Can a spaceship, say Musk's BFR, actually land on the ice surface of Titan, or Europa, or Enceladus?

It seems to me that the hot exhaust gases would make the surface melt where the rocket is trying to land, making it hard or impossible for it to stabilise on the surface. And even if it does land, the water would quickly freeze and imprison the rocket legs, making it hard or impossible for it to take off without expending too much energy.

I guess what's portrayed on the image, on Europa, is misleading...

BFR resting vertically on an icy surface with Jupiter in the background

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    Those icy bodies are very cold, the atmosphere of Titan is dense, but the other moon's is very thin. Heat transfer is very low in a thin atmosphere, the ice will not melt. The ice may sublimate partially from solid to vapor directly. – Uwe May 2 at 10:42
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    The exhaust gas is hotter then the melting point of many kinds of rock, or of the steel deck on the SpaceX drone ships. You rely on being quick enough not to heat the materials to their melting point. – Steve Linton May 2 at 12:51
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    @SteveLinton most of these bodies are small to tiny so the thrust during such landing can be quite low – jkavalik May 2 at 12:56
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    @SteveLinton The temperature is high in the combustion chamber, but because of the nozzle expansion, it can be surprisingly low at the exit plane of a vacuum-optimized engine. – Russell Borogove May 2 at 16:30
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    Not sure about the BFR's engineering details, but the Lunar Excursion Module left its legs behind when it left the moon. You don't want to go back up with anything you don't need anymore. So it wouldn't have mattered if they'd frozen on to the surface... – Oscar Bravo May 3 at 12:09
up vote 39 down vote accepted

Sorry for the length of this, but it brings up some interesting facts and possibilities.

The moons you mention, Titan, Europa, and Enceladus, are three very different places. Titan has a relatively large surface gravitational acceleration (as far as satellites go) and a very thick atmosphere; Europa has a relatively large surface gravitational acceleration and a very thin atmosphere; and Enceladus has a weak gravitational acceleration and a very thin atmosphere. This makes landing techniques different for the three.

At Titan the surface temperature is ~95 K (-180 C), and at the tropopause is ~77 K—really cold! And the surface atmospheric pressure is nearly 1.5 bars, for a mass density ~4 times that of Earth's. The gravitational acceleration is only ~1/7 of Earth's. I doubt that they would try to land on Titan with a BFR spaceship as currently envisioned. The cold, dense atmosphere makes for a tremendous convective cooling rate, so without prodigious heating many spacecraft parts would go below their allowable minimum temperatures, especially any exposed electronics and parts with lubricants. Handling the environment at Titan would require an extensive redesign. Titan's low gravity leads to a very large atmospheric scale height, the vertical distance over which the pressure changes by a factor of e. Couple that to the high surface pressure and you get an atmosphere that produces measurable aerodynamic drag nearly 1000 km above the surface (!), as Cassini and Huygens verified. When the hypersonic/supersonic deceleration phase is finished, there's still a long way to go to the surface, and that takes time. The Huygens probe took 2-1/2 hours to get down after opening its parachute, even with a change to a smaller 'chute on the way down. During that time the craft is exposed to even more intense convective cooling. That said, Titan's atmosphere and low gravity make aeronautics easy. It lets you glide most of the way down rather than having to burn precious propellants. As mentioned above, the duration of the part of the landing burn with the plumes impinging on the surface would be short enough that the amount of melting would be small. If the landing legs did indeed stick to the mostly-ice surface material, a quick blast of electrical or chemical heating on the pads would release them. There are all kinds of other options and issues to consider, such as: use of parachutes or a parafoil on the way down; use of a (really large!) balloon for the initial departure ascent so aerodynamic drag doesn't cost so much in ∆v; and use of rocket propulsion, or aerodynamics to land.

Europa's surface gravity is similar to Titan's, but its surface atmospheric pressure is 12 orders of magnitude smaller, so it's a vacuum landing. At such a low pressure the ice doesn't melt, it sublimates, going directly from solid to gas, so you can think of it as ablating. Again, the duration of the part of the landing burn with the plumes impinging on the surface would be short enough that the amount of ablation would be small. The main problem at Europa is the radiation intensity, far more intense (one to two orders of magnitude) than the Van Allen belts at Earth. The resolution of the image is too low for me to tell for sure, but it appears to show people (I assume in space suits!) with flashlights on the surface outside of the spacecraft. That's not going to happen! One other problem is the ∆V required for that mission, assuming it's not one-way. I suppose you could couple a cluster of big tanks to the BFR spaceship for the trip to Jupiter, Jupiter orbit insertion (maybe Ganymede and/or Callisto gravity assists helping there), pump-down to Europa approach (also with gravity assists), and Europa orbit insertion. Without the gravity assists the ∆V would be impossibly high, even with the auxiliary tanks. The tanks are separated and left in Europa orbit for the landing. Upon return from the surface the BFR would reconnect with the non-empty tanks for the flight back to Earth, necessarily involving more gravity assists. All this has to happen in a fairly short time or radiation spoils everything.

Enceladus is a much less demanding destination than Europa, except for doubling the heliocentric distance, which makes for long flight times (maybe in addition to the auxiliary propellant tanks you'd have some auxiliary food storage). The surface gravity is only 0.113 m/s^2, about 1/81 of Earth's, and the radiation is far more benign. Similar to the approach to Europa, upon arriving at Saturn and inserting into orbit (gravity assist or aerogravity assist from Titan?), you do a moderately ∆V-intensive pump-down to Enceladus approach and insertion into Enceladus orbit. But from there it's much easier: the total ∆V from a 100 km circular orbit to landing is only about 200 m/s. And the ice ablation situation is similar to that at Europa. Might Philae-style bouncing be a problem, landing in such low gravity with a big spacecraft? The south polar region is where all the action is, where the plumes are venting Enceladus's evaporated sea water into space, so that's the most attractive place to land. But with those plumes depositing thick layers of fluffy ice-grain material on the surface, it's hard to know what the surface topography is beneath the fluff, and that's a landing hazard.

This is a problem for all three destinations: finding the equivalent of a car parking lot to set down on. Titan would probably be the least risky in that regard, but you can't just set down anywhere. There are rugged mountain ranges, lakes and seas, river channels, etc. The only location where we know we could find a suitable landing spot is near the Huygens landing site. For Europa and Enceladus there are also rugged areas and other areas that look smooth and land-able at the resolution of the images we have. But the next level down in resolution might yield surprises, akin to what Armstrong and Aldrin found upon arriving at their Mare Tranquillitatis landing spot. And if you have fluff-filling of rugged terrain you can get other surprises, mostly unpleasant ones.

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    You can get a high resolution version of that image: livewallpaperswide.com/wp-content/uploads/2018/04/… , and yes! There are little tiny people waling around on a huge perfectly flat ice sheet (!) on Europa. Little tiny dead people. – Mark Adler May 2 at 19:44
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    I once calculated that humans behind 100 mils of Aluminum (a standard for measuring radiation effects) would get a lethal dose of radiation at Europa's distance from Jupiter in about 15 minutes. And we're not talking cancer here. We're talking direct and immediate nerve damage lethal, so stone cold dead in 15 minutes. Maybe 30 minutes with half the radiation blocked by Europa itself. They probably would have been killed long before that on their way to Europa. – Mark Adler May 2 at 19:48
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    I heard about 15 years ago that without any shielding at all (i.e., a naked human) at Europa's distance, the time to prompt lethal dose from nerve damage is similar to the time it would take exposure to the vacuum to kill you. I would guess that with the significant component of multi-Mvolt and tens-of-Mvolt particles in that radiation field, bremsstrahlung makes shielding beyond some fairly massive level essentially ineffective. – Tom Spilker May 2 at 21:09
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    Interesting about those tiny people hadn't spotted them :). It would be okay for Callisto, gets less surface ionizing radiation than Mars, also flatter than Europa which is one of the roughest objects in the solar system close up with crevasses, turned over ice bergs, probably penitentes, and the artist's impression would do just as well for Callisto which has the advantage of no planetary protection issues and has been proposed as a good site for a human base in the Jupiter system.. – Robert Walker May 3 at 10:00
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    Callisto also makes more sense as a refueling stop. Though you can get to any of the Galilean satellites via flybys with no extra fuel after Jupiter capture orbit, you can get to Callisto with fewer flybys than the others. I don't see anything at all in favour of Europa for humans, and huge negative of planetary protection, contaminating Europa's oceans with Earth life so you probably only find the life you brought there yourself. Why didn't he say it was Callisto? Because more people know where Europa is? – Robert Walker May 3 at 16:07

It's probably going to be less of a concern than you'd guess. The icy worlds of our solar system have essentially no atmosphere, so the surface materials will sublimate directly to vapor and be dispersed rather than melting and freezing the landing pads into place.

Fairly little of the surface will be disturbed to begin with. The gas expansion which occurs in a vacuum optimized rocket nozzle cools the exhaust substantially; while temperatures may exceed 3000 K in the chamber, the exhaust may be well below 1000K at the nozzle exit plane.

The exhaust gas will be relatively tenuous for the same reason, and it will disperse rapidly in vacuum, so it will only transfer a small amount of heat to the surface in the few seconds of landing.

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    The observation about vacuum-optimized nozzles is true for Europa and Enceladus, but Titan's surface pressure is nearly 1.5 times that at Earth's surface. If you try to run a vacuum-optimized nozzle in that environment without adding a lot of mass in stiffeners the nozzle skirt collapses. Even with the stiffeners the engine performance takes a hit, though with all the aerodynamic assistance available at Titan the ∆V you'd need for landing would be relatively small, and you'd just take that hit. – Tom Spilker May 2 at 19:01
  • On Titan you can simply elect not to land on ice. – Russell Borogove May 3 at 0:37
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    So far we've seen no evidence for anything other than water and organics on Titan's surface. When it formed, the silicates and metals (heavy stuff) sank to form the core, and it looks like no strong convection (cryovolcanism) has brought any of that up onto the ice crust. The organics are the result of sun-driven photochemistry of the methane (and a few other very minor species) in the upper atmosphere raining down onto the surface, "tholins" as Carl Sagan called them. At warmer temperatures most would be gas, liquid, or something akin to grease, not any better than ice to land on. – Tom Spilker May 3 at 1:35
  • The BFR will be able to take off and land on Earth, indeed he's going to start off by building a BFR before the first stage and he says it will be able to get to orbit (just) by itself. So it is rated for a full pressure atmosphere. The first stage booster increases its payload to orbit by an order of magnitude. space.stackexchange.com/a/24111/3038 – Robert Walker May 3 at 14:41
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    Agree, ice is likely to be the bulk material with the highest melting point on the Titan surface. Unless they build a launch platform of some other material. – Robert Walker May 3 at 23:20

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