Man falls from space

[Bob in a quantum-state-of-faith]

Now here's another question:  the organisers claimed that this will help prove that  astronauts stranded in orbit can get back down without a capsule.  Hmm ... don't they think it might get a bit warm on re-entry?

Some actually talked about that very issue, on some of the other news articles I read (when searching for "what happened to the capsule").

They pointed out that this was a very early first step towards orbital re-entry in a space suit (aka Robert Heinlein's Starship Troopers). 

Coming down from orbit has two factors: 

1) your relative surface-speed is incredibly high-- that is, you are moving with respect to the surface, many times the speed of sound (you must, to remain in orbit).
2) using the atmosphere as an air-brake involves some pretty high temperatures that would have to be addressed.

Now, if the potential re-entry was fixed relative to the surface, as was the case with this jump, then the suit he used could easily be used here--provided it had enough air to last until the traveler was low enough.  Simple gravity would bring the jumper down, and the gradual thickening of the atmosphere would serve for gentle braking-- as was exactly the case here.   The stickler would be if they jumper was too high, and could possibly generate too much speed before entering the thicker, lower atmosphere-- of course, some sort of small pre-chute could easily be employed to slow the reentry with sufficient gentleness.

I see two possible ways to land from a low earth orbit, in a suit:

1) some sort of solid rocket booster, which would kill your orbital speed down to one that was manageable with drag chutes before you re-enter the atmosphere
--or--
2) some sort of ablative heat shield compound in an outer casing (as what Heinlein envisioned with his Drop Troopers).

Likely a combination of each-- rockets to slow your orbital speed to something not-orbital, allowing gravity to do it's thing and an outer casing that would melt away (taking heat with it) to slow you even more.

This experiment/stunt ignored all of that, and concentrated in the last stage of the re-entry process:  keeping someone alive in near-vacuum and ultra-low temperatures.

[Sibling Zono (anon1mat0)]

Vertical vs Horizontal.

If you are falling in a vertical pattern you spend less time on the atmosphere, but while in orbit the approach vector is closer to horizontal and at a high speed, increasing the amount of friction and therefore heat. If you change the vector (ie, lower your orbital velocity) friction should be lower.

_{(this is me in high conjecture/out of my @$$ mode )}

[Bob in a quantum-state-of-faith]

Vertical vs Horizontal.

If you are falling in a vertical pattern you spend less time on the atmosphere, but while in orbit the approach vector is closer to horizontal and at a high speed, increasing the amount of friction and therefore heat. If you change the vector (ie, lower your orbital velocity) friction should be lower.

_{(this is me in high conjecture/out of my @$$ mode )}

Velocity has a great deal to do with either approach.  Meteors hit the atmosphere at all sorts of angles, including near-vertical-- but their relative speed (with respect to the atmosphere) is very high, so they typically burn up.

Orbital velocity with respect to the ground is quite high-- many, many times the speed of sound.   But it's a balancing thing, the velocity has to be just so, or you'll change orbits, as gravity is a fixed thing (well, apart from it's inverse-square-with-distance thing, it's fixed).

Change [reduce]* your orbital velocity enough, and you literally fall out of orbit-- but, you're still going many-many times the speed of sound.   It'd take a freakin' huge booster engine to kill that velocity enough, that you'd be falling at ordinary speeds (i.e. a speed just due to acceleration of gravity for example).   Any mass put into orbit cost lots of boost-energy to get it up there:  so, instead of a giant retro-rocket thing, engineers use the atmosphere instead [for braking].

Now, imagine a Star-Trek teleporting machine, one that preserves angular momentum (i.e. the speed of the earth's surface rotating with respect to it's whole mass).   Use the machine to teleport yourself directly up-wards into the blackness of actual space.  Then, you begin to fall (after a cartoonish pause to permit you to get your bearings, and realize you're really, really high up... naturally... ) accelerating due to the effects of gravity, but also traveling horizontally with respect to the whole earth's mass-- roughly the same speed as the surface.

But, the atmosphere is (more or less) also moving at the same speed, in roughly the same direction... mostly due to friction I'm told.

So in this hypothetical case, your only speed with respect to the atmosphere is due to gravity:  how high can you go, before you have enough time to accelerate to dangerous velocities before re-entering the atmosphere?



In truth, I do not know the answer.  Earth's gravitational pull is 32 feet per second per second, and is always "on".   According to the chart above, there's enough atmosphere at 280,000 feet to burn up meteors that are going too fast.

So I would surmise you'd likely not wish to start a whole lot higher than that figure--else you'd get sufficient velocity that once you did get to 280,000 feet, you'd burn up too.

Keeping in mind that "terminal velocity" only works inside of an atmosphere... such that your acceleration due to gravity theoretically has no upper limit--and given sufficient distance under constant gravity, you could theoretically exceed even light-speed.

Obviously, the earth's gravitational well is insufficiently deep for such a stunt.  But suppose you had you a massive black hole... one the size of a galaxy, say... hmmm.


_____________

* if you increase your orbital velocity, you move to a higher orbit-- increase it enough, you fall out of orbit into space.

[Swatopluk]

Of course there is also the difference between circular and elliptical orbits (the former is one extreme of the latter). Circular orbits are elementary, elliptical ones require a good deal more math. The lowest and slowest orbit would be a circular one at ground level with about 8 km/s (5 miles per second) constant orbital speed. In an elliptical orbit speed depends on the position. At least one space probe used long ellipses for a very energy effficient braking maneuvre where it would go twice per round through the high atmosphere losing only tangential speed, so the ellipsis grew shorter each time but without losing height until the oribit had become almost circular. It took ages but consumed very little fuel.
A 'cold' orbital entry would be possible but tricky by a combination of drag (reducing speed relative to the atmosphere) and lift (reducing the speed of descent). Both would have to be adjusted over time with probably not much freedom of variation. Not something one could put into a suit, I presume.

[Sibling Zono (anon1mat0)]

One case in point, imagine we have the space elevator up to the geosynchronous orbit, and a quarter of the trip you jump from it. Drag should be a function of atmospheric density at any height, and the question is at what point terminal velocity would make you burn in the atmosphere. That same principle applies to parachutes in Mars.

[Bob in a quantum-state-of-faith]

High enough, and you'd have to worry where the moon was at-- for there's an equilibrium point betwixt the moon and the earth- but that's most of the way to the moon, so no real worry here.

As for a space elevator up to geosynchronous orbit?  That'd be high enough, I would wager, that you'd reach dangerous velocities before the atmosphere was dense enough to be effective on a normal human-sized falling body.

Of course, orbital dynamics tells us that the hypothetical elevator needn't be quite that high-- anything above the atmosphere, and centripetal forces hold it up (the force that holds water into a bucket as you swing it on a rope around your head, misnamed "centrifugal force").

In fact, once you reach a certain altitude, those forces push you up to the elevator's terminus-- the rotational effect of that elevator being fixed to the earth at one point. Obviously, a person leaping from such a point would go into orbit, instead of falling.  And now, I don't know how high that would be.  Arther C Clarke explains it all in his novel, The Fountains of Paradise and also in the novel 2061.   But, to figure it out, you'd take the rotational speed of the earth, and compute how high an orbit that would be... hmmm... geosynchronous?  So maybe I'm all wet at presuming a lower elevator .. it's been awhile since I studied this stuff, actually.

[Swatopluk]

There is also a very readable paper (or two) in the Acta Astronautica. Don't know whether it is availabe online for free. It goes into great detail about how to actually build the thing, how far it has to extend etc.
To simplify it a bit: One line gets dropped from a satellite then a robot crawls up it drawing the next strand behind it. the next one crawls behind with two more and so forth. The robots can be used as the counterweights at the upper end, allowing for a shorter toital length of line. This way reduces the failure risk significantly and also the costs, should it happen anyway. Using multiple lines (actually bands) makes the whole thing also less sensitive to damage and easier to repair.

[Bob in a quantum-state-of-faith]

Clarke surmised using carbon in the form of stranded diamond fiber in 2061.  (His novel was written prior to the discovery of nano-carbon tubes.)