After a nearly 30-year odyssey navigating political shoals and weathering solar storms, a 900-pound robotic explorer dubbed Ulysses is slowly beaming back its final dispatches.
Sometime in the next few weeks, the craft will fall silent, ending an era of solar exploration that has given scientists their first 3-D view of the vast magnetic cocoon the sun builds around the solar system.
Ulysses helped unveil processes that have given scientists a deeper understanding of how solar storms form and move through the solar system. It made the first direct measurements of the interstellar dust that the solar system collects as it plows through interstellar space. And Ulysses helped scientists study distant gamma ray bursts – a phenomenon one astrophysicist has dubbed "the birth-cry of a black hole."
The craft "has been a terrific old workhorse," says Richard Marsden, the European Space Agency's manager for the joint ESA/NASA mission. "It's produced great science and lasted much longer than we expected." Late last year the mission got its third extension.
The International Solar Polar Mission, as it was formally called, also served as a crucible for developing and sustaining international partnerships in space exploration. Early on, the United States canceled the planned Ulysses twin, raising concerns that the US was an unreliable partner. The mission became a one-craft show.
But the relationship recovered. US scientists provided half the instruments on Ulysses. The craft was launched in 1990 from a space shuttle and has been controlled by a joint team based at the Jet Propulsion Laboratory in Pasadena, Calif. The National Aeronautics and Space Administration's Deep Space Network has served as the project's communications link.
Now, reinforced by the success of the Cassini-Huygens mission currently under
"After the problems we had in the beginning" of the Ulysses project, "we've gotten excellent support from NASA," Dr. Schwehm says. Word of Ulysses' imminent demise came toward the end of February. It was starting a third extended mission when it was hit with a peck of power problems.
The craft swings around the sun once every 6.2 years in an orbit that takes it as far out as Jupiter and as close to the sun as Earth. Its generators contain plutonium, whose radioactive decay yields the heat used to produce electricity. After 17 years in orbit, the generators no longer produce enough electricity to run the high-power transmitter, science instruments, and heaters at the same time. Now its high-power transmitter has quit. Designers were using the transmitter's warmth to keep fuel from freezing. So engineers are scrambling to download as much data as they can using the craft's slower, low-power transmitter before the fuel freezes and engineers lose the ability to keep the orbiter's big antenna aimed at Earth. Once the fuel freezes, it will be locked in its orbit around the sun.
For space physicists interested in the solar system, Ulysses has been a wild ride. Concepts for the mission emerged in the 1950s and '60s, notes Justin Kasper, a space physicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. The only view humans had of the sun was two-dimensional, along a line traced by Earth's orbit around the sun. But even back then, he says, people saw a need to orbit a spacecraft over the sun's poles and through several changes in the sun's 11-year activity cycle to get a more complete picture of our nearest star.
For instance, he says, physicists had noted that during the sun's quiet period, when some of its processes are simpler to unravel, the solar wind's speed gusts and eases as it flows past Earth. But no one knew if that was true all over the sun or merely from Earth's vantage point.
Ulysses' first pass over the sun's poles came during solar minimum. It found that over the poles and throughout the lower latitudes, the sun was blasting solar-wind particles into space at a steady 1.7 million miles an hour. The one exception was a narrow belt along the sun's equator, where the speed quickly dropped and was more variable.
"What Ulysses showed us is that we're rattling around" in that little belt along the equator, Dr. Kasper says. "It was one of the most dramatic results Ulysses has had."
Studies of solar wind have practical applications. Violent outbursts from the sun can damage satellites, disrupt communications and electricity distribution on Earth, and endanger astronauts on orbit. Ulysses helped researchers see that even storms that don't fall along the sun's equator may affect conditions on or near Earth.
Such information is critical to improving forecasts of "space weather," scientists say.
NASA's plan to return astronauts to the moon by 2020 is raising the stakes for better space-weather forecasting, researchers say. Late this year or early next, NASA plans to launch the Solar Dynamics Observatory, which will image the sun every 10 seconds over a wide range of wavelengths. NASA is also planning to launch a suite of six satellites dubbed Solar Sentinels beginning as early as 2012. Four will orbit the sun well inside Mercury's orbit, while two others take up positions near Earth – one in Earth's shadow, the other just sunward of Earth.
Meanwhile, ESA is planning a solar orbiter that will dip to within 21 million miles of the sun, well inside Mercury's orbit. It will work in concert with NASA's sentinels. These missions are timed to coincide with the last peak of solar activity before manned lunar missions leave Earth.
As for Ulysses, "it always provided excellent science," says ESA's Dr. Schwehm. And the fact that it's facing a natural end makes it easier to move on. "We're not facing a very tough decision to switch off a spacecraft; it switches off itself."