Those U2 missions out of Laverton in Victoria referred to in these posts were high altitude sampling missions. They were sampling for radioactive cloud from Soviet and Chinese nuclear tests. They were conducted in both Hemispheres at high altitude. They could tell by spectroscope analysis whose it was and trigger material used and a lot of other things.
US pilot Buddy Brown says the most pucker factor was from the thought of having to bail out over Antartica if the engine failed. He reckoned they'd last 5 minutes at those temps.
The highest bailout from a U2 was 55000ft. The guy was badly hurt.