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.
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.
(Cassini - Huygens thing, with carrier having extra fuel for lander ...)
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