Enos the chimpanzee orbited the Earth twice yesterday in the final unmanned Mercury test before John Glenn’s orbital flight. He performed well under difficult conditions.

Enos — a four-year-old chimpanzee from the Cameroons, trained for two and a half years — was supposed to do three orbits, but a malfunctioning thruster caused the capsule to overheat and Mission Control decided to bring him down after two. He was apparently uncomfortable and frustrated: the overheating combined with a lever-response problem (the device was misbehaving, delivering shocks when it shouldn’t) made the flight unpleasant.

He came back fine. After recovery he reportedly shook hands with everyone and seemed happy to be out of the capsule.

What does Enos’s flight prove? That the Mercury Atlas system works for orbital flight with a biological payload. Glenn’s flight will be the next use of the Mercury Atlas, and unlike the Redstone rocket used for Shepard and Grissom’s suborbital flights, the Atlas has a history of failures. Enos’s successful two-orbit flight (even with the thruster problem) validates the most important aspects of the orbital mission design.

Ham went up in January on a suborbital flight. Enos has now orbited. We’ve been extremely careful about validating each step before committing humans. I believe this is right even when it feels slow.

Glenn is next. The real thing. I’ve been following this since Gagarin and I’m ready.