Originally called The Young Sentinels but changed to The Space Sentinels this animated show by Filmation ran for only 13 episodes. The 3 main characters were Hercules, Mercury and Astraea, in charge was Sentinel One and they were helped by the maintenance operator robot M.O.
Below are some Talent Tests for a proposed Live Action version of the show that never happened. Evan Kim who provided the voice of Mercury can be seen here playing Mercury. Evan is known for the Fistful of Yen sketch in John Landis's Kentucky Fried Movie, he also played Tony the camera man in V: The Mini Series.
I imagine had this show gone ahead then the look would have been similar in tone to Filmation's other SF shows like Space Academy, Jason of Star Command and Ark 2.
A while a go I built a CG Model of M.O. he can be seen on my art blog Here