(The backup crew would later fly on Apollo 15.)
Apollo 12 launched on schedule, during a rainstorm. 36.5 seconds after lift-off from Kennedy Space Center, the Saturn V rocket body was hit by a bolt of lightning. The CM's instruments momentarily went off-line and Mission Control lost the telemetry feeds from the spacecraft for several seconds. When ground control regained telemetry lock with the spacecraft, the feeds were garbled and reported incomplete and possibly inaccurate information. EECOM John Aaron thought that the garbled telemetry might be caused by a malfunction in the launch vehicle's Signal Conditioning Equipment (SCE), since the SCE converted raw instrument data into forms usable by spacecraft instrument displays and ground telemetry equipment, and it would have automatically gone off-line in response to the kind of disruption to the spacecraft's electrical systems that a lightning strike would cause.
With this in mind, Aaron suggested the crew "Try SCE to aux" – thereby forcing the backup SCE on-line. The command was a relatively obscure one and neither the Flight Director, nor CAPCOM, nor Mission Commander Conrad could immediately recall how to implement it; however, lunar module pilot Alan Bean remembered that the SCE switch was on his panel because of a training incident a year prior to launch where just such a failure had been simulated. Aaron's quick thinking and Bean's memory were able to salvage what otherwise would have been an aborted mission (at the time of the failure, the flight had just entered abort mode One Bravo). With telemetry restored, the crew proceeded to parking orbit and was able to restore and verify the functionality of their spacecraft before re-igniting the S-IVB third stage for trans-lunar injection.
The S-IVB was originally intended to be put into a solar orbit by venting the remaining propellant. However, an extra long burn of the ullage motors meant that venting the remaining propellant in the tank of the S-IVB did not give the rocket stage enough energy to escape the Earth-Moon system and instead the stage ended up in a semi-stable orbit around the Earth after passing by the Moon in November 18, 1969. It finally entered into solar orbit 1971, but returned to Earth orbit (briefly) 31 years later. It was discovered by amateur astronomer Bill Yeung and he gave it the temporary designation J002E3 before it was determined to be an artificial object.
The Apollo 12 mission landed on an area of the Ocean of Storms that had been visited earlier by several unmanned missions (Luna 5, Surveyor 3, and Ranger 7). The International Astronomical Union, recognizing this, christened this region Mare Cognitium (Known Sea). The landing site would thereafter be listed as Statio Cognitium on lunar maps (Conrad and Bean did not formally name their landing site, interestingly enough, though the intended touchdown point was nicknamed Pete's Parking Lot by Conrad).
The second lunar landing was an exercise in precision targeting, using a Doppler Effect radar technique developed to allow the pinpoint landings needed for future Apollo missions. Most of the descent was automatic, with manual control assumed by Conrad during the final few hundred feet of descent. Unlike Apollo 11 where Neil Armstrong took partial control of the lander and directed it further down range when he noticed that the intended landing site was strewn with boulders, Apollo 12 succeeded, on November 19, in landing within walking distance (less than 200 meters) of the Surveyor 3 probe, which had landed on the Moon in April 1967.
Conrad actually landed Intrepid 580 feet short of Pete's Parking Lot because the planned landing point looked rougher than anticipated during the final approach to touchdown. The planned landing point was a little under 1180 feet from Surveyor 3, a distance that was chosen to eliminate the possibility of lunar dust (being kicked up by Intrepid's descent engine during landing) from covering Surveyor 3. But the actual touchdown point — 600 feet from Surveyor 3 — did cause a thin film of dust to coat the probe, giving it a light tan hue.
To improve the quality of television pictures from the Moon, a color camera was carried on Apollo 12 (unlike the monochrome camera that was used on Apollo 11). Unfortunately, when Bean carried the camera to the place near the lunar module where it was to be set up, he inadvertently pointed it directly into the Sun, destroying the vidicon tube. Television coverage of this mission was thus terminated almost immediately.
Conrad and Bean removed pieces of the Surveyor 3, to be taken back to Earth for analysis. It is claimed that the common bacterium Streptococcus mitis was found to have accidentally contaminated the spacecraft's camera prior to launch and survived dormant in this harsh environment for two and a half years. However, this finding has since been disputed: see the article Reports of Streptococcus mitis on the moon.
Astronauts Conrad and Bean also collected rocks and set up equipment that took measurements of the Moon's seismicity, solar wind flux and magnetic field, and relayed the measurements to Earth. (By accident Bean left several rolls of exposed film on the lunar surface.) Meanwhile Gordon, on board the Yankee Clipper in lunar orbit, took multispectral photographs of the surface.
The lunar plaque attached to the descent stage of Intrepid is unique in that unlike the other lunar plaques, it (a) did not have a depiction of the Earth, and (b) it was textured differently (the other plaques had black lettering on polished stainless steel while the Apollo 12 plaque had the lettering in polished stainless steel while the background was brushed flat).
Intrepid's ascent stage was dropped (per normal procedures) after Conrad and Bean rejoined Gordon in orbit. It impacted the Moon on November 20, 1969 at 3.94 S, 21.20 W. The seismometers the astronauts had left on the lunar surface registered the vibrations for more than an hour.
The crew stayed an extra day in lunar orbit taking photographs, for a total lunar surface stay of thirty-one and a half hours and a total time in lunar orbit of eighty-nine hours.
Yankee Clipper returned to Earth on November 24, 1969, at 20:58 UTC (3:58pm EST, 10:58am HST), approximately 500 miles (800 km) east of American Samoa. During landing, a 16 mm camera dislodged from storage and struck Bean in the forehead, rendering him briefly unconscious. He suffered a mild concussion, and needed six stitches.
The Surveyor 3 camera retrieved by the Apollo 12 astronauts now resides in the Exploring the Planets gallery at the National Air and Space Museum.
The Apollo 12 Command Module Yankee Clipper is on display at Virginia Air and Space Center, Hampton, Virginia.
The Lunar Module Intrepid impacted the Moon November 20, 1969 at 22:17:17.7 UT (5:17 PM EST)3.94 S, 21.20 W.