Yet given the high cost of venturing into the heavens – even with multinational wallets – the question persists: Why send humans aloft at all? To some, it's simply about choosing the right tool for the job.
Robots could do just fine in environments like the moon. Steven Squyers, a planetary scientist at Cornell University in Ithaca, N.Y., told a presidential panel in August that the lunar surface was shaped by relatively simple forces and an instrument-laden rover could answer basic questions about its history.
But Mars, his speciality, is another case. Over its 4.6-billion-year life span, glaciers, vast bodies of water, volcanoes, wind, and plate tectonics have sculpted the planet. "All of this complexity means that human explorers can, in principle, contribute more to the scientific exploration of Mars than they can to any other body in the solar system," Dr. Squyers said.
And if the hunt is for life-forms, current or past, rather than just rocks, it would require deep drilling – something also better done by humans. "What Spirit and Opportunity typically achieve in a day," he said of the current robotic rovers on Mars, "a human explorer could do in less than a minute." In 5-1/2 years, Opportunity has ventured 10 miles from its landing site. Apollo 17 astronauts in their moon buggy traveled farther in one day.