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Let's say I happen to find myself outside of earth's atmosphere [right at the Kármán line 100km up] one night and don't wish to cause a fuss as I descend to my home. I am stationary in regards to the dirt beneath me. Could I inflate a transparent non-reflective balloon of sufficient size say 1 million $m^3$ with hydrogen and descend into the atmosphere without glowing or causing a sonic boom, landing in my backyard using my impeccable ballooning technique? Releasing gas as needed.

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  • $\begingroup$ I assume you're starting out at orbital velocity? $\endgroup$ – DaaaahWhoosh Feb 17 '16 at 17:48
  • $\begingroup$ No, they let me out going the speed of the earth below. $\endgroup$ – King-Ink Feb 17 '16 at 17:50
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    $\begingroup$ Worth noting: the challenge of inserting yourself into that "orbit," stationary with regard to the dirt, may be quite a large effort. Orbital speeds at that altitude are roughly 8km/s. Speed of sound in the atmosphere at 100km is well under 0.2km/s, so you're goign to have quite the breaking operation to do! $\endgroup$ – Cort Ammon Feb 17 '16 at 18:16
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    $\begingroup$ @Samuel That velocity would be in the ECI frame (orbital velocities are typically phrased in that frame). Stationary with ground would be 0km/s in the ECEF frame. If you took real good advantage of the earth's rotation at the equator, you could state that "stationary with respect to the ground" is traveling at about 0.4km/s in ECI, but that's a maximum. At higher lattitudes, that value decreases. Either way, you're looking to bleed off around 7.5km/s if you happened to be at orbital velocities before engaging in this stealthy stunt. $\endgroup$ – Cort Ammon Feb 17 '16 at 19:28
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    $\begingroup$ @Samuel I point out the orbital parameters to frame the problem. If we aren't arriving from orbit (which accounts for many scifi scenarios), it strikes me the way we got to our present predicament would dramatically shape the answers. Somehow we had enough stealth to get to our present position, with an extremely unusual velocity, undetected. What we used to get there may pay a crucial part in how we get down. $\endgroup$ – Cort Ammon Feb 17 '16 at 19:46
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You'd be better off simply using a parachute.

You're not in orbit, you're just very high up, there is a big difference.

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Because of this, you're not going fast enough, nor will you get going fast enough, to burn up while you're falling.

The highest skydive thus far was done by Alan Eustace in 2014. He detached from a balloon at over 41 km up and after falling for a rather long time, he opened his fairly standard parachute. As @2012rcampion calculated in the comments, you'll probably want to use a drogue, just like Alan did for his jump. These are commonly used for tandem parachutists for stability and slightly increased drag; they're not hard to come by.

If you're an experienced parachutist, you can make some pretty tricky landings in very small spaces. You won't need to worry about bringing along all that lifting gas and a massive balloon. Just a regular parachute.

As for the stealthiness of the entry, this is the entire goal of HALO jumps. You might call yours a VHALO (Very High Altitude, Low Opening) jump. Using similar techniques, you can make an incredibly stealthy entry into your home country. As stealthy as military research can make you.

In a typical HALO exercise, the parachutist will jump from the aircraft, free-fall for a period of time at terminal velocity, and open their parachute at a low altitude. The combination of high downward speed, minimal forward airspeed, and the use of only small amounts of metal helps to defeat radar and reduces the amount of time a parachute might be visible to ground observers, enabling a stealthy insertion.

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    $\begingroup$ @NPSF3000 In all cases you'll be falling at terminal velocity, not higher. So there will be none of the compression that causes burning up on reentry. $\endgroup$ – Samuel Feb 17 '16 at 18:49
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    $\begingroup$ @Samuel At very low atmospheric densities, terminal velocity can be several kilometers per second. I did a quick simulation, which indicates that the maximum speed if you jumped from 100 km without a drogue parachute is about 1 km/s or Mach 3.5, at around 30 km altitude. $\endgroup$ – 2012rcampion Feb 17 '16 at 19:03
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    $\begingroup$ I used a ballistic ratio of 0.002 m^2/kg, which gives me a terminal velocity of about 60 m/s at sea level. $\endgroup$ – 2012rcampion Feb 17 '16 at 19:09
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    $\begingroup$ With a 2 m^2 chute I get a ballistic ratio of 0.015 m^2/kg; which results in a top speed of around 900 m/s or Mach 3.0 at around 50 km altitude. Peak acceleration is just under 4 g's, but the jumper only exceeds 2 g's for about 25 seconds. Peak stagnation point temperature is 400 deg C, but the atmosphere is so thin up there you won't feel it through your pressure suit. $\endgroup$ – 2012rcampion Feb 17 '16 at 19:19
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    $\begingroup$ @2012rcampion The initial estimate is probably a bit low for a ballistic ratio of a human in a suit, but I don't think either case is too much of a problem. I would certainly recommend using a drogue though, the increased stability alone is worth it there. $\endgroup$ – Samuel Feb 17 '16 at 19:28
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Samuel has covered this pretty well, a stealth re-entry is just fine since you're not in orbit. It is the orbital speed of many many thousands of mph that causes most of the problems when re-entering the atmosphere.

The main problem you will have is that there is no atmosphere to slow you down for the initial parts of the fall, this means that you will not have any terminal velocity to speak of.

Fortunately the atmospheric pressure is increasing gradually so use of a correctly sized small parachute should allow you to decelerate as the atmosphere thickens, what you don't want to do is hit the atmosphere at supersonic speeds and start overheating and/or creating sonic booms.

Once you've reached the atmosphere proper ditch the drogue and do a standard free-fall to HALO insertion to reach your target in a manor as stealthy as your parachutes will allow.

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If you're descending from Earth orbit at say, 17,000 mph, your large balloon (assuming it can take the reentry forces) is still going to heat up the atmosphere into a fiery plasma as it trades velocity for heat, and it's still going to go 'BOOM' because it's travelling faster than sound. It might be a sharper 'BOOM' rather than a 'BABOOM' that a more conventional reentry vehicle would make.

If you want to make a quiet reentry, you need to slow down to below the speed of sound before entering the atmosphere. Then you can make a nice quiet landing, maybe slipping into a general aviation flight pattern. You'll need extra long skirts on your rocket exhausts, to mask the flare of the engine.

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  • $\begingroup$ I am not in orbit just hanging out at earth speed. But the second paragraph answers the question. $\endgroup$ – King-Ink Feb 17 '16 at 18:01
  • $\begingroup$ @King-Ink "Hanging out at earth speed" at 100km is not a trivial task. If you can do that, there's a good chance you already have all of the tools you need to make any stealthy descent you please $\endgroup$ – Cort Ammon Feb 17 '16 at 18:18
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    $\begingroup$ @CortAmmon it was quite the night. $\endgroup$ – King-Ink Feb 17 '16 at 18:22
  • $\begingroup$ There won't be a boom. Upper atmosphere is thin, and without powered thrust you'll experience a steady deceleration and never exceed the local speed of sound. $\endgroup$ – Sean Boddy Feb 18 '16 at 1:16

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