With the first three crewed Apollo missions successfully meeting their objectives over the course of just five months, by the spring of 1969 it was beginning to look as though NASA would succeed in meeting the late President Kennedy’s call to land astronauts on the Moon before the decade was out. With growing confidence in their program, just one more mission was needed to test the Apollo hardware and procedures needed to land on the Moon. That mission, Apollo 10, would prove to be the most complex crewed mission ever flown to date and would set the stage for the success of Apollo 11 in July 1969.

Pushing towards the Moon

In the wake of the Apollo 1 tragedy in January 1967, Apollo program managers developed an aggressive, multi-step test flight plan leading up to the first Moon landing. The “A” missions were meant to test the new Saturn V Moon rocket during unmanned flight. While the first flight, Apollo 4 launched on November 9, 1967, was quite successful (see “Apollo 4: First Flight of the Saturn V”), the Saturn V experienced a number of significant problems during the Apollo 6 test flight launched on April 4, 1968 raising much concern among Apollo program officials (see “Apollo 6: The Saturn V That Almost Failed”). Fortunately, the causes of the problems were quickly identified and fixes made certifying the Saturn V to launch crewed missions without the need for a third unmanned test flight.

The “B” mission objectives to test the Lunar Module (LM) unmanned in low Earth orbit were successfully met by Apollo 5 launched on January 22, 1968 (see “Apollo 5: First Flight of the Lunar Module”). Next was the “C” mission to test the new Command Service Module (CSM) in low Earth orbit with a crew on board. The C mission objectives were fully achieved by the highly successful Apollo 7 mission launched on October 11, 1968 (see “Apollo 7: Rise of the Phoenix”). The objectives of the “D” mission centered on testing the LM with a crew as well as joint operations with the CSM in low Earth orbit. Once again, all the objectives for the D mission were fulfilled by Apollo 9 launched by a Saturn V on March 3, 1969 (see “Apollo 9: Giving the ‘Spider’ Wings”).

The next missions in the NASA’s original 1967 test flight plan were the “E” and “F” missions which would be flown in high Earth orbit and lunar orbit, respectively. The experience gained from each mission was meant to build confidence in the Apollo hardware at progressively greater distances from the Earth. The actual lunar landing would be the objective of the “G” mission. But as the string of successful Apollo missions unfolded, program managers were ready to depart from their original test flight plan which was now viewed as being too conservative. Apollo 8, launched on December 21, 1968 for the impromptu “C-prime” mission, had already met the CSM-related E and F mission objectives with its successful flight to lunar orbit (see “Apollo 8: Where No One Has Gone Before”). With these issues already addressed, it was decided that the E mission would be skipped and that the upcoming “Apollo 10” flight would follow the F mission plan to test the LM in lunar orbit. Confidence in this decision was only bolstered by the excellent performance of the Apollo 9 mission. The F mission “rehearsal” in lunar orbit, if successful, would be followed by the first G mission to attempt a manned lunar landing during the summer of 1969.

Mission Objectives

On November 13, 1968, NASA officially announced the all-veteran crew for the upcoming Apollo 10 mission. It consisted of USAF Colonel Thomas P. Stafford as the commander, US Navy Commander John W. Young as the Command Module Pilot (CMP) and US Navy Commander Eugene A. Cernan as the Lunar Module Pilot (LMP). Stafford, 38 years old, was a US Naval Academy graduate who served in the USAF and was the chief of the Performance Branch of the USAF Aerospace Research Pilot School at Edwards Air Force Base before he was selected as part of NASA’s second group of astronauts in September 1962. He had previously flown as the pilot on the Gemini 6 mission (see the Gemini 6 page) then as the command pilot for Gemini 9 (see “The Angry Alligator & The Snake: The Mission of Gemini 9”) for a total of just over 98 hours of experience in orbit. The 38-year old Young was a test pilot before being selected as part of NASA’s second group of astronauts along with Tom Stafford. He had previously flown as the pilot of the Gemini 3 mission (see “The Mission of Gemini 3”) and as the command pilot on Gemini 10 (see “Gemini 10: Dual Rendezvous in Orbit”) for a total of almost 76 hours in space. Gene Cernan was a naval aviator with over 1,900 hours of flight experience before being selected for NASA’s third group of astronauts in October 1963. The youngest member of the crew at 35, Cernan had previously flown as the pilot on the Gemini 9 mission with Stafford logging 72 hours in orbit.

For the Apollo 10 mission, the backup commander was USAF Colonel L. Gordon Cooper who was one of the original Mercury astronauts (see “Project Mercury: Choosing the Men and Their Machine”). Cooper had previously flown on the Mercury-Atlas 9 and Gemini 5 missions (see “Eight Days or Bust: The Mission of Gemini 5”). The backup CMP, USAF Lt. Colonel Donn F. Eisele, was a member of NASA’s third astronaut group and had previously flown as the CMP on the Apollo 7 mission (see “Apollo 7: Rise of the Phoenix”). US Navy Commander Edgar D. Mitchell, the backup LMP, was a member of the fifth group of NASA astronauts chosen in April 1966 and had not previously flown in space.

The primary objectives of the Apollo 10 mission were to continue the demonstration of the performance of the crew, space vehicle and mission support facilities for a crewed Saturn V mission to the Moon. The intent was to rehearse all aspects of a lunar landing mission without the actual descent towards the lunar surface. Even without an actual landing, this would be the most complex crewed spaceflight ever attempted. The launch vehicle for the Apollo 10 mission was the Saturn V designated SA-505 which sported a number of incremental improvements over SA-504 used for Apollo 9 as more experience was gained flying this huge rocket. The spacecraft for the mission were CSM-106 (Command service Module number 106) and LM-4. For this lunar mission, CSM-106 carried a full propellant load for its SPS (Service Propulsion System) and had a launch mass of 28,870 kilograms. LM-4, which was constructed from the start for an orbital test flight and was too heavy to attempt a lunar landing, had a fueled launch mass of 13,993 kilograms – about 500 kilograms lighter than LM-3 flown during Apollo 9 largely due to a lighter ascent stage propellant load. With a total docked mass of almost 42.9 metric tons, the docked CSM-106/LM-4 combination was the most massive crewed spacecraft ever launched (beating Apollo 9 by over six tons) as well as the largest payload ever sent to the Moon (beating Apollo 8 by 14 metric tons).

In order to avoid confusion during communications while the CSM and LM were flying independently, the crew chose the callsigns “Charlie Brown” and “Snoopy”, respectively, for their spacecraft after characters in the popular Peanuts cartoon created by Charles L. Schulz. Part of the reason for this choice was because of the tradition at NASA’s Manned Spacecraft Center (now Johnson Space Center) of giving Snoopy pins as a symbol of quality performance. While NASA officials greeted the choice of the callsigns with raised eyebrows (as they did the fanciful callsigns of Gumdrop and Spider used for the Apollo 9 mission), more dignified names would be selected for subsequent lunar landing missions.

The Mission Plan

Launch of Apollo 10 would take place from Pad B of Launch Complex 39 – the first (and, as it turned out, the only) Saturn V launch from LC-39B. The first two stages of the Saturn V along with an initial 145-second burn of its S-IVB third stage would place Apollo 10 into a temporary 185-kilometer parking orbit around the Earth. All systems of the spacecraft would then be checked out during the following two hours before the final decision to proceed to the Moon would be made. Assuming all is well, the S-IVB stage would reignite 2½ hours after launch about halfway through its second revolution for the TLI (Trans Lunar Injection). This burn would place Apollo 10 into an extended geocentric orbit with an apogee of about 597,000 kilometers which would intercept the Moon during the outbound leg. Following TLI, the CSM would extract the LM from the S-IVB stage and make a brief burn of the SPS to move away from the spent stage for the 73-hour trip to the Moon. Afterwards, the S-IVB stage would turn and vent its residual cryogenic propellants to deflect itself further away from Apollo 10 and eventually fly past the trailing edge of the Moon and into solar orbit.

During the three-day trip to the Moon, the Apollo 10 crew would perform their normal duties checking out and maintaining spacecraft systems. During the mission, a dozen live telecasts from the CSM were planned using a new 5.4-kilogram color television camera built by Westinghouse. A smaller RCA black and white camera used during the earlier Apollo missions was also carried as a backup. Using navigation readings taken by the crew in combination with tracking data from the ground, Apollo 10 would make a single midcourse correction using the SPS and up to three minor corrections using the CSM’s smaller RCS (Reaction Control System) engines during the transit to the Moon. Like the earlier Apollo 8 mission, Apollo 10 would follow a free return trajectory that would pass the leading edge of the Moon and fly about 110 kilometers above the lunar far side. In case of a problem with the SPS, Apollo 10 would simply loop around the Moon and return back to Earth after a total trip time of 150 hours without the need of any additional propulsive maneuvers. A similar free return trajectory had already been used by the unmanned Soviet Zond 5 and 6 missions launched in September and November of 1968 as test flights of the 7K-L1 variant of the Soyuz for a planned manned circumlunar flight.

Assuming that the SPS and other key systems on Apollo 10 were operating properly once the Moon was reached, the spacecraft would perform its Lunar Orbit Insertion (LOI) as it passed behind the Moon where it would be out of touch of ground controllers. The nominal 906 meter per second retrograde burn would then place Apollo 10 into an initial 111 by 315 kilometer lunar orbit. After two orbits, the SPS would perform a second burn for a delta-v of 42 meters per second to place the spacecraft into a circular 111-kilometer orbit. This two-step orbit insertion process was adopted to minimize the chances that Apollo would inadvertently crash into the Moon in the case of an excessive LOI burn.

Activities the day after LOI would center on testing the LM in lunar orbit. Initially it was proposed that the LM would separate from the CSM and fly some distance away before performing a rendezvous and docking similar to the Apollo 9 flight profile. But by December 1968, mission planners pushed for a more realistic test which would use the LM’s DPS (Descent Propulsion System) to perform the DOI (Descent Orbit Injection) burn to place the spacecraft into a descent orbit with a perilune of just 15 kilometers as would be done during an actual lunar landing mission. During the low pass over the Moon, the crew would photograph the proposed first lunar landing site, designated Site #2 in Mare Tranquilitatis, under similar lighting conditions expected for the upcoming landing mission. During this overpass, the LM’s landing radar would also be tested on the Moon for the first time to ensure that it could lock onto the lunar surface and give accurate readings.

Instead of firing the DPS for the final descent to the lunar surface near perilune, the engine would be fired for a 59 meter per second posigrade delta-v which would raise the apolune from 111 to 359 kilometers. This maneuver would allow the CSM to overtake the LM during the subsequent orbit and place the CSM ahead of the LM for its next perilune. During its second perilune, the LM would jettison its descent stage and then fire its APS (Ascent Propulsion System) for AOI (Ascent Orbit Insertion) to simulate a return to orbit from the lunar surface. Over the next 3½ hours, the LM ascent stage would perform a series of maneuvers and ultimately rendezvous and dock with the CSM in lunar orbit. During its eight hours of independent flight, the LM would be able to test the rendezvous radar system out to distances as great as about 500 kilometers (over three times the distance during the Apollo 9 tests) as well as the procedures to descend towards and return from the lunar surface.

After the LM crew returned to the CSM, the LM ascent stage would be jettisoned. By ground command, the LM APS would be ignited once again and burn until the depletion of its remaining propellants. The estimated delta-v of 1,170 meters per second would allow the LM ascent stage to escape the Moon and enter solar orbit. After an additional 29 hours in lunar orbit performing landmark sighting exercises and photographing future Apollo landing sites, the SPS would be fired again for the TEI (Trans Earth Injection) to return Apollo 10 to the Earth after a total of 61½ hours in lunar orbit. The TEI with a delta-v of 1,104 meters per second would start Apollo 10 on its 54-hour trip back to Earth. During that time, up to three midcourse maneuvers were planned to fine tune the spacecraft’s aim for reentry. The Apollo 10 CM would then splashdown in the mid-Pacific Ocean after a total mission time of 192 hours. With the successful completion of the Apollo 10 mission objectives, the way would be clear to attempt an actual lunar landing.

Getting Underway

The first piece of Apollo 10 mission hardware to arrive at Kennedy Space Center (KSC) in Florida was the descent stage of LM-4 on October 11, 1968 followed by its ascent stage five days later. Unlike LM-3, which had many problems after delivery, LM-4 was in good condition for flight. Its stages were mated on November 2 and LM-4 completed a combined system test four days later. In the meantime, CSM-106 had completed its factory testing by October 19 and was delivered to KSC on November 24. The modules were mated on November 25 to begin their preflight testing.

The first stage of the SA-505 launch vehicle, designated S-IC-5, arrived by barge at KSC on November 26 with delivery of the second stage, designated S-II-5, accepted on December 3. The S-IVB-505N third stage arrived by air the same day. With SA-504 being prepared for the Apollo 9 mission in Bay 1 of the giant Vertical Assembly Building (VAB) at LC-39, SA-505 was stacked on Mobile Launch Platform 3 (MLP-3) in Bay 2 – the first time this bay had been used since the then-new VAB started supporting Saturn V operations three years earlier (see “The Saturn 500F: The Moon Rocket That Couldn’t Fly”). The stacking of SA-505 was completed on December 30.

On January 10, 1969, NASA officials changed the launch date from its original placeholder of May 1 to May 17 to line up with the lunar launch window needed to simulate a landing at Site #2. CSM-106 completed its testing at Kennedy’s high altitude chamber on January 17 and was moved to the VAB on February 3 for stacking. With the spacecraft’s overall testing completed, Apollo 10 was moved out of the VAB and transferred to LC-39B on March 11 to continue launch preparations there. On March 17, the launch date was pushed out by a day to May 18 to allow a better view of Site #3 further to the west. With the final countdown demonstration test completed on May 6, Apollo 10 was clear to make its first launch attempt on May 18 during a window extending from 12:49 to 5:09 PM EDT.

The terminal countdown for Apollo 10 started at 9:00 PM EDT on May 16, 1969. The only major issue experienced was a failed primary pump for the LOX replenishment system at the T-8 hour mark. The problem took 50 minutes to diagnose and correct delaying LOX loading as a result. Still, LOX loading was completed at T-4 hours, 22 minutes with the lost time made up during a preplanned one-hour hold at the T-3 hour, 30 minute mark. The countdown resumed after this hold at 9:19 AM on May 18 as the crew was preparing for their flight.

With no further delays in the countdown, the 2,942 metric ton Apollo 10 lifted off from LC-39B right on schedule at 12:49:00 PM EDT (16:49:00 GMT) on May 18 into mainly cloudy skies. The ascending Saturn V under the power of its five F-1 engines hit Mach one 66.8 seconds after launch and encountered maximum dynamic pressure 15.8 seconds later. With the experience of five comparatively smooth Gemini-Titan II launches between them, the Apollo 10 crew began to experience increasing amounts of shaking as the Saturn V ascended. The center F-1 engine was shutdown 135.2 seconds after launch in order to limit the acceleration load to less than 4 gs. This event was followed by the shutdown of the remaining four outboard engines 26.5 seconds later at an altitude of 65.3 kilometers. After a nearly perfect performance, S-IC-5 was commanded to cut lose 0.7 seconds after engine shutdown where the S-II-5 stage ignited and continued the ascent under the power of its five J-2 engines. The spent S-IC-5 stage impacted in the Atlantic Ocean an estimated 539 seconds after launch about 646 kilometers downrange.

As S-II-5 was firing, Apollo’s launch escape tower was jettisoned 197.8 seconds after launch since it was no longer needed to support abort options during the rest of the ascent. As the climb to orbit continued, the crew experienced much more shaking than they had on earlier launches. In order to help smooth out the ascent and lessen longitudinal vibrations (an effect called “pogo”) encountered during the Apollo 9 ascent, the center engine of the S-II was shutdown early at seven minutes and 41 seconds after launch. The remaining four J-2 engines continued burning for an extra 17 seconds compared to previous flights with engine shutdown coming nine minutes, 12.6 seconds after launch at an altitude of 187.4 kilometers some 1,636 kilometers downrange. The S-IVB-505N stage then ignited its single J-2 engine after separation to continue pushing its payload to orbital velocity with its first burn of the mission. The spent S-II-5 stage then tumbled to Earth impacting about 20 minutes and 18 seconds after launch about 4,425 kilometers downrange in the Atlantic.

The S-IVB-505N stage finally shut down its single J-2 engine 11 minutes, 43.8 seconds after launch – just 0.3 seconds later than planned. Apollo 10 was now in a nearly perfect 184.7 by 185.8 kilometer parking orbit with an inclination of 32.5°. With an estimated in-orbit mass of 133.8 metric tons, Apollo 10 was the heaviest object ever placed into Earth orbit up until this time beating the previous record holder, Apollo 9 with its attached S-IVB stage, by 1.3 metric tons. The Apollo spacecraft alone, with a total mass of 46.5 metric tons at this point, was heavier than all ten crewed Gemini spacecraft launched in 1965 and 1966 combined.

Heading for the Moon

Once in orbit, the task of checking out all systems on Apollo 10 started in preparation for the go/no-go decision for TLI. Apollo 10 flew in a nose-forward attitude controlled by the pair of APS (Auxiliary Propulsion System) pods at the base of the S-IVB stage. As the first revolution was being completed, the CM’s docking probe was extended and the navigation computer received state updates. With the “go” given for TLI after 1½ revolutions, the orbit of Apollo 10 had risen to 189.1 by 201.0 kilometers prior to the preparation for TLI due to venting from the S-IVB stage. The J-2 engine on S-IVB-505N reignited at 19:22:27 GMT at an altitude of 197.9 kilometers. At the end of the five minute, 43 second TLI burn (just under a second longer than planned), Apollo 10 was at an altitude of 333.1 kilometers with an inertial velocity of 10,837 meters per second. With a velocity just shy of Earth escape velocity, Apollo 10 was now in an elongated 220.1 by 596,286 kilometer geocentric orbit on course for the Moon.

After the S-IVB had stabilized its attitude for the next phase of the mission, CMP John Young used a 3.3-second burst from the CSM’s RCS thrusters to separate from the S-IVB at 19:51:42 GMT as they were passing an altitude of 6,532 kilometers. He turned the CSM and came to a halt about 45 meters away (three times farther than planned) as the four panels of the spacecraft launch adapter (SLA) were jettisoned revealing the LM, Snoopy, for the first time. At 19:55:00 GMT, the crew began their first 22-minute color television broadcast as the CSM, Charlie Brown, made its approach and finally docked with Snoopy at 20:06:36 GMT for a worldwide audience to see. After the camera was shut off, the crew made sure that the spacecraft were properly latched together in preparation for extracting the LM from the spent S-IVB stage. The live telecast resumed at 20:45:00 GMT with separation of the S-IVB coming 25 seconds later. With Apollo 10 now in free flight, the second telecast was ended after almost 13½ minutes. At 21:28:09 GMT while passing an altitude of 33,222 kilometers, the CSM ignited the main engine of the SPS for a 2.7-second burn as part an evasive maneuver with a delta-v of 5.7 meters per second to move safely away from S-IVB-505N.

Afterwards, S-IVB-505N started a 47-minute sequence of events at 21:31:15 GMT to dump its residual propellants through the J-2 engine and vent its tanks followed by the firing the APS ullage engines until propellant depletion. The impulse from these actions resulted in a delta-v of 41.3 meters per second altering the course of S-IVB-505N so that it passed the trailing edge of the Moon at 23:40 GMT on May 21 (78.9 hours after launch) at a minimum altitude of 3,112 kilometers. The lunar flyby then increased the velocity of S-IVB-505N relative to the Earth by 805 meters per second sending the spent stage into a 0.908 by 1.017 AU solar orbit with a period of 344.88 days.

With Apollo 10 now well on its way to the Moon, the crew got out of their bulky A7L spacesuits they had been wearing and donned light weight flight suits. They then began the normal tasks of system checks, housekeeping duties and navigation checks that would dominate their schedule for the next three days. The crew also welcomed the opportunity to perform live telecasts so that Mission Control and television viewers worldwide could view this historic test flight. Two additional telecasts during the first day in space, one starting at 21:55 and the other two hours, five minutes later, lasted 13 and 24 minutes, respectively, and gave the crew a chance to show the interior of their cabin as well as the receding Earth out the CM windows. Before settling down for their first rest period of the mission, Apollo 10 was placed in a slow roll of about two revolutions per minute. This “barbeque mode” helped to even out the solar heating on the spacecraft aiding in thermal control.

Getting into Lunar Orbit

The morning after launch, the crew noted an issue with their drinking water as they were preparing breakfast. Apparently, Stafford had inadvertently doubled the amount of chlorine added to the drinking water generated as a byproduct of the electricity-producing fuel cells. While still safe to drink, the unpleasant taste (as well as the excess gas noted in the water during previous Apollo flights) combined with the sometimes less-than-appetizing quality of the rehydrated meals bothered the crew. Stafford, Young and Cernan were not known for their appetites to start with, so they ended up skipping some of their meals over the days to come. Aside from minor inconveniences, the crew was in good health compared to the previous Apollo flights. All three experienced stuffy heads once weightless but it passed in a day or so. Only Gene Cernan experienced a mild case of nausea from space sickness and commented how the exercise they were instructed to perform to lessen the severity of the symptoms actually made the symptoms worse. But the sensation subsided and never became a problem as it did for LMP Rusty Schweickart during the Apollo 9 mission 2½ months earlier.

At 19:21:56 GMT on May 19, the SPS was ignited for a 7.1-second burn 203,998 kilometers from the Earth. This midcourse correction with a delta-v of 15.0 meters per second set Apollo 10 on a course which would simulate the approach that Apollo 11 would use two months later. The navigation was so accurate that no other course corrections were required. During the trip to the Moon, the crew performed a half dozen more telecasts providing views of the spacecraft interior and the shrinking Earth. Unlike the Apollo 8 crew who did not see the Moon until they were about to go into orbit, the Apollo 10 crew were able to spot the waxing crescent Moon and even observe the unlit side by means of earthshine (although they could not spot any landmarks). Cernan was also able to spot the spent S-IVB-505N stage at a range of 7,400 kilometers.

With its velocity steadily decreasing since TLI, Apollo 10 reached the point where the gravitational attraction of the Earth and Moon are equal at 06:39:50 GMT on May 21. From this point onward, the velocity of Apollo 10 would start to increase as it fell into the lunar gravity well for the next 14 hours. At 17:26:26 GMT, the crew held a final live telecast before Apollo 10 reached the Moon. The crew was informed by Mission Control that at one time or another, more than one billion people around the globe had watched their activities.

As predicted, contact with Apollo 10 was lost at 20:37:24 GMT as the spacecraft slipped behind the Moon as viewed from the Earth. At 20:44:54 GMT, with Apollo 10 travelling at 2,508 meters per second, the SPS came to life for the third time in the mission for the start of LOI. After a burn which lasted just six seconds shy of six minutes, the velocity of Apollo 10 had been cut by 909 meters per second to settle into an initial 111.5 by 314.8 kilometer retrograde orbit around the Moon. Now that they were in orbit, the crew had a chance to look at the lunar landscape closeup up for the first time. Since all were veterans of low Earth orbit missions, they noted how much more slowly the spacecraft seemed to move over the lunar surface. Once Apollo 10 had reestablished contact with Mission Control, Stafford announced “Houston, tell the Earth that we had arrived.”

As planned, Apollo 10 fired its SPS again for 13.9 seconds starting at 01:14:08 GMT on May 22 near the end of the second revolution. The 42.5 meter per second delta-V placed Apollo 10 into its final, nearly circular 109.6 by 113.0 kilometer orbit. During their next pass over the lunar near side, the crew started a 29-minute live telecast at 01:33:40 GMT where they gave a running commentary about the lunar features being observed for their audience back home.

Six hours after entering lunar orbit, Stafford and Cernan began to check out the LM for the first time in the mission. At 02:19 GMT, the LM cabin was pressurized. The docking probe, drogue and the hatches were easily removed but a shower of small pieces of Mylar insulation was created requiring cleanup. Cernan entered Snoopy for the first time at 02:44 GMT to finish the cleanup and begin powering up the spacecraft. After testing key systems, Stafford and Cernan returned to Charlie Brown and sealed the hatches at 05:19 GMT. The crew would take a well-deserved rest period before the next day’s hectic schedule where the LM would be tested in free flight.

Testing Snoopy in Lunar Orbit

Mission Control had intended to let the astronauts sleep in as long as possible on May 22 to get as much rest as possible. But when they radioed to wake them up with the song, “The Best Is Yet to Come”, and reveille, Houston found that the Apollo 10 crew had already been up for some time, eaten breakfast and had begun work on their flight plan checklist. At 15:51 GMT, Stafford and Cernan entered the LM to begin activating the lander’s systems and configure them for the upcoming tests. They were concerned when they noticed that the LM had slipped about 3½° out of alignment at the docking interface. The fear was that undocking might shear the docking latches preventing a hard docking after Snoopy returned. They were assured by Mission Control that there was no concern as long as the misalignment was less than 6°. With Stafford and Cernan clad in their A7L spacesuits and Snoopy’s landing gear successfully deployed at 18:49 GMT, everyone was ready to go.

At 19:00:57 GMT, Charlie Brown and Snoopy undocked during the 12th revolution around the Moon. Young, now alone in the CSM, used the RCS thrusters to back off to a distance of nine meters to inspect Snoopy. He turned on the color television camera about 17 minutes after undocking for a 20-minute telecast so that Mission Control could monitor events. At 19:36:17 GMT, Young fired Charlie Brown’s thrusters again for 8.3 seconds for a delta-v of just 0.8 meters per second placing the CSM into a 106.9 by 116.5 kilometer orbit. This orbit would allow Charlie Brown to move 3.3 kilometers ahead of Snoopy after a half an orbit for DOI and the beginning of the LM’s descent. If DOI failed, this orbit would also automatically bring the CSM back in the vicinity of the LM an hour later.

As Charlie Brown and Snoopy were about to pass behind the Moon following further LM system checks and dealing with a CSM radar transponder issue, Stafford and Cernan were given the go-ahead to proceed with DOI on the lunar far side. At 20:35:01 GMT, Tom Stafford ignited the DPS first with the engine at 10% thrust then throttled up to 40% for the last 12 seconds of the 27.4-second DOI burn. With the velocity cut by 21.7 meters per second, Snoopy was now in a 15.7 by 112.8 kilometer orbit whose low point would be over Mare Tranquilitatis.

With Charlie Brown coming around from the lunar far side ahead of the descending Snoopy, Young was able to report to Mission Control that DOI had been successful. Young continued to relay radar ranging data as Snoopy closed in on its perilune. Meanwhile, Stafford and Cernan continued their system checks and gave running commentary of their progress as Stafford snapped photographs of the Moon below. Unfortunately, the picture taking came to a premature halt when Stafford’s camera jammed. As Snoopy approached the low point of its descent orbit, the LM’s landing radar was able to lock onto the lunar surface. The low point of the approach was measured to be 14.4 kilometers at 21:30:42 GMT. With a great view of Site #2, Stafford noted that the near side of the site appeared fairly smooth with the far end appearing somewhat rougher and potentially requiring more propellant to maneuver around rough areas. Aside from this, there were no problems with surface feature visibility or the sun angle.

After perilune, Stafford ignited the DPS for a second time at 21:47:25 GMT. The 40-second burn with the DPS throttled to 100% increased Snoopy’s velocity by 53.6 meters per second placing the LM into a 22.4 by 352.1 kilometer orbit. This phasing orbit would allow Charlie Brown to catch up and pass Snoopy over the following lap around the Moon getting both spacecraft into the proper relative positions for ascent and rendezvous.

The Ascent

All was going much as planned until Snoopy was approaching its second perilune pass when the descent stage would be jettisoned and the LM’s APS fired for AOI. Stafford had gotten the LM into the proper attitude for the upcoming events about ten minutes ahead of schedule. After experiencing some minor attitude issues, Snoopy unexpectedly began to wildly turn in pitch and yaw at 23:33:49 GMT. As Cernan commanded the LM to thrust forward, Stafford hit the button to jettison the descent stage at 23:34:16 GMT with the attitude some 30° out of the expected alignment. Stafford then took manual control pitching Snoopy’s ascent stage forward while reconfiguring the attitude control mode. The situation was resolved only about a half a minute after the rapid uncontrolled turning had started. It was found that the abort guidance system, which was being tested at this point, was accidentally switched from the “Attitude Hold” to “Auto” mode during a minor yaw correction. In the auto mode, Snoopy dutifully began to adjust its attitude to track the Charlie Brown overhead instead of holding still to jettison the descent stage.

With the ascent stage under control, the APS was ignited at 23:44:02 GMT for the 15.6-second AOI burn. With a delta-v of 67.3 meters per second, the burn reshaped Snoopy’s trajectory into a 20.4 by 86.1 kilometer orbit which would simulate the LM’s ascent from the lunar surface. Shortly after AOI, Stafford and Cernan got a radar lock on the CSM as the distance between them closed from 460 to 280 kilometers during the next half an orbit. Young, on board Charlie Brown, was able to sight the ascending Snoopy through a sextant at a range of 259 kilometers. After coasting for 50 minutes (as the GMT date moved from May 22 to 23), Snoopy’s RCS thrusters were ignited at 00:34:55 GMT for a 27.3-second burn called the “coelliptical sequence”. This 13.8 meter per second maneuver circularized the orbit somewhat to 75.3 by 90.2 kilometers allowing the LM to continue closing on the CSM but at a slower pace.

A half an orbit later with Charlie Brown now 140 kilometers ahead and 25 kilometers above Snoopy, the LM’s RCS thrusters were ignited at 01:32:53 GMT for just 1.7 seconds for the “constant differential height” maneuver which raised Snoopy’s perilune to 78.0 kilometers. Almost half an orbit later at 02:11:55 GMT, the LM’s RCS were fired again for 16.5 seconds for the “terminal phase initiation” which placed Snoopy on course to rendezvous with Charlie Brown. Two small course correction burns were performed using the LM’s RCS about 15 and 29 minutes later. Terminal phase braking using the LM’s RCS started at 02:54:49 GMT with Snoopy coming to stop only eight meters from Charlie Brown. Young was able to slip the CM’s probe into the LM’s drogue with a thruster firing by Stafford getting the two docked at 03:11:02 GMT. Although some problems were encountered, Snoopy performed well during its eight hours of independent flight clearing the way for an actual lunar landing.

Twenty minutes after docking, Stafford and Cernan rejoined Young in the CSM. At 04:09 GMT, closeout activities were started in the LM which was finally jettisoned at 05:13:36 GMT. For its last test, Snoopy ignited its APS by ground command at 05:41:05 GMT. The 249-second burn with its delta-v of 1,400 meters per second sent Snoopy into solar orbit. Contact with the derelict spacecraft was maintained until about 16:50 GMT when the ascent stage battery was depleted.

Heading Home

With the major mission milestones met, the crew of Apollo 10 had another day left in lunar orbit. They would continue to photograph sites of interest on the lunar surface and well as perform orbital navigation and landmark sighting exercises. At 04:56:12 GMT on May 24, the crew performed their last telecast from lunar orbit for 26 minutes. Two earlier broadcasts had been deleted from the schedule to give the crew time to rest. With all systems go, Apollo 10 went behind the Moon for the last time during its 31st revolution and ignited the SPS for the TEI burn at 10:25:28 GMT. The 164.8-second burn of the SPS main engine increased the velocity of Apollo 10 by 1,121.8 meters per second to begin the 54-hour trip back to Earth.

Now on their way back home, the mood of the Apollo 10 crew was much lighter. At 10:39:51 GMT (less than 12 minutes after the end of TEI), a 43-minute telecast was started at Gene Cernan’s urging. Almost immediately, the recorded voice of Dean Martin singing the familiar song, “Going Back to Houston”, was heard from Apollo 10 to start the broadcast. There would be five more telecasts from Apollo 10 during the next two days showing the receding Moon and the crew inside the CM cabin. Navigation fixes during the coast back to Earth showed that TEI was so accurate only a single midcourse correction maneuver was required just three hours before reentry – a 6.7-second burn of the RCS thrusters starting at 13:38:48 GMT on May 26 for a delta-V of just 0.7 meters per second.

After the SM was jettisoned at 16:22:26 GMT, the CM was maneuvered into the proper attitude for reentry. Apollo 10 reached the entry interface at an altitude of 122 kilometers at 16:37:54 GMT travelling at 11,069 meters per second – the fastest reentry of a manned spacecraft. With a maximum braking load of 6.78g reached 80 seconds later, Apollo 10 came out of its 4½-minute communications blackout and deployed its drogue chute at 16:46:18 GMT. After 47 seconds, the trio of main parachutes were deployed followed by splashdown at 16:52:23 GMT in the Pacific Ocean at 19.4°S, 173.37°W northeast of Australia. After a mission lasting eight days, three minutes and 23 seconds, Apollo 10 came down only 2.4 kilometers from its aiming point and 5.4 kilometers from the primary recovery ship, the USS Princeton – an Essex-class carrier just eight months from its decommissioning.

Rescue divers attached a floatation collar to the CM as it bobbed in 0.9-meter swells with the crew hatch opened 25 minutes after splashdown. The crew exited the CM into a life raft and were subsequently hoisted on board a helicopter which had them on the deck of the USS Princeton at 17:31 GMT. The CM was brought on board less than an hour later as the astronauts were greeted by the officers and crew of the recovery ship. The crew and spacecraft were returned to the mainland for debriefings and examinations. Although some minor issues had been encountered, Apollo 10 was a resounding success with virtually all the primary and secondary mission objectives achieved. With this successful rehearsal, NASA was ready to commit to a manned lunar landing for the Apollo 11 mission two months out.

After the Apollo 10 mission, Tom Stafford replaced Alan Shepard (who had returned to flight status) in July 1969 as Chief of the Astronaut Office. In 1972 he was promoted to brigadier general and assigned as the Commander of the American part of the ASTP (Apollo Soyuz Test Project) launched in July 1975. He then went on to command the Air Force Flight Test center at Edwards AFB in California before retiring in November 1979. John Young went on to be the backup Commander of Apollo 13 then the Commander of the Apollo 16 lunar landing mission flown in April 1972. In January 1974, Young began a 13-year stint as Chief of the Astronaut Office and went on to command the STS-1 and STS-9 Space Shuttle missions in April 1981 and November 1983, respectively. Gene Cernan was the backup Commander for the Apollo 14 mission and given command of the final Apollo lunar mission, Apollo 17, flown in December 1972. Cernan retired from NASA and the US Navy with the rank of captain in 1976 before moving on to a career in private business.

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Related Video

Here is an excellent NASA documentary about the Apollo 10 mission entitled To Sort Out the Unknowns.

Related Reading

“Apollo 9: Giving the ‘Spider’ Wings”, Drew Ex Machina, March 19, 2019 [Post]

“Apollo 8: Going Where No One Has Gone Before”, Drew Ex Machina, January 5, 2019 [Post]

General References

David Baker, The History of Manned Space Flight, Crown Publishers, 1981

Courtney G. Brooks, James M. Grimwood and Loyd S. Swenson, Jr., Chariots for Apollo: The NASA History of Manned Lunar Spacecraft to 1969, Dover, 2009

Alan Lawrie & Robert Godwin, Saturn V The Complete Manufacturing and Testing Records, Apogee Books, 2005

Richard W. Orloff and David M. Harland, Apollo: The Definitive Sourcebook, Springer-Praxis, 2006

Apollo 10 Press Kit, NASA Press Release 69-68, May 7, 1969

Saturn S-IVB-505N Stage Flight Evaluation Report, McDonnell Douglas Astronautics Company, August 1969

Saturn V Launch Vehicle Evaluation Report – AS-505 Apollo 10 Mission, MPR-SAT-FE-69-7, NASA Marshall Space Flight Center, 1969