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FOR IMMEDIATE RELEASE                                                October 19, 1990

       Magellan Project flight controllers at NASA's Jet Propulsion Laboratory plan to temporarily halt radar mapping of the surface of Venus in early November as Venus and Earth near their maximum distance with the sun in between.

       The maximum distance between the planets, called superior conjunction, occurs Nov. 1. The sun and Venus will be less than one degree apart. While Venus will not actually go behind the disc of the sun, its nearness to the sun from Earth's point of view will cause radio signals passing close to the sun to degrade.

       The time of superior conjunction was known and calculated for Magellan early in mission planning. There also is a second period when mapping will be lost to some extent. That is between Dec. 16 and Jan. 26 when Venus will be between the spacecraft and Earth for part of the playback portion of the orbit. It is called the occulted mapping phase. Some data will be returned to Earth, but there will be a "hole" in the Venus map in the northern latitudes.

       Loss of the radio frequency, called S-band, which transmits telemetry to Earth stations through the high-gain antenna at 1200 bits per second (bps) is predicted for Saturday when the angle between Venus and the sun narrows to 3.4 degrees.

       But another frequency, called X-band, is degraded less by the hot plasma around the sun and controllers said they expect to be able to command the spacecraft each day through more than a week of superior conjunction by X-band.

       Use of X-band for the uplink started on Saturday, Oct. 13. Its use both to and from the spacecraft is predicted to last until Oct. 27. The radio downlink rate is 268 kbps.

       Solar activity will determine how much the radio signals degrade and how long mapping will be halted. Magellan project officials will decide go or no-go decisions on mapping on a day by day basis.

       On Oct. 29, the spacecraft's computer will automatically halt mapping for at least nine days. The mapping could be halted for as long as two weeks, however, depending on solar activity. If solar activity is very heavy, fight controllers will switch earlier into the spacecraft's computer sequence by ground command.

       The project also was looking closely at its contingency plans in the event of a spacecraft problem during the superior conjunction period.

       Beginning Monday, Nov. 5, project officials will again make day by day go or no-go decisions to consider when mapping will be resumed, dependent on solar activity. A mapping sequence would be uplinked and mapping could resume as early as Nov. 7.

       The project expects it will regain the data rate frequency of 268 kbps on Nov. 9 and the telemetry frequency on Nov. 16.

       The first standard mapping sequence at 268 kilobits per second, with telemetry communication with Earth at 1200 bps, is scheduled to start on Nov. 21.


10/19/90 JJD