So long, Ulysses!

Ulysses upon deployment from Space Shuttle Discovery

Courtesy of NASA

Friday, June 13th, 2008--

The spacecraft Ulysses is about to end its 14-year study of the sun, almost 18 years after it was sent into space by the Space Shuttle Discovery. And what a mission it’s been; thanks to Ulysses--a joint European Space Agency (ESA) and NASA operation--scientists now know a lot more about Earth’s closest star, from its inner workings to the first-ever measurements of the unexplored environment above the sun's poles.

Ulysses’s trip to the sun was anything but direct-- it arrived there by way of Jupiter. The probe hitched a ride into space on board Discovery in October, 1990. An upper stage rocket sent it into Earth orbit and from there, it was injected into interplanetary orbit. More than a year later, Ulysses reached Jupiter, where scientists took advantage of the energy of the big planet’s gravity to launch the spacecraft into its present polar orbit over the sun.

Ulysses is as much an engineering marvel as it is a science whiz. Nigel Angold, ESA Ulysses Mission Operations Manager, calls the spacecraft an “engineering overachiever.” Ulysses has been going strong for more than 17 years, its 10 instruments still working perfectly, according to Angold. So far, Ulysses has orbited the sun three times, completed six polar passes, observed one and a half solar cycles (cyclical changes in the sun’s activity), and sent four times more data back to Earth than expected. Not a bad resume for a spacecraft sent to the sun on a five-year mission.

And as for the science, the data sent back to Earth from Ulysses have changed scientists’ understanding of the sun and the space around it. This week in Paris, mission scientists recapped the mission’s major discoveries. “The main objective of Ulysses was to study, from every angle, the heliosphere, the vast bubble in space carved out by the solar wind,” says Ed Smith, NASA’s Ulysses Project Scientist at the Jet Propulsion Laboratory.

What has Ulysses discovered about the sun and its immediate neighborhood? Some of the key findings center on the solar wind and the sun’s magnetic field. Because the mission lasted longer than its original goal of one orbit of the sun, scientists were able to study how the solar wind, a stream of charged particles coming out of the sun, changes over time. For instance, scientists thought the solar wind was a slow wind with occasional high-speed gusts. Ulysses showed that it’s primarily a fast wind originating from the sun’s magnetic poles, and that it’s not constant, but varies during the sun’s cycle of magnetic activity, which lasts about 11 years.

Ulysses also helped scientists learn some things about the sun’s magnetic field. Before the mission, they thought the magnetic field moved out with the solar wind and, because of the sun’s rotation, got dragged into a spiral, like a garden sprinkler, according to Richard Marsden, ESA Ulysses Project Scientist and Mission Manager. Ulysses showed that the magnetic field is much more complex than that, not following an ordered spiral, but a random course instead. That allows particles emitted by solar storms at low latitudes to climb up to higher latitudes, far away from the storms that spawned them. And that’s important, because astronauts in deep space may be exposed to radiation in areas once considered to be safe.

How will Ulysses die? In the end, it will simply run out of power. Despite its proximity to the sun, the spacecraft doesn’t run on solar power. Instead, it’s powered by a radio isotope thermal electric generator (RTG), a radioactive heat source that now, after so many years, is failing as those isotopes gradually decay. Over its lifetime, the spacecraft has lost almost one third of its available power, according the ESA.

Soon, says Angold, Ulysses will suffer a natural death as the RTG’s power continues to decrease. The further the spacecraft gets from the sun, the colder it will get, because the RTG can’t generate enough energy to keep it warm. Eventually, Ulysses will get so cold that its thruster fuel—what keeps the main antenna pointing towards Earth—will freeze and engineers will lose contact with the space probe. When that happens, Ulysses will keep orbiting the sun, though its mission will be over. That’s expected to happen around July 1.

Ulysses may be ending, says Angold, “but the odyssey will continue for the next generation, as scientists continue to use the data to better understand the workings of our closest star. So goodbye, Ulysses.”

--Karin Vergoth