Three pairs of moon's gravity keep them in a stable orbit. At least a dozen more moons have been discovered but we are not sure if they are true moons. The people who choose pick 100 people but only 20 of them pass. If you want to become an astronaut you have to work well with people, you have to communicate well, you have to have good work experience, have good grades, and a college degree. When you get picked you have to practice a lot, and you need to know how to do the things they want you to know for that mission.
Some astronauts learn to drive the space shuttle because they practiced and practiced.
Also some learn to space walk and do science experiments. All future astronauts must make good grades in school. More about becoming an astronaut Also, when you are picked you go to a space center called Johnsons Space Center. It is located in Houston, Texas. Also to become an astronaut applicants must be adaptable and understand the tremendous workload that is involved in becoming an astronaut.