When we flew to the US back in April this year, I was lucky enough to see a pair of B-2 Spirit bombers fly underneath our 777 at about 60 degrees of angle to our direction of travel. They were moving pretty fast but I spent about the next half an hour watching them – you could see the dots for a good while and the contrails (black and sooty ones) for ages.

I can imagine how other things might appear across long expanses of sky after seeing those.