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There are many places in the world where hundreds of hot air balloons are flown in close proximity to each other. How do the pilots steer them away from each other to avoid mid-air collisions?

Wind blows all of them in one direction and I'm sure there are plenty of calculations done by these expert pilots prior to taking off, but I am also sure that not all the balloons will react to wind at exactly the same rate.

What happens when they are getting too close for comfort? Do they have horizontal movement control?

image: Adirondack Balloon Festival Credits: NewYorkUpstate.com

Ironically this image already has one collision in it on ground.

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    $\begingroup$ "I am also sure that not all the balloons will react to wind at exactly the same rate." Wind is just air moving, and the balloons just move with the air. If you imagine all the air to be moving North at 2 miles per hour, all the balloons will move North at 2 miles per hour. $\endgroup$ – David Schwartz Oct 10 '17 at 17:57
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    $\begingroup$ Surely if they do collide, it can't be that serious, right? $\endgroup$ – E.P. Oct 10 '17 at 18:04
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    $\begingroup$ I'm curious where you see the collision on the ground. $\endgroup$ – FreeMan Oct 10 '17 at 20:42
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    $\begingroup$ @David regardless of their surface area and mass and also some shape differences? $\endgroup$ – Hanky Panky Oct 11 '17 at 1:27
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    $\begingroup$ @DavidSchwartz I'm afraid that's very simplified. Physics won't agree to it per see. It is true that the balloon will eventually reach the speed of wind and move as fast as the wind but the time it takes to reach there is going to be different for balloons of different mass and different drag coefficient and different surface areas $\endgroup$ – Hanky Panky Oct 11 '17 at 4:34
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I grew up in a family hot air ballooning business, and while I haven't been involved in a few years, I can answer your question in two words: They don't!

Taking Off

As far as collisions go, the other answers have mostly addressed this: hot air balloons only control vertical movement, so with all the balloons experiencing the same wind, they'll move at approximately the same speed.

When taking off, we always angled the balloon so that the wind would blow from basket to envelope. In a large group, such as at festivals, we would maintain the same practice as much as possible, and have a line of balloons perpendicular to the direction of the wind. Often, this would restrict the space enough that inflating balloons would roll slightly into adjacent ones, but with a competent/strong person on the crown-line (the rope attached to the top of the balloon, used to maintain stability in the event of crosswind), the small gusts which caused localized rolling could be overcome.

In large groups, when a balloon is ready to lift off, the pilot will typically ensure nobody is going to be directly overhead, then apply more heat to lift out quickly. This prevents the basket from catching any of the envelopes of the balloons still on the ground. As mentioned, once in the air, collisions are not really a problem.

EDIT: I believe you can see two balloons on the ground but vertical which are pressed up against each other in the picture you've provided. Try looking near the middle, just to the top left. You can see that the envelopes are harmlessly pushing away from each other while the baskets are still quite far apart.

In The Air

Balloons are huge and move slowly, particularly slowly relative to each other. If two balloons are exactly level and one directly blocks the wind to the other when a gust comes, it's possible for one to suddenly gain speed. I've never seen gusts strong enough or long-lasting enough to create a speed difference of more than a couple miles per hour, so when the balloons do touch, the envelopes gently 'kiss,' then bounce back. This can, at worst, create a slow rocking of the basket.

Now, the last situation, if a pilot is unaware of another balloon directly overhead and starts going up, it could be a problem. However, pilots typically have radios and can communicate with each other if it's necessary. The pilot in the unseen balloon above would start applying heat to try to get out of the way as well as radio some (likely unkind) choice words to the pilot below him.

Landing

This is more of an after-thought, but if a pilot wants to land and another balloon is already there, the pilot will simply stay in the air a little longer to find another landing site.

Competitions

A second after-thought, but I know about 10+ years back there were a couple competitions at festivals where pilots would receive a special ribbon device and a partner-balloon. The device would mark the longest the ribbon had to extend. Each of the pilots would attach one end to their basket, then the duo who stayed the closest to each other would win a prize.

Another type of competition also occurred in which the pairs of pilots would take off about 15-30 minutes apart and the second pilot would have to try to land as close to the landing site of the first one as possible. Sometimes, this meant landing at roughly the same time and in uncomfortable proximity. Safety is always the first concern of a good pilot, though! If a pilot wasn't sure it could be done safely, he or she wouldn't do it!

Summary

As longwinded as this is, the gist of the answer is that balloon-balloon collisions aren't very serious unless the basket is involved. Typically, they move slowly enough to make it easy to avoid unwanted collisions, despite their lack of horizontal control.

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    $\begingroup$ This! This is what I was about to start a bounty for. I wished to see an answer from someone that did it for a living $\endgroup$ – Hanky Panky Oct 11 '17 at 13:14
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    $\begingroup$ And welcome to the site:) a very nice first post $\endgroup$ – Hanky Panky Oct 11 '17 at 13:44
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    $\begingroup$ Thanks! I was around hot air balloons most afternoons from the age of 5 to 15, feel free to message me if you have any other questions about ballooning or if you feel my answer could be improved! $\endgroup$ – N. P. Oct 11 '17 at 14:44
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    $\begingroup$ @HankyPanky After a certain amount of time, you can still post a bounty. One of the reasons given for bounties is to reward exceptional answers. I, myself, have done this once. $\endgroup$ – CGCampbell Oct 11 '17 at 19:52
  • $\begingroup$ You can also see very clearly in the bottom right there's two balloons still on their trailers with their envelopes touching. $\endgroup$ – Clonkex Apr 18 at 3:21
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The perspective on that photo makes it hard to tell, but it looks like many of the balloons are still on the ground. So it's not quite as chaotic as it appears.

There is no direct horizontal control. All they can do is ascend or descend to catch the preferred wind.

How do they steer those away from each other to avoid mid air collisions?

For the most part, they don't. The same wind is blowing them all, so if you launch into a clear space, it's pretty difficult for some other balloon at the same altitude to join you.

I am also sure that not all the balloons will react to wind at exactly the same rate.

Exactly? No, but pretty close. It also means that if two craft do come together, it will be at a pretty low closing speed. When they are at the same altitude, the envelopes might bump, but that shouldn't hurt anything. The danger that they want to avoid is ascending or descending into another balloon.

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    $\begingroup$ If the envelopes bump, how much hot air is displaced, to be replaced by cold (dense) air after the collision? How much altitude is lost due to this, and at what rate? Might this happen to one of the balloons involved more than the other, such that one balloon will be above the other soon after the collision? $\endgroup$ – dotancohen Oct 10 '17 at 12:11
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    $\begingroup$ @dotancohen Yes the smallest one would have the greatest displacement, and since it had less hot air volume to start with, it would lose the most hot air in your scenario, and would sink faster. $\endgroup$ – Koyovis Oct 10 '17 at 13:15
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    $\begingroup$ @dotancohen, It would depend on the relative speed, but it's not like squeezing a bottle, think more like squeezing a pillow. There can be a bit of deformation before there's a significant volume reduction. Here's a small one that has little effect. youtube.com/watch?v=z0BSR0r9Mng $\endgroup$ – BowlOfRed Oct 10 '17 at 17:29
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The balloons just float, there is no thrust so no wake etc. If there is a constant wind, all balloons have exactly the same speed. Only local phenomena can create differences in horizontal relative distances, such as:

  • Wind gusts and wind direction changes
  • Wind shear
  • Local updrafts

A local difference in wind speed will accelerate first one, then the second balloon to the new wind speed due to differences in inertia from mass. But only momentarily.

Wind velocity does change with altitude, and the best chance of a horizontal speed difference between two balloons is when there is a vertical speed difference. This was the cause of the deadliest balloon mid-air collision, the one near Alice Springs in 1989, killing 13 people. It was the result of one balloon climbing into another balloon: a difference in vertical speed initiated by the lower balloon. The company practise was for the upper balloon to give way to the rising lower balloon, which was not followed in this case.

But if I may make my choice of colliding vehicles, I would definitely choose to be in one of two horizontally colliding balloons in full flight - slowly wafting against each other.

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    $\begingroup$ Seeing straight up would be pretty hard in most balloons - How would you defend against this? clear panel in the top of the balloon? Small cameras? Or just keeping your eyes open and remembering? $\endgroup$ – Criggie Oct 10 '17 at 10:09
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    $\begingroup$ How do they give way? Just rise as fast or faster than the lower balloon all while trying to get their attention? $\endgroup$ – Brad Oct 10 '17 at 15:09
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    $\begingroup$ @Brad good point, yes i guess the only way is up. $\endgroup$ – Koyovis Oct 10 '17 at 15:14
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    $\begingroup$ You can always go up. And eventually, when you go up too far, you hit the jetstream and then you get out of the way really quickly ;-) $\endgroup$ – Cort Ammon Oct 10 '17 at 19:16
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    $\begingroup$ @CortAmmon, of course everyone has expired from hypoxia and/or hypothermia by that point, but let's not quibble over details. ;) $\endgroup$ – FreeMan Oct 10 '17 at 20:45

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