Landing on other planets/moons

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CalliArcale

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jamied_uk asked this question in another thread:
what about other than the moon isit posable to land on another planet such as saturn for example?

I'd like to answer that one at length without cluttering the Apollo answers thread, and in a way where he's likely to see the answer and realize it's to his question. ;-)

Celestial bodies (planets, moons, etc) where spacecraft have soft-landed (so, excluding crashes and spacecraft intended to impact at high speed):

* The Moon (Suveyors, Apollos, Lunas, Lunokhods)
* Mars (Viking 1 and 2, Soviet Mars spacecraft, Pathfinder/Sojourner, the twin Mars Exploration Rovers, Mars Phoenix Lander)
* Venus (Veneras)
* 433 Eros (NEAR Shoemaker, which was not intended to be landable, but some careful piloting did the trick)
* 25143 Itokawa (Hayabusa, sort of; the soft-lander failed, but the main spacecraft did bump the asteroid before separating again)
* Titan (Huygens)

In theory, you can land on any rocky body, and splashdown in any liquid body. The gas giants can't be landed on; they have no land, and their interiors are very exotic -- gases that become severely compressed but are too energetic to solidify or even liquify until you get extremely deep. (Many planetary scientists suspect that there is liquid metallic hydrogen at the core of Jupiter, but this is not proven and indeed not very testable at present. Experiments have at least shown it to be plausible, though.) Some targets are trickier than others. Mars should be one of the easier ones, but it's got a reputation as sort of the Bermuda Triangle of the solar system. I don't think this reputation is very fair, though; Mars has been the target of way more missions than any other body besides Earth and Moon, and it's definitely trickier to get to Mars than either Earth orbit or the Moon. Statistically, it *should* have more failures than any other body. It has an atmosphere that you can use to help slow your spacecraft, and against which you need to protect your spacecraft during descent -- that's kind of unfortunate, because it means it's got enough atmosphere to be a problem, but not enough to help you very much; parachutes don't slow a spacecraft down enough. Spacecraft end up needing both parachutes and descent motors, which makes them more complicated, which gives them more opportunities to fail catastrophically.

Mars' moon Phobos has been a target. The Soviets made two attempts to visit this tiny moon, but both failed. The Russians will make a much more ambitious attempt soon, called Phobos Grunt ("Phobos Ground"), now in the design stages. The idea is to land on Phobos, collect samples, and then return them to Earth. Phobos is easier to launch from than Mars, so it would (in theory) be easier than a Mars sample return mission.

Venus is tougher, though. Its atmosphere is definitely thick enough to slow down a probe, but its *so* thick that the probe may actually be crushed before it lands! Seriously! It's also insanely hot. You can fry eggs on the pavement on a summer day on Earth, but on Venus you could melt *lead* on the pavement. This is a serious problem for landers, because their electronics will tend to overheat rapidly. The early Veneras overheated before they even landed; in the end, the Soviets decided just to throw everything they had at the problem. Too much heat and pressure? Fine! We'll build it like a submarine! So they did; a design bureau that normally builds submarines was assigned to build the lander. They made it incredibly tough, and insulated the hell out of it. That's normally a bad thing for spacecraft; if it can't shed heat, it'll cook itself to death. But they knew that Venus would cook it even faster. So the first Veneras to land successfully and return data from the surface were heavily insulated so they wouldn't fail until they'd cooked themselves to death, rather than failing before even landing. They still didn't last long, but they returned the only data we have of the surface of Venus. Odds are we're not going to be sending a lot more landers there....

Mercury is tricky because of its position. In order to get your spacecraft to Mercury in one piece, you'll need to make sure its closing velocity is slow enough that landing isn't suicide. Mercury has much less orbital energy than the Earth, though, and your spacecraft will initially have about the same energy as Earth. The first challenge is to get rid of all of that. The MESSENGER spacecraft is currently on a long trip to Mercury. It launched August 3, 2004. A year later, it encountered Earth again, using Earth's gravity to reduce its perihelion so it could encounter Venus. Two flybys of Venus got it set up for its first Mercury flyby, which came January 14, 2008. It encountered Mercury again in 2008, and will make another flyby in September of this year. It will encounter Mercury again in 2011, and will finally have the right closing velocity to get itself captured by Mercury's gravity. A lander wouldn't have to worry about orbital insertion, necessarily, but it would have to shed a lot of energy before it could safely land, so it would have to do something similar. Another problem would be either heat or power (or perhaps both); Mercury is quite close to the Sun, and has very long days and nights.

Various asteroids and comets have been discussed as targets for a lander. The European Rosetta spacecraft is currently on course to visit Comet 67P/Churyumov-Gerasimenko in 2014. It carries a lander named Philae which will actually land on the comet's nucleus. Comets appear insubstantial, but several flyby missions to comets have revealed that they are in fact very much like asteroids, albeit generally rather icy ones. The huge clouds surrounding them only appear when they get near enough to the Sun for the ice to start boiling away.

Jupiter's large Galilean moons are often talked about as possible targets. Dreams of a "cryoprobe" that would land on Europa are often raised, but seldom get very far because there just isn't enough known about Europa yet to design them correctly. Europa is mostly made of water with a bit of rock, but since it has a substantial magnetic field and a very young and obviously active surface, it is believed that its icy crust covers a deep subsurface ocean. But how deep is the ice? And is the "ocean" actually liquid or just slushy? These are critical questions before designing a cryoprobe that can melt through the surface to deploy some kind of submarine robot to explore. The dream is to find hydrothermal vents at the bottom of Europa ocean, and maybe even marine life....

Galileo carried an atmospheric probe to Jupiter, which is the closest any spacecraft has gotten to "landing" on Jupiter. The probe was crushed to death long before it stopped falling. Years later, the Galileo orbiter followed the probe into Jupiter, deliberately deorbited to prevent the possibility of an out-of-control Galileo crashing into one of the Galilean moons (particularly Europa) and potentially seeding it with Earth life forms.

Saturn's large moons are also often discussed. Titan is the only object in the outer solar system that has had a soft-landing. Cassini carried the Huygens probe to the Saturn system, and then dropped it towards Titan. Huygens was designed primarily as an atmospheric probe -- Titan is the only moon with a significant atmosphere (pressure actually higher than Earth's). Huygens drifted on the wind, taking lots of measurements and a number of pictures, before descending all the way to the surface. Since nobody knew what Titan's surface was like (rocky? hydrocarbon ocean? what?), it was useless to try to design a lander like Viking. Instead, they focused on the atmospheric phase of the mission and then made sure that it would land gently in whatever was there, and would float if it happened to be liquid. The probe tipped on its side when it touched down on the surface of Titan, which proved to be covered in ice frozen so hard it might as well have been rock, but worn into pebbles in a very particular way -- Huygens appears to have dropped into a dry streambed, which probably fills with ethane and/or methane during flash floods in a monsoon season, similar to many dry streambeds in desert regions of the American southwest.

So far, that's it for actual soft-landings on other worlds. (And the Galileo probe is a bit of a stretch.) Soft-landing is tricky, and the probes tend not to be as versatile as orbiters. So space agencies usually only send them if they have something very specific in mind. It's certainly possible to land on other worlds. But landing on the gas giants, such as Saturn, is something that cannot be done with current technology.
 
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Payloadcontroller

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Very nice, Calli. You have great detail in there, hon.

Yes, it's me. :mrgreen:
 
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dangineer

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Sombody mentioned this in another thread, but another important thing about landing on other planets is the distance required to get there. This goes more into engineering than science, but is definitely something I am very interested in (as an aerospace engineer). One of the most important paramters in spacecraft design is the change in velocity, or Delta V. This tells the spacecraft designers how much fuel they will need to bring. The more Delta V required, generally the more expensive the mission.

Missions to the outer planets (beyond Mars and the asteroid belt) require more Delta V because they need to overcome the sun's gravity more than for closer planets. Also, it takes a lot longer to get to the outer planets, so the spacecraft needs to be built to last longer.

Now if you want to send people to the outer planets, that introduces a whole other set of problems...
 
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CalliArcale

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Delta V is actually also a big problem with inner solar system planets; the MESSENGER probe is having to work very hard to get into orbit around Mercury because of having to shed all of the orbital energy it got by launching off of the Earth.
 
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silylene

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CalliArcale":16vfh7rb said:
Celestial bodies (planets, moons, etc) where spacecraft have soft-landed (so, excluding crashes and spacecraft intended to impact at high speed):

* The Moon (Suveyors, Apollos, Lunas, Lunokhods)
* Mars (Viking 1 and 2, Soviet Mars spacecraft, Pathfinder/Sojourner, the twin Mars Exploration Rovers, Mars Phoenix Lander)
* Venus (Veneras)
* 433 Eros (NEAR Shoemaker, which was not intended to be landable, but some careful piloting did the trick)
* 25143 Itokawa (Hayabusa, sort of; the soft-lander failed, but the main spacecraft did bump the asteroid before separating again)
* Titan (Huygens)
...
And planet Earth, the celestial body we live on ! (which is not so easy to land on either)
 
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RachaelWhitney

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i dont understand how we cant build a cool flyer that can go anywhere and land on any type of solid land...you know like in star wars.

ok ok, i know science fiction, but seriously, how can it not be possible to make a 2 person flyer?
 
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CalliArcale

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Oh, it's totally possible (though it won't behave quite like the ones on Star Wars; those follow the law of plot rather than the laws of physics). The main problem is finding someone to pay for it. ;-)

EDIT: To elaborate a bit on the "won't behave like the ones on Star Wars", those spacecraft have imaginary propulsion systems that don't seem to require any propellants or other fuels. (And they apparently don't need bathrooms either; movies are able to dispense with certain necessities which unfortunately eat up a lot of payload margin in the real world.) Whether using Star Wars propulsion or modern chemical propulsion, you need to get adequate velocity in order to get off of a planet. There's no propulsion system currently buildable which can do it like the ones on Star Wars; we need to expend some kind of propellant. And since there aren't any SuperAmerica stations in outer space, we have to bring all the propellant for the entire trip along with us. Sure, we can use some tricks to save prop -- the interplanetary equivalent of "hypermiling". And somethings these tricks can save huge amounts of propellant, though they cost a lot in terms of time. But until someone figures out a practical method of manufacturing fuels from raw materials found on other worlds, we have to bring all the stuff with us from Earth. That's why they needed that freakin' huge Saturn V to do Apollo. Most of what they were launching wasn't people and equipment. It was the propellant needed to get them there and back again.

So really, in addition to money, there is the rather large technical problem of developing engines that won't need such an unwieldy amount of propellant to do their jobs. In theory, that's an engineering problem, and in theory, achievable. But in reality, it's going to need new technology, and there's no way to predict how much time and money will be needed to find that new tech. Somebody has to think of it first.
 
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dangineer

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Good post CalliArcale! The two main parameters when looking at launch vehicles is specific impulse and thrust. Specific impulse is pretty much the same is exhaust velocity, so we'll just deal with it that way. In order to launch into space using very little fuel, both of these need to be pretty high. The problem with current technology is that both of these are essentially inversely proportional - if you increase one, you decrease the other. So if you increase exhaust velocity in an attempt to use less fuel, generally you lose so much thrust that the spacecraft can't lift off anymore, even with the decrease in fuel. This is all because as you increase the amount of fuel going into the system, you need a lot more energy to accelerate that fuel to higher and higher speeds. If the fuel itself doesn't have enough energy, then the energy has to come from somewhere else, which increases the mass of the overall system considerably. There comes a point where once you reach a certain exhaust velocity, the system will always be too heavy, no matter how much you scale it up.

This is why there's so much research in making lighter materials and lighter power systems. This is also the reason why there is so much research in developing systems that use fuel from outside of the vehicle , or no fuel (air breathing propulsion, laser power, space elevators, etc.).
 
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matthewota

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That posting is so informative and concise that I saved it as a text file.

When spacecraft engineers design probes for landing on other planets or moons, they take what they know about the planet or moon's characteristics and engineer for it. For example, the Apollo Lunar Module's landing gear was actually overengineered for the conditions that they found after the initial landings. However, the landing gear was designed to work in 1/6G, the reduced gravity of the Moon. A full up Lunar Module set up on earth would collapse under it's own weight.

The Russians had to try many times before they got a probe to function on Venus. Earlier attempts literally melted on the way down.

The European Rosetta Probe will reach comet 67P/Churyumov-Gerasimenko in May 2014 and will release a "lander" called Philae that is designed to "land" on the comet. However, the gravity level is so weak on the nucleus that the probe will use harpoons to hard attach itself to the surface.

I have been studying the Saturnian Moon Hyperion. It is built like a sponge, with over 40% of its volume as voids. The surface gravity is small, and the crust may be too frangible for a person to navigate on it easily.

There are asteroids such as 253 Mathilde that are noting more than loose aggregates of rocks...orbiting rubble piles.
 
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EarthlingX

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I wonder why powered landing is so hard, considering that most of modern jets use some kind of vector thrusting and fly-by-wire systems, which i think, could be at least partially applied to solve this problem. Advances in robotic walking are another example that could probably be applied. OK, got it, we don't have fuel depots yet ... I m sorry, but this fuel depots really started my imagination :) (Cassini - Huygens thing, with carrier having extra fuel for lander ...)
Most of celestial objects in our Solar system, at least by number, have no atmosphere, so one would assume, landing without atmosphere braking of some sort, would be considered most general solution.
Shouldn't we be searching the easy targets first (low delta V, 'short' travel time) for what we need (mass for radiation shielding, oxygen, water, ...) ?
Send people there when we would need to oversee some more complex robotic operation like In-Situ Resource Utilization ? Develop landers with more thrust, evolve them for deeper wells as we go ?
It sounds like money and adventure ... :) :roll:
 
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CalliArcale

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EarthlingX":369qghhz said:
I wonder why powered landing is so hard, considering that most of modern jets use some kind of vector thrusting and fly-by-wire systems, which i think, could be at least partially applied to solve this problem.
This is exactly what they do. ;-) Lunar landers and Martian landers both use descent thrusters. On Earth, you can use airbreathing jet engines (which are basically rockets that use atmospheric oxygen instead of an onboard oxygen supply), propellers, or ducted fans. On the Moon or Mars, you're stuck with rockets of some kind, and controlling them is an interesting engineering challenge. But it's a fairly straightforward challenge, and of course it has been accomplished many times.

The biggest problem is that you don't know ahead-of-time what the ground looks like. That problem is getting less with the better imaging available with Mars Reconnaissance Orbiter and its new sister-ship, Lunar Reconnaissance Orbiter. These have adequate resolution to pick out most obstacles. That was not previously true. Even Mars Global Surveyor couldn't resolve boulders a meter across, and a boulder that size is certainly large enough to scotch a robotic landing, if the lander tries to land on the edge of it.

Fortunately, computers have improved tremendously in the past few decades, allowing engineers to mitigate this hazard. At the time of Viking, mission controllers had to just pick a relatively smooth area and cross their fingers. Now, it's possible to give the lander "eyes" and a brain capable of detecting and avoiding some obstacles. In the beginning, they were just programmed to maintain the proper attitude during descent. The Mars Exploration Rovers, on the other hand, were able to optically scan the terrain to maneuver away from obstacles. It's very cool, but consider this: Opportunity still wound up in a crater barely larger than the landing platform, which was extroardinarily valuable scientifically, but very nearly a disaster. If Oppy hadn't been able to climb out of the crater, the mission would've been a failure, and the descent system had neither detected the crater nor aimed for it. (It couldn't have; the landing system required the spacecraft to bounce freely and uncontrolled for quite some distance.) This is because the main objective of the descent system was to make sure the first bounce didn't hit something sharp enough to puncture the airbags. After that, it was all random.

But it did work, and that's the wave of the future for these spacecraft. More and more of the descent will be intelligent, not only keeping the spacecraft level and within the target ellipse but looking at the terrain (visually, by radar, or whatever) to choose the precise landing spot.
 
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3488

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Hi Calli,

That's very true.

Viking 1 lander on Mars was originally supposed to land near the mouth of Ares Vallis, based on Mariner 9 imagery. Viking Oribter 1 was able to reimage that area, to confirm that being a safe site, but it was deemed too rough. True, the VO1 was unable to view the individual boulders, but could ascertain that the area was not as smooth as deduced from Mariner 9 (Mariner 8 was to have orbited Mars in a polar orbit barely above the atmosphere unlike Mariner 9's more eccentric orbit with a higher Periaerion, but Mariner 8 was lost in a launch failure).

Viking 1's landing site was repositioned approx 836 KM to the NW in Chryse Planitia.

In 1997, the airbag encased Mars Pathfinder successfully landed at Viking 1's original rejected site, & continued to deploy the mini rover Sojourner & also performed many observations of the surrounding terrain, an ancient flood plain, so what goes around, comes around so they say.

Looking at the Mars Pathfinder images, it makes me wonder if Viking 1 would have safely landed at Mars Pathfinder's site. I would guess maybe 25% yes , 75% no. There are many large boulders & the ground was certainly more rolling than at Viking 1's site in Chryse Planitia.

I think even in Chryse Planitia, Viking 1 was very lucky to have successfully landed, given the fact that there sill many large rocks, the ground was not smooth & there was one large boulder only 8 metres away (Big Joe) & some dunes only a short hop away.

Viking 2 IMO had an easier secondary site in Utopia Planitia, (the original in Cydonia rejected as considered too rough) still rock strewn, but not really any large boulders & the general terrain was much smoother.

Really I think the airbag approach is the better one if the target area is thought to be either rough or steep. Phoenix Mars Lander was different as MRO HiRISE showed that area within Scandia Colles was very smooth with only a few large rocks. Phoenix images post landing certainly confirmed that, the first ever martian polar imagery & other results from ground level.

The MER's, MER A Spirit's site in Gusev Crater certainly was rocky, some rocks being sharp angular basalts, which Spirit missed fortunately (could have punctured the airbags) during the bouncing landing.

MER B Opportunity landed on terrain as smooth as a car park in Meridini Planum, apart from the final rolling when she rolled over a rock (Bounce Rock) & into a shallow crater, since named Eagle Crater.

Myself, I hope for at two Mercury landers or rovers, one at least on 1 Ceres, perhaps one each on 2 Pallas & 4 Vesta, one each at least on all four of the Jupiter Galilean moons, one each at least on Titan, Enceladus & Dione.

Andrew Brown.
 
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