Ulysses Solar Probe Nears Death

The scoop: NASA and the European Space Agency's Ulysses spacecraft has been long overdue for death en route around the sun. But as of July 23, 2008, its electronic heart is still ticking. Mission manager Ed Massey gives his take on the life of this intrepid robotic explorer.

The Ulysses spacecraft -- a joint ESA and NASA mission to study the sun's environment -- has led to an astounding amount of science. But in a strange twist of fate, the cold of space will eventually silence the probe forever.

I began work on Ulysses when I arrived at NASA's Jet Propulsion Laboratory in 1987. Despite launch delays in 1983 and again in 1986 after the Challenger accident, the 814-pound spacecraft had to wait four more years because two other missions were patiently waiting their turns: Magellan to study Venus, and Galileo to observe Jupiter.

Ulysses finally left Earth aboard space shuttle Discovery in 1990, and I was there to watch it depart. It soon became the fastest manmade object of its time and the fifth mission to get a gravitational assist at Jupiter, after the two Pioneer and two Voyager spacecraft.

The boost around the gas giant was necessary to send Ulysses on a first-ever loop below and above the sun's poles. We have now wrapped up three passes around the sun over nearly 18 years in space -- 13 years beyond its originally scheduled lifetime. How's that for overachieving?

Getting Ulysses this far, however, has been anything but easy. Solar heating caused the spacecraft to wobble, or nutate, and it was launched with a mortal power supply.

The wobbling was at its worst above and below the sun, but we enlisted the help of the Deep Space Network to give Ulysses a beacon on Earth to help dampen its nutation. Getting Ulysses' power supply to last took switching off some redundant equipment, using more efficient units and micro-managing the on-off cycles of the scientific instruments.

But it is the diminishing electrical power that will spell the end for Ulysses. As power levels drop and the spacecraft gets farther and farther from the Sun, the hydrazine fuel keeping it pointed toward the Earth will freeze.

Ulysses may be chillier than ever before, but, contrary to our expectations, the hydrazine remains unfrozen.

The spacecraft's scientific legacy will live on after we lose contact, which will happen any day now. Ulysses confirmed some theories about our backyard star, but blew a lot more out of the water, and has redefined our view of the sun's heliosphere -- the "bubble" of solar particles ejected into space.

Here's only a sample of findings described in more than 1,300 scientific papers:

• The sun casts both fast and slow winds of solar particles; the speedier from the sun's poles, and the slower from the equatorial region.
• The fast solar wind persists throughout the solar cycle of roughly 11 years, vanishing only at the cycle's violent peak when coronal holes disappear.
• The sun's magnetic field rotates in tune with the solar cycle, from minimum to maximum.
• Its radial field, on the other hand, remains constant.
• Cosmic rays from interstellar space don't have easy access to the heliosphere in the polar region.
• Ulysses directly measured interstellar gas for the first time, hinting at the presence of a "bow shock" for interstellar radiation beyond the heliosphere.
• Large interstellar dust particles make up about 2 percent of all mass between stars.

Over the years, a lot of people have been involved with Ulysses to make this science possible.

Some retired and some moved on to other projects. Some ESA personnel returned to Europe while others married and raised families here in the United States. Yet a few of us are still involved with the project, and unfortunately, a few are no longer with us at all.

During my own time on the project, I've worked with most of the people involved and am truly impressed with their dedication and professionalism. So, when Ulysses sends its final signals to Earth, it will be a sad but truly triumphant day for all of us.

Ed Massey is the manager of the joint NASA/ESA Ulysses project at the Jet Propulsion Laboratory (JPL) as well as manager of the Voyager project. The views expressed are the author's alone and do not represent the official position of JPL, NASA, or the Discovery Channel.

Got something to say? E-mail your questions, comments or concerns to discoveryspace@discovery.com. Your words may appear on Discovery Space.

MORE EXPERTS