NASA's Magellan spacecraft has begun a unique experiment designed to return data about the upper atmosphere of Venus and the behavior of a spacecraft entering it. The experiment marks the beginning of final activities for the spacecraft, which is expected to burn up in the atmosphere of Venus by October 14.
"This is the next to last act of a truly magnificent performance by Magellan and its science and operations teams," said Dr. Wesley T. Huntress, associate administrator for space science at NASA Headquarters, Washington, D.C. "Magellan has far surpassed all of its original mission goals and, in the process, revolutionized our understanding of a planet that represents what Earth might be like with a runaway greenhouse effect."
With its primary mission of mapping the surface of Venus successfully accomplished, Magellan has been used for a series of experiments that were unanticipated before its launch. In the latest maneuver, known as the "windmill" experiment, the spacecraft's wing-like solar arrays are turned in opposite directions -- like windmill sails -- to encounter pressure from molecules in the upper atmosphere of Venus.
The experiment is measuring how much torque will be needed to keep the spacecraft from spinning on its axis, said Project Manager Doug Griffith at NASA's Jet Propulsion Laboratory. These data will allow engineers and scientists to better understand basic gas-surface interactions and to gain additional aerodynamic and atmospheric data on Venus for future mission designs.
The windmill experiment is scheduled to last until September 14. Two weeks later, more orbit trim maneuvers are scheduled to lower the spacecraft's altitude to prepare for the final termination experiment. Three further trim maneuvers will change the altitude by 8 to 9 kilometers (5 to 6 miles) each on October 10, placing the altitude of periapsis -- or closest approach to the planet -- at 155 kilometers (96 miles). The spacecraft's orbit will be lowered finally to 136 kilometers (85 miles) on October 12, with Magellan again put in a windmill attitude to collect more atmospheric data during its final entry. Gravity data acquisition will continue during all these periods up until October 10.
"After October 12, Magellan will permanently enter the atmosphere in about two days, possibly in one day," Griffith said. The atmosphere will drag the spacecraft toward the surface of the planet, but it will burn up high in the skies over Venus, he said.
There are two primary possibilities leading to NASA's final loss of contact with Magellan, Griffith said. Either the spacecraft will overheat and its communications systems will be damaged, or Magellan's control thrusters will be unable to maintain pointing control toward ground-based receiving dishes on Earth as the spacecraft spins to its demise.
In recent weeks, the performance of the spacecraft's solar arrays has begun to degrade due to the extreme temperature changes as the spacecraft passes from direct sunlight to shadow during its orbit. The thermal stress after more than five years in space and several weeks in low orbit has caused degeneration of its solar cell connections and has brought the spacecraft near the end of its useful life, Griffith said.
"It is a race to the finish," said Betsy Beyer, Magellan program manager at NASA Headquarters. With the continuing loss of power due to the solar cell degeneration, the spacecraft may shut down even earlier than projected, before a planned entry experiment. "Magellan has done more than its duty," Beyer said. "If it goes in its own way, instead of how we planned to end it, it is still a winner."
Controllers sent commands to Magellan in late August for orbital trim maneuvers that reduced its altitude from a near-circular orbit of 197 by 541 kilometers (123 by 338 miles) to an orbit of 172 by 390 kilometers (107 by 242 miles). These altitude reductions were required to set up conditions for the final experiment phases.
Magellan was launched in May 1989, and began mapping the surface of the planet using synthetic aperture -- or side-looking -- radar in September 1990. In three cycles, each comprising one Venus day or 243 Earth days, the spacecraft mapped 98 percent of the planet's surface, providing unprecedented views of its unique pancake domes of lava, strange volcanic structures, craters and high mountains.
In three subsequent cycles, it has measured Venusian gravity over 95 percent of the planet, gathering data so that scientists can map the planet's interior. Magellan also has contributed to ongoing study of the planet's massive atmosphere of carbon dioxide and high sulfuric acid clouds. This period included the first-ever "aerobraking" of a spacecraft into a near-circular planetary orbit, for Magellan's final operations.
The data which have streamed back from Magellan's radar imager, its atmospheric studies and its gravity data acquisition maneuvers have built a vast data base of new knowledge about Venus and the formation of the solar system that will be studied by scientists for decades to come, project officials said.
JPL manages the Magellan project for NASA's Office of Space Science, Washington, D.C.
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