Well, apart from the two Russian cosmonauts also on board. He was the sole American, not the sole human. Cassidy, however was part of the largest group in space, 13 people, when a Shuttle with 7 crew joined the 6-person ISS Expedition 20.
[Edit] The astronauts who have been genuinely alone and far from the rest of humanity were the Apollo command module pilots who stayed in lunar orbit while the other two crew descended to the surface.
Well, apart from the two Russian cosmonauts also on board. He was the sole American, not the sole human. Cassidy, however was part of the largest group in space, 13 people, when a Shuttle with 7 crew joined the 6-person ISS Expedition 20.
[Edit] The astronauts who have been genuinely alone and far from the rest of humanity were the Apollo command module pilots who stayed in lunar orbit while the other two crew descended to the surface.