There are actually pretty strict rules for the high-altitude balloon people in terms of payload weight/volume, and being visible to aircraft, both visually and

There are actually pretty strict rules for the high-altitude balloon people in terms of payload weight/volume, and being visible to aircraft, both visually and on radar. http://www.eoss.org/pubs/far_annotated.htm http://www.eoss.org/pubs/far_annotated.htm Basically they have to be made so that on any collision the payload will break off safely, and that when it falls it won't seriously damage anything that it hits.