SpaceX and Blue Origin LLC are competing to launch satellites and take humans to the moon. They are also paying big salaries to hire so many young and tireless engineers that old-line aerospace employers like Boeing Co. and NASA are finding it harder to fill positions.

Most aerospace students really covet jobs at SpaceX and Blue Origin, recruiters say. The private firms are run by two of the three richest men in the world, Elon Musk and Jeff Bezos, who ultimately imagine people living and working in Earth's orbit and on the surface of Mars.

Their private firms also often pay more than established space operations. SpaceX is currently listing starting aerospace engineer positions at $95,000 to $115,000 a year.

NASA, which follows the federal government's General Schedule pay scales, offers starting salaries along a range that starts at $54,557 for engineers with bachelor's degrees, $66,731 for master's degrees and $73,038 for doctorates at the Kennedy Space Center in Florida.

Helping SpaceX or Blue Origin build towering rockets, orbiting labs or moon landers can also mean serving at the whims of mercurial executives.