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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 '19 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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Just to add some more insight here. The question of aerodynamics and size keeps being asked. It is for the most part not relevant. Balloons move slow, at the speed of the wind near the equator basically. They relatively quickly get up to speed.

During events like this it may appear to be chaos to you from one photo, but it is not, the crews are experienced, and there are organizers with launch directors who are experienced. They wont let you launch until you have a clear space. As pilot in command though it is ultimately up to you and once in the air to deal with what you see as your improves when you rise above the balloons up wind on the ground blocking your view. You are also dealing with any in front of you.

On an normal-ish day the balloons would be launched down wind balloons first to get them out of the way, you normally dont see up wind or middle unless it is a slow day and there isnt enough horizontal wind to cause collisions. the exception being balloon fiesta where three waves go at once, and you have ballons from 2/3rds and 1/3rd already up to speed when they pass over the furthest down wind row of balloons. they used to have one wave, but that took too long and limited the number of balloons in the event total. they are VERY experienced at getting balloons up we get plenty of pilot briefing instructions, new pilots simply dont go to fiesta or dont fly if concerned, likewise experienced. If it is windy you will see collisions at fiesta the balloons up wind are supposed to climb at a limited rate, no more than 200fpm to clear the downwind balloons but also not create a problem above. from the sheer volume of launches relative to issues over the years this works fine. you want envelope to envelope not envelope to basket you bounce off and keep flying, envelope to basket can rip the lower envelope and/or snag it on the upper creating an issue, but this is as rare as getting in an accident on the way to the field.

back to shape the shape of the balloon is generally irrelevant. while on the ground is when it is most relevant, special shapes twist and turn and have a hard time of it, regular shapes do not. The other time is when you pass through a shear, but you dont often have those, depends on where you are flying, ABQ yes and other places along the rockys, but other places not as much not on days you would actually fly that is. but as you pass through the shear then how it affects you is based in part on aerodynamics, but even with a regular shape you want to move through those shears quickly to avoid getting swung around. and you know they are there because you read your weather report looked at winds aloft and probably set a pi ball off (latex helium balloon) so you can visually see what the wind is doing, in an event like the above or fiesta you have essentially dozens to hundreds of other balloons up and you can see what the wind is doing to them. I have seen beer can balloons get rocked badly by going through a shear, at the same time the organizers of the event were not launching per their own rules launching into 25mph or higher winds for an event whose rules were 15 or less, closing the field immediately after everyone saw what happened to that balloon (well the navy wanted to show off so they got very hot and climbed so fast that their balloon deformed)

there are (american) football shaped balloons which are sport balloons or competition balloons their aerodynamics are not for horizontal wind issues but for vertical they are small, light and the shape is not efficient for flight compared to the typical pear shape, but can slide up and down easier to quickly get into different direction wind to fine tune direction for competitions (dropping a baggie onto a x on the ground or grabbing an item off of a tall pole)

From that picture it was probably a light wind day which creates the issue of the balloons not getting out of the way, you can see most are on the ground, the others are in the way but that is either because of light wind or "the box" that is not ABQ so it is likely not a box effect, so that is probably light wind and the folks launching were okay with mid pack launches, also that is a very small rally. ahh you tagged that as new york. on second review the picture is from a balloon which is already down wind so from that angle you often will see chaos, there are some launches from the middle but many are from the down wind end of the group. I dont see any chaos in that picture, normal small rally flight.

Short answer you dont get a license and cant fly in a rally without experience, some require 100 hours some less, all require a license (not student), that you are current, etc. How you avoid a collision is first not launching directly in front of a balloon that is in flight. Next we have a large capacity to climb and descend, obviously in a rally you dont want to just hit it and climb as now you are coming up under someone else possibly they have to climb and a chain reaction, the odds of being right under someone is quite small though likewise being right in front of someone for a direct hit during launch. With very very rare exceptions once in flight everyone is at the same speed (get to speed quickly) so you just hold your position if too close to someone then you or they climb or descend. You only have vertical movement control, sometimes the wind is different directions at different altitudes so you can climb or descend and get some more space from someone you were too close to.

I do not see any chaos nor concern in that picture have flown countless rallies like that and that picture is part of the fun/beauty of ballooning nothing to be concerned about there. (okay the fog in the background is a concern, is there more fog above or upwind, can you find an open landing spot, usually it burns off quickly as the sun continues to rise. With how that looks in that picture I wouldnt have any concerns about fog). Another thing to think about is small rallies like this usually have the same pilots year after year, even across the whole country you recognize many of the same balloons, I have never been to that rally but recognize and have flown with a number of those balloons. Its your friends it is not strangers, you often fly the same rallies every year in different towns the same folks so you know each other, you know who the crazy pilot is but also know his/her habits. All of these factors are taken into account, it is not as careless of a sport as you are indicating with your questions...

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  • $\begingroup$ Where do I say that this is chaotic or careless? I just asked an aviation question. The question does not offer needless commentary. $\endgroup$ – Hanky Panky Jan 20 at 10:00

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