Controllers of the Magellan mission at Venus started final mission activities Thursday with two orbit trim maneuvers. The maneuvers lowered the altitude of periapsis (the closest approach to the planet) from 200 kilometers to 182 kilometers (124 miles to 113 miles). That activity began the setup stages for the "windmill" experiment planned for Sept. 6-9, and the final "termination" experiment activities planned for Oct. 10-14. Magellan's final gravity data acquisition continues successfully and will conclude on Oct. 9. The "windmill" and "termination" experiments will gain additional aerodynamic and atmospheric data for future mission designs.
Controllers are also managing the spacecraft power as the solar array performance continues to degrade. The loss of power appears to result from thermal fatigue caused by the many solar occultation periods during the past year. As it went in and out of sunlight, depending on the planet's position relative to the sun, the frequent changes from high to low temperature and back again caused some of the solar cells to fail. The solar cycles and effect on solar cell strings have exceeded previous lifetime estimates. In many areas the spacecraft design life was two to three years and Magellan has operated for more than five years.
Controllers are hopeful the power situation can be managed through mid-October, but it is apparent that the Magellan spacecraft is close to the end of its useful life. Beginning on Sept. 6, the "windmill" experiment will reconfigure the solar panels to see how much torque is applied by Venus' very thin upper atmosphere. Controllers will see how much control torque they have to apply to prevent the spacecraft from spinning on its axis.
The "termination" experiment will be performed at three lower altitudes to gather data similar to the "windmill" experiment. In the final phase, the spacecraft's altitude will be lowered starting on Oct. 10. The spacecraft is expected to burn up in Venus' atmosphere by Oct. 14.