I don’t have a specific one in mind but nuclear winter, or a catastrophic problem spreading through the atmosphere, or something bioengineered would be concievable.
It seems unlikely to me that those would kill all of the earthbound population while still leaving the earth habitable enough for our returning astronauts. (If they kill, say, 99.99% of the population then the survivors will far outnumber the astronauts.)
What kind of shock do you have in mind?
I don’t have a specific one in mind but nuclear winter, or a catastrophic problem spreading through the atmosphere, or something bioengineered would be concievable.
It seems unlikely to me that those would kill all of the earthbound population while still leaving the earth habitable enough for our returning astronauts. (If they kill, say, 99.99% of the population then the survivors will far outnumber the astronauts.)