Magellan Project officials at NASA's Jet Propulsion Laboratory today explained the successful first-of-a-kind experiment of "aerobraking" a spacecraft by dipping it into the atmosphere of a planet.
The Magellan spacecraft's orbit was changed from widely elliptical to nearly circular by dragging it through the top of the thick Venusian atmosphere repeatedly over a period of more than 75 days.
Magellan was the first orbiting planetary spacecraft to use atmospheric drag, or aerobraking, to change the orbit. Launched in May 1989, Magellan was placed in an orbit with a closest approach, or periapsis, of 300 kilometers (186 miles). Its furthest distance from the planet, or apoapsis, was 8,500 kilometers (5,270 miles).
By lowering the spacecraft into the top of the atmosphere with closely controlled maneuvers, project engineers were able to lower the periapsis to about 140 kilometers (87 miles), which is just skimming the thin upper atmosphere.
The purpose was to reduce the orbital high point, apoapsis, using the atmospheric drag to slow the spacecraft rather than use the limited fuel available for the small rocket thrusters.
The orbit of Magellan was successfully modified from a 3 hour, 15 minute elliptical orbit to a nearly circular 94 minute orbit, about the same as orbital periods of space shuttle flights in Earth orbit.
Additionally, the project was able to gather significant new information about the planet's atmosphere.
In its new orbit, Magellan is positioned to profile the planet's gravity at the mid and higher latitudes and the poles, and give scientists a better picture of its interior.
"An historic first for planetary spacecraft has been achieved by demonstrating the innovative aerobraking technique to change orbits," said Project Manager Doug Griffith. "The Magellan flight team has done this on a shoestring budget in the best spirit of `cheaper, better, faster.'"
Project Scientist Steve Saunders said that with the circular orbit, "We will begin collecting valuable gravity data around the poles for the first time."
By mapping key areas at the higher latitudes, he said, scientists will be able to compare gravity anomalies for surface features to understand how those features are caused by interior processes.
"We will see global patterns that will help us understand origin of major surface features such as mountains and plateaus," he said.
The aerobraking in the atmosphere of Venus also provided a better understanding of planetary atmospheric response to the 11-year sun spot cycle, said Dr. Gerald Keating, senior research scientist from NASA's Langley Research Center in Hampton, Va.
"We are learning from Venus about greenhouse heating near the surface, and exceptionally strong cooling of the upper atmosphere, processes which may affect Earth in the future," Keating said.
During the aerobraking, he said, it was found that aerodynamic heating of the spacecraft was much less than expected and that the Venus atmosphere was less disturbed than expected.
"These findings indicate that future spacecraft may be able to fly safely lower in carbon dioxide atmospheres than previously believed, making aerobraking a more effective technique and thus improving the designs of future Mars and Venus missions," Keating said.
Magellan finished its radar mapping of the surface of Venus on September 14, 1992, returning images of 98 percent of the planet. It subsequently mapped the gravity of Venus with high resolution in the equatorial band for a full cycle, one Venus day, or 243 Earth days.
The aerobraking experiment began on May 25.
Presenters at today's news conference included Griffith, Saunders, Keating and Deputy Mission Director Ann Tavormina.
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