Apollo 10, the fourth crewed mission in the Apollo program (NASA’s project to land humans on the moon), and the second (after Apollo 8) to orbit the Moon was launched on On May 18, 1969.

Today’s (May 18) story of what happened this day in Science, Technology, Astronomy, and Space Exploration history.

Apollo 10

The mission of Apollo 10 was to test all aspects of an actual crewed lunar landing, including testing the components and procedures, except the landing itself. It was a “dress rehearsal” of a moon landing.

It was the first flight of a complete, crewed Apollo spacecraft to operate around the Moon (Apollo 8 was a command-module-only lunar orbital flight). The crew members were:

  • Commander: Thomas P. Stafford (third spaceflight)
  • Command Module Pilot: John W. Young (third spaceflight)
  • Lunar Module Pilot: Eugene A. Cernan (second spaceflight)
Apollo 10 launch
The Apollo 10 (Spacecraft 106/Lunar Module 4/Saturn 505) space vehicle is launched from Pad B, Launch Complex 39, Kennedy Space Center at 12:49 p.m. (EDT).

Apollo 10 to “Sort Out the Unknowns”

As the mission Commander Thomas P. Stafford stated during a preflight press conference, Apollo 10 was planned to “sort out all the unknowns” to make the Moon landing possible.

In other words, the mission was a “dress rehearsal” for the lunar landing mission, a goal President John F. Kennedy set for the nation just eight years earlier.

Apollo 10 was the first mission to carry a color television camera inside the spacecraft and made the first live color TV transmissions from space.

Mission Objectives included:

  • A scheduled eight-hour lunar orbit of the separated lunar module, or LM
  • Lunar Module’s descent to about 9 miles (14.5 km) above the moon’s surface before ascending for rendezvous. This was the point where powered descent would begin on the actual landing.
  • Lunar Module docking with the command and service module, or CSM, in about a 70-mile (112.5 km) circular lunar orbit.
  • Pertinent data to be gathered in this landing rehearsal dealt with the lunar potential, or gravitational effect, to refine the Earth-based crewed spaceflight network tracking techniques, and to check out LM programmed trajectories and radar, and lunar flight control systems.
  • Twelve television transmissions to Earth were planned.

All mission objectives were achieved.

Apollo 10 crew
Apollo 10 crew of (left to right) Eugene A. Cernan, John W. Young, and Thomas P. Stafford pose in front of their Saturn V rocket at Kennedy Space Center’s Launch Pad 39B. Photo: NASA

Apollo 10 mission highlights

Earth orbit phase

  • Apollo 10 was launched on a Saturn V SA-505 rocket on May 18, 1969, at 16:49:00 UTC.
  • 11 minutes and 53 seconds later, the three stages of the mighty Saturn V placed the Apollo 10 spacecraft, at 98,273 pounds (44,576 kg) the heaviest ever launched, into a temporary parking orbit around the Earth. The apogee and perigee of parking orbit were 100.32 by 99.71 nautical miles (115.5 by 114.75 miles or 185.8 by 184.6 km). The orbital speed of the parking orbit was 7793 meters per second (about 17,430 mph or 28,000 km/h).

Translunar phase

  • After inflight systems checks, the 343.08-second translunar injection maneuver (second S-IVB firing) was performed at 2:33:27.52. The S-IVB engine shut down at 2:39:10.58 and translunar injection occurred ten seconds later, after 1.5 Earth orbits lasting 2 hours 27 minutes 16.82 seconds.
  • At 3:02:42.4, the Command and Service Module (CSM) was separated from the S-IVB stage. It was transposed and then docked with the Lunar Module (LM) at 003:17:36.0.
  • The docked spacecraft were ejected at 3:56:25.7 and a separation maneuver was performed at 004:39:09.8.
  • A ground command for propulsive venting of residual propellants targeted the S-IVB to go past the Moon. The closest approach of the S-IVB to the Moon was 1,680 nautical miles (1,933 miles or 3,111 km), at 078:51:03.6 on May 21 at 23:40 GMT.
  • A preplanned, 7.1-second, midcourse correction was executed at 26:32:56.8 and adjusted the trajectory to coincide with a July lunar landing trajectory (Apollo 11 lunar landing trajectory). The maneuver was so accurate that two additional planned midcourse corrections were canceled.
  • At 075:55:54.0, at an altitude of 95.1 nautical miles (110 miles or 176 km) above the Moon, the service propulsion engine was fired for 356.1 seconds to insert the spacecraft into a lunar orbit of 170.0 by 60.2 nautical miles (195.6 by 69.3 miles or 314.8 by 111.5 km). The translunar coast had lasted 73 hours 22 minutes 29.5 seconds.

The translunar phase was televised to Earth starting at 3:06:00 for 22 minutes and from 3:56:00 for 13 minutes 25 seconds.

Lunar Orbit Phase

  • After two revolutions of tracking and ground updates, a 13.9-second maneuver was performed at 80:25:08.1 to circularize the lunar orbit at 61.0 by 59.2 nautical miles (70.2 by 68.1 miles or 113 by 109.6 km).
  • At 81:55, Apollo 10 lunar module pilot Eugene A. Cernan entered the LM at 081:55 for two hours of “housekeeping” activities and some LM communications tests.
  • At 95:02, mission commander Thomas P. Stafford and the lunar module pilot Eugene A. Cernan entered to activate LM systems and discovered that the LM had moved 3.5 degrees out of line with the CM. The crew feared that separating the two spacecraft might shear off some of the latching pins, possibly preventing redocking. But mission control reported that as long as the misalignment was less than six degrees, there would be no problem.
  • Undocking occurred at 098:29:20 and was televised for 20 minutes and 10 seconds starting at 98:13:00. During this period, the LM landing gear was deployed and all LM systems checked out.
  • At 98:47:17.4, an 8.3-second CSM reaction control system maneuver separated the CSM to about 30 feet (9.1 meters) from the LM. The CSM was in an orbit of 62.9 by 57.7 nautical miles (72.4 by 66.4 miles or 116.5 by 107 km) at the time. Stationkeeping was initiated at this point while the command module pilot visually inspected the LM. The CSM reaction control system was then used to perform the separation maneuver directed radially downward toward the Moon’s center. This maneuver provided a separation at descent orbit insertion of about 2 nautical miles (2.3 miles or 3.7 km) from the LM.
  • Following stationkeeping, a 27.4-second LM descent propulsion system burn at 99:46:01.6 inserted the LM into a descent orbit of 60.9 by 8.5 nautical miles (70 by 9.78 miles or 112.8 by 15.75 km) so that the resulting lowest point in the orbit occurred about 15° from lunar landing site 2 (Apollo 11’s landing site). Numerous photographs of the lunar surface were taken.
  • An hour later, the LM made a low-level pass over Apollo landing site 2. The pass was highlighted by a test of the landing radar, visual observation of lunar lighting, stereoscopic strip photography, and execution of the phasing maneuver using the descent engine. The lowest measured point in the trajectory was 47,400 feet (14,448 meters) above the lunar surface at 100:41:43.
  • The second LM maneuver, a 39.95-second descent propulsion system phasing burn at 100:58:25.93, established a lead angle equivalent to that which would occur at powered ascent cutoff during a lunar landing and put the Apollo 10’s LM into an orbit of 190.1 by 12.1 nautical miles (218.76 by 13.92 miles or 352 by 22.4 km).
  • At 102:44:49, during preparations for rendezvous with the CSM, the LM started to wallow off slowly in yaw and then stopped. At 102:45:12, it started a rapid roll accompanied by small pitch and yaw rates. The ascent stage was then separated from the descent stage at 102:45:16.9 at an altitude of 31.4 nautical miles (36.1 miles or 58.15 km) and the motion was stopped eight seconds later. A 15.55-second firing of the ascent engine at 102:55:02.13 placed the ascent stage into an orbit of 46.5 by 11.0 nautical miles (53.5 by 12.7 miles or 86 by 20.4 km). The descent stage went into lunar orbit.
  • At the orbital low point, the insertion maneuver was performed on time using the LM ascent propulsion system. This burn established the equivalent of the standard LM insertion orbit of a lunar landing mission (45 x 11.2 nautical miles, 51.8 x 12.89 miles, or 83.3 x 20.74 km). The LM coasted in that orbit for about one hour. The terminal maneuver occurred at about the midpoint of darkness, and braking during the terminal phase finalization was performed manually as planned.
  • The rendezvous simulated one that would follow a normal ascent from the lunar surface. It started with a 27.3-second LM coelliptic sequence initiation maneuver at 103:45:55.3, which placed the spacecraft into an orbit of 48.7 by 40.7 nautical miles (56 by 46.83 miles or 90.1 by 75.38 km). This was followed by a 1.65-second constant differential height maneuver at 104:43:53.28 which raised the perigee to 42.1 nautical miles (48.45 miles or 78 km). The 16.50-second terminal phase initiation maneuver at 105:22:55.58 then raised the orbit to 58.3 by 46.8 nautical miles (67 by 53.85 miles or 108 by 86.67 km). Docking was complete at 106:22:02 at an altitude of 54.7 nautical miles (63 miles or 101.3 km) after 8 hours 10 minutes 5 seconds of lunar flight.
  • Once docked, the LM crew transferred the exposed film packets to the CM. The LM ascent stage was jettisoned at 108:24:36. A 6.5-second separation maneuver at 108:43:23.3 raised the LM orbit to 64.0 by 56.3 nautical miles (73.65 by 64.8 miles or 118.5 by 104.3 km). This was followed at 108:52:05.5 (about one revolution after jettison) by a 249.0-second remote control firing to depletion of the ascent engine. This burn, commanded as planned, utilized the LM ascent engine arming assembly and was targeted to place the LM into a solar orbit. Communications were maintained until LM ascent stage battery depletion at about 120:00. The ascent stage batteries lasted about 12 hours after LM jettison.
  • Prior to transearth injection, views of the lunar surface and spacecraft interior were transmitted to Earth for 24 minutes 12 seconds starting at 132:07:12.
  • After a rest period, the Apollo 10 crew conducted landmark tracking and photography exercises. During the remaining lunar orbital period or operation, 18 landmark sightings, and extensive stereo and oblique photographs were taken. Two scheduled TV periods were deleted because of crew fatigue.
  • The transearth injection was achieved at 137:39:13.7 at a velocity of 2739 meters per second (6,127 mph or 9,861 km/h), following a 164.8-second engine firing at 56.5 nautical miles (65 miles or 104.6 km) altitude. The Apollo 10 spacecraft had been in lunar orbit for 31 lunar orbits lasting 61 hours 37 minutes 23.6 seconds.
Apollo 10 Lunar Module Ascends
After dropping down to 47,400 feet (14,448 meters) above the moon’s surface, Thomas P. Stafford and Eugene A. Cernan aboard the ascent stage of the Apollo 10 lunar module, return to John W. Young in the command module on May 22, 1969. Photo: NASA, John W. Young
A nearly averted disaster

During descent stage separation, the lunar module began to roll unexpectedly because the crew accidentally duplicated commands into the flight computer which took the LM out of abort mode, the correct configuration for this maneuver. The live network broadcasts caught Cernan and Stafford uttering several expletives before regaining control of the LM. The commander took over manual control to reestablish the proper attitude.

Decades later, Cernan said he observed the horizon spinning eight times over, indicating eight rolls of the spacecraft under ascent engine power. Recordings from the flight do not support this dramatic memory.

While the incident was downplayed by NASA, the roll was just several revolutions from being unrecoverable, which would have resulted in the Lunar Module (LM) crashing into the lunar surface.

Transearth Phase

  • Transearth activities included a number of star-Earth horizon navigation sightings and the CSM S-band high gain reflectivity test which was conducted at 168:00. The passive thermal control technique and the navigation procedures used on the translunar portion of the mission were also used during the return trip.
  • The only midcourse correction required was a 6.7-second maneuver at 188:49:58.0, three hours before CM/SM separation.
  • The service module was jettisoned at 191:33:26, and the CM entry followed a normal profile. The command module reentered Earth’s atmosphere (400,000 feet or 121,920 meters altitude) at 191:48:54.5 at a velocity of 11,068.5 meters per second (24759.5 mph or 39,846.6 km/h), following a transearth coast of 54 hours 3 minutes 40.9 seconds. The service module impacted the Pacific Ocean at a point estimated to be latitude 19.4° south and longitude 173.37° west.


Using its parachute system, Apollo 10 Command Module (CM) performed a soft splashdown in the Pacific Ocean at 16:52:23 GMT (12:52:23 p.m. EDT) on May 26. Mission duration was 192:03:23.

Apollo 10: “Tell the world, we have arrived” (NASA video). Producer/Editor: Amy Leniart

Apollo 10 mission conclusions

The Apollo 10 mission provided the concluding data and final environmental evaluation to proceed with a lunar landing. The following conclusions were made from an analysis of post-mission data:

  1. The systems in the command and service modules and the lunar module were operational for a piloted lunar landing.
  2. The crew activity timeline, in those areas consistent with the lunar landing profile, demonstrated that critical crew tasks associated with lunar module checkout, initial descent, and rendezvous were both feasible and practical without unreasonable crew workload.
  3. The lunar module S-band communications capability using either the steerable or the omnidirectional antenna was satisfactory at lunar distances.
  4. The operating capability of the landing radar in the lunar environment during a descent propulsion firing was satisfactorily demonstrated for the altitudes experienced.
  5. The range capability of the lunar module rendezvous radar was demonstrated in the lunar environment with excellent results. Used for the first time, VHF ranging information from the CM provided a consistent correlation with radar range and range rate data.
  6. The lunar module abort guidance system capability to control an ascent propulsion system maneuver and to guide the spacecraft during rendezvous was demonstrated.
  7. The capability of the Mission Control Center and the Manned Space Flight Network to control and monitor two vehicles at lunar distance during both descent and rendezvous operations was proven adequate for a lunar landing.
  8. The lunar potential model was significantly improved over that of Apollo 8, and the orbit determination and prediction procedures proved remarkably more precise for both spacecraft in lunar orbit. After a combined analysis of Apollo 8 and Apollo 10 trajectory reconstructions, the lunar potential model was expected to be entirely adequate for the support of lunar descent and ascent.


M. Özgür Nevres

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