Cassini Probe Will Dive Through Enceladus's Water Jets (nasa.gov)

An anonymous reader writes: NASA's Cassini probe has a daring mission tomorrow: dive through the water jets spraying from the south pole of Saturn's moon Enceladus. The probe will be a mere 30 miles above the surface, traveling at a relative speed of 19,000 mph. Researchers hope to gain insight into the chemical composition of the jets. "[T]he plumes are more than just gas and water: samples show that they also contain many of the building blocks essential to Earth-like life. This lends itself to the exciting possibility that organisms similar to those that thrive in our own deep oceans near volcanic vents exuding carbon dioxide and hydrogen sulfide might exist on Eceladus." The molecules suspended among the water may tell us whether Enceladus's oceans are capable of harboring life. "The spacecraft's sensors will pick up gases in the plume searching for the presence of molecular hydrogen (H2). The amount of H2 found could reveal how much hydrothermal activity is occurring in the ocean."

Comments

Hmm

[BigSlowTarget]

Cool science but it sounds like "Hold my beer and watch this" on a planetary scale.

Re:Hmm

[Anonymous Coward]

It sort of is, but Cassini only has a year or so left in its mission before it is out of propellant needed for adjusting its orbit around Saturn. From here on, missions will get riskier, finishing with the Grand Finale, where it orbits between Saturn and the innermost ring a couple dozen times, before it plunges into the planet to keep from possibly contaminating Enceladus

Re:Hmm

[nospam007]

"It sort of is, but Cassini only has a year or so left in its mission before it is out of propellant "

That's why they drive it through the probe wash. Clean probes bring more money with aliens.

Re: Hmm

[Muad'Dave]

Being Plutonium-powered, the remnants of the probe (even if reduced to its atoms) will leave an indelible* mark on Saturn that someone with nuclear technology was here.

*The half-life of Pu-238 is 75k years and doesn't occur naturally. Its daughter product, U-234, is only 55ppm in naturally-occurring Uranium (at least here on Earth) and has a half-life of 246k years. U-234 decays to Th-230, and its half-life is another 75k years.

Re: Hmm

[GrumpySteen]

You don't seem to understand what Saturn is. With wind speeds up to 1800 km/h and almost no solid surface to speak of, the remnants of the probe will be dispersed among the 80+ trillion cubic kilometers of gas and liquids that make up the bulk of what we call Saturn. The amount of plutonium aboard the Cassini probe would be rapidly spread out so much that it won't be detectable within a few weeks, which is hardly indelible.

Re: Hmm

[Muad'Dave]

You may be right. My thinking was this: The windspeeds may be high, but that doesn't preclude the probe's nuclear fuel from surviving relatively intact. First, the RTG is hardened against damage, including that of re-entry. Second, absolute windspeed isn't the issue, it's the windshear over the size of the RTG. I didn't take into account the creamy liquid center, so that's an issue. I assumed that the core was far enough down that the probe would 'float' in the dense gas.

Re: Hmm

[Lumpy]

Earth high speed reentry is like a down filled pillow compared to conditions just 1000 miles down in Saturn's gravity well.

Re: Hmm

[calque]

I assumed that the core was far enough down that the probe would 'float' in the dense gas.

Even when hydrogen is compressed to millions of atmospheres, plutonium is still hundreds of times denser. So it seems to me that the remnants would keep sinking until they hit the rocky core and not float or disperse. That would be very, very deep indeed, since the core is thought to only be about twice the size of Earth.

Rings

[Anonymous Coward]

This is great. They're incurring a degree of risk to investigate Enceladus.

My dream is that one day they'll risk Cassini to get a better look at this.

Re:Rings

[dotancohen]

My dream is that one day they'll risk Cassini to get a better look at this.

It's too late. That photo was from when Saturn was at equinox, 2007 or so, and therefore the Sun cast long shadows at it's equator (the ring plane orbits Saturn above the equator). Saturn orbits the Sun about once every 30 years, so you've got 15 years between equinoxes (equinoii?). That means the next opportunity to catch those moonlets' shadows at their longest, and to have the leading faces properly illuminated, will be in 2022 or thereabout. Cassini won't last that long.

kinetic energy

[Rinikusu]

How much kinetic energy is in a drop of water at 19k mph?

Re:kinetic energy

[Anonymous Coward]

At "pharmacist drop" of 20 drops per milliliter its mass is 50mg
19000mph is 8493.76m/s
The kinetic energy is 1803J.

Energy of a typical NATO rifle round, 5.56x45 mm is 1796J

Re:kinetic energy

[Rei]

But of course these aren't like Earth "water drops", they're more like frozen dust grains.

Still, potentially destructive.

There's interesting potential for future missions (almost assuredly ion-powered) to do Enceladus sample return by doing flybys with a carbon collector (better than the silicon aerogel used by Stardust). But for that you have other options than just doing a straight flyby - you could enter a highly elliptical orbit around Enceladus with the apogee - or the ascent - positioned over the geysers. The latter may be better because even if you had no relative velocity, the ice grains still have significant speed of their own, even at Enceladus escape. - so by ascending relative to Enceladus, you're further reducing (to a limited degree) the relative velocity at impact (some grains move slower than others, of course, you'll likely have some collisions in any circumstance at almost zero relative velocity). The craft could sit in orbit for a very long period of time, even years, until its collector is basically saturated, before sealing it, breaking orbit (little energy required if you're already in a highly elongated orbit), thrust-and-gravity-boost your way out of Saturn via flybys of other moons (giving additional science in the process), and return to Earth.

Regardless of the form, an Enceladus sample-collection mission wouldn't have to be limited to Enceladus. Other Saturnian moons have at times presented evidence of lesser cryovolcanism, and Saturns' many different rings themselves could prove to be potential collection targets if mission planners felt confident enough about the safety of the trajectory.

Cassini Probe Will Dive Through ... Water Jets

[grep -v '.*' *]

I guess I didn't realize that satellites would come with an umbrella option. Windshield wipers are standard I guess?

Slashdot

[wonkey_monkey]

molecular hydrogen (H2)

Okay, so you think Unicode is too hard. But why can't we even have <sub> and <super>?

And yet we can have <code>...

Electric Geysers

[trout007]

The EU guys have an interesting take on this. Will be cool to see how charged this geyser is.
https://m.youtube.com/watch?v=...

Contamination

[MichaelSmith]

There might be life on Ecelandus, and after this close pass, there might be life on the Cassini probe too. So when cassini reaches end of life, it will most likely be crashed in to something, probably Saturn, but does this risk transferring organisms from Ecelandus to Saturn?

Re:Contamination

[Jason Levine]

There might be life on Ecelandus, and after this close pass, there might be life on the Cassini probe too. So when cassini reaches end of life, it will most likely be crashed in to something, probably Saturn, but does this risk transferring organisms from Ecelandus to Saturn?

Organisms might be transferred to Saturn, but given the conditions there they wouldn't be alive for long.

Re:Contamination

[beernutmark]

I am thinking more about the risk of earth organisms being rinsed off Cassini and falling back to Ecelandus during the pass. I wonder what the risks of that are.

Re:Contamination

[Anonymous Coward]

Sorry, but you can't possibly know that.

30 miles up ..at 19,000 km/hr... Saturn Descending

[fygment]

is 30 miles even within the margin of error for the trajectory calculations?

just seems like an extreme risk for a fully functioning probe ... UNLESS ... it's all been an elaborate hoax that JPL is tired of perpetrating so to end it, the probe will be 'lost' in a crash on the moon. The graphics artists who have been involved in the ruse will be 'taken care of', the file will be closed ... until it's discovered one of the artists somehow survived and has decided to go public ... Tom Cruise starring role in Saturn Descending

But contaminating Saturn is ok?

[Viol8]

Seems to me there's just as much chance of interesting chemistry possibly leading to some form of life happening deep down in a gas giant as there is on a small moon.

Re:But contaminating Saturn is ok?

[MightyYar]

Pretty nasty conditions for life on Saturn

Re:But contaminating Saturn is ok?

[Viol8]

We have no idea what the limitations for self reproducing biochemistry are. I so no physical or chemical reason some form of it couldn't exist further down in the cloud deck.

Re:But contaminating Saturn is ok?

[necro81]

Consider the end scenarios:

1) Cassini crashes into Enceladus. Because it has no atmosphere to speak of and a solid surface, the spacecraft will impact on the ice and make a real mess. Fragments of the spacecraft may survive, more or less in the condition that they left Earth (although much older), including the plutonium RTGs. Eventually, these may work their way through the ice and into the subsurface ocean, contaminating a fairly interesting environment (the ocean-ice interface and the ocean-crust interface).

2) Cassini crashes into Titan. Because there is a significant atmosphere, Cassini will burn up to some extent, but some of it, surely, will survive re-entry, distributed over a large area, and thump into the surface. Due to the thick atmosphere and low gravity, the terminal velocity is quite modest (slower than Earth's), so any bits of Cassini that survive re-entry will have a pretty soft landing. This, too, is contamination of a fairly interesting environment (the surface-atmosphere interface, or in the hydrocarbon lakes).

3) Cassini is intentionally de-orbited into Saturn. Saturn is basically all atmosphere and has no surface to speak of: it'll burn up pretty much all the way down, eventually floating in the deepest parts of the planet that are especially dense enough so that even metals are buoyant. These deep reaches are also really hot, which will at least kill anything still alive or viable on the spacecraft, and probably just melt everything in some extreme chemistry. Compared to permanently scattering the spacecraft across a moon, the amount of time Cassini passes through the various layers of Saturn before reaching its hot death is quite brief. Finally, Saturn is the 2nd most massive planet in the solar system, 10^3 -to- 10^6 times the size of its moons, so any contamination from Cassini will be much more diluted.

So, considering that getting Cassini out of the Saturn system is not possible, tossing it into Saturn itself seems the best option.

Re:But contaminating Saturn is ok?

[Viol8]

"tossing it into Saturn itself seems the best option."

I would have thought putting it in a parking orbit would have been the best option. That way it could still be used.

Re:But contaminating Saturn is ok?

[MightyYar]

Engineering is all about tradeoffs. Sure, the ideal would be to not crash the probe into any planet. Given what we know, steering into the least-likely place to have life is the least bad decision. What is your proposal?

Re:But contaminating Saturn is ok?

[Coren22]

Elements other than hydrogen and helium might be a good thing. Saturn doesn't have much in the way of heavier elements, so how exactly would it work to have life without heavy elements?

Re:But contaminating Saturn is ok?

[Lumpy]

Pls the fact that the radiation from saturn on the moons there is pretty freaking nasty. Living inside a 1000watt microwave would be more enjoyable.

Re:But contaminating Saturn is ok?

[MightyYar]

I'm sure it has plenty of fuel for that...

Re:But contaminating Saturn is ok?

[calque]

By "not much", you mean not much in terms of percentages. But Saturn is kind of large in an absolute sense.

Re:But contaminating Saturn is ok?

[MightyYar]

They have some fuel right now, but not in 2 years after they've done many more experiments and maneuvers. The tradeoff would be to do less science.

I'm wondering what kind of life you are imagining on Saturn that can withstand the intense radiation there, but cannot handle a foreign body slipping deep into the interior until it is melted by heat and pressure, then rapidly dispersing in the intense winds of the planet. I'm also going to remind you that we already fired the other half of this probe into Saturn - so the damage is done.

Re:But contaminating Saturn is ok?

[MightyYar]

"What do we want to do with the probe after the science program has ended? And lets make the plans general enough in case the program end date changes."

I'm not sure how that changes the argument. It's still an engineering trade off between more science vs putting it into a permanent, safe orbit somewhere.

Also, the 5-20 m/s delta-v for going into heliocentric orbit would be virtually the same as the delta-v used to hit Saturn

That may very well be true, but that's not what they are doing here. They already used the fuel to get closer to the moon. They are depending on another close encounter to sling the probe into Saturn. It may be true that they could sling it off in another direction, but I'm not sure they could guarantee that it would not eventually hit something else that could harbor life. Saturn almost certainly does not, and if it does it will almost certainly not be harmed by a probe that will effectively melt away in a part of the planet that could not support any kind of life because of radiation, heat, and pressure.

I am not imagining any kind of life on Saturn.

So then why are you arguing? If one is concerned about Earthly contamination of life somewhere, using the last bit of fuel to slam into Saturn is a pretty safe bet. What purpose would slinging it into an uncontrollable orbit around the sun serve?

No, the Huygens probe was fired at Titan, not into Saturn.

Yes, my mistake. The point remains - the damage is done.

Should you be posting with such confidence about the mission constraints if you aren't even familiar with the mission in general?

Why would you be wasting your time arguing with me if I'm such an idiot?

Re:But contaminating Saturn is ok?

[MightyYar]

No, it is an argument about cost vs. science,

Engineers care about costs. Costs are an engineering problem. We're saying the same thing.

Because, as already clearly stated (again...), I'm simply saying you were wrong about fuel being an issue.

Please re-read the thread. When I made the snarky "I'm sure it has enough fuel for that..." comment, I was replying to a post that suggested crashing it into the sun or sending into deep space. I stand by my statement - it does not have the fuel to do either of those things, unless "sending it into deep space" is defined so widely as to include orbits around the sun that will bring it back from deep space. This probe is stuck in the solar system.

Re:30 miles up ..at 19,000 km/hr... Saturn Descend

[Big Hairy Ian]

It's the only way NASA could give it a jet wash

I bet ya $10

[NominalLoss]

... a trout hits the windshield.

Jumping off a Tall Bridge

[Anonymous Coward]

I seem to remember reading somewhere that what kills most people who jump off tall bridges is hitting the water, its sort of like wet concrete at high speeds. If these are really water jets and not jets of water mist then hitting them with anything traveling at 19,000 mph ( ~8.5 km per second in real numbers) just might cause a bit of a splat. Sort like a giant, high tech bug hitting a windshield.

Re:Jumping off a Tall Bridge

[tehcyder]

I seem to remember reading somewhere that what kills most people who jump off tall bridges is hitting the water

Well yes, what else would it be?

Re:This is just corporate welfare

[B33rNinj4]

Neither does building roads, or maintaining bridges and dams. Still, science is worth the minimal costs involved, because we should always be actively trying to learn.

They should probe...

[Anonymous Coward]

They should probe Uranus!

Re:This is just corporate welfare

[tehcyder]

And?

This is about knowledge, the single most important thing in all of existence. If you don't like learning, you might as well kill yourself now because there is no point to your life.

But it's a bit difficult to care about learning when you're starving to death, don't you think?