340 days

NASA billed it as a year in space, but Mr. Kelly’s trip will actually be a few weeks short of that. (An unexpected delay could extend his stay slightly.) Still, it dwarfs the previous NASA record, held by Michael López-Alegría, who spent 215 consecutive days in space in 2006 and 2007.

But it is far short of the all-time record of almost 438 days. That was achieved by Valeri Polyakov on the old Russian Mir space station back in 1994 and 1995.

This is Mr. Kelly’s fourth mission, so his cumulative time in orbit will be 540 days, assuming an on schedule return.