Cassini's Saturn Mission Goes Out in a Blaze of Glory | Hacker News Reader

Cassini's Saturn Mission Goes Out in a Blaze of Glory

npr.org

235 pointsokket82 comments

"It's 13-year mission to explore the strange world of Saturn went on nearly a decade longer than planned."

Another testament to NASA's approach of relying on extremely well tested, older technology.

When I was younger I didn't entirely agree with that approach (imagining what could be done with the latest & greatest camera tech etc etc). Having lived long enough to see a few of these very extended duration missions, has entirely put me in the corner of agreeing with their approach.

This is also why it's ok, more often than not, that NASA missions cost so much (a more frequent criticism these days). 20 years in space; 13 engaged in a highly functional, active mission; truly incredible.

Don't they use the latest and greatest tech that has been made spaceworthy? It's not just a matter of whether it's been tested a lot. Off-the-shelf electronics just won't work for these things, because they're not sufficiently hardened against radiation. From what I can dig up, it looks like Cassini uses 20-30MHz 16-bit CPUs, which is really not bad at all for a probe whose design started around the early 1990s.

> Don't they use the latest and greatest tech that has been made spaceworthy?

Sure, and by the end of that process (testing, construction, launch, coast, arrival), it winds up being "extremely well tested, older technology".

But that implies that being "extremely well tested, older technology" is an afterthought, not a conscious choice. In other words, if brand-new hardware were magically radiation-hardened, they wouldn't disqualify that hardware for consideration by dint of being new. Especially since newer hardware tends to be more energy-efficient than older hardware, which is a very important consideration for these missions.

And no, this isn't implying that they're going to launch a mission without any testing at all. But spacecraft are specced and designed years before they actually launch; if you want to launch a craft in 2030, you need to pick your hardware in 2025. The intervening years are plenty of time to gain assurance in the reliability of the hardware, regardless of whether that hardware was released in 2025 or 1995.

No, in 2025 you're going to be picking from the hardware that's been already approved for space flight at that point. Reduces likelihood of discovering several years into building the probe that you're going to have to redesign everything to work with a different CPU.

NASA does things this way but SpaceX explicitly does not. Rather than using 1 space hardened part, SpaceX usually uses 3 non-space hardened parts (processor, memory, bus, etc) that all do exactly the same calculations simultaneously and then compare every single answer and ignore it when one out of 3 parts disagree. This allows them to be both cheaper and much less power hungry because they operate at least 2 process nodes smaller than NASA hardware. I've seen data that says that while each individual processor will experience space related errors quite often, the whole 3 part voting system is at least as accurate as the way NASA does it.

There's also a difference between a space probe, built once and having a mission that spans years or decades, and a rocket that gets built more often and operates only in Earth orbit (especially the first stage has to worry far less about radiation flipping bits).

This isn't solely a difference between NASA and SpaceX, but also a difference in mission requirements.

It's not a SpaceX-only scheme, nor is it novel technology. It's got a long history in fighter jets, as well as modern passenger planes for example (Boeing 777).

Triple Mode Redundancy: https://en.wikipedia.org/wiki/Triple_modular_redundancy

Also, the longest life span SpaceX has ever really needed is maybe the Dragon, which is a month-long mission. Cassini has been in space for 20 years. Very different design requirements.

NASA must be well aware of the triple redundancy logic you're describing since they themselves used it on the Saturn V in the 60's.

https://en.wikipedia.org/wiki/Saturn_Launch_Vehicle_Digital_...

Was talking with a JPL engineer at a party a few weeks back, he happened to mention that current state-of-the-art is the RAD750, essentially a radiation-hardened powerpc G3.

The RAD750 might be flight proven but I wouldn't call it the 'current state-of-the-art' in radiation hardened computing. Its technology is over a decade old.

Linking my comment to a sister submission [1]:

Final Images ( choose Grand Finale )

https://saturn.jpl.nasa.gov/galleries/images/

I wonder if it got destroyed or just lost the signal due to the atmosphere. Amazing times to live.

[1] https://news.ycombinator.com/item?id=15256753

I'm not seeing a "Grand Finale". Did it move?

Blue dropdown menu on the left side under the main graphic.

Considering the final speed of the probe I don't think there's any likelihood of not breaking and burning up. The whole end of its mission was designed to do exactly that.

>75,000 MPH at atmosphere contact.

> Insulating gold-colored blankets will char and break off first, followed by Cassini's antenna, 30-foot-long magnetometer boom, and other loose or fragile parts. Carbon blocks full of plutonium-238 fuel will last the longest.

> "It will basically disintegrate ... long before we hit any real surface of Saturn," Webster said.

http://www.businessinsider.com/time-cassini-will-be-destroye...

Sounds like an explosive goodbye ;)

Watching final coverage, I didn't notice anyone mention that the crash actually happened an hour earlier - speed of light being what it is, the signal being watched taking around an hour to arrive. Spent some time trying to grok the dichotomy while waiting for "signal lost".

https://10middletonp.files.wordpress.com/2015/09/watchmen-20...

They definitely did, at least in the JPL 360deg stream (can't say if the commentary was identical across all locations, but probably not).

Pictures from these kind of NASA events always show rows and rows of people sitting at computer screens. I have always wondered why there are so many of them, and what they are doing. Obviously there is a lot of work to be done to process the data that is received, but why so many real-time people? Is it just organizational bloat?

Found this[1] answer. Basically, each of them is watching/controlling a small part or a subsystem of a such mission. Some organisational overhead is possible ( dunno, chief scientist, mission director/assistant, etc ).

[1] http://www.slate.com/blogs/quora/2014/05/25/how_do_nasa_s_mi...

They were ahead of the curve with the open office plan. I always assumed each person was responsible for paying attention to or watching some specific system or stream of data to alert any possible issue.

I'd also be fine with everyone just showing up because they worked hard and long on the project.

Look at the CERN control room shots too http://home.cern/sites/home.web.cern.ch/files/image/update-f... http://today.slac.stanford.edu/images/2008/lhc-control-room3...

There are a lot of moving parts

A nitpick would be that open office is probably pretty great for mission control/control rooms where some complex event is being coordinated as it unfolds in real time. But I don't think NASA engineers necessarily develop in such settings.

You're absolutely right I was flippant with the open office remark.

Also a lot of champagne on the back wall!

Similar open control room with lots of screens at LIGO.

I've never worked in a mission operation context, but I assume in this case it's mostly an assembling of the team as a sort of last celebration for the end of the (very successful) mission, than any major technical requirement.

I've been in the Ops Room of a UK warship (Type 45 air defence destroyer) during training. Lots (~20?) of people looking at screens, each with a different system. Plus team leaders, coordinators and senior decision makers. On the bridge more people driving and fighting the ship, and checking local seaspace for intrusuion. In the damage control centre, more guys. Propulsion? More crew.

On a complex system with multiple sensors and effectors you need a lot of people.

And with critical systems where the downside of any mistake is high cost, in life or money, it helps reduce risk if people only watch a small number of data points, which they can concentrate on fully. So, similar to your navy example, I have heard of early warning radar operators being asssigned to only 10 degrees of horizon on their screen, so you now need 18 people to cover east to west fully, but each person is completely focussed on their little task.

Each person is responsible for a different subsystem on the spacecraft, as well as supervisors like mission directors.

the netflix documentary Mission Control: The Unsung Heroes of Apollo describes the early days when someone had to invent the very concept of a "mission control center" after it became clear that astronauts were going to need a lot of realtime, outside support.

wikipedia says "One of [Christopher C.] Kraft's most important contributions to manned spaceflight would be his origination of the concept of a Mission Control Center."

https://en.wikipedia.org/wiki/Christopher_C._Kraft_Jr.

It was rewarding to have made a contribution, however small, to Cassini. I can't believe that beautiful object I last saw in the clean bay all those years ago (decked out with a jack-o-lantern face for Halloween) is gone. Good bye Cassini!

Don't miss NASA's incredible CG video of what this was like: https://www.youtube.com/watch?v=xrGAQCq9BMU

That was incredible. Really moving :'(

I'm truly sad that Cassini's gone; more than I thought I'd be.

I didn't fully realize until today that I'd been keeping an eye on it for roughly 1/4 of my life.

Amazing mission.

But I expect some of us (probably not me) will get to see it close-up for ourselves.

It isn't "gone", by any stretch.

Its images and the research directions it spawned will live on far longer than anything most of us will hope to every be able to bring into the world, or otherwise accomplish.

> Scientists [were] worried that when [Cassini] loses power, it could crash into a pristine moon, contaminating a place where we might someday search for life

Why is this so bad? Moons are pretty big compared to spacecraft, why are they so worried about the environmental damage of a single spacecraft weighing a few tons?

They are worried Earth microbes on the satellite might eventually be confused for life on the moon.

Think fifty years in the future. The Herschel probe lands on Enceladus, armed with instruments that can sample the water and test it for presence of biological life. A hit! A world-changing discovery, we've detected DNA!

...but wait, hang on a second, check the records. Isn't this really close to where we predict that Cassini crash-landed after it ran out of fuel and contact was lost in 2023? Shit. Shit shit shit. We have no idea whether this DNA is actually from the oceans of Enceladus, or if it's from a hardy bacteria that hitched a ride on Cassini all the way from Earth, and thrived in the oceans of Enceladus.

If it's thriving in Enceladus, it might be out-competing the life that's already there. We may never know, we can't know what native Enceladus life was like, or if it even existed!

They're not worried about knocking down some space-trees; they're worried about it being impossible to assess scientific results with accuracy.

> We may never know, we can't know what native Enceladus life was like, or if it even existed!

To be fair we don't even know if life on Earth is native to it in the first place or not...

True, in fact I think a dominant theory is that life came from Somewhere Else, hitching a ride on an asteroid. Another one is that it formed around the volcanic vents in the ocean.

Solar wind makes live seeding from outside the solar system significantly less likely. It's possible, but considered far less likely than you might think. On the other hand life going from Earth to Mars/other moons etc is expected.

Finding life on Saturn's moons that looks eerily related to life on Earth would provide some weight to panspermia theories. But only if we could be sure that we didn't bring it there ourselves.

Why not worrying about the same thing about Saturn though? The best thing would be to slingshot Cassini in outer space like Voyagers but I guess they wouldn't have this option here.

The moons of Saturn are small even compared to the earth. One could expect far more "wreckage" to survive the entry to a moon.

Getting into Saturn, however, is vastly different. Casssini has broken up into many smallish, white-hot pieces only to eventually merge into clouds of ammonia at -200C that are blowing at extreme velocities. Further down, there are clouds of water at 0C and then metallic liquid hydrogen. Perhaps pieces could end up there or on the rocky core? Saturn is a weird place.

How much worse than an autoclave is entry to Saturn?

Titan is actually larger than Mercury. Just something I learned yesterday while reading Cassini posts. It really puts the size of Saturn into perspective.

Mercury is also real damn small. Just a tiny bit bigger than Earth's moon.

Titan is larger, but also significantly less dense. Mercury is more massive than Titan.

Mercury is pretty much just a leftover iron core

Gas giants are a much less obvious place for native life to evolve, at least from our understanding of life. Enceladus appears to have liquid water, which makes it much more likely (still slim).

That said, Arthur C Clarke included giant city-sized lifeforms floating in the clouds of Jupiter in his 2001, 2010, 2061 series. And Isaac Asimov wrote "Victory Unintentional" in the robots series.

I think the assumption is that any onboard life would be destroyed by the descent into Saturn.

This is the key point and deserves more attention. Mission control was not concerned with space junk impacting the moons. They were concerned with biological contamination altering potential evolutionary trajectories on any of the moons. Saturn's atmosphere was deemed to be sufficient to cause the requisite friction and head to cleanse Cassini from any such potential contaminants.

With Cassini in a close orbit around Saturn, there is not enough propellant to escape once again. And it would be going much slower than the Voyagers.

So practically speaking, leaving Saturn once it got there was never really an option.

Now, if you could find a way to scoop up some ring material or gas from the planet to refuel...

This is not true. You're right that it would quite slow, but end-of-mission options were investigated that would have sent Cassini all the way to Uranus or Neptune:

http://www.sciencedirect.com/science/article/pii/S0094576512...

Yep. I forgot that the Saturn system has lots of opportunities for energy boosts from the moons.

Titan specifically (none of the other moons are big enough to have any significant effect). Even if you're aware of that, it's a pretty surprising result, since it took so much more delta-v to capture into Saturnian orbit than it would to escape (mostly because we wanted to fly Cassini to Saturn and capture it into orbit in a reasonable amount of time; the trajectories that could've been used to escape from Saturn were quite slow).

Some of the options were pretty crazy. This paper: https://arc.aiaa.org/doi/abs/10.2514/1.42893?journalCode=jsr

noted that it would've been possible to escape (in the year 2014) to Jupiter (2021), then use gravity assists to also visit Uranus (2029) and Neptune (2061), or use Jupiter or Uranus to escape the solar system. Cassini also could've used any of Io/Europa/Ganymeda/Callisto's gravity to be captured in orbit around Jupiter, or entered into orbit around Neptune using Triton. I assume the main reason none of these options were chosen was that, with limited fuel remaining and the spacecraft not expected to remain operational forever, it was seen as more valuable just to stay at Saturn until end of mission (as well as pick up a bit more science diving the probe into the atmosphere).

They actually did have this option, see

http://www.sciencedirect.com/science/article/pii/S0094576512...

However, it was evaluated as inferior to the option chosen (there was apparently no concern at all about contaminating Saturn itself); see the slide titled "EOM Options with Science Evaluation":

http://www.lpi.usra.edu/opag/march_08_meeting/presentations/...

It seems pretty unlikely that Earth life could accidentally seed any moon of Saturn, and extremely unlikely that it could out-compete life that's been adapting to that environment for eons. As in, even less likely than Earth's surface bacteria hitching a ride on probes down to volcanic vents on Earth and out-competing the life there. Maybe it would find a small niche to fill.

No matter how careful we are, if we do find life on another planetary body, genetically similar enough to be confused with present-day Earth life, the assumption should be that it is from Earth and there had been no other life. Unfortunately it leaves "are we alone?" unanswered, but it answers "can life exist beyond Earth?"

Cassini has an RTG, so a crash into something like Encaladus could potentially create a nice warm little oasis for Earth bacteria to live in. A few decades and some on the fringes of this oasis might adapt - there's believed to be liquid water there.

Don't have to out-compete if there's nothing there, and being similar to Earth life wouldn't let us rule it out - rocks from asteroid impacts are known to have travelled between Earth and Mars, so it's not outrageous for one to have travelled between Earth and Saturn.

If we find life on Enceladus that looks just like Earth life, the null hypothesis is that it came from Earth. That won't change, whether we crash a probe there or not. Finding extant life anywhere beyond Earth would still be a paradigm-shattering discovery, even if we must assume it arrived from Earth on a probe or ejecta.

The probe we send to test for life must, of course, be completely sterile to ensure it's not detecting its own hitchhikers.

> If we find life on Enceladus that looks just like Earth life, the null hypothesis is that it came from Earth.

Sure, and ruling that out is easier if we can say for certain we didn't crash an old space probe nearby.

The only way it would help is if we had some other way to rule out asteroid impact ejecta but not probe contamination. So we'd have: 1) Life on Enceladus that's 2) genetically indistinguishable from Earth life, which 3) we know didn't travel on a rock, but 4) could have travelled on a probe. We're getting into teapot-orbiting-Mars territory.

"no idea" and "impossible" is a stretch of the truth. Hyperbole.

There are lots of tools in the biologist's shed for this sort of thing, and we know how to use them (think of all the field work done anywhere on Earth, such as Antarctica... if the task of sorting life was impossible, we should defund all of that immediately).

We have an idea. It just makes it harder in the extremely remote chance that it crashes in such a way that it could contaminate an icy moon and have any passengers start growing wildly.

Personally, I think that, fundamentally and cosmically, the whole point of Gaia producing a species which can send probes to other planets is contamination. We're Gaia's reproductive system.

Shouldn't we expect life on Enceladus to be different enough to tell the difference?

I'd say when we can ensure no contamination there isn't any reason not to ensure no contamination.

What if it turned out that the DNA found was similar to ours? Now think of the difference in chances if we'd dropped Cassini on the surface or if we hadn't.

> Shouldn't we expect life on Enceladus to be different enough to tell the difference?

We should likely have as few expectations as possible. The discovery of an entirely different form of life on Enceladus would be as incredible as the discovery that life on Enceladus uses the same DNA building blocks as we do.

>contaminating a place where we might someday search for life

Should it have crashed into a moon that we later find life on (especially should the life be remarkably similar to that here on earth), whose to say we didn't send it there via Cassini in the first place? Avoiding this problem entirely seems to be the best bet, also allowing them to collect atmospheric data from Saturn on the way out.

In some cases we could still estimate a probable answer.

Biologists can separate two cryptic species of living beings placed into an species complex, just taking a look to its DNA. The mutation rate among two isolated bacteria from earth and saturn would be noticeable. Unless we discover an active saturnian's nano-turism to pass the holidays on earth's oceans, the only reasonable answer to explain two identical or almost identical bacteria in both planets is a contamination event.

Cassini's mission profile was to do progressively more dangerous activities as the time allowed. Nearing propellant exhaustion, they investigated close to the rings and the planet's upper atmosphere before destruction. Contamination prevention was only a part of the decision.

"All solar systems missions are subject to constraints designed to minimize the unintentional impacts of Mars and, when applicable, the contamination of icy satellites such as Europa, Enceladus, and Ganymede." [1], also see [2]

"The Huygens probe which landed on Titan was not sterilized as the chances of finding life were considered insignificant. From further investigations it is evident that chances for life on Titan are higher than initially thought. Although forward contamination in this case is still considered unlikely, it reinforces the need to ensure we protect extra-terrestrial chances of life." [3], also see the second page of [4]

[1] https://planetaryprotection.nasa.gov/requirements [2] https://saturn-archive.jpl.nasa.gov/faq/FAQHuygens/ [3] http://www.spacesafetymagazine.com/space-exploration/extrate... [4] http://www.esa.int/Our_Activities/Space_Science/Cassini-Huyg...

> "We don't have a gas gauge. It would be really nice if we did," Molly Bittner, a systems engineer at JPL who has worked on Cassini for the past four years, tells NPR. Instead, mission controllers had to estimate the amount of fuel used by each maneuver. And there had been lots of maneuvers since 2004.

Does anyone know why they couldn't have a way to measure the amount of fuel left? This seems like a relatively easy engineering problem from a non-expert perspective.

If you know the thrust of the engine, then you can calculate the mass of the spacecraft based on how the orbit changes given a certain engine burn. I assume the dry mass of the craft is known to high certainty, so (almost) everything else is propellant.

The conventional method is a float valve, which obviously doesn't work in space. Putting a sensor in the tank also involves drilling a hole, which compromises its strength. I think the simple answer is that they didn't need it, so they left it out and relied on highly accurate predictions of fuel usage.

True, they probably didn't need to solve the problem of directly measuring fuel in low gravity, given the thousands of other problems they needed to solve.

Apparently this is a common problem with satellites as well:

> “Traditional gauging methods tend to have wider error bands as the tank empties"

> Measuring how much fuel remains in an operating spacecraft has been a matter of educated guesswork. It is calculated either by book-keeping – estimating the fuel used on every thruster burn – or using a pressure sensor to estimate from headspace gas pressure how much liquid fuel is left. Unfortunately, book-keeping can be as much as 10% too high or too low, while the gas method consistently overestimates fuel reserves.

https://www.newscientist.com/article/dn11254-new-spacecraft-...

There's not actually gas, or a tank. Most space probes like this are powered by radioactive decay of a plutonium core.

Cassini is far too old to have ion thrusters -- and even ion thrusters use reaction mass.

There is a plutonium RTG that generates electricity. That's used for heaters and powering the science instruments and communications gear.

That's used to generate electricity. The spacecraft still use propellant when they need to generate thrust.

Orbital maneuvers are so precise that when they say 'estimate', they are probably still talking about a small margin of error.

What is the time latency between taking a photo and transmitting the complete data for it?

> "Congratulations to you all," Maize announced to applause. "It's been an incredible mission, incredible spacecraft, and you're all an incredible team."

And one final note: we have just about run out of budget on this, so back at your desks you'll each find a security guard and an empty, commemorative JPL cardboard box and if you can just fill that by 4 pm and ...

JPL actually does a pretty good job of taking care of their employees, with many projects in action at once.

I went to grad school with a JPL employee who was getting her master's degree in astronomy during the cruise part of Voyager 2's flight between Uranus and Neptune.

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