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This idea came to me while playing Kerbal Space Program. I noticed that the larger my parachute was, the slower my rocket would fall back down to Kerbin. I would like to know if it is possible to create a parachute so large in the real world that it might stop all velocity, essentially making whatever is attached to it float in mid-air. Common sense is telling me "no," but I could always be wrong, and I would love some explanation behind whether or not it is possible.

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    $\begingroup$ No, because air resistance only acts when you're moving. But the terminal velocity could in principle be made arbitrarily small, I think. $\endgroup$ – Javier Nov 18 '14 at 2:19
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    $\begingroup$ I think it is called a kite. Wait, I haven't seen one large enough for a person. $\endgroup$ – LDC3 Nov 18 '14 at 2:22
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    $\begingroup$ Hot air balloon comes to mind, though it's not quite the same as a kite. $\endgroup$ – Kyle Kanos Nov 18 '14 at 2:25
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    $\begingroup$ Of course @JavierBadia's comment applies to a simple, static atmosphere. If you have any action you can take advantage of thermals. Of course, you'll want to have active steering at which point you have a soft-wing glider. I think we have a site for that: Aviation. $\endgroup$ – dmckee Nov 18 '14 at 2:58
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    $\begingroup$ Yes, a spherical parachute which encompasses the whole planet. Air cannot escape from the edges. $\endgroup$ – superluminary Nov 20 '14 at 8:15
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No. All parachutes, whether they are drag-only (round) or airfoil (rectangular) will sink. Some airflow is needed to stay inflated, and that airflow comes from the steady descent.

Whether your net descent rate is positive or negative is a different question. It is quite easy to be under a parachute and end up rising (I have done it myself), you just need an updraft in excess of your descent rate. Never lasts though, as a permanently floating parachute would violate a couple of laws of nature.

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    $\begingroup$ However, if you are very small and light, like dandelion seeds or balooning spiders, you can sometimes "parachute" for weeks! $\endgroup$ – szulat Nov 18 '14 at 10:17
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    $\begingroup$ @szulat: if you're small enough (microscopic), gravity becomes essentially irrelevant. $\endgroup$ – Michael Borgwardt Nov 18 '14 at 10:25
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    $\begingroup$ which laws of nature are violated $\endgroup$ – coburne Nov 18 '14 at 15:30
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    $\begingroup$ @coburne: What goes up must come down :) $\endgroup$ – slebetman Nov 19 '14 at 3:28
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    $\begingroup$ +1 This is the correct answer. However, while a particular updraft may not last forever, it's quite possible to find enough updrafts to stay aloft more or less indefinitely. It's also not inconceivable for a particular geographic feature or atmospheric phenomenon to produce a more-or-less persistent updraft. The heating from an active volcano, for instance. Also, the core updraft at the center of a large low pressure center can last for quite a long time. Hadley cells (due to global solar heating patterns) also lead to more-or-less persistent updrafts near the equator. $\endgroup$ – reirab Nov 19 '14 at 7:15
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It would be possible in theory, but only in a very side-thinking way: if you make a parachute so large it encapsulates the whole Earth, it will in effect act as a balloon and not fall down, due to the internal pressure of the atmosphere.

This wouldn't work in practice for obvious reasons, but maybe in Kerbal you might be able to do something like it..

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    $\begingroup$ I shouldn't be upvoting this, but I was thinking the same thing as I read the question. $\endgroup$ – Mr Lister Nov 18 '14 at 13:04
  • $\begingroup$ Even in KSP you couldn't make a parachute around the entire atmosphere. One lesson you learn from KSP is just how big the world is compared to your tiny craft. $\endgroup$ – Joe Nov 19 '14 at 21:45
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A parachute is a device specifically designed to create viscous friction. Viscous friction generates a force that:

  1. is oriented opposite to the velocity;
  2. is proportional to (a certain power of [*]) the velocity.

So the falling velocity will increase until the drag force (pointing upwards) becomes equal to the weight of the falling object (pointing downwards). This equilibrium velocity can be reduced increasing the drag, but cannot be killed completely (unless you have infinite drag) because this would kill the drag force.

If you want to stop the motion you need another force, this can be the buoyant force (but then you have an aerostat). Another possibility is to have an upward air flow, then you will be falling with respect to air being steady with respect to ground.

[*] Typically $F\propto v$ for small Reynolds number and $F \propto v^2$ for big Reynolds number

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    $\begingroup$ Surprised this answer does not have more up-votes - it is the most direct answer here and the only one specifically noting the key fact that the drag requires some (air) velocity. $\endgroup$ – Keith Nov 20 '14 at 1:56
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It could be possible if the parachute was very large, rigid, shaped like a floating object, and you started descending from the vacuum of space. In this case the parachute would float on top of the atmosphere. It's easier to visualize if you imagine the parachute being a boat and you fell into some water; the boat would float on top of the water and reduce your velocity to zero.

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  • $\begingroup$ Hmm, interesting idea, might be a good invention! $\endgroup$ – Jonathan Nov 18 '14 at 20:54
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    $\begingroup$ Actually, this has already been invented, except that it doesn't need to start from space and actually works much better at lower altitudes than at the edge of space. It's called... a balloon. :) Also, blimps and dirigibles. This principle actually works better at lower altitudes because the on which you're floating is more dense. Water, on the other hand, is not appreciably compressible, so that's why you don't float much better at lower depths in the water than you do on the surface. Also, water has surface tension that helps with floating. $\endgroup$ – reirab Nov 19 '14 at 7:07
  • $\begingroup$ @reirab is that the same? A balloon is closed and contains, eg, hot air or helium to make its overall mass less than that of the air it displaces (aka "bouyant"). I think Thomas is suggesting something with no top, which "contains" empty space in the way that a rowboat on a lake contains air. I guess the trick is to build it out of something light and strong enough, a material which is to air as wood is to water, so the shell + its internal vacuum doesn't weigh more than the air it displaces. $\endgroup$ – user3764 Nov 19 '14 at 17:23
  • $\begingroup$ @user3764 I meant more the concept of 'floating on the atmosphere' than his actual particular suggested design. Certainly, buoyancy has been used by aviation for quite a long time. In order for the 'open top' design to work, the top of the hull would have to remain outside the atmosphere, just as the top the hull of a boat has to remain above the water level. Finding a material strong enough to not collapse and light enough that it actually could float on the extremely sparse atmosphere at high altitudes would be very difficult. A balloon is much easier and accomplishes the same goal. $\endgroup$ – reirab Nov 19 '14 at 17:48
  • $\begingroup$ The atmosphere gains in pressure too gradually for you to float anything that is reasonably sized on it $\endgroup$ – Jim Nov 20 '14 at 15:10
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The closest you are going to get is a parachute large enough to slow your descent to the point where you can find lift in rising air and climb away.

They exist and are called paragliders! Strictly speaking they are still falling at 1 to 2 metres per second but rely on rising air ( thermals, ridge etc ) to 'fall' slower than a parcel of surrounding air.

Some large cumulonimbus clouds are big enough to even 'suck' up a normal parachute and I have flown under big storms that appear to be sucking so hard that a bag of cement would stay airborne!

But in still air you are stuck with air flow causing drag so movement of some sort is required.

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Without the ability to change the shape of the parachute, no. With the ability, yes - briefly.

A modern square parachute acts as a wing, producing enough lift to slow the descent of the vehicle, but it relies on forward momentum to do so and to remain inflated. If the trailing edge of the parachute is pulled down quickly, the air moving under the wing will be deflected down, generating a lot of lift for a brief moment. This is called "flaring". With the right training, under the right conditions, a parachutist can stop themselves dead in the air relative the ground. Doing this right before you hit the ground is the trick to landing on your feet.

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  • $\begingroup$ Flaring sounds suspiciously like flapping... $\endgroup$ – curiousdannii Nov 20 '14 at 7:38
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I will answer "yes" if you think out of the box for a parachute, which is a way for a person ejected from a plane to fall on the earth safely.

Theoretically, one might design a parachute with a layer of helium so as to match the parachute and person downward gravitational force at a certain height, possibly 4 km above ground so as to avoid mountains, with an upward buoyant one. The helium could be compressed and released in the parachute when pulling a cord while ejected or falling.

Of course it would be of no use as a parachute , unless one slowly emptied the helium controlling the rate of fall. (It should also have the parachute usual shape and strength as a backup in case the system did not work for some defect).

Helium is very expensive. One could ignore the explosive danger and use hydrogen , after all it could be compressed in a strong enough module not to present problems during normal flights. Such a parachute will ensure a safe landing as the person could choose the spot to land by slowly deflating the parachute/balloon and drifting to better spots.

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    $\begingroup$ It would be an aerostat in fact in this case, not a parachute. $\endgroup$ – Ruslan Nov 18 '14 at 9:19
  • $\begingroup$ @Ruslan that is why I defined Parachute, it is a method to preserve the life of the ejected pilots/passangers. It is not suppose to stay in the air , but help in the safety, which is why parachutes were invented. An aerostat serves a differnt function. $\endgroup$ – anna v Nov 18 '14 at 12:42
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Yes it is possible. The trick is to have a parachute which is large enough that it's Schwarzschild radius extends down to the object it is lifting.

Under such a circumstance, the parachute would stop ALL motion of the object it is lifting.

PS I just watched Interstellar :D

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  • $\begingroup$ I was about to upvote as this is a nice idea. Then I saw that you seem to just cite Interstellar and decided to think about it for a bit longer. Finally, have an upvote, but a smaller one than initially planned. $\endgroup$ – phresnel Nov 19 '14 at 16:34
  • $\begingroup$ @phresnel Would you prefer I cite Schwarzschild's Metric? To show that a 4 vector at the Schwarzschild Radius would be completely rotated? $\endgroup$ – Aron Nov 19 '14 at 16:54
  • $\begingroup$ I am not into this topic, but this would be cooler than citing popular science fictions :D $\endgroup$ – phresnel Nov 20 '14 at 13:35

protected by dmckee Nov 18 '14 at 19:49

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