Gemini 4 saw the first U.S. spacewalk, was the first U.S. mission to last multiple days, and made an attempt to rendezvous and fly in formation with the rocket's second stage in orbit. The rendezvous was unsuccessful and revealed the counter-intuitive elements of orbital mechanics, where pointing your craft at a target and firing your engines doesn't always move you towards the target. The rendezvous was called off to focus on the more-important spacewalk, which was a success.