After 17 years, the Ulysses mission is coming to an end. The LANL-produced radio-thermal generators on board the spacecraft, featuring a little bit of cracklin' hot 238Pu, have decayed to the point that there is no longer enough electrical power on board to keep the hydrazine fuel from freezing in the pipes. Attitude control will be lost sometime in the very near future, and with it the ability to keep the antenna pointed back towards Earth.
Ulysses falls into a rare category for spacecraft—those that exceed their predicted mission lifetimes several times over. Ulysses was launched from the payload bay of Space Shuttle Discovery, back in October of 1990 with an expected mission duration of five years. I had just started high school. In February of 1992, it successfully performed a gravity slingshot around Jupiter to propel it up above the ecliptic and give it a polar orbit of the Sun-Jupiter system. Its unique vantage point has led to an abundance of data on the magnetic environment at high latitudes near the Sun, and a greater understanding of the heliosphere in general.
I am fascinated by long-mission spacecraft that go beyond low Earth orbit because of the demand for creative usage of limited resources. Once it is launched, there are no resupplies, no repairs, and no hardware upgrades. When things break, the system must be reconfigured to make the best of the remaining hardware. The US space program is replete with examples of mission teams overcoming seemingly insurmountable problems with launched hardware. Probably my favorite example is when Galileo's main antenna failed to deploy, reducing the data bandwidth from 134,000 bps to 16 bps. To combat this bottleneck in communications, mission engineers uploaded image compression algorithms that didn't exist at the time of Galileo's design, essentially causing Galileo to transmit JPEGs instead of bitmaps. This fix saved the mission, which turned out to be one of the most successful planetary science missions ever.
The Mars Exploration Rovers are certainly the most recent example of spacecraft who operate beyond their expected lifetimes. They were launched in 2003 and expected to have a 90 day operational lifetime once they arrived at Mars. After four and a half years on the surface (1584 days for Spirit), they're still cruisin' around.
But all things come to an end, and the end is very predictable for spacecraft powered by RTGs. Ulysses, though its technology is somewhat antiquated by today's standards, is still a triumph of modern science. It's achievements should be celebrated as we turn our focus to the long-mission spacecraft still out there and the upcoming missions soon to be launched.
I hope we're alive when the historic preservationists are out retrieving these types of craft. It's funny to think that one day we'll probably bring Voyager I and II back and put them in the Smithsonian.
You don't watch enough movies. We don't bring Voyager back, Voyager comes back to us greatly enhanced and speaking through a bald Indian hottie.
In truth, the energy expenditure for sample return from such a tremendous gravity well as the solar system is unlikely to ever be considered worth while. Getting Voyager on a path that escapes the Sun's gravitation pull required a lot of force— a big multistage rocket followed by two monster planetary slingshots. To get it back, you'd have to go out there again, with more speed, capture it, then lose *all* of that energy, turn around, and re-enter the gravity well... only to lose all of the energy *again* as you attempt to decelerate and return your payload to Earth safely. Massive massive energy expenditure.
And I'm absolutely convinced that our ability to produce that amount of energy will become trivialized (by comparison) in the future. It's just a question of when.