PUBLIC INFORMATION OFFICE JET PROPULSION LABORATORY CALIFORNIA INSTITUTE OF TECHNOLOGY NATIONAL AERONAUTICS AND SPACE ADMINISTRATION PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011 FOR IMMEDIATE RELEASE February 12, 1991NASA's Magellan spacecraft, which is now mapping Venus with imaging radar, was commanded on Tuesday to shorten its mapping swath by 10 minutes on every other orbit. This change was made to control the spacecraft's temperature now that the orbit is in full sunlight.
Flight controllers at the Jet Propulsion Laboratory saw Monday that the spacecraft's battery compartment was nearing its temperature limit because of the orientation of the spacecraft to the Sun during the mapping portion of its orbit, said Project Manager Tony Spear.
The shortened mapping order will only be needed for about the next 17 days, while Venus and the spacecraft are in their present geometry relative to the Sun, Spear said. Magellan will still acquire data over the Aphrodite region, the large continent near Venus' Equator.
The peak battery compartment temperature Tuesday was 25 degrees Celsius (77 degrees Fahrenheit). The alarm limit is now set at 28 degrees C (82.4 degrees F). At 32 degrees C, (89.6 F), one of the two batteries would be disconnected.
Although the angle of the Sun is now decreasing as Venus moves in its orbit, the temperature in the battery compartment has continued to rise. Spear said there are two possible explanations for the continued heating. The exterior of the battery compartment may be turning darker with exposure to the Sun and therefore absorbing more of the Sun's heat, or the battery compartment is being illuminated by extras sunlight reflected from the spacecraft's solar panels.
In order to keep the battery compartment temperature within acceptable limits, the Magellan team decided to prepare a set of modified commands to be transmitted to the spacecraft Tuesday and start operating Wednesday.
The new commands will shorten the period of mapping on every other orbit by 10 minutes by turning the spacecraft to point its high-gain antenna toward Earth earlier in its orbit. This maneuver will put the spacecraft in the shade of its antenna for a longer period.
The shortened mapping time will decrease the mapping coverage in Venus' southern hemisphere by a small amount for the next 17 days or so, Spear said. The 10-minute mapping decrease will be reassessed on Friday, Feb. 15, and may at that time be lengthened or shortened.
Magellan began its 243-Earth-day primary mapping cycle of Venus last Sept. 15. The first cycle will complete one full rotation of Venus beneath the spacecraft's orbit. The second mapping cycle begins May 16. Similar spacecraft maneuvers to control temperatures are expected during the second cycle.
To date, Magellan has mapped more than 55 percent of the planet with its synthetic aperture radar.
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