@hobs I'm not sure what the plane is, but the Canberra and the U2 were/are both over 70,000 feet cruising altitude. I think one of the big USAF drones can also fly close to that high.
@hobs https://www.militaryfactory.com/aircraft/aircraft-ranked-by-ceiling.php
If the balloon was cruising at FL60, there are a whole bunch of aircraft capable of getting there.
@hobs good theories out there that it was the training version of the U2
@hobs Definitely a U2. Shadow looks right for a side view. Also the reinforced fatter bit at the end of the slender wing is pure U2: it's a skid for when the plane lands.
To save weight the U2 only has undercarriage in the central fuselage (for takeoff it has jettisonable wheels under each wing), so when it stops after landing it tips over sideways: hence the reinforced skid wingtips.
Can be seen in this video about landings https://youtube.com/watch?v=HS0wbv78gl0&t=132