Apollo 10

Apollo 10 was the fourth manned mission in the United States Apollo space program, and the second (after Apollo 8) to orbit the Moon. Launched on May 18, 1969, it was the F mission: a "dress rehearsal" for the first Moon landing, testing all of the components and procedures, just short of actually landing. The Lunar Module (LM) came to within 8.4 nmi of the lunar surface, the point where the powered descent to the lunar surface would begin. Its success enabled the first landing to be attempted on Apollo 11 in July, 1969.

According to the 2002 Guinness World Records, Apollo 10 set the record for the highest speed attained by a manned vehicle at 39,897 km/h (11.08 km/s or 24,791 mph) during the return from the Moon on May 26, 1969.

Due to the use of their names as call signs, the Peanuts characters Charlie Brown and Snoopy became semi-official mascots for the mission. Peanuts creator Charles Schulz also drew some special mission-related artwork for NASA.

Support crew

 * Charles M. Duke, Jr
 * Joseph H. Engle
 * James B. Irwin
 * Jack R. Lousma
 * Bruce McCandless II

Flight directors

 * Glynn Lunney, Black team
 * Gerry Griffin, Gold team
 * Milton Windler, Maroon team
 * Pete Frank, Orange team

Crew notes
Apollo 10 was the first of only two Apollo missions with an entirely flight-experienced crew (the other being Apollo 11). Thomas P. Stafford had flown on Gemini 6 and Gemini 9; John W. Young had flown on Gemini 3 and Gemini 10, and Eugene A. Cernan had flown with Stafford on Gemini 9.

In addition, Apollo 10 marked the only Saturn V flight from Launch Complex 39B, as preparations for Apollo 11 at LC-39A had begun in March almost immediately after Apollo 9's launch.

They were also the only Apollo crew all of whose members went on to fly subsequent missions aboard Apollo spacecraft: Young later commanded Apollo 16, Cernan commanded Apollo 17 and Stafford commanded the US vehicle on the Apollo–Soyuz Test Project.

The Apollo 10 crew holds the distinction of being the humans who have traveled to the farthest point away from home, some 408950 km from their homes and families in Houston. While most Apollo missions orbited the Moon at the same 111 km from the lunar surface, timing makes this distinction possible as the distance between the Earth and Moon varies by approximately 43000 km (between perigee and apogee) throughout the year, and the Earth's rotation make the distance to Houston vary by another 12000 km each day. The Apollo 10 crew reached the farthest point in their orbit around the far side of the Moon at approximately the same time Earth had rotated around putting Houston nearly a full Earth diameter away. The Apollo 13 crew holds the distinction of being the farthest any human has traveled from the Earth's surface.

By the normal rotation in place during Apollo, the backup crew would have been scheduled to fly on Apollo 13. However, Alan Shepard was given the Apollo 13 command slot instead. L. Gordon Cooper, Jr., Commander of the Apollo 10 backup crew, was enraged and resigned from NASA. Later, Shepard's crew was forced to switch places with Jim Lovell's tentative Apollo 14 crew.

Deke Slayton wrote in his memoirs that Cooper and Donn F. Eisele were never intended to rotate to another mission as both were out of favor with NASA management for various reasons (Cooper for his lax attitude towards training and Eisele for incidents aboard Apollo 7 and an extramarital affair) and were assigned to the backup crew simply because of a lack of qualified manpower in the Astronaut Office at the time the assignment needed to be made. Cooper, Slayton noted, had a very small chance of receiving the Apollo 13 command if he did an outstanding job with the assignment, which he did not. Eisele, despite his issues with management, was always intended for future assignment to the Apollo Applications Program (which was eventually cut down to only the Skylab component) and not a lunar mission.

Mission parameters

 * Mass: CSM 28,834 kg; LM 13,941 kg

Earth orbit

 * Perigee: 184.5 km
 * Apogee: 190 km
 * Inclination: 32.5°
 * Period: 88.1 min

Lunar orbit

 * Perilune: 111.1 km
 * Apolune: 316.7 km
 * Inclination: 1.2°
 * Period: 2.15 hours

LM – CSM docking

 * Undocked: May 22, 1969 – 19:00:57 UTC
 * Redocked: May 23, 1969 – 03:11:02 UTC

LM closest approach to lunar surface

 * May 22, 1969, 21:29:43 UTC

On May 22, 1969 at 20:35:02 UTC, a 27.4 second LM descent propulsion system burn inserted the LM into a descent orbit of 60.9 by so that the resulting lowest point in the orbit occurred about 15° from lunar landing site 2 (the Apollo 11 landing site). The lowest measured point in the trajectory was 47400 ft above the lunar surface at 21:29:43 UTC.

Mission highlights
This dress rehearsal for a Moon landing brought the Apollo Lunar Module to 8.4 nmi from the lunar surface, at the point where powered descent would begin on the actual landing. Practicing this approach orbit would refine knowledge of the lunar gravitational field needed to calibrate the powered descent guidance system to within 1 nmi (LR altitude update lock) needed for a landing. Earth-based observations, unmanned spacecraft, and Apollo 8 had respectively allowed calibration to within 200 nmi, 20 nmi, and 5 nmi. Except for this final stretch, the mission went exactly as a landing would have gone, both in space and on the ground, putting NASA's flight controllers and extensive tracking and control network through a rehearsal.

Shortly after trans-lunar injection, Young performed the transposition, docking, and extraction maneuver, separating the Command/Service Module (CSM) from the S-IVB stage, turning around, and docking its nose to the top of the Lunar Module (LM), before separating from the S-IVB. 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.

After reaching lunar orbit three days later, Young remained in the Command Module (CM) Charlie Brown while Stafford and Cernan entered the LM Snoopy and flew it separately. The LM crew performed the descent orbit insertion maneuver by firing their descent engine, and tested their craft's landing radar as they approached the 50000 ft altitude where powered descent would begin on Apollo 11. They surveyed the landing site in the Sea of Tranquility, then separated the descent stage and fired the ascent engine to return to Charlie Brown.

The ascent stage was loaded with the amount of fuel it would have had remaining if it had lifted off from the surface and reached the altitude at which the Apollo 10 ascent stage fired. The fueled LM weighed 30735 lb, compared to 33278 lb for the Apollo 11 LM which made the first landing. Craig Nelson wrote in his book Rocket Men that NASA took special precaution to ensure Stafford and Cernan would not attempt to make the first landing. Nelson quoted Cernan as saying "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. So had we literally tried to land on the Moon, we couldn't have gotten off." In his own memoir, Cernan wrote "Our lander, LM-4...was still too heavy to guarantee safe margins for a moon landing."

Upon separation of the descent stage and ascent engine ignition, the Lunar Module began to roll violently due to the crew accidentally duplicating 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. Cernan has said he observed the horizon spinning eight times over, indicating eight rolls of the spacecraft under ascent engine power. While the incident was downplayed by NASA, the roll was just several revolutions from being unrecoverable, which would have resulted in the LM crashing into the lunar surface.

Splashdown occurred in the Pacific Ocean on May 26, 1969, at 16:52:23 UTC, approximately 400 nmi east of American Samoa. The astronauts were recovered by the USS Princeton, and subsequently flown to Pago Pago International Airport in Tafuna for a greeting reception, before being flown on a C-141 cargo plane to Honolulu.

Hardware disposition
The LM Snoopy's descent stage was left in orbit, but eventually crashed onto the lunar surface because of the Moon's non-uniform gravitational field; its location was not tracked.

After being jettisoned, Snoopy's ascent stage engine was fired to fuel depletion, sending it on a trajectory past the Moon into a heliocentric orbit. The Apollo 11 ascent stage was left in lunar orbit to eventually crash; all subsequent ascent stages were intentionally steered into the Moon to obtain readings from seismometers placed on the surface, except for the one on Apollo 13, which did not land but was used as a "life boat" to get the crew back to Earth, and burned up in Earth's atmosphere. Snoopy's ascent stage orbit was not tracked after 1969, and its current location is unknown. In 2011, a group of amateur astronomers in the UK started a project to search for it.

The Command Module Charlie Brown is currently on loan to the Science Museum in London, where it is on display. Charlie Brown's Service Module (SM) was jettisoned just before re-entry and burned up in the Earth's atmosphere.

After Apollo 10, NASA required astronauts to choose more "dignified" names for their command and lunar module. The requirement was unenforceable: Apollo 16 astronauts Young, Mattingly and Duke chose Casper, as in Casper the Friendly Ghost, for their Command Module name. The idea was to give children a way to identify with the mission by using humor.

After the insertion into trans-Lunar orbit, the Saturn IVB third stage became a derelict object where it would continue to orbit the Sun for many years. , it remains in orbit.

Mission insignia
The shield-shaped emblem for the flight shows a large, three-dimensional Roman numeral X sitting on the Moon's surface, in Stafford's words, "to show that we had left our mark." Although it did not land on the Moon, the prominence of the number represents the significant contributions the mission made to the Apollo program. A CSM circles the Moon as an LM ascent stage flies up from its low pass over the lunar surface with its engine firing. The Earth is visible in the background. On the mission patch, a wide, light blue border carries the word APOLLO at the top and the crew names around the bottom. The patch is trimmed in gold. The insignia was designed by Allen Stevens of Rockwell International.