How can people launch balloons privately (filled with helium or hydrogen) that reach the altitude of 30,000 feet? Isn't that a safety risk in the same way as geese?
Unmanned balloons can indeed create a safety risk for aircraft. An aircraft could collide with the balloon itself (as described in the case that Jan Hudec posted in his comment), or with the balloon's payload (see the FAA's AIM section 7-5-4 for general comments on this).
But it's important to note that unmanned balloons are (or should be) launched only after appropriate preparation and communication; most countries have some sort of regulations to follow. In the US (for example), 14 CFR 101 governs unmanned balloon launches and it includes the following requirements:
- Launches must be in (mostly) clear skies and away from airports and built-up areas
- The balloon must have a radar reflector and lights (if launched at night)
- ATC must be informed in advance of the balloon launch, expected path etc.
- The operator must track the balloon's position and descent and report them to ATC as needed
In other words, the balloon should be as visible as possible - both visually and on radar - and ATC should know where it is. That gives pilots a way to see and avoid the balloon and ATC can provide warnings and/or avoidance instructions to aircraft directly where possible, which is actually fairly similar to how aircraft avoid other aircraft.
Of course, there may be cases where someone launches a balloon without following those rules (or the local equivalents in another country). That would create a much higher safety risk but it would be illegal and the operator would be risking penalties for doing it.
High altitude balloons reach more like 30km rather than kft. This means that there's a limited time in which they can interfere with normal aviation traffic (which, during cruise, is usually in a fairly narrow band between 8-12km).
Outside of cruise can be more of a problem (i,e: When landing or taking-off). For that reason it's essential to launch from somewhere far from an airport, so that the balloon can ascend in a location where it will not affect air traffic. Most countries also require you to inform air traffic control in advance of a balloon launch so that they can issue a warning (NOTAM) to pilots that lets them know to be aware of balloon activity.
Lastly, one can reduce the risk to aviation by constructing the balloon payload carefully. The balloon itself doesn't pose much of a risk but a heavy/dense payload can. To reduce the risk lightweight components should be used and dense items like batteries kept to a minimum. Most countries also have rules governing the size and weight of the balloon that you can launch.
For balloons that can't meet the rules (too heavy, need to launch near an airport, etc) there's two choice: 1) Don't launch. 2) Perform a much more extensive risk analysis and actively divert air traffic away from the balloon. This requires air traffic control to be able to see the balloon (using a radar reflector or, more usually, a transponder). Because of the cost of this option it's normally only very important or high-value payloads that are sent this way (such as NASA's research balloons).
As for the TAM flight, that does not sound like a High Altitude weather balloon. That sounded like an unmanned hot air balloon carrying a big banner. I am guessing, based on the article that it was designed to stay at a lower altitude so people could see the banner. High altitude weather balloons are like giant party balloons, very thin latex usually. They generally rise at 1000 ft/min and are out of busy airspace quite quickly. The payloads are limited to 12 lbs, but usually much lighter. Mine was just over 2 lbs. If the regulations are followed there is very little risk.Good luck, do your research, and have fun.
High altitude balloons going into commercial airspace that are large enough to be a threat to a plane require a NOTAM to be issued and even small balloon launches are encouraged to have a NOTAM. Also, a weather balloon will not spend very much time in the danger zone but will drift right through it, so the exposure is brief. Most commercial fights occur along well-known corridors and flight paths. Balloons are often launched far away from those lines of travel. Just that alone makes it pretty safe. Small planes do the same thing: stay away from commercial flight corridors.