GROVER, which stands for both Greenland Rover and Goddard Remotely Operated Vehicle for Exploration and Research, is an autonomous, solar-operated robot that carries a ground-penetrating radar to examine the layers of Greenland's ice sheet. Its findings will help scientists understand how the massive ice sheet gains and loses ice.

R2, which has been on the ISS since 2011, has a mission: clean handrails, vacuum air filters and take air-flow measurements. The problem is it doesn't yet have the ability to learn and complete the work. So NASA is looking for someone to teach the bot. The Robonaut Challenge calls on contestants to write algorithms that allow R2 to interact with a training dashboard the space agency built.

At 72, he said, he feels a moral obligation to step up his activism in his remaining years. "If we burn even a substantial fraction of the fossil fuels, we guarantee there's going to be unstoppable changes" in the climate of the earth, he said. "We're going to leave a situation for young people and future generations that they may have no way to deal with."