At any given astro-gathering, I'll sometimes try to stump the attendees with this question: "How many spacecraft have visited Jupiter?"

Most people get the easy ones: Pioneers 10 and 11 (1973-74), Voyagers 1 and 2 (1979), and Galileo (1995). A few recall that Cassini (2000) and New Horizons (2007) have zipped by en route to their final destinations.

Ulysses spacecraft

Launched in 1990, Ulysses sent its final transmission to Earth on June 30, 2009.

European Space Agency

But only True Space Cadets can cite the 1992 flyby of Ulysses, a craft designed jointly by the European Space Agency and NASA to study the Sun.

Ulysses needed to fly closely past Jupiter so that the planet's gravity could radically alter the craft's orbit — yanking it up and out of the ecliptic plane so that it could loop back over and under the Sun's poles. This helios incognita is unobservable from Earth, and space physicists first started dreaming about a solar polar mission in the late 1950s. Only with the advent of planetary gravity assists in the 1970s did such a mission become realistically attainable.

Once in its highly inclined, highly elongated 6.2-year orbit, Ulysses was ready to explore the Sun as never before. In all, it made three swings over the solar poles: in 1994-95, when the Sun was near the minimum of its 11-year activity cycle; again in 2000-01, during the most recent solar maximum; and a third pass from late 2006 through early 2008. (Ulysses never really got close to the Sun; its perihelion is 1.4 astronomical units.)

Ulysses' solar orbit

The Ulysses spacecraft has sailed over both of the Sun's polar regions every 6.2 years as it circles in a looping orbit that extends out to Jupiter and is almost perpendicular to the ecliptic. Click on the image for a larger view.

Source: European Space Agency

I was musing about all this yesterday while watching a webcast of Ulysses' final hour of contact with its home planet. The craft is nearly out of hydrazine fuel for its stabilizing thrusters, and there's not enough money to continue the mission for another year. With the Sun mired in a prolonged activity slump and Ulysses once again headed away from it, NASA and ESA agreed to pull the plug once and for all.

The webcast was full of nostalgic reminiscences by the graying team members. Most expressed sadness at the mission's termination — yet they all admired the spacecraft's plucky durability. A major communication glitch in early 2008 appeared to doom the mission, but clever engineers found a fix. Given the craft's delicate state, the final shutdown was supposed to occur a year ago, but the funding and hydrazine managed to last for one more year.

Here are the final few entries in a blog posted yesterday by the operations team at the Jet Propulsion Laboratory:

UTC Timestamp: 30-Jun-2009 19:59
Telemetry is back in lock at 64 bits per second engineering format. The command counter confirms reception of the final command.

UTC Timestamp: 30-Jun-2009 20:10
Telemetry loses lock as the spacecraft switches to use the low gain antennas.

UTC Timestamp: 30-Jun-2009 20:15
Ground station can not find the carrier. Transmitter is off on the spacecraft. Goodbye Ulysses.

About the size of a large golf cart, Ulysses operated for 18 years, 8 months, 24 days — and in doing so it managed to eclipse (by just 20 days) the previous endurance record for an ESA spacecraft, that being the International Ultraviolet Explorer. (Those space lifetimes, though remarkable, are still well short of the Voyagers' longevity, currently at just under 32 years.)

Heliosphere diagram

This artist's concept shows structures in the tenuous gas where the expanding solar wind meets the interstellar medium far beyond Pluto. Voyager 1 has apparently crossed the first boundary layer, as described in the text. Voyager 2 still has a way to go.

By studying the Sun all those years, Ulysses has paid big scientific dividends. Most notable was the realization that the solar wind, the tangle of hot, ionized gas and magnetic-field lines rushing outward from the Sun's lower atmosphere, has a distinct character over the poles. There the flow tends to be less turbulent and about twice as fast — some 500 miles (800 km) per second — than what's coming from equatorial latitudes.

Moreover, fast flows arise from relatively cool areas in the solar corona and slow ones from hot areas, counter to expectations. As U.S. project scientist Edward J. Smith (Jet Propulsion Laboratory) explained yesterday, this means that the solar wind must somehow be magnetically driven and not simply pushed out into space because the corona is hot. "The solar wind is closely related to the magnetic flux of the Sun," Smith emphasized, "which means the whole concept of its origin needs review."

Not bad for a spacecraft that has no camera! Click here for a nice summary of the mission and here to read more about Ulysses' scientific legacy.

So will we ever hear from Ulysses again? Probably not. Its instruments and radio transmitter have been switched off, and the systems that keep its main antenna pointed toward Earth have been disabled. Once it gets farther away, the remaining hydrazine will probably freeze, rupturing its fuel lines. However, engineers did activate two omnidirectional receivers, which would allow them to someday resume contact with the soon-to-be-tumbling craft.

Never say, "Never."


Image of Justin S

Justin S

July 2, 2009 at 11:27 am

Does hydrazine expand when frozen? Few substances do.

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Image of James Barnes

James Barnes

July 7, 2009 at 5:48 am

Hydrazine does not expand when frozen.In any case, frozen fuel is of no use.

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