Ulysses’s odyssey comes to an end

Contributed by
Feb 26, 2008
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You may have already heard that scientists have decided that is it time for the solar satellite Ulysses to shed this mortal coil.

Ulysses was launched from the Space Shuttle back in 1990, and was designed to operate for 5 years. Now, over 17 years later, its radioactive power source has finally decayed to the point where power is a serious issue. They've decided that in a few months they'll shut it off, after an extraordinary mission.

Ulysses didn't take pictures, so you may never have heard of its breakthrough science. It was the first machine to directly detect interstellar dust particles and helium atoms in our solar system, literally, interlopers from another star. It took unprecedented data of the Sun and its magnetic field, and did so continuously for so long that we now have an excellent baseline for such measurements, including over an entire sunspot cycle*.

For me, the most interesting aspect of the mission was that it was in a solar polar orbit: instead of sticking to the orbital plane of the planets like most probes, it was actually sent into an orbit nearly perpendicular to the orbit of the planets, so that it could peer straight down over the solar poles, an aspect we had never witnessed before.

Getting a probe into an orbit like this is hard. Why? Because the Earth orbits the Sun pretty quickly, at 30 km/s (18 miles/second). You need to mostly negate that velocity for a probe to end up perpendicular to the plane of Earth's orbit, and then you need to give it a huge velocity "down", south if you will, to get it in that orbit (or up, of course, but in this case Ulysses was sent down). No rocket we have now (or in 1990) could do that.

So we borrowed energy from one of the biggest sources we have: Jupiter. Ulysses was launched toward the giant planet, and using a slingshot maneuver launched itself down, down, and away, into the polar orbit around the Sun. While it was at Jupiter it took lots of scientific measurements, and has been sending back data ever since.

But now that's over. With the power source dying, it cannot keep energy flowing to its instruments, communication devices, and also be able to heat the hydrazine fuel it uses for maneuvering (this is the same stuff the spysat that was recently destroyed -- and many other satellites -- use for fuel). When Ulysses's orbit takes it out to Jupiter's distance once again, it's so cold that the probe has a hard time keeping its fuel from freezing. All of these together mean it's time for Ulysses to say its goodbyes.

My only regret for this mission? It didn't swing by the asteroid 201 Penelope.

*Pedantically, you could say it's half a cycle since the Sun's magnetic field reverses every 11 years, and therefore a full cycle is 22 years. But it's a full sunspot cycle of minimum numbers of sunspots to maximum and back to minimum, so that counts.