Friday, February 22, 2008

The end of a heroic spacecraft

The Ulysses Spacecraft is tested in 1985
Image Credit: NASA / JPL

The ancient Greek epic "The Odyssey" follows the adventures of Odysseus (known in Latin as Ulysses), who wanders the Mediterranian for ten years after the Trojan War, trying to get home to his wife and son. Today, it was announced that the modern Ulysses, a spacecraft studying the sun, will soon fall silent and be doomed to wander the Solar System for aeons to come.

Ulysses is a space probe studying the sun. It is unique in that it used Jupiter's gravity to get into an orbit that goes over the north and south poles of the sun. Without a little muscle from Jupiter, it isn't possible to launch a spacecraft into such an orbit -- all of our other solar probes orbit the sun at its equator, just as the Earth and all the planets do.

Ulysses was launched by the space shuttle Discovery in 1990, and after an 18-month trip to Jupiter, began its mission. It takes Ulysses 6 years to orbit the sun once, and it is most of the way through its third trip around the sun. Since the sun goes through a cycle of activity every 11 years, Ulysses has seen one and a half complete solar cycles.

The Ulysses mission was designed not to take pictures of the sun (we can do that from Earth!), but to study the sun's radiation and wind and magnetic fields from vantage points that the Earth can never see. Ulysses has studied how the sun's wind and magnetic field vary from the equator to the poles, information we could only guess at before. We've learned that the solar wind (a stream of particles coming out of the sun) is much faster near the sun's poles, moving along at over 450 miles per second! Here at the sun's equator, the wind blows at a much slower 200 miles per second. Ulysses also found that the sun's magnetic field is not much stronger near the poles than at the equator; a simple magnetic field would be twice as strong at the poles. This tells us that the sun's magnetic field is quite complicated. And, among many other discoveries, Ulysses found that the sun's magnetic field is very good at protecting us from cosmic rays, powerful radiation from the Galaxy and beyond. no matter what direction they are coming from.

Is there any saving Ulysses? No. Ulysses is powered by a radioactive power plant -- although the spacecraft is studying the sun, it is too far away from the sun for solar panels to work very well. In the power plant, radioactive fuel decays, creating heat that is converted to electricity. But as the fuel decays, the energy released steadily drops. Within the next month or two, the energy released will drop below the levels needed to keep the spacecraft pointed at the Earth.

So, although Odysseus was able to find his way back home after 20 years abroad, the Ulysses spacecraft will never get to come home. It will soon fall silent, continuing to orbit the sun for ages to come.

My congratulations to the Ulysses team on a long and successful mission!

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