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 July 4th."
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.
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