month approach to the red planet.
Mars Pathfinder, a lander carrying a small rover and science
instruments to Mars, has less than half of its total distance to
complete now, said Dr. Robin Vaughan, Pathfinder navigation team
member at NASA's Jet Propulsion Laboratory. The spacecraft will
overtake Mars Global Surveyor at 0100 Universal Time on March 15
(5 p.m. Pacific Standard Time tonight, March 14).
At the time of the event, Mars Pathfinder will be 43.7
million kilometers (27 million miles) from Earth and 69.7 million
kilometers (43.2 million miles) from Mars. The spacecraft is
more than halfway along its arcing flight path, on which it will
have traveled a total of 497 million kilometers (309 million
miles) by the time it reaches Mars.
"Although Pathfinder was launched about a month after Mars
Global Surveyor, it is traveling faster than Surveyor and is on a
shorter flight path to the red planet," said Brian Muirhead,
Pathfinder project manager at JPL. "Whereas Mars Global Surveyor
will take 10 months to reach Mars, Pathfinder takes only seven
months. Once we reach Mars, we dive directly into the Martian
atmosphere. The descent will only take about four minutes, and we
should be on the surface of Mars by about 10 a.m. Pacific time on
Pathfinder is on a different type of trajectory to Mars
than Mars Global Surveyor. Called a "Type 1" trajectory, the
spacecraft does not have to travel as far to intercept Mars. Mars
Global Surveyor will log a total of 700 million kilometers (435
million miles) in its flight path toward the red planet.
"The advantage to using a Type 1 trajectory is that you
have to travel less than one-half of the way around the Sun to
intercept Mars," Vaughan said. "So Pathfinder takes 212 days to
reach Mars, while Mars Global Surveyor will spend 309 days to
reach the planet."
Mars Global Surveyor is on a "Type 2" trajectory, taking it
more than 180 degrees around the Sun to intercept the planet. A
major difference in this type of trajectory is that the
spacecraft travels at a slower velocity with respect to the Sun.
Subsequently, the craft requires less fuel to slow down at Mars
than if it had followed Pathfinder's trajectory. For instance,
Pathfinder is currently traveling at 27 kilometers per second
(60,700 miles per hour), while Mars Global Surveyor is traveling
at about 26.75 kilometers per second (59,800 miles per hour).
"Less fuel translates into simpler, smaller spacecraft and
less expense," said Glenn Cunningham, Mars Global Surveyor
project manager. "Mars Global Surveyor also will employ a fairly
new technique requiring very little fuel to drop down into its
mapping orbit. The technique is called 'aerobraking,' and takes
advantage of the drag of the Martian atmosphere. As the
spacecraft dips down into the top of the atmosphere at its
closest point to the planet each orbit, the drag from the
atmosphere on the spacecraft will reduce its orbital speed. This
drops the altitude of the highest part of the orbit, changing it
from the initial elliptical shape to the circular shape required
for mapping the planet."
Aerobraking was first demonstrated successfully with the
Magellan spacecraft, which mapped the surface of cloud-covered
Venus using a sophisticated radar-imaging system. Magellan
aerobraked into the Venusian atmosphere in October 1994, sending
back data about Venus' thick sulfur and carbon dioxide-choked
atmosphere until it burned up in the planet's sizzling
temperatures. Mars Global Surveyor, however, will not dip so far
into the much thinner Martian atmosphere that it would burn up.
Pathfinder is scheduled to perform two more flight path
corrections and, possibly, a fifth maneuver to keep it on course
for landing on Mars on July 4. The last two maneuvers will occur
near the end of the cruise phase, on May 7 and June 24, when the
spacecraft is close to Mars. If necessary, a fifth maneuver will
be executed just a few hours before entry into the Martian
atmosphere on July 4. Mars Global Surveyor will perform its
second trajectory correction maneuver on March 20. Engineers are
continuing to explore possible ways of freeing a broken damper
arm that is wedged in the joint of one of the solar arrays, so
that the panel locks in place.
The Mars Pathfinder and Mars Global Surveyor missions are
managed by the Jet Propulsion Laboratory for NASA's Office of
Space Science, Washington, D.C. Pathfinder is the second in
NASA's Discovery program of low-cost spacecraft designed to carry
out highly focused science goals. Mars Global Surveyor is the
first spacecraft in a decade-long program of robotic exploration,
called the Mars Surveyor Program.