Cassini's Primary Mission Ends, Two-Year Extension Begins

wooferhound points out recent news that the Cassini probe has completed its original four-year mission and is beginning a two-year extended mission, which was authorized earlier this year. Cassini's first mission brought us a treasure trove of information about Saturn and its various moons. The new mission will target two of those moons in particular for further study: Titan and Enceladus. Quoting: "The spacecraft is extremely healthy and carries 12 instruments powered by three radioisotope thermoelectric generators. Data from Cassini's nominal and extended missions could lay the groundwork for possible future missions to Saturn, Titan or Enceladus. [The two moons] are primary targets in the two-year extended mission, dubbed the Cassini Equinox Mission. This time period also will allow for monitoring seasonal effects on Titan and Saturn, exploring new places within Saturn's magnetosphere, and observing the unique ring geometry of the Saturn equinox in August of 2009 when sunlight will pass directly through the plane of the rings."

Comments

what the hell?

[davester666]

Authorized? What exactly would NASA/whomever have talked about when they were deciding this?

Is it still working? Yes.
How well is it working? Everything seems to be responding within operational parameters.
How much does it cost to keep listening for another couple of years, while it continues exploring? We estimate it to be maybe 0.01% of the cost of sending a new probe to do the same thing.

Sir, since we are at the end of it's planned lifespan, do we have authorization to keep listening to it or shall we send it the destruct signal? I'm leaning towards self-destruct, but maybe we can get some good PR showing how reliable some of our stuff is w.r.t. those shuttle disasters, so I guess we'll keep listening.

Re:what the hell?

[bazald]

I suppose they mean that a number of extended missions were proposed and evaluated, and this happens to be the mission that was authorized, beating out the other proposals.

Somehow I doubt that issuing a self-destruct signal was one of the other proposals ;)

Re:what the hell?

[jd]

There are thousands of things NASA wants to do, but costs keep going up and funding (in real terms) keeps going down. The costs of buying time on the Deep Space Array, renting a control room, paying for the mission specialists with the skills needed, etc, costs a lot more than running tourist centres. The Government doesn't want facts, it wants PR stunts.

Re:what the hell?

[Anonymous Coward]

Authorized? What exactly would NASA/whomever have talked about when they were deciding this?

The people controlling: the budget for paying the salaries of the technicians supporting the mission, paying for costs of equipment and its upkeep, scheduling of necessary assets on this end of the telemetry chain and a lot of other things.

Ultimately, it's up to the President and Congress as ultimate budget-makers, but I bet it only needed a cursory nod from higher-ups at NASA in this case.

No money means no listening. The people in charge of the Cassini mission can't exactly dig into their own pockets to keep it going ("My uncle has this old deep space radio network, and Bobby's dad has a fully staffed mission control center he's not using...").

Thankfully, it's not that hard to get those controlling the purse strings to spring for additional funding when you can point to what you've already done and tell them the probe is still good to go and that you're still getting outstanding results. There's also the unwritten rule where you more or less can be sure of automatic approval for extension if the probe is still going after the initial "planned" lifetime- the two Mars Rovers a perfect case in point.

Re:what the hell?

[RMingin]

So you're expecting the Martians to tag all the cameras and then tip over the probe soonish? The probe doesn't STOP when the crew goes home, by your own link the few people coming in are just supervising while the probe works on pre-scheduled work. Also, the dude you linked who was leaving sounds like an intern. He did his years, he saw a lot, learned some stuff, and he's ready to move on. Not everyone is as worked up over Phoenix as you seem to be.

I'm not seeing the strange.

Also, while we no longer build them like we used to, the Viking landers and orbiters grossly outperformed their expected MTBF. Viking 1 Lander went over SIX YEARS on the surface of Mars, taking pictures and so on, until some tool on Earth told it to point the main antenna elsewhere, making it hard to get back in touch.

Go over that again... SIX FREAKING YEARS. What was the expected duration? Quite a lot less. It's not unreasonable for us to expect Phoenix to possibly beat it's expiration date as well.

Re:what the hell?

[jd]

You're right, we no longer build them like we used to. Viking 1 lasted 6+ years, the Voyager and Pioneer probes all lasted decades despite extreme radiation. In contrast, there is a next-to-zero chance this probe will survive the Martian winter (no idea when that is, though). Yes, the probe is downloading data on automatic, the programs are about as tested as they're going to get, and the batch files the probe is running through will take a long time to complete, but Mars is a dangerous environment. It's not as geologically inactive as had been thought, the dust devils are nasty devils, and very little is understood about the polar regions. Some of those are events NASA can't deal with. A tremor would likely damage or destroy the probe. Even if the probe survived, it would not be pointing at Earth and would likely have no means to correct itself to do so.

(The Giotto probe that flew into Halley's Comet got blasted by pea-sized lumps of rock but had software designed to cope with such a contingency, and the armour to withstand it. Two layers of kevlar, interleaved with two layers of anti-meteorite shielding.)

NASA has grown poorer, is rushing these missions as much as possible, and can't afford to build systems as robustly as they should, but more importantly, missions have become tougher and involve more hazardous environments.

Re:what the hell?

[dlevitan]

Personally I find it strange that a member of the Mars Phoenix team left the project while the vehicle was still operating on Mars, and that the most of the team are taking the July 4 holiday off as if Phoenix will still be there in a years time.

With regards to the first post, the author is apparently a master's student who has completed his degree and is now leaving. More specifically, he's interested in science education and while I confess I'm not sure what E/PO Coordinator & Evaluator stands for, there's a good chance (with his background) that it means Education/Public Outreach. In which case its not a critical position and his leaving, if it was already planned (which it seems to have been), this is not a serious issue.

As for taking the weekend off, the people on the project have probably been working almost continuously ever since Phoenix landed. If they take 2 days off out of 90 its not the end of the world.

Re:what the hell?

[khallow]

It's not as geologically inactive as had been thought, the dust devils are nasty devils, and very little is understood about the polar regions.

I don't know what you've been hearing, but first couple of things are bunk. The spacecraft takes a big jolt when it lands. Sure it doesn't have all the instruments out then, but most earthquakes are as insignificant on Mars as they are on Earth. Second (googling around a bit), I find that the probe doesn't point at Earth. It has a non-directional UHF antenna with hemispherical coverage of the sky. This antenna allows communication with two satellites currently in orbit around Mars. That's how it communicates with Earth.

Second, the dust devils aren't nasty because the martian atmosphere is no more than 1% of Earth's atmospheric pressure.

NASA has grown poorer, is rushing these missions as much as possible, and can't afford to build systems as robustly as they should, but more importantly, missions have become tougher and involve more hazardous environments.

My take is that NASA's approach to science missions is deeply flawed. Any serious exploration of space is going to require space probes with development cycles of say four years or less. There are two big problems with longer development cycles. First, the longer the project, the more likely that it'll be redesigned, have funding trouble, or be cancelled. If you can get the development to launch in under 4 years, then you can launch missions under the same administration. Second, long development cycles greatly reduce how fast you can react to new information. If you find out something interesting but not currently in the schedule, it's unacceptable to have to wait another ten years or more before another probe can be sent to investigate this. You are wasting a lot of peoples' time.

Then we get to robustness. Currently, NASA launches expensive one off missions that are pretty robust as a single unit, but don't take advantage of economies of scale. My take is a few highly robust missions are more expensive for the data gained and dollars spent than a large number of cheaper, less robust missions. And that's including the higher failure rate of the less robust missions. There are various tricks. Instead of making one or two probes, you can make three to ten. Instead of building probes from scratch, work on developing a suite of "off the shelf" infrastructure and instruments so you can put together a probe for a particular mission fast. Also, I think that a counterintuitive result is that an organization which launches a lot of cheap probes is eventually going to get those to be more reliable than the organization which continues to launch a few, highly "robust" probes. The reason is that a lot of probes means a lot of exploration both of failure modes for those probes and data on the harsh environments that these probes end up in. While the "robust" probe makers don't know what they're up against and have to overengineer their probes.

Re:incredible

[Anonymous Coward]

They learned it from Scotty.

The classic case was on the Voyager Grand Tour. Voyager 2 was only 'designed' to go to Jupiter and Saturn. But the scientists and engineers doing the designing wanted it to also go to Uranus and Neptune. Once Voyager 1 successfully completed its flyby of Titan, the operations managers of Voyager 2 requested permission to skip that encounter with Voyager 2 so that they could do a gravity assist maneuver to go to Uranus.

Re:incredible

[ouachiski]

Prepare for the worst and hope for the best. With projects that cost as much as these and in outer space this reins more true than ever. Over engineer every thing. That is why I am not surprised when these things last past there expected life span. It means the engineers are doing there job.

Re:incredible

[DerekLyons]

That would be 'mostly correct'. Voyager 2 was the backup for Voyager 1, but it was planned from fairly early on that if Voyager 1 sucessfully flew by Titan (and thus completed the primary mission of Voyager 1/2) then Voyager 2 would be diverted to Uranus and Neptune.

Re:incredible

[Anonymous Coward]

The actual timing of that option was after the Voyager program was funded for two 4 year missions (as Mariner 10 and 11). The engineers ensured that the spacecraft were capable of 12 years missions so that the Grand Tour would be an option. The original Grand Tour option was 4 spacecraft with 2 Jupiter-Saturn-Titan flybys and 2 gas giant-ice giant tours (with one potentially being sent directly to Pluto). This was rejected for funding issues (along with the NERVA program) in favor of fully funding the Viking program. The Voyager program after much public outcry ended up with 2 cheaper probes to flyby Jupiter and Saturn with special emphasis on studying Titan. After Voyager 1 survived its cross plane maneuver and flyby of Titan, permission was granted to perform a Grand Tour with Voyager 2.

How much fuel (propellant) is left?

[wisebabo]

I've looked all over the Cassini web site and I can't seem to find out anything that says how much fuel is left. Not the plutonium in the RTGs (that should last another decade or so) but rather the (bi-propellant?) in the main fuel tanks or the (mono-propellant? hydrazine?) used in the thrusters.

Not only is the lack of thruster propellant typically what ends the life of otherwise useful satellites (Cassini is not spin stabilized so attitude correction is ultimately performed by the thrusters) but in addition to keeping Cassini pointed in the right direction, it REQUIRES a significant (compared to most space probes), delta-v capability. This is because it must be able to change its trajectory to take advantage of gravity assist from Titan, if it cannot precisely hit the "window" it is aiming for, it will be sent into a totally different (and unrecoverable) orbit. (Of course if it were not for the "space billiards" gravity assist maneuvers there is no way Cassini would have been able to achieve more than a tiny fraction of its current mission. This, to me is the most impressive and gratifying part of the entire mission).

These relatively large changes in delta-v are what Cassini needs its two main engines for (one's a backup). Unfortunately due to a mistake in design, Cassini had to unexpectedly use up about a third(?) of its propellant because it had to carry the Huygens probe into Saturn orbit with it (rather than releasing it on its inbound trajectory). This was because the engineers neglected to design the radio to handle the large doppler shift that would have occurred had Cassini whizzed by on its hyperbolic trajectory before the orbital insertion burn while Huygens slowly parachuted to Titan's surface. By decelerating Cassini into Saturn orbit, THEN releasing the probe they were able to receive MOST of the data transmitted (another goof lost one channel) so it saved that part of the mission but at the expense of fuel.

Now if the orbital planners have been very careful (are you reading this Sherman? ;) they may have been able to use less fuel than planned by being very accurate with their burns. So the question is: What will run out first? The main propellant used for orbital changes or the hydrazine used for attitude adjustments? (Also remember that Cassini doesn't have a separate instrument platform so every instrument pointing activity requires turning the entire spacecraft, most of this is done with reaction wheels but there is friction and sometimes the wheels must be desaturated).

Or is something else going to run out first? (Kodak film, videotape, mailing envelopes, postage stamps ;) Or am I completely wrong about this? (Perhaps they've figured out how to use Saturn's magnetic field to help stabilize the spacecraft).

I hope that after this mission extension (and the next) they'll do something really crazy like use up a ton of fuel for a really really risky low slow flyby THROUGH the rings. Imagine seeing thousands and thousands of boulder-to-mountain sized rocks in a vast plain far as the eye can see with Saturn looming in the background! Just like the paintings by Chesley Bonestell (and lots of science fiction shows like Voyager). Good science too.