Hi inclement and welcome to the forum.
I personally would advise you to skill your sims a bit in athletic to, pushing weights. Because if you fullfill the Body Builder challenge, your sims will never get tired from a powerweight training. Breaking spacerocks also do count as something sims can get tired from resulting in a forced break for the next 8 hours in which they can't break space rocks.
If you fullfill the Body Builder challenge in combination with the Timber Terminator, your sims can cut space rocks until their energy bar is low.
For more information you could look at the site, under Timber Terminator. I can not link it here now, because I'm using my mobile to type this.
Good luck!