Ulysses satellite's heroic journey comes to an end

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."

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