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.
February 26th, 2008 at 11:27 am
You thought the leaden winter would bring you down forever,
But you rode upon a steamer to the violence of the sun.
February 26th, 2008 at 11:50 am
Why wasn’t Ulysses fitted with a solar panel?
February 26th, 2008 at 12:01 pm
Pieter,
Solar panels aren’t of much use out by Jupiter. Out there, the panels would only get about 4% of the light they would get if near the Earth.
February 26th, 2008 at 12:02 pm
I went to the probe’s site a few times, looking for pictures. I was always disappointed and I supposed it didn’t have a camera. They could say it on the main page at least: “people looking for pictures beware: the probe does not carry a camera”.
I wonder how it navigates. Isn’t a star tracker some kind of camera?
Pieter: solar panels are nearly useless at Jupiter’s distance from the Sun.
February 26th, 2008 at 12:10 pm
Phil,
I was thinking about writing a little homage to Ulysses because it was one of the two spacecraft whose data I used in projects that helped put me through grad school.
We took advantage of Ulysses’s extreme polar orbit to sample the varying regimes of the solar wind and use that data to project how those regimes would affect comet plasma tails. We were able to characterize the high and low-speed streams and predict what those streams would do the plasma tails of comets of interest to our team. Sweet stuff!
I’m sorry to see the spacecraft go, but it has had a good long run. May its data archives live forever!
February 26th, 2008 at 12:16 pm
I am always amazed by the ingenuity shown by the scientists that design, build, and fly these robotic probes. They are an inspiration for us mere mortals.
fos
February 26th, 2008 at 1:50 pm
darn moist, i was gonna put some cream references
February 26th, 2008 at 3:26 pm
@moist and morley
Me too, oh, well its not too late, I guess.
Tiny purple fishes swim, laughing
through your fingers
and you want to take them with you
to the hardlands of the winter
February 26th, 2008 at 4:31 pm
There’s hydrazine on board? Quick! To the missile launchers!
February 26th, 2008 at 7:17 pm
I’ve always wondered why polar orbits were so rare … even among planets.
So now I know!
February 26th, 2008 at 8:43 pm
dear david,
You rock
February 26th, 2008 at 8:56 pm
Obligatory “Operation: Annihilate” retro-reference.
I suppose there’s no feasible way to somehow brake the probe enough to send it — in a reasonable amount of time — hurtling back down into the sun, so that it might be “finally free”?
I dunno, I’ve always enjoyed watching NASA (intentionally) de-orbit their probes in spectacular fashion. The pics might not always be the greatest, but the concept is great — I likes me some fireworks, I does.
February 26th, 2008 at 9:32 pm
Moreover, it didn’t hang around here for a few years as part of the mission. Bummer.
February 26th, 2008 at 10:45 pm
When the MESSENGER data was delayed because of a com issue with Ulysses I was SHOCKED that it was still going. I’m surprised that the radioactive supply is shot after only 17 years though since the Pu 238 half life is something like 85 years. Voyager is still communicating more than 30 years later. My understanding is that it isn’t the amount of radioactive materials, rather it is the degrading of the thermocouple that harvests the energy.
February 27th, 2008 at 12:00 am
A Science@NASA article from 2004 mentioned that the power level is down to 207 W from 285 W (30W of which should be due to decay if Pu 238 is used) which already then caused problems with the propellant heaters if they tried to use all the instruments and communicate at the same time. My guess is that the Voyeger probes where more overengineered and that they usually don’t run all their instruments.
February 27th, 2008 at 4:31 am
Props to the Cream fans, but surely Ulysses 31 is the more appropriate reference?
” Ulysses, Ulysses, soaring through all the galaxies, in search of Earth, flying in to the night.”
February 27th, 2008 at 6:09 pm
The article I read (spaceflightnow.com) was a little confusing, but I think they turned off one of the transmitters to save power so they could redirect that power to the heaters to keep the hydrazine from freezing, but then when they tried to turn the transmitter back on, it didn’t work, so they figured without the transmitter and with the general wearing out of the other systems, it wasn’t worth keeping it going. (The article didn’t say, but I presume Ulysses is still communicating with the Earth using a low-power transmitter, so they know it’s still alive, but the high-power transmitter is needed to send back significant quantities of scientific data, rather than just “yup, I got that message” stuff.)
February 27th, 2008 at 9:26 pm
Buzz,
I don’t remember the source, but I think you’re right about the transmitters. That’s not to say that you can’t get science data from a low-gain antenna (see Galileo mission), but it certainly puts a crimp in things.
February 28th, 2008 at 1:35 pm
Are we sure it stole energy from Jupiter to get into the tighter orbit? There’s less energy in a tighter orbit, so maybe Jupiter stole energy from the probe rather than the other way around?