What would a human need to survive re-entry on their own?

What sort of protection would a human need to survive reentry into the atmosphere without a space ship or other vessel? Preferably with parachute and sans parachute scenarios.

By the time you finish protecting someone from re-entry, you will have basically built a little person-sized spacecraft around them. Re-entry is a technologically challenging thing to survive, and even the smallest problem can escalate quickly, as the Columbia disaster taught us only too well.

The main source of the problems with re-entry is that if you’re orbiting the earth, you’re going extremely fast. The ISS travels at just under 8 km/s, which translates into 17,224 mph, or 27,720 km/h. When landing, generally we want our sideways velocity to be as close to zero as physically possible, so we’re going to have to slow down by more than 17,000 mph.

The atmosphere itself is a pretty good set of brakes- it’s a much thicker medium to go through than space, so it will slow you down, just like walking knee-deep in water is slower going than walking on land. The trouble with using the atmosphere is that you tend to exchange your speed for heat. The force of being dragged through the air is a force of friction, and as all the air particles collide with the re-entering craft, they donate a little bit of heat with each collision. Unfortunately for a poorly protected person, the atmosphere’s friction generates so much heat that the air itself turns into a 3000F (~1650C) plasma. The job of the Space Shuttle’s heat protecting tiles is to resist this intense heat (there’s something similar on the bottom of every object that has re-entered our atmosphere). When this plasma builds up, the craft is effectively cut off of communications with the ground, since all radio waves are blocked - this is known as the plasma blackout period, and lasts for a little over ten minutes. If you’re trying to protect a person from re-entry, step one is going to be to make sure that you have surrounded your person in some seriously intense insulation to keep several thousand degree plasma from roasting them to a powder.

On top of being incredibly heat-resistant, the insulation is going to have to be incredibly resistant to cold, since we’re starting in the frigid temperatures of space. For the survival of our human, it’s not enough to have a plasma-protecting layer that doesn’t crack in the insane cold of space; our person must also be kept in the very narrow range of temperatures in which we humans are comfortable. Fortunately, temperature regulation is usually built into another piece of equipment he’s going to need - a pressurized suit. Since we’re starting in space (zero air pressure), and the descent goes through a lot of extremely thin atmosphere (very little air pressure), we need our person to be cocooned in a pressurized suit to keep the gasses in his blood from boiling from the lack of atmospheric pressure.

Even with these considerations taken care of, we can’t just wrap our unlucky space-jumper in some kind of high test, pressurized, internal temperature-regulating bubble wrap and fling them out of the International Space Station. The human body is a very delicate thing and does not handle large accelerations well, and this includes spinning. If our jumper lost even a little bit of stability as he fell through the atmosphere, he could begin to tumble. The chaotic rotation of a tumble can cause strong forces - several times the force that gravity normally exerts (abbreviated 1G). A force of 6Gs for more than a few seconds can cause even seasoned pilots to black out. At the point when our pilot has blacked out, there’s almost no hope of recovering from the tumble, and if the force on the body is not reduced quickly, your chance of death increases rapidly. Tumbling was one of the major concerns with Felix Baumgartner’s jump from 24 miles up, and he did in fact tumble for some time, but managed to pull out of it - if the tumble had continued or had been harder to escape, he would have been in serious trouble.

So now, in order to be safe, we need an aerodynamically stable pressurized plasma-proof coating for our space jumper, just to survive early re-entry. This is effectively a small craft built around our person, and we’re not even close to the ground yet.

Parachutes aren’t very useful until you get reasonably close to the ground; the air needs to be thick enough to exert a strong drag force to help you slow down when it catches in the chute. You also need to be going sufficiently slowly that the air is not being heated into a several thousand degree plasma around you. But once you get down to this level, parachutes are a fantastically useful and reliable method of slowing yourself to the point where you probably will not die upon impact with the ground. Because they’re so reliable, we have attached several of these to nearly every single object we land on a surface, which includes on Mars. The Space Shuttle had a trio of parachutes deployed upon touchdown to help it slow down, and all the Gemini & Apollo class missions had parachutes deployed before splashdown. The Soyuz capsules still land this way - it’s a tried and true method.

Trying to land without a parachute is a lot harder. The space shuttle did most of its slowing down (once it made it past the plasma stage) by gliding. The shuttle was an impressive feat of engineering; once the guided portion of re-entry was over, the pilot of the shuttle manually landed a completely unpowered craft, slowing it down to a touchdown speed of ~220 mph. The runway for the shuttle is phenomenally long (about 15000 feet) and made of high traction concrete, just to give it enough time to roll to a stop.

Other than gliding, the technology to control a landing is only now being developed. SpaceX is working on what they call the Grasshopper engine, which is meant to be able to do vertical takeoffs and landings, and just a few days ago managed to take off, lean over to one side, hover, and then safely land again. So presumably, if a person’s structured protective re-entry gear came with rockets on the bottom of it, it could control the descent and slow them down automatically to a smooth landing. This is probably a lot kinder of a landing than the Mars Pathfinder method of landing, which is to have downward facing rockets slow you most of the way down, and then surround yourself in airbags and bounce to a stop. (Here again we have the problem of the human body being a lot more delicate than the Mars rovers - the Mars rovers hit the ground going slightly over 50 mph, which caused accelerations on the rovers themselves of over 50Gs - not particularly good for a human body.)

It’s by no means a simple task to make it back down to the ground from Earth orbit. We may someday have the materials and the technology to protect someone from the many forces involved, but at the moment, it would be a plunge to certain death.

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