The U.S.-European Ulysses mission to study the solar wind with a satellite orbiting around the
sun’s poles is expected to cease operations in the coming weeks, putting an end to one of the first and most successful major trans-Atlantic collaborations in space science, U.S. and European officials said.
Launched in October 1990 on a planned five-year mission, the Ulysses satellite’s plutonium-238 fuel supply has decayed to the point where it is unable to provide minimal heating to the satellite.
In what program managers conceded is an irony of the mission, Ulysses orbits too far away from the sun to be heated by conventional solar arrays. Its radioisotope thermoelectric generator (RTG), which uses the steady decay of plutonium-238 to provide onboard power, is now able to generate no more than 195 watts, compared to 285 watts at the mission’s start.
The dwindling supply of electric power means Ulysses is unable to keep its hydrazine fuel from freezing, an event that will prevent the satellite from keeping its antennas pointed toward the Earth to send data.
“That will be the end of the Ulysses mission,” said Nigel Angold, Ulysses mission operations manager at the European Space Agency (ESA). “But all science instruments are still functioning perfectly 17 years after launch. This is phenomenal. The old-fashioned tape recorders have delivered four times more data than planned.”
ESA and NASA ground teams are scheduled to end Ulysses operations July 1.
The 17 years of data-producing life has meant that Ulysses has observed the
sun over 1.5 solar cycles instead of just half a cycle. Solar activity ebbs and flows over an 11-year cycle known as the solar cycle.
In a June 12 press briefing at ESA headquarters here, Angold said the satellite will remain in its current orbit – flying over the poles
sun at altitudes of between 150 million and 810 million kilometers – for several hundred years.
ESA Science and Robotic Exploration Director David Southwood, pointing to the fact that Ulysses has operated more than three times longer than planned, said: “People say: ‘Over-engineered.’ Don’t believe it. This is really one beautiful spacecraft.”
What subsequently became known as Ulysses was begun in 1977 and was the first significant collaborative effort between ESA and NASA. Including the scientific instruments provided by individual European nations, ESA and NASA divided about equally the cost
�of the mission, which in today’s currency is estimated at about 1 billion euros ($1.53
The satellite’s longevity and continued output of data on the heliosphere – the bubble created by the solar wind – obscure the fact that Ulysses has faced more than one near-death experience since its launch aboard the U.S. Space Shuttle Discovery.
The first occurred only weeks after launch. Mission managers discovered to their surprise that the deployment of a 7.5-meter boom caused the satellite to wobble, making it impossible for the high-gain antenna to remain stable enough to send data to Earth.
“A month or so into the mission we were not even sure it was going to continue much beyond that,” Angold said. Engineers subsequently came up with a way to use Ulysses’ onboard thruster motors in an unconventional way to reduce the wobble.
Depending on where Ulysses is in its orbit, its radio transmissions
take 50 minutes to travel to Earth.
In 1997 – two years past its originally scheduled retirement – Ulysses’ RTG decay forced ground controllers to adopt a series of power-saving measures. Redundant units were switched off. Instruments that required lots of power were not run simultaneously.
Since 2002, operations have been modified to permit different instruments to share power. Keeping the hydrazine fuel from freezing was already a concern, and instrument use was reordered so that the heat generated from their operations could warm the coldest areas on the satellite.
In February 2003, the main transmitter was lost. Transmissions were switched to a smaller backup transmitter, but this forced NASA and ESA ground teams to deploy larger ground antennas to take down Ulysses data to compensate for the lower-powered transmissions.
In January, a secondary transmitter failed to switch back on after having been turned off to save power when the satellite was not being tracked.