Science Questions

Skydiving from Space

Sun, 2nd Dec 2007

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Paul Kingston, Queensland asked:

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?


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

Astronaut"OK so our astronaut bails out of his rocket.  What would happen then?  Assuming heís 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, thereís no atmosphere at all then that would mean the astronaut would stay in orbit forever.  It happens that thereís 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, itís not great news for the astronaut because the atmosphereís so thin it would take him about a year to get slow enough that he would just fall to Earth.  Because the astronautís 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 youíd have to do is slow yourself down from that 24,000m/h to essentially 0.  Even the shuttle canít 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, itís travelling at 17,000 m/h. Even the shuttle canít 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 arenít equipped with such devices.

Unfortunately for our stranded astronaut I think itís pretty close to impossible, or at least very difficult in our space suit that we have to survive re-entry.  I think our astronautís 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 youíre only going to 100-110,000 feet, thatís a project that Iím working on right now.  Iím not re-entering into the atmosphere.  You can freefall from that altitude and youíre going to get up to a speed of 900m/h but you will not feel that speed because the atmosphere is so thin.  Youíd 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.  Youíll never be able to enter back in."


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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. Rdn00b, Thu, 29th Nov 2007

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., Thu, 29th Nov 2007

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., Thu, 29th Nov 2007

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? clemjonze, Fri, 30th Nov 2007

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., Fri, 30th Nov 2007

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 :) Dave T. Haslam, Fri, 30th Nov 2007

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. another_someone, Fri, 30th Nov 2007

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 chris, Sat, 1st Dec 2007

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

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. another_someone, Sat, 1st Dec 2007

No idea on that one, Chris. But, the fastest freefall to date is 614 mph from 102,800 feet by Joseph Kittinger., Sat, 1st Dec 2007

Same numbers, just convert from imperial to metric.

But the follow on was:

The problem is still that 60Km barrier. another_someone, Sat, 1st Dec 2007

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., Sun, 2nd Dec 2007

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. Bored chemist, Sun, 2nd Dec 2007

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.

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. another_someone, Sun, 2nd Dec 2007

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. Bored chemist, Sun, 2nd Dec 2007

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, Sun, 2nd Dec 2007

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. another_someone, Tue, 11th Dec 2007

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.

(hey if something is wrong so tell me i'm new here from india)  duke, Wed, 12th Dec 2007

Hello Duke, from India, and welcome.

An astronaut that is close to the Earth will fall to Earth like anything else.  The reason why most astronauts do not fall to Earth is because they are in fact in orbit around the Earth (i.e. they are spinning around the Earth fast enough so as to compensate for the gravitational pull of the Earth), but if they stopped orbiting, they would just fall down.

Yes, a tiny pebble can go through his space suit - but it all depends upon the speed of the pebble.  The problem is that in space, because there is almost a total vacuum, things can travel very, very fast, and so can be very dangerous, even if they are small.

You are right that the space suit will not tolerate much temperature, or at least, it will not tolerate the thousands of degrees of temperature that most space vehicles re-entering the Earth's atmosphere will be subjected to.  So if the astronaut is to survive re-entry, we have to find a means to prevent him coming in so fast as to create such high temperatures, because it he is subjected to those temperatures, he will fry up very quickly.

The point is that is a total vacuum, the only way to slow things down (or speed things up) is by use of rockets; but space is not a total vacuum (actually, nowhere exists a total vacuum - many places in the universe are very close to a total vacuum, but it is physically impossible for anywhere to be an absolute vacuum).  So the question was whether a very, very, large parachute could not still be deployed in space in order to make use of the very, very, low density of matter that does exist there, in order to slow the falling astronaut down sufficiently so that he does not enter the atmosphere very fast, and so he will no burn up as he enters the atmosphere. another_someone, Wed, 12th Dec 2007

Didn't somebody jump out a balloon from a great hight, a long time ago, and didn't have any problems? turnipsock, Wed, 9th Jan 2008

Yes, and it was mentioned in the 8th post in this thread 30/11/07. Bored chemist, Wed, 9th Jan 2008

I think we discussed something like this before.
There are two lots of energy to get rid of for an astronaut in orbit. He (or she) will have KE, due to the orbital speed and PE, due to height above the surface. Both the KE and PE,  per kg are  about 3e7J at 300km altitude.
Getting rid of the initial KE is more of a problem, I think, than dealing with extra KE transferred from the PE, which builds up only gradually.
To dissipate the energy safely, without excessive heat generation, it is necessary to do it over a long time.I am surprised that 'they' don't seem to consider a very long glide for re-entry (lone astronaut or shuttle)- taking many orbits, possibly. If there is enough atmosphere to produce drag / friction / heat, then there should be enough to produce lift. This could keep the craft / person to an altitude at which there is a much lower power dissipation, compared with the Shuttle, which plunges in - brute force and ignorance - and gets white hot.
I seem to remember objections on the grounds of the problems involved with supersonic flight control but this would seem to be a 'detail' to me. You would need variable geometry and computer control but it sounds preferable to the roasting tiles method which is even more brown trouser stuff.
I guess the lone astronaut bit is a no no, although the original re-entry vehicles weren't much bigger than the astronaut, so you could regard them as just a big space suit(?).

P.S. The balloonist had no initial KE to get rid of.
lyner, Thu, 10th Jan 2008

why do we have to orbit...? cant we just get high enough above to just hang loose in space....then enter very slowly in a straight line? i guess earth would just zoom off and away right?! :-P haha Paul, Sun, 30th Aug 2015

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