Tangent: Seveneves by Neal Stephenson is a hard sci-fi novel that explores this very situation. Present day humanity (i.e., no manned interplanetary missions,

Tangent: Seveneves by Neal Stephenson is a hard sci-fi novel that explores this very situation. Present day humanity (i.e., no manned interplanetary missions, minimal manned presence in orbit, etc.) discovers total-extinction cataclysm will happen in two years. Scramble to figure out short (how to get genetically viable population into orbit) and long (how to keep them alive for a thousand years until the surface is habitable again) term solutions to preserve humanity.