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Magellan Begins Termination Activities


From the "JPL Universe"
September 9, 1994

Magellan begins termination activities

JPL's Magellan spacecraft on Sept. 6 began a "windmill" experiment, testing the amount of pressure needed to keep it from spinning in Venus's upper atmosphere. The experiment begins the final termination activities for the spacecraft, expected to burn up by Oct. 14.

Magellan was launched in May 1989 and began mapping the surface of the planet in September 1990. In three cycles, each comprised of one Venus day or 243 Earth days, it mapped with imaging radar 98 percent of the planet's surface.

In three subsequent cycles, it has observed Venus' gravity over 95 percent of the planet, gathering data so scientists can map the planet's interior. This period also included the "aerobraking" of the spacecraft into a near-circular orbit for final gravity operations in the last two cycles.

In recent weeks, the spacecraft's solar arrays have experienced losses due the extremes of temperature as the spacecraft went from direct sunlight to shadow in its orbit. The thermal stress after more than five years in space has caused degeneration of solar cells and has brought the spacecraft near the end of its useful life, said Project Manager Doug Griffith.

Controllers sent commands in late August to begin orbit trim maneuvers to reduce the altitude of the spacecraft from a near- circular orbit of 197 km by 541 km (123 by 338 miles) to an orbit of 172 km by 390 km (107 by 242 miles). These altitude reductions were required to setup conditions for the final experiment phases.

For the "windmill" experiment, the spacecraft's wing-like solar arrays are turned in opposite directions, like windmill sails, to receive pressure of upper atmosphere molecules. The experiment is testing to see how much torque will be needed to keep the spacecraft from spinning on its axis, Griffith said. These data will allow engineers and scientists to better understand gas- surface interactions and gain additional aerodynamic and atmospheric data for future mission design.

The windmill experiment is scheduled to last until Sept. 14. Two weeks later, more orbit trim maneuvers are scheduled to lower the spacecraft altitude to prepare for the final termination experiment.

Three further trim maneuvers will change the altitude by 8 to 9 km (5 to 6 miles) each on Oct. 10, which will place the altitude of periapsis at 155 km (96 miles). It will be lowered finally to 136 km (85 miles) on Oct. 12. Gravity data acquisition will continue during all these periods up until Oct. 10.

"After Oct. 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 added.

Two things may happen, Griffith explained. The spacecraft will overheat, or its control thrusters will be unable to maintain pointing control. In either circumstance, project controllers will lose communication with Magellan.

Using synthetic aperture radar, or side-looking radar, the Magellan project has mapped almost all of the surface of Venus at high resolution, providing maps of its unique pancake domes of lava, strange volcanic structures, craters and high mountains.

Additionally, the project has provided a gravity map of the planet and contributed to an ongoing study of its massive atmosphere of carbon dioxide and high sulfuric acid clouds.

The data that streamed back from its radar image and gravity- data acquisition during six cycles has built a gigantic database of new knowledge about Venus and the formation of the solar system which will be studied by scientists for decades to come, project officials said.

Magellan coming to the end of a long and productive life

Magellan will end its mission operations in mid-October, when the spacecraft enters the atmosphere of Venus and burns up. The mission has far exceeded all of its primary and extended-mission objectives and has produced more image data than all previous U.S. planetary missions combined.

By Phil Allin, Magellan Project

In recent months it appeared that Magellan might become the first functioning planetary spacecraft to be abandoned in space. Due to the failure of the on-board tape recorder "A" and the subcarrier modulation of transmitter A, and the degraded performance of transmitter B, Magellan is incapable of resuming radar mapping.

But Magellan is not "dead." It continues to maintain precise attitude control and a strong carrier signal for communication with Earth.

It has been returning global gravity data from circular orbit since early August 1993 and will complete a high-resolution gravity map covering 95 percent of Venus on Oct. 9. Then it will be commanded to lower its orbit in a series of steps that will cause it to enter the atmosphere. Thus it may become the first planetary spacecraft to be terminated intentionally.

On other planetary missions the spacecraft usually quit operating due to the failure of a critical system or the depletion of some limited resource such as attitude-control propellant.

For example, when Mariner 2 went to Venus in 1962, several of the critical components became hotter than their design limits. When it went silent about a month after the successful flyby, it was believed that the battery or earth sensor had failed. Many of the later Mariners failed when they ran out of the nitrogen gas used for attitude control.

When the Pioneer Venus Orbiter ran out of propellant in October 1992 after 14 years in space, controllers could not raise the periapsis out of the Venus upper atmosphere and it burned up as it entered.

Some planetary spacecraft have been designed for long, quiescent cruise periods, maintaining a fixed attitude with reference to the Earth and reference stars. In this way, Voyagers 1 and 2 can continue their search for the heliopause while expending very little propellant. The demise of the Voyagers is expected to come when the output from the radioisotope thermoelectric generators (RTGs) is insufficient to power the critical on-board electronics--in 20 to 30 years.

Magellan is a different kind of bird. Due to the almost constant maneuvers required during the radar-mapping phase of the mission, the spacecraft is controlled by reaction wheels. These electrically driven momentum wheels allow for smooth movement in each of the three axes. This system requires frequent "desaturations" (adjustment of spin rate in the wheels) and "starcals" (verification of the attitude by scanning reference stars).

During brief periods in which the forces on the spacecraft might exceed the control of the reaction wheels--such as the Venus orbit insertion or the recent aerobraking passes--the spacecraft is switched to thruster control. The thrusters are also used during desaturations, but Magellan carries far too little propellant to remain on thrusters for an extended period.

Perhaps the greatest risk to Magellan is the failure of one of the remaining gyros. Gyro A2 is presently at 52,000 hours of operating life. The gyros are designed to have an operating lifetime between 40,000 and 60,000 hours. However, gyro B2 failed with less than 20,000 hours. If either of the remaining gyros fail, the spacecraft would be unable to maintain its attitude in space and would tumble out of control.

Another threat is the degradation of the solar panels. Magellan is in a period of solar occultation, passing through the shadow of Venus on each orbit.

Fifteen times a day the panels are subjected to a 190 C temperature cycle. As a result, the electrical connections between the solar cells are breaking, reducing the power output. The power has to be managed by periodically turning off some of the spacecraft electronics.

Analysis of the present propellant reserves show that, even if all the remaining propellant were used to raise the orbit, the spacecraft will enter and burn up in the Venus atmosphere in about five years.

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