As NASA has demonstrated repeatedly, unmanned space probes can return a significant amount of data about our solar system and the universe beyond. But even state-of-the-art machines such as Curiosity, the one-ton, instrument-laden machine due to arrive on Mars in August 2012, can only accomplish so much without arms, legs and a brain.

That's why astronauts, even in the post-shuttle era, will remain essential personnel in the near and not-so-near future of spaceflight. In fact, we may see more astronauts than ever before as private companies loft their own crews into the heavens. That's good news for anyone who has dreamed of taking one giant leap for mankind, but becoming an astronaut -- for the government or industry -- will remain a challenging process. Let's start first with how the U.S. space program finds astronaut candidates.

NASA manages the Astronaut Corps through the Flight Crew Operations Directorate at Johnson Space Center in Houston, Texas. The Astronaut Corps includes those men and women who are qualified to fly into space, but excludes astronauts who have transitioned to management positions within NASA and no longer operate spacecraft. In recent years, the number of astronauts in the corps has been reduced substantially -- from approximately 150 in 2000 to 61 in 2011 [source: National Research Council]. But that number could rise again if U.S. budget and policy makers follow the recommendations of the National Research Council, which issued a report in September 2011 warning NASA it could face an astronaut shortage if it doesn't ramp up recruitment.

The space agency took an initial step to make up the shortfall when it began accepting applications for a new class of astronaut candidates on Nov. 15, 2011. The deadline to apply is Jan. 27, 2012, and in early 2013, NASA will select up to 15 qualified men and women to enter the Astronaut Corps. By June 2013, those future spacefarers will be in Houston, training for the missions in NASA's next chapter. Those missions will likely include trips to the International Space Station (ISS) aboard Russian Soyuz rockets, but could also involve missions beyond low-Earth orbit using U.S.-built commercial vehicles or NASA's Orion spaceship and heavy-lift rocket.

Still ready to explore the outer reaches? We'll get into the qualifications you'll need to have to become an astronaut for NASA next.