Outside the Allegheny Observatory, an orange X marks the spot for a soon-to-be-planted sweetgum tree that’s seen the recesses of the cosmos. During the third Apollo mission to the moon in 1971, astronaut Stuart Roosa brought a pocketful of seeds. Before joining NASA, Roosa had been a smokejumper — a firefighter who parachutes from airplanes to douse wildfires — and was a lover of forests and trees…