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Magellan Prepares for Venus Orbit Insertion


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FOR IMMEDIATE RELEASE           May 4, 1990

The Magellan spacecraft, on a radar-mapping mission to Venus, has traveled 607 million miles and has cruised one and one-fourth times around the sun since it was launched from space shuttle Atlantis a year ago May 4.

It will arrive at Venus on August 10 and be placed in a near polar orbit by a solid rocket motor.

Today, Magellan is in an orbit around the sun, 103 million miles from Earth and 17 million miles from Venus. Its velocity, relative to the sun is 68,000 miles per hour.

It takes nine minutes for a radio signal, at the speed of light, to reach the spacecraft from Earth and another nine minutes for a signal to return.

The Magellan cruise has been relatively quiet, but the spacecraft team has learned to cope with some unexpected problems.

The star scanner unit, used to check the position of the spacecraft by scanning pairs of reference stars, went through a period of seeing unexpected glints of light during its daily calibrations. Such events are called spurious interrupts and cause the attitude reference units to abort the updating process.

Engineers narrowed the likely causes to two conditions. One was bombardment of protons during large solar flares. (Magellansaw three major solar events in its first six months). The second was when the star scanner unit went from the shade to sunlight during calibrations which caused small particles to be shed by the spacecraft cover.

The first cause was fixed with a software patch that narrowed the tolerances associated with a correct star fix; the second was fixed by ensuring that the star scanner unit remained in the shade before and during star calibrations.

Magellan had another persistent problem early in its cruise. The rocket engine modules, solid rocket motor and two equipment bays experienced temperatures hotter than predicted. In all cases, the components never got hot enough to trigger red alarms, but it was important to understand the higher temperatures to assess their impact throughout the mission.

The fix to this problem has been to use the high gain antenna to shade the components from the sun, and temperatures are expected to remain in the acceptable range.

Other spacecraft teams have also solved problems involving one of the gyroscopes and radar engineering data.

In the weeks ahead, all the teams will have a busy schedule as they plan for a radar mapping test, trajectory correction maneuver, going into orbit around Venus, testing and the beginning of mapping.

During the last week of May, the spacecraft will be programmed to simulate three days of radar data taking, including turns. All the teams will be at their stations as the data comes through the Deep Space Network to the Radar Processing Teams and the Data Management and Archive Team.

Although the radar will be pointed at deep space during the simulated mapping orbits, for the purposes of the test, data will be treated as real so that all aspects of the Ground Data System can be tested.

A final trajectory correction maneuver is scheduled for July 25. It will change the spacecraft's velocity 0.7 meters per second. An earlier correction maneuver in March added just under 0.5 meters per second.

On the morning of Aug. 10, the spacecraft will fire its solid rocket motor to go into Venus orbit. The firing will take place while Magellan is behind Venus as viewed from Earth, and then the rocket motor will be jettisoned.

After it is verified that Magellan is in orbit, a period of 20 days has been set aside to verify the spacecraft's health, turn the radar on, and make adjustments. This period is called In-Orbit Checkout (IOC).

Real data will be taken at various times during this period and processed by the Imaging Team, but the main purpose is to assist the radar team in adjusting the radar instrument's parameters.

At the end of the IOC, mapping of the planet will begin and continue for 243 days, one rotation of Venus. The first products of the mapping phase will be available in early October.

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