First, it is easy to penetrate the atmosphere of course, if you don’t care about the speed. As you say the problem is to enter it slowly. It would be possible to do this too, but it would take a HUGE amount of fuel. The basic reason is that to stay in low Earth orbit, in “free fall”, a spacecraft has to travel at a high speed, at least 7.8 km / second, or about 17,500 mph in any atmosphere skimming orbit.
When a spacecraft is near to Earth, it falls towards it continuously, just as a ball does when you throw it. If you throw something at thousands of miles per hour, from above the atmosphere, gravity still pulls it down in the same way that it does for a ball, but it travels so fast that the gravity of Earth just pulls it into an curve all the way around the Earth and back to its starting point. That’s basically what an orbit is, and is how satellites such as the ISS stay in orbit.
If you slow your spacecraft down below that 17,500 mph, even by a small amount, say a few hundred mph, it will fall too far towards Earth before it completes its orbit and will hit the atmosphere at thousands of miles per hour, usually half an orbit later.
Example to show how only a small change in velocity is needed to de-orbit: the Soyuz TMA 17M mission on its return to Earth on December 11, 2015, started from an altitude of 416.7 km, and re-entered with a change in velocity of 128 meters per second or 286 miles per hour.
The Earth, with its atmosphere, is rotating in the same direction as satellites orbit. However, that only helps a little, as the surface of the Earth moves eastward at around 1,000 mph which may sound a lot, but it is not much compared to the re-entry speed of 17,500 mph.
To avoid such a fast re-entry into the atmosphere, the spacecraft needs to do two things at once. It needs to slow down and at the same time, it needs to apply thrust away from Earth to counteract the effect of gravity. If you can imagine some device with unlimited fuel, then it would be easy for a spacecraft to slow to a hover above the atmosphere. It would need to thrust continually at 1 g upwards to maintain position - and then it could lower itself down to the surface, as slowly or as quickly as the pilot wishes.
However, a one g acceleration takes a lot of fuel. To carry enough to slowly lower a reasonable sized payload to Earth, your spaceship would need to be nearly as large as the rockets that take our spacecraft into orbit. It would make an enormous difference, if we could use fuel with a high power density, such as antimatter, which lets you convert matter directly into energy, or perhaps use nuclear fusion, or whatever. Then, we could do it easily. But we don’t have such fuels yet. So is there any other way to do it?
SPACE ELEVATOR
We could lower a spacecraft slowly if we had a space elevator. This is basically a giant lift, with the top of it way above geostationary orbit, attached to the surface of Earth near the equator. It is held in place, and tensioned, by a counterweight above geostationary orbit. This video gives an idea how it works - and it also explains orbits at the beginning.
https://youtu.be/qPQQwqGWktEIf we had that, your spaceship could dock with it at geostationary orbit. That would be easy, so long as you make sure that your orbit takes you around the earth in the same direction that it rotates, once every 24 hours. That way you are hovering above the same point on Earth all the time in your orbit, so will be stationary relative to the top of the elevator.
Once you’ve done that, then you would get out of your spacecraft, or attach your spacecraft to a lift, and then just slowly travel down the elevator at, say, 200 mph, or whatever speed you find comfortable and safe, until you reach the Earth’s surface. You’d feel gravity gradually increase from zero g to full g as you descend.
The space elevator is a mega engineering project, with many issues to sort out before it can be built. However as well as that, at the moment there is one issue that is a deal breaker for it. It requires cables able to hold up their own weight for distances of thousands of kilometers.
We don’t quite have that technology yet. Our best cables can hold up hundreds of kilometers of their own weight, which is rather remarkable, but not quite good enough. Carbon nanotubes are strong enough in theory, but if there is just a single atom out of place they won’t work. So far, our technology isn’t up to the task of making thousands of kilometers long perfect carbon nanotubes with not an atom out of place. Other materials also just aren’t quite strong enough yet. So it remains an idea at present, although some enthusiasts think it may be practical in the near future, possibly even in the very near future a decade or two away. Arthur C. Clarke’s science fiction story The Fountains of Paradise is based around this idea.
ROTOVATOR
You could also use a rotovator - this diagram shows the idea
Cycloid - zortig (wikipedia)
The line there represents half of a tether. A space tether should actually span the full diameter of the circle, but it’s the same idea.
As the tether orbits Earth, it rotates, and if you arrange the rotation rate carefully you can make the tip closest to Earth stationary. That could be used to take a spacecraft traveling at faster than orbital velocity, e.g. in a transfer orbit from the Moon, and slow it down so it is stationary, momentarily hovering above the Earth’s atmosphere. Once you’ve done that then you can let it go and it falls down vertically. In the other direction, you could use the same approach to launch a payload to orbit from a position stationary in the atmosphere (for instance, attached to a balloon).
However we don’t have the materials to do that yet. It’s the same problem as the space elevator. It needs a thousands of kilometers long tether, which would be too long to hold its own weight under the artificial gravity generated during the spinning motion. Until we have those materials, we need to use a “watered down version” but it could still be enough to make a difference. Normally, you need to travel at about Mach 20-25 to go into low Earth orbit (depending on how high the orbit is). The launch assist tether reduces that to Mach 12 or less. For more details, see Launch Assist Tethers. You could use the same process in reverse to de-orbit a spacecraft to Mach 12 in the upper atmosphere, and then it glides down from there.
REBOOSTING THE TETHER AFTER THE GRAVITATIONAL ASSIST
The tether loses speed, every time it is used to boost a payload into orbit, dropping into a lower orbit, very time it does one of these gravitational assists. But it can get back into position between the gravity assists using solar power and then sending electric current along the tether, using the Earth's magnetic field for a motor to accelerate back into orbit.
Or, rather elegantly, it can also do it by de-orbiting a similar payload (spacecraft returning to Earth or materials exported to Earth from the Moon or asteroids). This was fully worked out in a plan called the Hypersonic Airplane Space Tether Orbital Launch (HASTOL) System.
Hoyt has suggested that we could use the same approach for exports from the Moon. Our materials are already easily strong enough to make a rotovator which can be stationary relative to the Moon each time it briefly touches the lunar surface.
If we had two such tethers, one orbiting the Moon and one orbiting the Earth, then the whole thing could be powered by movement of materials from the Moon to Earth, since the Moon is high in the Earth’s gravitational well. It’s rather like the way movement of water downhill powers a water wheel. See Exporting materials from the Moon. This system is not nearly as massive as a space elevator, massing less than thirty times the mass of a typical payload, so there is much less construction needed. Those thirty payloads are few enough so that it would soon pay for itself, if there was need for frequent transport to and from the Moon. In his scheme the Earth rotovator just slows down payloads to LEO, but the materials could also be returned to a HASTOL type tether and into the Earth’s atmosphere.
TRADITIONAL ROCKET
Or you could use a large rocket refueled from orbit of course, to slowly lower your spacecraft through the atmosphere. This would require a lot of fuel though. It’s not like the lunar module.
It’s easy to carry enough fuel to land on the Moon, in a slow controlled de-orbit. The lunar module descent stage had a total mass of 15,200 tons, and of that, 8.355 tons was propellant (these figures varied depending on the mission). So around 45% was payload, including as the payload there, the lander itself, the ascent stage and its fuel, and the crew.
Apollo 11 lunar module. The fuel for the landing was only 55% of its total weight.
Apollo 9’s Spider’s ascent stage seen from below, showing the ascent nozzle (this was a test in LEO as you can see from the Earth behind in the photo)
The lunar module’s ascent stage had a total mass of 4,780 kg, of that crew was 144 kg and the propellant was 2375 kg. The fuel amounted to less than 50% of its total weight. A little under a quarter of the original lunar landing returned from the surface to orbit.
If Earth had as little gravity as the Moon it would be easy to get into orbit and back again and we wouldn’t need to use the atmosphere at all. But we need rather a lot more. Though the gravity is only six times greater on Earth, we need far more than six times the amount of fuel because of the way rockets work. For every few tons of fuel at the end of the journey, you may many extra tons early on, which is just fuel to accelerate fuel.
By way of example only 4% of the Saturn V was payload, so 96% of the launch mass is either burnt or discarded on the way to orbit. For the Ariane V the payload fraction is 2.5%, and it’s similar (slightly less) for the Soyuz 2 used to launch the crewed Soyuz MS.
For the space shuttle, only 1% was payload because most of the mass put into orbit was the shuttle itself which was returned to Earth. For more on this, see The Tyranny of the Rocket Equation and for some more example figures, this Payload fraction table
So, if we had no atmosphere, you’d need something as large as the Soyuz rocket already in orbit, to get a crew of three back to Earth, because it takes about the same amount of fuel to get something into orbit as to de-orbit it. That’s 312 tons total mass to launch a payload of 7.08 tons for the new Soyuz MS with the crew of three. You would have to keep launching those 7.08 ton loads until you have 312 tons in orbit before you have enough fuel in orbit to return your crew of three safely. That would take around 44 launches. It would take six launches of a Falcon Heavy - which is not quite ready yet, but which will be able to send 54.4 tons to LEO. It would take three launches of the Space Launch System once ready.
You could do it more easily with the Saturn V. This had a payload to orbit of 140,000 kg (after boosts in payload capacity for the last two missions). So three launches would be more than enough at least in terms of the total mass.
Back in the 1960s NASA studied even larger rockets, the NOVA, with the eye to a mission to Mars. They would be able to send hundreds of tons into LEO.
Nova - studied from 1959 to 1962. Finally cancelled 1964. Figures show payload to LEO in metric tons. Image © Mark Wade
Rockets designed for a humans to Mars mission such as the Saturn V-4X(U), but never built, could have sent 527,600 kg (1,163,100 lb) to a 486 km orbit at 28.00 degrees. That would be much more than enough to send the mass of a Soyuz 2 fully loaded with fuel + payload to LEO in a single launch.
So, it’s not impossible, but this would make things tough for anyone living on a planet with Earth gravity and no atmosphere or very little atmosphere to slow down spaceships for re-entry, but not impossible. They could send robotic spacecraft into orbit easily but returning their citizens from orbit would be tough. It’s no wonder that we use our atmosphere to slow down our spaceships for re-entry.
One thing that could change all this however is if you don’t have to carry the fuel on board the spaceship. If it is beamed to the spaceship from elsewhere, say from Earth, then you don’t need to use fuel to carry more fuel, and then it becomes more feasible to land by hovering in the atmosphere.
FLYING TO ORBIT AND BACK ON A BEAM OF LIGHT
This is a neat idea, but so far has only been tried in small scale demos, raising models a hundred feet or so on laser beams
Who knows, it might become the standard way to get into space some time in the future, but is a long way from achieving that potential right now. A related idea is a mixed system with a laser or microwave system supplying energy, and fuel on the rocket. See Laser Propulsion Could Beam Rockets into Space, and Jordin Kare's talks to the Space Show.
If you can send a spacecraft into orbit that way you can also return it from orbit the same way if you want to. However we don’t have any spacecraft yet that can do this.
USING THE ATMOSPHERE AS A BRAKE - SPACE SHUTTLE
This is the way it is done today, we to use the upper atmosphere as a brake, then it slowly parachutes to the surface or glides down in the lower atmosphere. How easy that is depends on the spacecraft.
If it is a heavy one like the Space Shuttle then it can only slow down deep in the atmosphere where it is dense, and so it gets very hot. That’s why the space shuttle had to have ceramic tiles able to withstand temperatures up to 3000 °F (1,650 °C)
Space shuttle Enterprise - high density, can only slow down in the lower atmosphere gets very hot
LOWER TEMPERATURE REENTRY - SKYLON
Skylon is a plane being developed by the British company Reaction Engines with funding from the UK government and ESA. It will be able to fly to orbit from a conventional runway (though reinforced to carry the extra weight of all the fuel needed), return back to Earth, and then take off again within a couple of days with a crew of 200 to assist.
Its design is much lower in density than the space shuttle, once it has used up its fuel to get into orbit, so it slows down in the atmosphere at higher altitude on the way down.
What really matters is the mass per cross sectional area it presents to the atmosphere or more exactly, it’s Ballistic coefficient - Skylon could slow down even higher in the atmosphere if it presented a large blunt face like an aeroshell, but it has to be streamlined for the later stages of its flight. However it is also able to compensate for that to some extent by steering during the early part of the flight to slow down more quickly.
Skylon (future design being developed by UK / ESA). It flies to orbit from a normal length runway, reinforced to take the weight of fuel on lift off and may fly in the 2020s - once it has used up most of its fuel, during the landing it is low density and so slows down much higher in the atmosphere than the space shuttle
As a result, it will reach lower temperatures than the space shuttle on re-entry though higher than a supersonic jet at Mach 3. Here are a few figures for skin temperatures for comparison, hottest first, these are the figures for the hottest parts of the spacecraft or plane:
SKYLON’S ZEPELLIN-LIKE TRUSS CONSTRUCTION, WITH A REINFORCED GLASS CERAMIC AEROSHELL
Modern planes have “stressed skin” structures, where all, or most of the external load from the wings, tail, other stabilizing structures and heavy components such as the engine are taken up by the skin itself (See Fuselage for details). But the Skylon uses a structure much more like a zeppelin or small plane; girder-like with a thin silicon carbide reinforced glass ceramic aeroshell. It’s outermost shell is just a heat resistant covering and doesn’t take any stress.
Structure of the Skylon - internal truss framework made from carbon fibre reinforced plastic composite held together with kevlar ties. It has aluminium propellant tanks suspended inside it. Covering that, it has a thin outer aeroshell of a high temperature SiC fibre reinforced glass ceramic material. For details see page 2 of this report
This ceramic outer skin is black, which is why Skylon is shown as black in most of the artist renderings.
This is an animation to show the concept for a mission to orbit, and back, by Reaction Engines who developed the idea. Re-entry starts about seven minutes into the video
https://youtu.be/3H7BeyQOnPkVERY LOW TEMPERATURE RE-ENTRY - ORBITAL AIRSHIPS
This approach of reducing the density of the spacecraft to reduce its re-entry temperature is taken much further with the orbital airship idea in the plans of JP Aerospace.
This is very low density, in a kilometer scale airship filled mainly with hydrogen. It’s not only lower density than a plane, and the Skylon; it’s also much lower density than a normal airship. It only ever operates above 200,000 feet and is balanced for the upper atmosphere. It also has a huge cross section which it presents to the atmosphere.
This spaceship design consists of a near vacuum of hydrogen floating in a near vacuum of normal air. If they succeed in building it, then it will be able to slow down already, just through friction in the upper atmosphere not far below where the ISS is. By the time it gets to the levels of the atmosphere, where it is dense enough to heat the skin up significantly it’s already slowed down hugely so temperature of the skin is much less of a problem.
JP Aerospace orbital airship - kilometer scale, very very low density - this has the least temperature of all during re-entry, If it works out, it would be a very low cost way to get cargo to orbit, and passengers too. It would be a leisurely journey as you would get there slowly over several days. It would cost even less than the space elevator and has much less development cost.
On the way up it gradually accelerates to supersonic speeds, then to hypersonic speeds (by which time it is already in a near vacuum). It has solar panels over the upper surface to generate power, and uses these to power ion thrusters. These let you accelerate with low mass for the fuel, and very high exhaust velocity, so long as you have plenty of power, as it would have with such a large are of solar panels.
ORBITAL AIRSHIP CONSTRUCTION
It has no internal girders. The outer shell covers an interior consisting of many large bags of hydrogen to give it rigidity and to stop the hydrogen bunching up at its nose, and inflatable trusses, with nitrogen filling the gaps in between these components. The nitrogen is vented if necessary and then replaced from liquid nitrogen tanks.
You might think it’s impossible for a airship to go hypersonic. But this huge V shape is designed to be aerodynamic at hypersonic speeds in the upper atmosphere. To test this they have done the modeling and calculations and wind tunnel tests with scale models. They think they will be able to do it,
On the way down, the orbital airship is balanced to float at 200,000 feet altitude in the atmosphere. But since it is aerodynamic, it also behaves like a glider as they decelerate. So as the atmosphere catches it, it slows down - but doesn't fall as much as you'd think because it’s in a very long glide. It doesn't look much like a glider to our eyes perhaps, but that big voluminous V shape makes a great glider in the very tenuous upper atmosphere during re-entry.
So what keeps it up is partly aerodynamic lift and partly buoyancy. To start with it’s mainly aerodynamic, and as it slows down on its long glide through the upper atmosphere then finally it’s held up by buoyancy. The aerodynamic effects keeps it higher in the atmosphere for longer, and so keep it cooler on the way down.
Many details of the design are given in their Floating to Space: The Airship to Orbit Program They don’t actually give expected skin temperatures. But the design uses nylon rip-stop polyethylene (page 111) which suggests that they expect external skin temperatures well below 100 °C (212 °F) for continuous use.
(Most commercial grade Polyethylene starts to soften at 60 °C (140 °F) and has a maximum continuous use temperature of 65 °C (149 °F), High Temperature Polyethylene can retain its properties up to 100 °C (212 °F) )
On page 109 they say
"By losing velocity before it reaches the lower thicker atmosphere, the reentry temperatures are radically lower.... This makes reentry as safe as the climb to orbit"
It’s an interesting company - in Sacramento, California, JP Aerospace, America's OTHER Space Program. Their idea is that they don’t do any big expensive succeed or go bankrupt tests like SpaceX did in their early years. Instead every stage along the way pays for itself. At present they pay for the tests through pongsats and other ways to lift material to the edge of space. Later on they plan a “dark sky” station at the edge of space which will be of a lot of interest for itself both scientifically and for tourists. Next, they plan small airships doing test glides back to Earth, and then thrusting to orbit, then the first human pilots to orbit, and then huge orbital airships with passengers and cargo.
It’s probably going to take them a fair while, maybe decades but it’s interesting: “watch this spot”.
You might wonder what happens if the airship is hit by a meteorite or orbital debris. From page 112 of the book:
"One of the most common questions asked about ATO is about meteorites. "What happens if a meteor popped the airship?" The answer is very little would happen. A balloon pops because the inside is at a higher pressure than the air on the outside. The inner cells of the airship are "zero pressure balloons". ... There is no difference in pressure to create a bursting force. All a meteorite would do is to make a hole. The gas would leak out staggeringly slowly... "
To find out more about this see their book Floating to Space: The Airship to Orbit Program
The JP Aerospace airships can only descend to a height of 200,000 feet. They are so lightweight they could never survive at ground level. The slightest wind would tear them apart. So in their plan, they have conventional airships that take passengers up to a docking station in the upper atmosphere, which they call the “Dark Sky Station” because at that level the daytime sky is black, where they then transfer to the orbital airships.
ATMOSPHERIC RE-ENTRY FOR TITAN AND VENUS
There are two other places in our solar system with thick atmospheres like Earth, Venus, and Saturn’s moon Titan. Mars also has a very thin atmosphere. Then the gas giants have thick atmospheres too.
JP Aerospace hope the same idea can be used for Venus, with a high altitude staging post again. Perhaps they could use it for Mars too. The atmosphere of Mars is so thin that you could land an orbital airship like this on the surface. The strongest winds on Mars would only barely move an autumn leaf, fast though they are.
VAMP ORBITAL AIRSHIP RE-ENTRY - FOR VENUS AND TITAN, ALSO EARTH
If you want to fly all the way down to ground level on Earth in one go, then you need a more massive airship. Northrop group’s “VAMP” project to study the Venus atmosphere uses an airship design like the JP Aerospace, and they would inflate it outside of the atmosphere, so again that’s very like the JP Aerospace idea. It enters the Venus atmosphere already inflated, and because it is so large (55 meters in diameter) and low density, it doesn’t need an aeroshell.
However, unlike the JP Aerospace design, it’s able to fly in an Earth pressure atmosphere, so it’s not nearly as low density as an orbital airship. It still gets quite hot during the descent.
https://youtu.be/0EjgoEFiKkoIt inflates before it enters the atmosphere (see Patent for details), and as for the JP Aerospace idea it decelerates slowly in the upper atmosphere, so generating much less heat, because of its low ballistic coefficient. So it doesn’t need an aeroshell, though its outer envelope is reinforced to withstand up to 1200 °C (2192 °F) along leading edges
They hope it can be used for Venus, and also Titan, possibly Mars.
It would only descend as far as the Venus upper atmosphere, at the cloud tops, where temperatures and pressures are the same as for Earth. The cloud tops have natural protection from cosmic radiation, and nearly all the ingredients for life, indeed there are suggestions that it could be a good place for humans to settle outside of Earth. See my Will We Build Colonies That Float Over Venus Like Buckminster Fuller's "Cloud Nine"?
The first tests of VAMP would use the Earth’s atmosphere. So it could also be used for Earth re-entry. It might be useful for surveillance, photographing the Earth from above, and also for scientific studies of the upper atmosphere.
The same ideas could also be used for Titan - a moon of Jupiter with an extremely cold atmosphere at -180 °C, but it’s also dense, with the pressure as Earth’s. This means that humans could go out of doors there without needing a pressurized spacesuit, though of course they would need protection from the extreme cold and they would need air to breathe. See Let's Colonize Titan.
VAMP flying over Titan to sample and explore the upper atmosphere - Titans atmosphere is similar in density to Earth’s though much colder, so you have similar methods for re-entry for Titan and for Earth. Though it’s gravity is much less - indeed a human falling from a plane or aerostat on Titan would easily survive the landing without a parachute.
AEROSHELL - HIGH TEMPERATURES
The normal way to re-enter Earth’s atmosphere at present is to use an aeroshell which absorbs most of the heat, all the way through the early stages of re-entry, until the spacecraft is traveling slowly enough to drop the aeroshell and deploy parachutes. The spacecraft hits the atmosphere at many kilometers per second, so there is a lot of heat to dissipate. They have to use various techniques to get rid of it to keep the temperatures within reasonable bounds. The main methods are
So the temperatures reached depend on how effective these thermal protection systems are. Temperatures still typically reach of the order of 2000 °C upwards (well over 3500 °F). It was much more challenging for the Apollo return from the Moon, as their re-entry was at a higher velocity.
Artist’s rendering for Apollo command module re-entry. Temperatures reached 5000 °F on the outside of the capsule, or around 2760 °C. How the Apollo Spacecraft Worked (and old Apollo Flight Tests fact sheet )
Re-entry speeds are
Some materials can withstand even higher temperatures easily, for instance Hafnium diboride melts at 3,250 °C (5882 °F). It is a useful material as it also has good thermal and electrical conductivity. It’s a grey metallic looking material. It’s used for ICBM re-entry shields and leading edges. For more about it se Hafnium DiBoride (HfB2). Titanium and zirconium diboride have similar properties
TECHY DETAILS ABOUT ORBITS
You might wonder why the astronauts returned at such a high speed when they returned from the Moon. After all the Moon orbits the Earth at a velocity of only 1.02 km / sec. A satellite or third stage skimming the upper atmosphere at 180 km height has a velocity of 7.8 km / sec, yet, when returning from the Moon they hit the atmosphere at 11 km / sec. How does that work?
Let’s try this out for the ISS instead, which orbits at a height of between 400 and 410 km travels at a slightly slower speed of 7.66 km / sec (see online calculator here to work it out). It used to orbit at a height of 350 km, but in 2011 it was moved to a higher orbit to save fuel.
You might think that this higher and slower orbit mean that a spacecraft leaving the ISS could enter the atmosphere a little more slowly, but it actually works the other way around, to get into an elliptical orbit to transfer from a 180 km orbit to the ISS requires an extra 67 meters per second, so in the other direction if you come back from the ISS and were to target a 180 km high orbit, you have an extra 67 meters per second when you hit the atmosphere (in practice they would drop more delta v than that). If you come back in a transfer from the Moon you hit the atmosphere at 11.1 km / second. There’s an online transfer orbit calculator here.
So starting from a higher orbit just makes things worse for you. However, there is one thing you can do to help with re-entry speed, and that is, to orbit the Earth in the same direction that it spins. The Earth’s surface (and so its atmosphere too) moves at 460 meters per second, or about 1,000 miles per hour) because of its rotation. So if your satellite is orbiting in the same direction as the Earth in an equatorial orbit, then it has 0.92 km / sec less delta v relative to the atmosphere than if it orbits in the opposite direction to the Earth’s rotation. This makes re-entry just a little easier.
An orbit in the same direction as the Earth’s rotation also makes the launch easier. You need around 0.92 km / sec less delta v to get to orbit if you launch in the same direction as the Earth’s rotation, i.e. from west to east (it’s spinning towards the rising sun).
That’s quite a huge saving in fuel, which is why all the US launches are from Florida and why all the Soyuz launches from Russia are from West to East too
Shows the direction of the launches of Soyuz from West to East
This is why it was such a major gaff for the Gravity film when it showed all the orbital debris orbiting Earth from East to West, as Neil deGrasse Tyson tweeted.
BALLUTE - INFLATABLE AEROSHELL
But you can achieve a much gentler re-entry using a ballute - a cross between a balloon and a parachute. It works like an aeroshell but decelerates much higher in the atmosphere
In the first ever re-entry test of a ballute by the ESA for instance, the maximum re-entry temperature on its skin was 200 °C (392 °F).
Measured temperature reached a maximum of 200 °C (392 °F).
BALLOON RETURN FROM ORBIT
Then there’s an idea from 1966 to return a human being from orbit, in an emergency, using a balloon to dissipate most of the heat, though the seat for the astronaut helps as an additional aeroshell. So it combines some of the approaches of the previous ideas. It’s never actually been tested in space.
The space engineers in the early 1960s explored many other such ideas detailed here: Rescue. Some seem rather hair-raising including the Paracone - the astronaut just sits in a seat, with their back towards the Earth, and aims towards the center of a large continent, as its margin of error is 600 kilometers. When it comes to re-entry then a large inflatable aeroshell deploys which has a crushable cone. There is no parachute - it relies on the aeroshell crushing during landing to protect the astronaut.
Paracone. The astronaut has an inflatable aeroshell stowed away in the seat. During re-entry this deploys. They have no parachute - the aeroshell has a crushable cone which protects them during impact at the terminal velocity of 42 km / hour - a bit like a deliberate slow car crash into a brick wall with a shock absorbing crumple zone.
For more on these ideas see also Robert Frost's answer to Is it possible to upgrade space suits so that they can survive re-entry and then parachute down like Felix Baumgartner?
PARAGLIDING FROM ORBIT
This is another idea originally developed for Gemini in the early 1960s. For a while, before they settled on the familiar parachutes, the engineers thought that after the fiery stage of re-entry, the capsules would descend hung beneath a parasail or paraglider. Those tests were quite promising, though they ran into many issues, for instance getting it to unfold. Eventually this line of research ended in 1964, when they changed to the parachutes idea as used by Apollo and ever since. For details see: Coming Home
Anyway at around the same time, in 1960, the engineers came up with the idea of using the paraglider approach to go all the way from orbit, without an aeroshell. This was the inflatable paraglider (Rogallo wing), called “FIRST” (Fabrication of Inflatable Re-entry Structures for Test)
It could be folded up into a small cylindrical package which would be docked to a space station, much as our modern Soyuz TMA is. In an emergency, the crew enter this cylinder, and separate. The paraglider then inflates and deploys. It would re-enter at an angle of 1 degree, with the paraglider angle of attack of 70 degrees. They found that the deceleration would not exceed two g’s, and that there would be minimal heating because of the way it glides down to the surface. It would approach the speed of sound at 43 km, and from there it would be able to glide 345 km before eventually landing. Details here: FIRST Re-Entry Glider
CHANGING SHAPE FOR RE-ENTRY
The Spaceship-One uses a different idea for re-entry. It changes the position of its wings to the “feather” position which also tilts the spacecraft so that it presents as much surface area as possible so slowing it down. This is only for a sub-orbital hop at present. The first demonstration of the feather system was in 2011
https://youtu.be/Lqhzlq7UReARETURNING THE FINAL STAGE OF A CONVENTIONAL ROCKET
What about returning a final stage? That also is light weight and it presents quite a large cross section compared to the weight if you fly it backwards, rocket motors first, with supersonic retropropulsion.
First, some background. Every time a spaceship goes into orbit, it needs a final stage, a thin container full of fuel which is burnt to get it into orbit. It has to do that, because the spaceship itself is far too small to have enough fuel to get to orbit by itself ,even with the help of the first stage.
It then discards the final stage, which normally orbits Earth a few times and finally falls back to Earth (though with missions to the Moon or to Mars or other interplanetary missions the final stage has to boost the spaceship so much, it may end up in interplanetary space, for instance nearly every mission to Mars also sends a final stage in the general direction of Mars too).
So, could a final stage be reused in the same way that SpaceX re-used the Falcon first stage? Well it’s easier to return than the Space Shuttle, because it’s got so little mass (after it has burnt nearly all the fuel), and quite a blunt cross section when descending rocket motors first. When SpaceX returns the first stage of the Falcon 9, it slows down partly through friction in the upper atmosphere. The landing legs alone reduce its terminal velocity by a factor of two. It also has a burn in orbit and another burn just before it reaches the barge. You can see the first stage at the beginning and end of this movie (most of it is for the second stage). But it only has to shed one kilometer per second of delta v, and much of that is done with the two burns.
https://www.youtube.com/watch?v=XdtK0poWp20This could be done with a second stage too. Elon Musk has said he plans to re-use the final stage in the future, though it’s probably not going to work for the current Falcon 9. Here he answers the question: “Any plans for a reusable second stage?”
https://youtu.be/-pQ8M9mTw0M?t=782“The next generation vehicles after the Falcon architecture will be designed for full reusability.
I don't expect the Falcon 9 to have a reusable upper stage, just because with a kerosene-based system, the specific impulse isn't really high enough to do that, and a lot of the missions we do for commercial satellite deployment are geostationary missions. So, we're really going very far out. These are high delta-velocity missions, so to try to get something back from that is really difficult. But, with the next generation of vehicles, which is going to be a sub cooled methane oxygen system where the propellants are cooled closed to their freezing temperature to reduce their density, we could definitely do full reusability”
The basic idea of all these designs is that the less mass per cross sectional area presented to the atmosphere during re-entry (or more precisely, the lower the ballistic coefficient), then the higher it is in the atmosphere when it slows down, and so the lower the temperature of its skin during re-entry.
If the spaceship can glide to stay high in the atmosphere, this helps. It also helps if it can use retropropulsion to reduce its velocity before it enters the atmosphere and as it descends. Then, as in the case of an aeroshell, if it can radiate or absorb heat or ablate, or (perhaps in the future) use active cooling, this also helps.
