Answer by Robert Frost, Trained NASA, ESA, JAXA, CSA, and RSA astronauts for the International Space Station program, on Quora,

The NASA astronaut corps is a mix of civilians and active duty military officers, so what happens when they leave first depends on which of those camps they fit into.

If they are active duty military, and do not retire from the military prior to leaving their astronaut position at NASA, they will go on to whatever assignment the military wishes to give them. For example, Susan Helms was a member of the Expedition 2 crew. She returned to Earth in August 2001. By 2002, she had completed her post-flight duties such as debriefs and PAO activities and she decided to leave the astronaut corps. At that time, she was a Colonel in the US Air Force. The Air Force assigned her to become a division chief at the US Space Command in Colorado. Since then, she has become a Lieutenant General and is commander, 14th Air Force.

English: Susan Helms is a United States Air Force Major General and a former NASA astronaut. This image is the most recent published by the U.S. Air Force, following her promotion to Major General (Photo credit: Wikipedia)

Astronauts that are not active duty military at the time of retirement from the Astronaut office have a tendency to move on to executive positions in the aerospace industry. Commercial space companies often hire astronauts for the vast experience and network of contacts that they can provide. For example, Garrett Reisman left NASA and became the Program Manager for SpaceXs Dragon-Falcon 9 crew vehicle. His AsCan classmate Chris Ferguson left NASA and is now Garretts competitor, as he is the Director of Crew and Mission Operations for Boeing's Boeing's Commercial Crew Program.

English: NASA astronaut Garrett Reisman, STS-132 mission specialist, is pictured in the Cupola of the International Space Station while space shuttle Atlantis remains docked with the station. (Photo credit: Wikipedia)

Gregory H Johnson is an example of an astronaut that was active military, but retired from the military while still an astronaut. He retired from the Air Force in 2009, but stayed with NASA until this month. In September, he will become the Executive Director for the Center for the Advancement of Science in Space (CASIS). CASIS is a nonprofit that is responsible for managing payload science utilization of the ISS ISS.

Some astronauts do indeed go into politics. The most famous is, of course, John Glenn. Glenn went on to be a senator for the state of Ohio for 25 years. He also ran for President in 1984. Apollo 17 astronaut Harrison Schmitt became a senator for New Mexico. Most recently, Jose Hernandez left NASA in 2011 to run for a House of Representatives seat in his home state of California.

John Herschel Glenn Jr. (born July 18, 1921, in Cambridge, Ohio,) is a former American astronaut, Marine Corps fighter pilot, and United States Senator. He was the third American to fly in space and the first American to orbit the earth. This photo for his second space flight on October 29, 1998, on Space Shuttle Discoverys STS-95. (Photo credit: Wikipedia)

Astronauts are overachievers and some astronauts go on to very eclectic things. One of the most interesting astronauts is Story Musgrave. He was an active astronaut for over 30 years and holds the distinction of being the only astronaut to fly on all five space shuttles. While he was an astronaut, he obtained 7 graduate degrees math, computers, chemistry, medicine, physiology, literature, and psychology. In his spare time, he was a trauma surgeon, pilot, and parachutist. Today he operates a palm farm in Florida, a production company in Australia, and a sculpture company in California. He is a landscape architect. He has worked for Disneys Imagineering team as a concept artist. He teaches design at the Art Center College of Design in Pasadena. And hes a public speaker with 20 honorary doctorates.

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What Do Astronauts Do Once They Leave NASA?