Apollo 10

• 1. Introduction

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  Apollo 10 came within 7.8 nautical miles of the Moon's surface in May 1969 and then turned back. The crew had a working spacecraft, they had the training, and they were orbiting the Moon. What stopped them from landing was a deliberate choice NASA had made months earlier. And to make absolutely sure that choice would hold, the agency short-fueled the ascent stage of the lunar module. If Thomas Stafford and Gene Cernan had tried to touch down anyway, they could not have gotten back up. This is the story of the mission that was engineered to stop just short of history, and of why that restraint made the first Moon landing possible.

• 2. The Dress Rehearsal Debate

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  By 1967, NASA had mapped out a sequence of mission types, each designated by a letter, that had to be flown before anyone could set foot on the Moon. The letter "G" was reserved for the first lunar landing itself. Apollo 10 would be the "F" mission: a full rehearsal of every procedure and every system, stopping only at the moment of powered descent.

  The debate over whether to even fly the "F" mission was real and pointed. George Mueller, NASA's Associate Administrator for Manned Space Flight, favored skipping the rehearsal and attempting the landing on Apollo 10. Some within the agency agreed with him, arguing it was senseless to bring astronauts so close to the surface only to turn away. The Apollo 11 lunar module could have been substituted for the heavier one assigned to Apollo 10, at the cost of a one-month delay.

  Standing against Mueller was Christopher C. Kraft, Director of Flight Operations. Kraft argued that rendezvous procedures in lunar orbit were still unproven and that NASA lacked complete data on the Moon's mass concentrations, which could deflect a spacecraft from its intended path. Lieutenant General Sam Phillips, the Apollo Program Manager, heard both sides and came down firmly in favor of the dress rehearsal. His decision shaped everything that followed.

• 3. The Crew

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  NASA announced the Apollo 10 crew on the 13th of November 1968. Commander Thomas Stafford was 38 years old and had already flown twice: as pilot of Gemini 6A in 1965 and as command pilot of Gemini 9A in 1966, the latter alongside Gene Cernan.

  John Young, the command module pilot, was also 38. A 1952 graduate of Georgia Tech who became a Navy test pilot and was selected as a Group 2 astronaut alongside Stafford, Young had flown in Gemini 3 with Gus Grissom in 1965, becoming the first American outside the original Mercury Seven to reach space. He later commanded Gemini 10 with Michael Collins.

  Gene Cernan, the lunar module pilot, was 35 and a commander in the Navy. A 1952 graduate of Purdue University, he had been selected in the third astronaut group in 1963. With five spaceflight credits among the three of them, the Apollo 10 crew was the most experienced to reach space up to that point, and the first American mission to carry an all-veteran crew.

  The backup crew tells its own story. Gordon Cooper and Donn Eisele were assigned despite reservations. Deke Slayton, Director of Flight Crew Operations, felt Cooper had not trained as hard as he could. Eisele carried the reputation of the difficult Apollo 7 mission and had gone through a messy divorce. Under normal crew rotation they would have flown Apollo 13, but Slayton replaced them with Alan Shepard and Stuart Roosa. The crew that flew Apollo 13 was ultimately switched to Apollo 14, where Shepard and Edgar Mitchell walked on the Moon.

• 4. Charlie Brown and Snoopy

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  Stafford, Young, and Cernan named their command module Charlie Brown and their lunar module Snoopy, drawing from Charles Schulz's comic strip. Schulz approved the names but admitted he was uncertain about it, since Charlie Brown was, in his own conception, always a failure.

  The choice irritated some at NASA. Public relations chief Julian Scheer pushed for a change before the lunar landing mission, wanting more dignified call signs. Cernan later wrote that "the P.R.-types lost this one big-time", because the names connected instantly with audiences worldwide. Apollo 11 would use "Columbia" and "Eagle" for its command and lunar modules.

  Snoopy had a longer history with NASA than most people realized. Workers who performed with distinction were awarded silver Snoopy pins, and images of the cartoon dog appeared throughout NASA facilities, often shown wearing a space helmet in place of his World War I aviator's gear. Stafford said the choice of Snoopy as the lunar module's call sign was a way of acknowledging the contributions of the hundreds of thousands of people who supported the mission. Schulz pointed out that Snoopy had already journeyed to the Moon in the comic strip the previous year, thus defeating, in Schulz's words, "the Americans, the Russians, and that stupid cat next door".

  Peanuts creator Schulz also drew mission-related artwork for NASA to mark the occasion. The mission patch, designed by Allen Stevens of Rockwell International, depicted a large Roman numeral X resting on the lunar surface. Stafford described it as showing "that we had left our mark".

• 5. Short-Fueled by Design

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  The lunar module assigned to Apollo 10 was designated LM-4. Its total weight came to 30,735 lb, compared to the 33,278 lb of the Apollo 11 lunar module that would make the first landing. The critical difference was not the descent stage but the ascent stage propellant: LM-4 carried 2,631 lb, while Apollo 11's carried 5,238 lb.

  The disparity was intentional. The ascent stage was loaded with only the fuel it would have had remaining had it already lifted off from the surface and climbed to the altitude where Apollo 10's ascent engine would fire. That was roughly half the amount needed for a full launch and rendezvous from the Moon. If Stafford and Cernan had somehow touched down, they could not have returned to Young in the command module.

  Craig Nelson's 2009 book Rocket Men quoted Cernan directly on the calculation: "A lot of people thought about the kind of people we were: 'Don't give those guys an opportunity to land, 'cause they might!' So the ascent module, the part we lifted off the lunar surface with, was short-fueled. The fuel tanks weren't full." Mueller later confirmed there was no ambiguity: it was an early, heavy test module, built for the dress rehearsal only, and that was how it was used.

• 6. Flight of Snoopy

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  Stafford and Cernan re-entered Snoopy at 95 hours and 2 minutes into the mission and undocked from Charlie Brown three hours later, at 98 hours, 11 minutes, and 57 seconds. Young, left alone in the command module, became the first person to fly solo in lunar orbit.

  Snoopy's descent orbit insertion maneuver fired the descent engine for 27.4 seconds at 99 hours, 46 minutes, and 1.6 seconds, dropping the craft toward the Moon. The crew came within 8.4 nautical miles of the surface on a first pass, then performed a phasing burn to allow a second pass, reaching the closest point of 7.8 nautical miles. Stafford described the surface near the planned Apollo 11 landing site in the Sea of Tranquility as smoother than expected, resembling the desert around Blythe, California. His observations gave mission planners enough confidence to confirm the site for Apollo 11.

  The return was violent. As Stafford and Cernan prepared to jettison the descent stage, the lunar module began tumbling out of control. Cernan's alarmed response, caught on a live broadcast, included language that generated complaints back on Earth. The tumble traced to a guidance system switch that both crew members had moved in sequence, canceling each other's actions and returning the switch to its original position. Stafford jettisoned the descent stage about five seconds into the gyration and regained manual control within roughly eight seconds, then fired the ascent engine to begin the climb back to Charlie Brown.

  Snoopy rendezvoused and re-docked with Charlie Brown at 106 hours, 22 minutes, and 2 seconds, just under eight hours after undocking. Once Stafford and Cernan were back aboard, the ascent stage was sealed off, and its remaining fuel was burned to send it into a heliocentric orbit. It is the only Apollo lunar module to have been placed on that trajectory and the only once-crewed spacecraft known to still be in outer space without a crew.

• 7. Speed Record and Return

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  After 31 orbits spanning 61 hours and 37 minutes around the Moon, the Apollo 10 crew fired the main service propulsion system engine for about 2.5 minutes to begin the journey home, achieving the departure trajectory at 137 hours, 39 minutes, and 13.7 seconds into the mission.

  The return trip was deliberately shortened. The trajectory was designed to take 42 hours rather than the standard 56, which meant the spacecraft entered Earth's atmosphere at a higher speed than any crewed vehicle before or since. On the 26th of May 1969, the Apollo 10 crew reached 39,897 kilometers per hour, equivalent to 11.08 kilometers per second or 24,791 miles per hour, relative to Earth's surface. That record has never been broken.

  Splashdown occurred in the Pacific Ocean roughly 400 nautical miles east of American Samoa at 16:52:23 UTC. The crew spent about four hours aboard the recovery ship, during which President Richard Nixon called to congratulate them. Because they had not touched the lunar surface, the crew was not required to enter quarantine, unlike the Apollo 11 crew two months later. They were flown to Pago Pago International Airport in Tafuna for a reception, then boarded a C-141 cargo plane to Ellington Air Force Base near Houston.

  About two weeks of post-mission data analysis led a NASA flight readiness team to clear Apollo 11 for its scheduled July 1969 flight. On the 16th of July 1969, the next Saturn V lifted Neil Armstrong, Buzz Aldrin, and Michael Collins toward the Moon.

• 8. What Became of Snoopy

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  The command module Charlie Brown has been in the Smithsonian's custody since 1970. It traveled to several countries before being placed on loan to the London Science Museum in 1978.

  The ascent stage of the lunar module Snoopy took a different path. Placed into a heliocentric orbit after the mission, it was not tracked after 1969. For decades its location was unknown. In 2011, a group of amateur astronomers in the United Kingdom started a project specifically to find it. In June 2019, the Royal Astronomical Society announced that a small Earth-crossing asteroid designated 2018 AV2 was likely the long-lost spacecraft, with 98% certainty. If confirmed, Snoopy is the only once-crewed spacecraft still drifting through outer space with no crew aboard.

  Snoopy's descent stage met a quieter end. Jettisoned in lunar orbit, it was never recovered. Planetary scientist Phil Stooke, who studied lunar crash sites, wrote that it "crashed at an unknown location". Richard Orloff and David M. Harland, in their Apollo sourcebook, noted that gravitational distortions from the Moon's mass concentrations would have decayed its orbit and sent it into the surface. The same mass concentrations that Christopher Kraft had warned about in the landing debate ultimately claimed one of the mission's two modules.