Have a look at the Urban Dictionary - it does seem to carry that meaning: Urban Dictionary: space cadet
Student or Learner
I have a vague idea that a space cadet is the counterpart of our "heavenly soldier. In our language, "a heavenly soldier" is a sacastic term teasing a person who is out of place as well as ill-informed, can't do their job well, and doesn't care about anything too much. Is it close to "a space cadet?" Thanks.