It is surprising that weather balloons don't have ADS-B out (or did this ballon have that and something about the system didn't work?). If it did work, ADS-B would have made this collision very avoidable.
ADS-B, as regulated, is a terrible solution for this stuff. EIRP requirements make it extremely impractical as a transmission solution for small devices, most ADS-B In equipment isn't designed to correctly alert for separation with non-fixed wing devices, and (due in no small part to the very high EIRP), there are concerns about both air-time saturation and management plane saturation (ie - ADS-B In equipment also wasn't designed to track very many entities).
Some do. Edge of Space Sciences (EOSS) is group of citizen high altitude scientists, and their large balloon flights include certified ADS-B transponders [0] and radar corner reflectors. They also file their flights with the FAA to publish NOTAMS. They have significantly larger payloads than this, but are designed to quickly ascend to ~100kft and pop reducing the loiter time in congested airspace.
Project Loon balloons also show up on Flightaware, so they either have ADS-B or TIS-B.
A situation like this will almost certainly cause some congresspeople to fret and write bills that would require ADS-B on all balloons, which would be a death knell for amateur ballooning unless ADS-B (or "legacy" Mode A/C/S) transponders become significantly smaller and more affordable. Mode C/S transponders are already available in miniaturized form factors thanks to the UAS industry, and are designed to be interrogated by aircraft equipped with TCAS (i.e. all 10+ passenger aircraft) that provides pilots deconfliction commands automatically and with no ATC support. But they're still priced for industry, not amateurs.
Even a radar reflector would have helped a lot. ADS-B is off-limits for balloons, ultralights, hang-gliders, etc, and it seems like now that radio beacons can be manufactured very cheap & low power all those non-commercial aerial vehicles should be equipped.
For a 2.5 lb ground weight balloon, a radar reflector is likely still too heavy. The lightest weight marine radar reflector [1] I could find is about half a pound.
That being said, detection isn't everything; primary radar cannot make accurate altitude measurements, only bearing and range. While that's enough to route traffic around, it could be also mistaken for a false return.
Marine applications care a lot about durability and pretty much zero about weight, so I wouldn’t expect that to be representative of how light they can be. You could make a radar reflector from a bit of cardboard and aluminum foil.
It's not off limits in the regulatory sense. Balloons are generally exempted due to their size and power constraints. Per the submission, the balloon weighs 2.4 lbs at launch. That doesn't give them much room to add a transponder and battery for ADS-B while staying within the target weight limit.
47 CFR 87.107 requires that radios transmitters in aircraft broadcast an identification number, and the allowed forms of identification aren't permissive enough for non-registered aircraft.
ADS-B out is still relatively new (especially in aviation terms) so I expect we'll see this continue to evolve.
It's not off-limits. But, it's not required (for most balloons, ULs, etc). And due to cost and/or weight, people don't always use it when it's not mandatory.
The comments I was reading at ArsTechnica suggested that adding transponders was off-limits to balloons. The argument was that balloon operators wanted to add transponders but that it was resisted by FAA worrying about there being too many non-plane transponders for pilots/ATC to keep track of.
I obviously don't know which is right, but it does show that there is definitely confusion out there about the issue.
The uAvionix EchoESX [1] claims 4W continuous and with antenna probably adds 400g (0.8lb.)
WindBorne claims "12+ days typical flight, with demonstrated capability for 75+ day missions." So 1150Wh minimum (80Ah at 4S, which is probably like 16lb.) But you're up in the atmosphere and probably need to heat that battery so... more. But we're already at 18lb additional weight... Maybe you could offset with solar panels...
But, given that the entire balloon and payload weighs 2.5lb we're already way off the edge of feasibility for an active ads-b out.
Maybe there's something that would only listen and then respond when it heard something and that would reduce the power draw. But we're needing something 2 orders of magnitude less massive.
About the same as a transponder, I think. According to the FAA many weather balloons operate their transponders (if equipped at all) intermittently to preserve battery.
