Controllers working to keep Ulysses sun orbiter alive
BY STEPHEN CLARK
Posted: April 15, 2008
Scientists continue to extract bits of data trickling to Earth from the Ulysses solar probe as ground controllers employ new strategies to extend the life of the 17-year-old spacecraft, including a "long shot" plan to put the observatory in hibernation until the sun's activity reaches its next peak in 2013.
"The mission is still operating, with a subset of the instruments providing data," said Richard Marsden, Ulysses project scientist and mission manager for the European Space Agency.
Marsden said instruments on Ulysses are still providing data on the solar wind, cosmic rays, energetic particles, and the sun's magnetic field. But reduced power production from the craft's aging radioactive isotope generator means managers must ration electricity to keep the probe running.
"We are unable to operate the full payload because of the current power and thermal limitations," Marsden said. "No instrument failures have occurred since launch."
Officials declared Ulysses was nearing the end of its mission in February, citing the dwindling power supply, frigid temperatures threatening to freeze the probe's fuel pipes, and the diminishing return of valuable science data.
Ulysses is not able to send back copious amounts of data due to the failure of the orbiter's X-band transmitter, which did not respond to commands to turn on in January. Officials switched off the transmitter late last year in an attempt to conserve power for Ulysses' science instruments and critical heaters.
Controllers now stay in contact with Ulysses through a backup S-band system capable of downlinking science data at just 128 bits per second for a few hours per day, Marsden said.
The heaters are used to keep important parts of the spacecraft warm, such as plumbing inside the probe's propulsion system. Ulysses still carries nearly seven pounds, or three kilograms, of hydrazine susceptible to freezing as temperatures reach about 36 degrees Fahrenheit, according to Marsden.
Ulysses is using about one pound of hydrazine per month through normal space maneuvers and a novel tactic known as "bleeding" to slowly move hydrazine through the coldest part of the craft's plumbing. The technique is similar to homeowners' efforts to drip water from faucets to keep unprotected pipes from busting.
Enough fuel remains aboard Ulysses to keep the observatory operational until late this year, but Marsden said the mission is currently baselined to end July 1.
"Right now, we're focusing on getting as much science as we can out of the current phase of the mission," Marsden said.
Managers must soon decide whether to keep the spacecraft alive past July 1 using the fuel bleeding tactic, or to save propellant by putting the probe in a deep sleep for up to five years before reviving Ulysses to study the next solar maximum in 2012 and 2013.
"We are still considering whether or not to reserve some fuel in order to slew the spacecraft into a known hibernation attitude so that the high-gain antenna would be pointing towards the Earth when we pass through perihelion again in October 2013," Marsden said.
Higher temperatures as Ulysses passes about 125 million miles from the sun could warm the spacecraft enough to rouse it from its slumber.
"At that point, there is a good chance the remaining fuel will have thawed sufficiently to make it technically possible to reactivate the spacecraft for a few months," Marsden said.
Ulysses is currently moving away from the sun at a distance of about 250 million miles. The probe will reach the furthest point in its orbit in 2010.
According to Marsden, the main threat to such a plan is securing enough funding to keep ground stations and a control team ready to operate Ulysses after hibernation.
"It's probably a long shot," Marsden said.
An ESA spokesperson said the agency has not determined how much such a hibernation plan would cost. A decision on when to turn off the spacecraft will likely be made on the basis of the spacecraft's health, not by a management directive, Marsden said.
Ulysses is currently funded through March 2009 in a mission extension approved in November by the committee governing ESA's science program.
Ulysses operations cost ESA about $4 million each per year. NASA and ESA share operations from a control center at NASA's Jet Propulsion Laboratory. ESA is responsible for the spacecraft and NASA provides the ground segment, according to a NASA spokesperson.
The total cost of the mission for both agencies, including development, launch and operations, surpasses $1.2 billion.
Ulysses launched aboard the shuttle Discovery in October 1990. After a 16-month journey through space to Jupiter, Ulysses used the giant planet's gravity to slingshot into a path taking the probe above the sun's poles.
The spacecraft has completed two-and-a-half orbits of the sun since 1992, observing the ebbs and flows of solar activity through nearly two cycles.
Originally slated to last for five years after launch, Ulysses has tripled its guaranteed lifetime by completing more than 17 years of continuous operations.
Marsden said the impending end of Ulysses marks the end of his 30-year career working on the mission. He began working on the mission more than a decade before the probe launched.
"It's hard to realize that it will soon be over. In a way, it's like saying goodbye to an old friend. On the other hand, Ulysses has been a fantastic mission that has produced amazing science for many more years than we ever expected when we launched in 1990," Marsden said.
Once the spacecraft succumbs, Marsden said scientists will spend the next few years putting together a data warehouse for the mission.
"Even though the spacecraft may stop operating soon, that doesn't mean that the work stops. We still have the task of preparing the data archive so that future generations of scientists can exploit the measurements made by this unique exploratory space mission," Marsden said.
Ulysses observed a period of calm in the sun's activity called "solar minimum" in the first few years of its mission. When Ulysses returned to the sun's high latitudes in 2000 and 2001, activity had drastically increased.
"Active regions on the solar surface abounded, and solar storms were the order of the day," Marsden said.
Instruments aboard Ulysses also found high concentrations of energetic particles far removed from the solar storms that produced them. Scientists did not expect to discover the particles to disperse as easily as Ulysses discovered.
"Ulysses has revealed that previous ideas as to how particles are transported in the heliosphere need a thorough revision," Marsden said. "Astronauts in deep space could be exposed to radiation from sources that were previously considered to be at a safe location."
Ulysses provided the first direct measurements of interstellar dust and neutral helium gas, materials that fill the voids between stars. The solar system is currently believed to be moving through a warm dust and gas cloud, Marsden said.
The dust and cosmic rays penetrate deep into the heliosphere, or the sun's area of influence, above the solar poles. Ulysses is the first mission to study the sun from a polar orbit outside the ecliptic plane, home of the solar system's planets.
Ulysses also detected new sources of unique ions that help open another window to determine the composition of Milky Way galaxy's interstellar medium. Ulysses has observed "pickup" ions left more than 300 million miles in the wake of comets, which emit the building blocks of such ions.
"Areas where Ulysses data are providing new and exciting insights range from the origin of the solar wind itself and the way the sun's magnetic field reverses polarity, to the nature of the boundary of the heliosphere and the interstellar medium. Ulysses data have even provided important constraints on fundamental cosmological concepts like the evolution of matter in the universe," Marsden said.
Many of Ulysses' scientific capabilities will be replaced and augmented by the Solar Orbiter mission scheduled for launch in 2015. Solar Orbiter will move within 22 million miles of the sun and use repeated flybys of Venus to gradually boost the spacecraft into an orbit covering the solar poles during a mission lasting more than six years.