Understandably, the process for how to become an astronaut isn't easy. Going into space is an incredibly dangerous act, requiring the utmost mental and physical strength, and ability to cope with the wide variety of things that could go wrong at any moment.
Despite seemingly being the most popular choice for a 5-year-old's desired career path, a minuscule portion of the population actually attempts to be a professional space explorer, let alone get through the hoops necessary to do so. With respect to American hopefuls, NASA starts with a "basic" set of qualification requirements — this is where almost every single person falls short. If you have the time to work on them, though, go for it, because NASA allows any civilian to apply.
For starters, candidates have to have a bachelor's degree in engineering, biological science, physical science, or mathematics. After that, they need at least three years of related professional experience or no less than 1,000 hours of pilot-in-command time in a jet aircraft. Depending on what type of advanced degree a candidate might have, that can be substituted for a portion of the work-related requirement.
Next up are the physical requirements, including a long-duration spaceflight physical that calls for 20/20 vision in each eye, blood pressure below 140/90 while seated, and a heigh between 62 and 75 inches.
Currently, there are 47 active astronauts in NASA's program, with the newest having been selected in 2013. This next round of applications will be accepted from mid-December through mid-February, and those who have been selected will be announced sometime in mid-2017.
Once picked, astronaut candidates start by reading training manuals and taking computer-based lessons on the various vehicle systems they'll be expected to operate. A big portion of that includes not only learning all the necessary maneuvers, but also recognizing malfunctions and how to fix them on the fly. They also undergo zero-G training in the Neutral Buoyancy Laboratory (NBL), which simulates the weightless conditions in space with underwater tank facilities. Eventually, everything they learn should prepare them well enough to experience spaceflight on the International Space Station, Russian Soyuz spacecraft, NASA's Orion vehicle, and future spacecraft.