Fifty years ago and a quarter of a million miles away, three astronauts carried out the dress rehearsal for one of the greatest events in human history. On May 18, 1969, Apollo 10 lifted off from Launch Complex 39B at the Kennedy Space Center. Its mission: to return to the Moon and make the final flight tests that would pave the way for the Apollo 11 lunar landing two months later.

Half a century on, the Apollo 10 mission seems like an oddity – a kind of half step bookmarked by the historic firsts of Apollo 8 and Apollo 11. But this penultimate flight to the Moon before Apollo 11 touched down in the Sea of Tranquility was vital to the success of the first Moon landing.

Though Apollo 8 had already orbited the Moon and Apollo 9 had tested the Lunar Module (LM) in Earth orbit, there were still many questions that had to be answered and tests to be conducted. Until Apollo 10, the Apollo spacecraft, crew, mission support facilities, procedures, and communications systems had never been fully tested under actual lunar mission conditions. Until this was done, the success of Apollo 11 was very much in doubt.

For one thing, the Lunar Module had never flown in lunar gravity, nor had it been tested in actual descent or ascent trajectories. In fact, up to that time, only two Lunar Modules had ever flown and only one of these was manned. NASA engineers were still uncertain as to how the navigation and guidance systems would react to lunar gravity. Would the descent engine fire properly? What about the risky abort procedure?

Then there was the new deep space communications network. Was it up to the task? What about tracking? The latter was especially important after Apollo 8 discovered how irregular the Moon's gravitational field was. If mission trajectories were to be properly plotted and executed, then these anomalies had to be better understood.

And last, but by no means least, more detailed photos had to be taken of prospective landing sites to choose the best one for Apollo 11 and later missions.

Lift off

May 18, 1969 was an overcast day at what was then called Cape Kennedy and has now reverted to its original name of Cape Canaveral. It was 80° F (27° C), relative humidity was 75 percent, the barometer held steady at 30.09 in, and the wind was blowing from the southwest.

On pad 39B stood the gigantic Saturn V rocket, ready to blast into space. Atop the skyscraper-sized booster was the Command Service Module (CSM) with the call sign "Charlie Brown." Below this and shrouded by a protective fairing was the Lunar Module "Snoopy." Inside Charlie Brown were Mission Commander Thomas P. Stafford (Colonel, USAF), Command Module Pilot John W. Young (Commander, USN), and Lunar Module Pilot Eugene A. Cernan (Commander, USN). All three were spaceflight veterans who had flown on Gemini missions.

At 12:45 pm, the first stage of the Saturn V roared into life and Apollo 10 lifted off only half a second behind schedule. Nearby at Launch Complex 39A, a duplicate Saturn rocket was already being prepped for Apollo 11. In less than 12 minutes, the S-IVB third stage carrying the Apollo spacecraft went into a parking orbit about 100 nm (115 mi, 185 km) in altitude, where it circled the Earth twice while the astronauts and mission control made detailed systems checks to make sure the moonship was ready to proceed.

TLI

As the booster stage flew over Australia, the GO order was given and the S-IVB fired a second time for five minutes and 42 seconds. Its velocity jumped from 17,400 mph (28,000 km/h) to 24,250 mph (39,030 km/h), setting it on Translunar Insertion (TLI) orbit. Shortly thereafter, the crew of Apollo 10 separated the CSM from the S-IVB, turned around, and docked with the Lunar Module. Once it was free, the paired spacecraft used the CSM thrusters to move clear of the S-IVB and mission control ordered the now redundant stage to vent its propellants, giving it an extra boost that would send it slingshotting past the Moon and into an endless orbit around the Sun, where it is to this day.

Meanwhile, Stafford, Young, and Cernan had a relatively quiet three-day voyage. The spacecraft was set on a trajectory so precise that they only had to fire the Service Module's engine once for a course correction, leaving them little to do except eat, sleep, and study the flight plane.

Color TV

Actually, there was one more thing. Apollo 10 was the first manned space mission to carry a color television camera aboard. Built by Westinghouse, it was (for the 1960s) a marvel of miniaturization weighing a mere 12 lb (5.5 kg) and boasting a 3-in (7.5-cm) monitor so the astronauts could see what they were shooting as they transmitted standard 525-line, 30-frame-per-second color broadcasts.

As an estimated one billion people watched, the Apollo 10 astronauts sent back live TV broadcasts of the LM docking, life aboard the Command Module, and, most spectacular of all, the first live color images of Earth itself. It would later send similar images from lunar orbit that would connect the space travelers with the people back home in a way never experienced before.

Lunar orbit

On May 21, Apollo 10 arrived in lunar orbit. Because it was on a ballistic trajectory, it had been continually slowing down as it climbed out of the Earth's gravity well. Its 24,250-mph velocity had slowed to a mere 2,000 mph (3,220 km/h) before the Moon's gravity started to dominate and sped it up to 5,500 mph (8,800 km/h) just before the CSM fired its engine, pushing the spacecraft into a trajectory that was soon trimmed to a circular orbit about 60 mi (97 km) above the lunar surface. Here it would stay for the next 73 hours, circling the Moon once every two hours.

As Apollo 10 moved out from behind the Moon, Stafford radioed to mission control, "Houston, tell the Earth we have arrived."

Undocking

The first orbits around the Moon were dedicated to housekeeping, receiving updates from mission control, and gathering tracking data through the Manned Space Flight Network (MSFN) to learn more about the effects of gravitational anomalies on spacecraft trajectories. Apollo 8 had already given NASA a wealth of information, but more was needed. Earth observations had reduced the navigational margin of error down to 200 nm (230 mi, 370 km). Unmanned probes had brought this down to within 20 nm (23 mi, 37 km) and Apollo 8 down to 5 nm (6 mi, 9 km). Apollo 10's job was to bring it to within 1 nm (1.15 mi, 1.85 km).

As to the next step, it was really very simple. Stafford and Cernan were to enter the Lunar Module, leaving behind Young in the Command Module. They would then undock and carry out all the steps of a lunar landing descent except for the actual touchdown.

Not that there weren't incidents. When the trunk to the Lunar Module was first opened Mylar insulation spewed out, making a mess of both spacecraft. The crew also discovered that the LM and the Command Module weren't properly aligned, raising fears that some of the latching pins might shear, preventing redocking. However, NASA engineers determined that the misalignment was less than six degrees, so it was within tolerance.

After a thorough checkout, the Lunar Module's four landing legs were deployed. Then, with the new television camera recording the action from the Command Module, Snoopy released itself from Charlie Brown and moved away, making a slow spin to allow Young to make sure the legs had deployed properly and that there was no damage to the lander.

Young was also enjoying the novelty of being the first astronaut to ever have a Command Module all to himself. Stafford told him not to get lonesome – and to not head back to Earth without them.

Descent

Snoopy now fired its Descent Stage engine for almost 30 seconds, killing its speed and sending it into a descent orbit headed toward the future Apollo 11 landing site. Because NASA knew full well that the Apollo 10 crew were highly motivated, the engineers took the temptation to actually land away from Stafford and Cernan by providing the Ascent Stage with only as much fuel and oxidizer as it would have had if it had already lifted off from the surface. In other words, if they did land, they'd never be able to take off again.

Down went the Lunar Module, eventually reaching an altitude of only 47,400 ft (14,400 m). As they descended, Stafford and Cernan took photos of the landing site for future study and navigated by landmarks. Meanwhile, the landing radar was tested under landing conditions for the first time. Despite communication glitches, the astronauts sent back a running commentary, though it lacked the light banter that it usually contained.

Then things went very wrong very fast.

Trouble

The next phase of the mission was to simulate a landing abort. That is, to escape and return to the CSM as if an emergency had occurred by jettisoning the lower Descent Stage and firing the Ascent Stage engine. This was the most dangerous maneuver because it was what was called "fire in the hole" – firing the engine while it was still inside the Descent Stage – and no one was sure what would happen.

But Stafford and Cernan suddenly had another problem. Right before separation and as the engine fired, the Lunar Module when wild, gyrating and bucking about. The astronauts started using language that they weren't supposed to use while the public is listening as they fought to regain manual control. Stafford thought they were going into gimbal lock. That is, the engine swiveling hard over to one side and sticking. If that happened, the Ascent Stage would go into a spin that would knock the men unconscious and likely kill them.

Cernan later described the situation as like being in a fighter jet. As the Ascent Stage turned eight times Stafford managed to gain control within three minutes and though NASA downplayed the incident, it was a close run thing. Later analysis showed that the problem was caused by human error when the LM abort guidance system was set for AUTO instead of ATTITUDE HOLD. As a result, the computer tried to home in on the Command Module when it shouldn't have.

Rendezvous

Despite the drama, there was still work to do. The Ascent Stage carried out the same rendezvous procedure that a returning surface mission would use, matching orbits with the Command Module in a maneuver that was so carefully planned that it even took into account keeping the sun out of the astronauts' eyes. Young saw what was left of the LM through the Command Module's sextant at a distance of 161 mi (259 km). Stafford and Cernan locked on with radar as the orbits of the two synched and then the Ascent Stage halted when the two craft were within 26 ft (8 m) of one another. Stafford gave a final burp of the thrusters and Snoopy was home.

Goodbye Ascent Stage

After Stafford and Cernan transferred back to Charlie Brown, the hatches were sealed and Snoopy was jettisoned. Mission control now took over the Ascent Stage remotely and ordered it to fire its engine again. Snoopy blazed away, consuming the last of its fuel as it burned for 249 seconds. It reached escape velocity and went into orbit around the Sun. Mission control maintained radio contact until Snoopy's batteries died about 12 hours later.

Photos and back home

The final orbits around the Moon were spent taking photographs. These were very valuable because real film emulsion was much more detailed than the crude electronic images that unmanned probes of the time could send back. This detail was vital if future missions were to land safely.

Then, on May 24, 137 hours into the mission, the CSM's engine fired again, sending it on a 61-hour trajectory back to Earth. The crew sent back television transmissions during this time and on May 16, 1969, Charlie Brown separated from the Service Module and returned home as a blazing meteor that splashed down in the Pacific Ocean 400 nm (460 mi, 740 km) east of American Samoa before being recovered by the USS Princeton.

Firsts and records

Aside from completing its mission, Apollo 10 was also notable for a long list of firsts and records: it was the first flight of a color TV camera; it broke the record for the fastest speed anyone has ever flown – 24,791 mph (39,897 km/h); it was the only Apollo flight where all of the crew went on to command their own Apollo missions – Young would command Apollo 16, Cernan Apollo 17, and Stafford the Apollo-Soyuz Test Project; they had traveled farther from home than any other astronauts before or since; and they were the only Saturn V crew to lift off from LC 39B.

Next, the landing

But for all its accomplishments, Apollo 10's biggest slice of history was that, like a true pathfinder, it opened the way for bigger things to come. Thanks to Charlie Brown, Snoopy, and the three men who flew them, it was now possible for two more spacecraft called Columbia and Eagle to make one small step for man and one giant leap for mankind.