[thedoc (OP)]

If an astronaut is stuck in space, would it be possible for them to project themselves towards Earth and re-enter the atmosphere with only a spacesuit?  Could their spacesuit handle the re-entry temperatures?
Asked by Paul Kingston, Queensland

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[thedoc (OP)]

We put this question to Dr Phil Rosenberg from the Open University, who worked on the Huygens probe:

"OK so our astronaut bails out of his rocket.  What would happen then?  Assuming he’s in orbit around the Earth, actually not a lot!  Both the astronaut and the rocket are in orbit around the Earth going at about 11km/s or 24,000m/h.  They basically both orbit together.  If the atmosphere there was a vacuum, there’s no atmosphere at all then that would mean the astronaut would stay in orbit forever.  It happens that there’s a little bit of atmosphere up there.  A tiny amount about a thousand, trillion, trillion times thinner atmosphere than there is at the surface of the Earth.  That tiny amount of atmosphere will put a tiny bit of drag on the astronaut and as time went on that drag would slow the astronaut down.  As he slowed down he would start to descend until eventually the atmosphere is thick enough that he could just fall to Earth.

Unfortunately, it’s not great news for the astronaut because the atmosphere’s so thin it would take him about a year to get slow enough that he would just fall to Earth.  Because the astronaut’s travelling so quickly, 24,000m/h, as he starts to slow down and go into the thicker atmosphere the friction caused by the drag will heat him up and burn him in the atmosphere.  In order to avoid that essentially what you’d have to do is slow yourself down from that 24,000m/h to essentially 0.  Even the shuttle can’t do that at the moment.  The shuttle has a special thermocoating on the underneath it to re-enter the atmosphere.  As it enters the atmosphere, it’s travelling at 17,000 m/h. Even the shuttle can’t slow itself down enough.  Having said that, the astronaut is a little bit lighter than the shuttle so a decent sized rocket engine will slow him down enough but unfortunately most astronauts aren’t equipped with such devices.

Unfortunately for our stranded astronaut I think it’s pretty close to impossible, or at least very difficult in our space suit that we have to survive re-entry.  I think our astronaut’s going to need some sort of escape pod or some sort of space vehicle to get back down form space to earth and survive the really difficult conditions that are involved in that."

We also spoke to Cheryl Stearns, Pilot and record-breaking skydiver:

"You could do this if you’re only going to 100-110,000 feet, that’s a project that I’m working on right now.  I’m not re-entering into the atmosphere.  You can freefall from that altitude and you’re going to get up to a speed of 900m/h but you will not feel that speed because the atmosphere is so thin.  You’d only feel maybe 150m/h of actual wind resistance against your body.  The outside layers of your suit would only be maybe 200 degrees temperature with the friction that you have in the thin air.  So yes, that is possible to do, a parachute jump from that height, but definitely not possible to do a parachute jump or exiting out of a space craft.  You’ll never be able to enter back in."

[Nobody's Confidant]

Well, in space if you project yourself in one direction, you keep going that way till you hit something.
IF your spacesuit could stand the heat, then yes I beileve you could.

[paul.fr]

Spacesuits have many functions, but i don't think one of them is to survive re-entry or temperatures that high. So i would say it was not possible.

[paul.fr]

Didn't the Russians do this with a suit packed with old clothes or other material? Sent it floating towards earth and watched it burn up.

[clemjonze]

I think that if the space suit was specially designed for protecting the wearer during re entry into Earths atmosphere, (making the space suit essentially a space craft!), It would work.  A suit made for "normal" activity, (like making repairs and stuff outside the space station), would burn up in a most interesting and excruciating way....unless your atmosphere entry speed was controlled to reduce the heat/friction associated with high speed. 
Questions: How do you land? Must you arrive on the surface of the planet alive? Does landing as a fine black powder and mixture of various gases count?

[paul.fr]

I think that if the space suit was specially designed for protecting the wearer during re entry into Earths atmosphere, (making the space suit essentially a space craft!)

i don't think so, the heat shields on the shuttle absorb heat, surely even if the suit did survive some of that heat would transfer to the wearer, and cook him.

[Dave T. Haslam]

Hey there,

I'm a recent graduate of a (sounds weird, but really recommended) pan space sector masters programme, in which we do management and business as well as science and engineering. Whether that legitimises my answer is up to you but, after reading about this in a popular off the shelf science mag, I did some digging with my spacey contacts.

Indeed there is a venture to start a new extreme sport of 'SpaceDiving'. The idea is to develop safer space suits for suborbital and orbital space tourism on the basis that it's been done from 31km back in 1960 by Captain Joe Kittinger of the USAF, he jumped from a balloon, reached a maximum of 988km/h, which gets you as warm as a supersonic aircraft, at high altitude.

But that's not answering the question because 31km is still in the stratosphere and we claim 100km to be the legal beginning of space. Plus, the balloon is buoyant in the atmosphere, not flying uber fast to achieve lift or rocket powered to attain a ballistic trajectory, so hence his velocity was relatively low. That said, space tourism will place non civil servants in fast moving vehicles and they're scared about people getting toasty, particularly a problem for atmospheric re-entry from orbital tourism.

However, the Shuttle also uses expensive high temperature resistant metals on its pointy surfaces, leading edges etc. These are too expensive to cover the full underbelly, which is why the ceramic tiles are used. These metals could be more economical for something as small as a space suit though, so, in theory, yes. Especially if active cooling is used too, whereby fluids or heat exchangers transport heat from the leading edges to the cooler parts, reducing the thermal degradation. Anyway, woo, I'm going on and on

Dave Haslam
MSc Space Studies (Science and Engineering Stream - hence I'm shady about the business case - Prev. MSc Laser Physics/BSc Physics), oh, and looking for a job
newbielink:http://www.yuwie.com/davethaslam/ [nonactive]

[another_someone]

If one is talking about a space suit that follows the contours of the human body, I would have thought the complex airflow over that suit would be fairly difficult to manage if it is happening sufficiently fast to need an ablative coat.  Furthermore, if the space diver is travelling that fast, then I would imagine it would not be possible to deploy chutes until there has been sufficient loss of velocity to not burn up the chutes on deployment, but there is still the risk of the space diver (in the absence of stabilising chutes) going into a very dangerous flat spin.

Ofcourse, the use of compact, one man, space re-entry modules, that do not follow the contours of the human body, but provide a more aerodynamically manageable shape (even if only very slightly larger that the size of the human body it encases) would be a different matter.

The only other way I could see of doing it would be to use a gargantuan chute that would prevent the buildup of sufficient descent speed to cause a heating hazard.

[chris]

So how fast would someone travel, at peak velocity, falling from space towards Earth? I had heard claims of speeds exceeding 20 miles a second. Obviously you would accelerate to a very high speed due to the thin atmosphere and then decelerate again as the atmosphere increased in density. What does everyone think?

Chris

[another_someone]

So how fast would someone travel, at peak velocity, falling from space towards Earth? I had heard claims of speeds exceeding 20 miles a second. Obviously you would accelerate to a very high speed due to the thin atmosphere and then decelerate again as the atmosphere increased in density. What does everyone think?

Chris

In the absence of aerodynamic drag, all things will accelerate at the same speed, whether they be human sized or space shuttle sized.

http://en.wikipedia.org/wiki/Heat_shield#Thermal_protection_systems

An approximate rule-of-thumb used by heat shield designers for estimating peak shock layer temperature is to assume the air temperature in kelvins to be equal to the entry speed in meters per second. For example, a spacecraft entering the atmosphere at 7.8 km/s would experience a peak shock layer temperature of 7800 K. This method of estimation is a mathematical accident and a consequence of peak heat flux for terrestrial entry typically occurring around 60 km altitude.

The only way I can see out of this is to have a parachute large enough to start providing effective drag long before the 60Km altitude mark was reached.

[paul.fr]

No idea on that one, Chris. But, the fastest freefall to date is 614 mph from 102,800 feet by Joseph Kittinger.

[another_someone]

No idea on that one, Chris. But, the fastest freefall to date is 614 mph from 102,800 feet by Joseph Kittinger.

it's been done from 31km back in 1960 by Captain Joe Kittinger of the USAF, he jumped from a balloon, reached a maximum of 988km/h, which gets you as warm as a supersonic aircraft, at high altitude.

Same numbers, just convert from imperial to metric.

But the follow on was:

31km is still in the stratosphere and we claim 100km to be the legal beginning of space.

The problem is still that 60Km barrier.

[paul.fr]

Ah, I missed your post Dave...sorry about that, and thanks for pointing it out George.

Not too sure that it really matters, to the astronaut anyway. Because he must surely burn up on reentry, knowing what speed he may travel at, may allow him to calculate how long it is until he burn up but thats one calculation he may wish not to know.

hope that hides the fact that i don't know the answer.

[Bored chemist]

I don't think a space suit on its own could do it but I think a parachutist ought to be able to survive.
The heat that you have to lose comes from the potential energy you have from being so far up. At 100Km for a 100Kg spaceman and his kit (air bottles etc) you have M *g*h = about 100*10*100000 =100MJ of energy.
Imagine a big parachute that will slow you down so it takes a million seconds to fall (OK that's absurd because it's a week or two). At that rate you would have to dissipate 100W of power as heat. People generate about that much heat metabolocally so that would be no problem.
If you make the 'chute smaller so the person falls faster the mean power you need to dissipate gets higher- the question is how much can you get away with and how slowly can you fall. Another factor is that most of the heat would be dissipated by the parachute rather than you.
Also, if you can fall 31Km and get away with it then, if you fall at the same speed, you can dissipate the same power falling from 100Km. There might be problems with heat soaking through insulation but you only need roughly 3 times the thickness and you should be OK.

[another_someone]

Also, if you can fall 31Km and get away with it then, if you fall at the same speed, you can dissipate the same power falling from 100Km. There might be problems with heat soaking through insulation but you only need roughly 3 times the thickness and you should be OK.

The problem is that the speed is dependent on your terminal velocity, which depends on your aerodynamic drag.  At 100Km altitude, there is almost zero aerodynamic drag, so there is very little upper limit on your terminal velocity.  Of itself, this is not a problem, but as you hit the denser atmosphere, and you have already gathered this massive speed as you were travelling through near vacuum, that is when you hit the heat problem.

I don't think a space suit on its own could do it but I think a parachutist ought to be able to survive.
The heat that you have to lose comes from the potential energy you have from being so far up. At 100Km for a 100Kg spaceman and his kit (air bottles etc) you have M *g*h = about 100*10*100000 =100MJ of energy.
Imagine a big parachute that will slow you down so it takes a million seconds to fall (OK that's absurd because it's a week or two). At that rate you would have to dissipate 100W of power as heat. People generate about that much heat metabolocally so that would be no problem.
If you make the 'chute smaller so the person falls faster the mean power you need to dissipate gets higher- the question is how much can you get away with and how slowly can you fall. Another factor is that most of the heat would be dissipated by the parachute rather than you.

The reason I suggested an ultra big chute is not to slow down the descent to a ridiculously slow level, but to create any meaningful drag in a near vacuum.  Once the parachutist hits denser atmosphere, then such a large chute (maybe even kilometres across) would be far too large to be useful at lower altitudes.

[Bored chemist]

I realise that it would need to be a big 'chute- particularly in the outer atmosphere. I was just pointing out that, in principle, it can be done.

[another_someone]

I realise that it would need to be a big 'chute- particularly in the outer atmosphere. I was just pointing out that, in principle, it can be done.

I was going to add, there is another issue I can see here.  Since these ultra-big chutes are not going to be usable in the lower atmosphere, the parachutist would probably have to discard it, and deploy a second chute.  Since this ultra big chute is going to be too big to burn up in the atmosphere, it holds the potential to remain a long term liability in the upper atmosphere.  How would this be addressed?  Even if one looks to have the chute disintegrate under UV light, that will still take some finite time, and you then have to concern yourself with how the breakdown products would affect the upper atmosphere.

[another_someone]

I was going to add, there is another issue I can see here.  Since these ultra-big chutes are not going to be usable in the lower atmosphere, the parachutist would probably have to discard it, and deploy a second chute.  Since this ultra big chute is going to be too big to burn up in the atmosphere, it holds the potential to remain a long term liability in the upper atmosphere.  How would this be addressed?  Even if one looks to have the chute disintegrate under UV light, that will still take some finite time, and you then have to concern yourself with how the breakdown products would affect the upper atmosphere.

OK, I was thinking about this a little more, and came up with the following scenario.

It is conceivable that as the chute reaches lower altitudes, where a chute of many kilometres in diameter would simply be too wide, rather than discarding the large chute (which, as I indicated, would represent a navigational hazard), one simply collapses the outer parts of the chute, so it becomes a very long tail, but the core region that represents most of the drag is now much smaller is diameter.

Still, I'd rather prefer to see this tested on an unmanned reentry vehicle before a human being tries it.

[duke]

ammmm.... i dont think that the astronaut cant travel in space if he got stuck there.
          and supposingly he managed to move towards earth so his spacesuite will definanety burned. you know that if only a tiny pebble will srtike his spacesuite so it will pass through it. so imagine how will his suite can tolerate this much temp. due to friction force of earth's atmosphere if an big meteorite got evaporated.


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