It's not quite as good as being able to take a pet dog or cat into orbit, but Kirobo - 13 inches tall, weighing 2kg - is a pretty good alternative. He went up to the ISS on a Japanese resupply mission in August , but is only beginning his research mission today.
He - well, "it" if we're being accurate - isn't meant to help astronauts with technical tasks, or carry out his own spacewalks. He's an emotional resource. Space can be lonely, and having someone to talk to can be comforting.
Voice and face recognition combined with natural language processing give him the ability to converse with humans. His name is a merger of "kibo" - "hope in Japanese, and the name of the nation's research module on the ISS - and robot.
While he flew up to the ISS in August, he's only being joined today  by Japanese astronaut Koichi Wakata, who originally trained Kirobo to speak. Wakata is part of a three-man crew launching from Kazakhstan, and when he arrives he'll enjoy the distinction of two new records - the first astronaut to chat with a robot in space, and the ISS's first Japanese commander.
But of course you actually want to see Kirobo in action, so here he is:
Kirobo's mission will end in December 2014, when he'll return to Earth. When humans travel to Mars, data on how to cope in isolation for long periods is going to be essential for keeping astronauts emotionally stable - and, unlike 2001: A Space Odyssey's HAL, Kirobo may prove a friendlier model of companion.