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Solar Explorer Fires Thrusters in Trajectory Correction Maneuver

October 15, 1990

PASADENA, Calif. (AP) _ The Ulysses spacecraft fired its thrusters Monday, starting a four-day maneuver to fine-tune its path toward Jupiter and put it in better position to study the sun’s poles during 1994 and 1995.

″The thrusters began pulsing on and off at noon (PDT),″ said Franklin O’Donnell, a spokesman for NASA’s Jet Propulsion Laboratory. ″Everything is proceeding as planned.″

Ulysses was carried into Earth orbit aboard space shuttle Discovery on Oct. 6. Six hours later, it was released from the shuttle’s cargo bay. Then three rocket stages ignited to send Ulysses hurtling on its five-year, 1.86 billion- mile mission to study the sun’s polar regions.

Discovery, which landed at Edwards Air Force Base on Wednesday, left early Monday, strapped atop a modified jumbo jetliner for a day-long ride back to its launch site at Cape Canaveral, Fla.

Meanwhile, Ulysses was 6.1 million miles from Earth on Monday, traveling toward Jupiter at a speed of 25,370 mph relative to the Earth and 92,050 mph relative to the sun.

The spacecraft will use Jupiter, the solar system’s largest planet, as a ″gravity slingshot″ to leave the plane in which all the planets orbit the sun. That will allow Ulysses to fly almost over the sun’s south pole in 1994 and north pole in 1995.

However, the spacecraft will never get closer to the sun than it was when it was launched. To study the sun’s poles, Ulysses doesn’t need to get close to the sun, only to leave the plane of the planets.

Monday’s thruster firing was Ulysses’ first trajectory correction maneuver, although it used its thrusters last week to turn its main antenna toward Earth so engineers could communicate with the spacecraft at a faster rate.

The $250 million Ulysses is the centerpiece of a $750 million joint project of the European Space Agency and the National Aeronautics and Space Administration.

The first part of the maneuver will end early Wednesday. O’Donnell said it consists of repeated pulses in which the spacecraft’s axial thrusters - small jets that point opposite the direction Ulysses is traveling - fire for a total of 26 hours.

Ulysses’ radial thrusters, which point sideways, will fire in repeated pulses on Thursday.

The overall maneuver was designed to increase Ulysses speed by 222 mph and adjust its aim toward Jupiter. As a result, Ulysses will make its closest approach to Jupiter at 4 a.m. PST on Feb. 8, 1992, about 60 hours earlier than originally planned, O’Donnell said.

The early arrival will give Ulysses more time to study the sun’s polar regions. Instead of spending 228 days above 70 degrees solar latitude, it will be there for 235 days, O’Donnell said.

Ulysses will not fly directly above the sun’s poles, which would be 90 degrees latitude, but will reach a maximum solar latitude of more than 80 degrees. O’Donnell said that before the maneuver, it would have reached 79 degrees.

Engineers will start turning on Ulysses’ nine scientific instruments on Friday, a process that will last 6 1/2 weeks.

Ulysses’ thrusters use hydrazine fuel. The fuel flows over a catalyst that makes the hydrazine gas expand rapidly in a flame-like reaction, O’Donnell said.

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