At 17:21:50 UTC on July 19, 1969, a retrograde burn of the Service Propulsion System (SPS) on the Command/Service Module (CSM) lasting 5 minutes and 57 seconds placed Apollo 11 into a safe, elliptical orbit of 111 by 306 km around the Moon.
The LOI burn is one of the most complex parts of a mission to the Moon. The primary requirement for the burn is that the spacecraft needs to be able to achieve a free return trajectory should the SPS fail. Additionally, the burn should place the spacecraft into an orbital plane above the landing site at an acceptable approach azimuth (the angle of the approach path relative to lunar north). Ideally, the LOI would also put the spacecraft as close as possible to the surface but to avoid any possible collision scenarios the first orbit should not be lower than 110km.
Combining all the constraints it is impossible to execute the LOI to achieve all mission objectives. Therefore, the flight dynamics officer (FIDO) and his team prepare ten different scenarios and then the FIDO selects the “target” which violates the least amount of requirements. The selected program is then sent to the on-board computer as LOI starts while the astronauts are on the far side of the Moon.
At 21:43:36 UTC a 17 second long burn will put Apollo 11 to the final orbit and about an hour later the Lunar Module Pilot (LMP) will enter the Command Module (CM) for for initial power-up and system checks. The following day, after a good sleep, both Neil Armstrong and Buzz Aldrin will enter the Lunar Module (LM) at 12:52 UTC to conduct final preparations for descent. The LM will undock at 17:44:00 UTC, at 19:08:14 UTC the LM descent engine will fire for 30 seconds to provide retrograde thrust for the descent orbit insertion and at 20:05:05 UTC the descent engine will burn for 12 minutes and 36 seconds to put the crew on the path to the Tranquility Base.