When Marcus is heading for the sun, it is said he will get there in a week. That would require Marcus to 'float' there at an average speed of almost 900.000 km/h. Pretty impressive, even for a space-chimp.
Interestingly, you can fire a rifle in outer space. The cartridge is so self-contained that it contains an oxidizer; this would enable the round to fire off regardless of the lack of atmosphere.
Dr. Mallory refers to Marcus as Murderous George. This is a reference to Curious George for the children's book.
This episode was Emmy nominated for Sound Editing.