Lunar orbit rendezvous

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Diagram of LOR Lunar-orbit rendezvous.jpg
Diagram of LOR

Lunar orbit rendezvous (LOR) is a key concept for efficiently landing humans on the Moon and returning them to Earth. It was utilized for the Project Apollo missions in the 1960s and 1970s. In a LOR mission, a main spacecraft and a smaller lunar lander travel to lunar orbit. The lunar lander then independently descends to the surface of the Moon, while the main spacecraft remains in lunar orbit. After completion of the mission there, the lander returns to lunar orbit to rendezvous and re-dock with the main spacecraft, then is discarded after transfer of crew and payload. Only the main spacecraft returns to Earth. [1]

Spacecraft manned vehicle or unmanned machine designed to fly in outer space

A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, planetary exploration, and transportation of humans and cargo. All spacecraft except single-stage-to-orbit vehicles cannot get into space on their own, and require a launch vehicle.

Lunar orbit orbit of an object around the Moon

In astronomy, lunar orbit is the orbit of an object around the Moon.

Space rendezvous orbital maneuver

A space rendezvous is an orbital maneuver during which two spacecraft, one of which is often a space station, arrive at the same orbit and approach to a very close distance. Rendezvous requires a precise match of the orbital velocities and position vectors of the two spacecraft, allowing them to remain at a constant distance through orbital station-keeping. Rendezvous may or may not be followed by docking or berthing, procedures which bring the spacecraft into physical contact and create a link between them.


Representation of the lunar gravity well, illustrating how resources needed only for the trip home don't have to be carried down and back up the "well" LOR Gravity Well.png
Representation of the lunar gravity well, illustrating how resources needed only for the trip home don't have to be carried down and back up the "well"

Lunar orbit rendezvous was first known to be proposed in 1919 by Ukrainian Soviet engineer Yuri Kondratyuk, [2] as the most economical way of sending a human on a round-trip journey to the Moon. [3]

Ukraine Sovereign state in Eastern Europe

Ukraine, sometimes called the Ukraine, is a country in Eastern Europe. Excluding Crimea, Ukraine has a population of about 42.5 million, making it the 32nd most populous country in the world. Its capital and largest city is Kiev. Ukrainian is the official language and its alphabet is Cyrillic. The dominant religions in the country are Eastern Orthodoxy and Greek Catholicism. Ukraine is currently in a territorial dispute with Russia over the Crimean Peninsula, which Russia annexed in 2014. Including Crimea, Ukraine has an area of 603,628 km2 (233,062 sq mi), making it the largest country entirely within Europe and the 46th largest country in the world.

Soviet Union 1922–1991 country in Europe and Asia

The Soviet Union, officially the Union of Soviet Socialist Republics (USSR), was a socialist state in Eurasia that existed from 1922 to 1991. Nominally a union of multiple national Soviet republics, its government and economy were highly centralized. The country was a one-party state, governed by the Communist Party with Moscow as its capital in its largest republic, the Russian Soviet Federative Socialist Republic. Other major urban centres were Leningrad, Kiev, Minsk, Tashkent, Alma-Ata, and Novosibirsk. It spanned over 10,000 kilometres (6,200 mi) east to west across 11 time zones, and over 7,200 kilometres (4,500 mi) north to south. It had five climate zones: tundra, taiga, steppes, desert and mountains.

Yuri Kondratyuk ukrainian soviet scientist

Yuri Vasilievich Kondratyuk was a Ukrainian Soviet engineer and mathematician. He was a pioneer of astronautics and spaceflight, a theoretician and a visionary who, in the early 20th century, developed the first known lunar orbit rendezvous (LOR), a key concept for landing and return spaceflight from Earth to the Moon. The LOR was later used for the plotting of the first actual human spaceflight to the Moon. Many other aspects of spaceflight and space exploration are covered in his works.

LOR using Shuttle-Derived Heavy Lift Launch Vehicle Shuttleclunar.svg
LOR using Shuttle-Derived Heavy Lift Launch Vehicle

The most famous example involved Apollo Command and Service Module and Apollo Lunar Module, where they were both sent to a Translunar flight in a single rocket stack. However, variants where the landers and main spacecraft travel separately, such as the lunar landing plan proposed for Shuttle-Derived Heavy Lift Launch Vehicle and Golden Spike, are also considered as Lunar Orbit rendezvous.

Apollo Lunar Module A lander used in the Apollo program.

The Apollo Lunar Module, or simply lunar module, originally designated the Lunar Excursion Module (LEM), was the lander spacecraft that was flown from lunar orbit to the Moon's surface during the U.S. Apollo program. It was the first crewed spacecraft to operate exclusively in the airless vacuum of space, and remains the only crewed vehicle to land anywhere beyond Earth.

Shuttle-Derived Heavy Lift Launch Vehicle

The Shuttle-Derived Heavy Lift Launch Vehicle, also known as the High Confidence Heavy Lift Launch Vehicle was an alternate Super heavy-lift launch vehicle proposal for the NASA Constellation program. It was first presented on June 17, 2009 by John Shannon, NASA's Shuttle Program manager, to the Augustine Commission which is tasked to review NASA's human spaceflight program.

Golden Spike Company American space transport startup company

The Golden Spike Company was an American space transport startup active from 2010–2013. The company was chartered for business in Colorado with the objective to offer private commercial space transportation services to the surface of the Moon. The name of the company is in reference to the ceremonial final spike placed in the First Transcontinental Railroad upon its completion. The company's Internet site was taken offline in September 2015.

Advantages and disadvantages


Comparison of lunar lander sizes, from an early Langley study Comparison of Lander Sizes - Direct vs LOR.gif
Comparison of lunar lander sizes, from an early Langley study

The main advantage of LOR is the spacecraft payload saving, due to the fact that the propellant necessary to return from lunar orbit back to Earth need not be carried as dead weight down to the Moon and back into lunar orbit. This has a multiplicative effect, because each pound of "dead weight" propellant used later has to be propelled by more propellant sooner, and also because increased propellant requires increased tankage weight. The resultant weight increase would also require more thrust for lunar landing, which means larger and heavier engines. [4]

Another advantage is that the lunar lander can be designed for just that purpose, rather than requiring the main spacecraft to also be made suitable for a lunar landing. Finally, the second set of life support systems that the lunar lander requires can serve as a backup for the systems in the main spacecraft.


Lunar-orbit rendezvous was considered risky as of 1962, because space rendezvous had not been achieved, even in Earth orbit. If the LEM could not reach the CSM, two astronauts would be stranded with no way to get back to Earth or survive re-entry into the atmosphere. The fear proved to be unfounded, as rendezvous was successfully demonstrated in 1965 and 1966 on six Project Gemini missions [Note 1] with the aid of radar and on-board computers. It was also successfully done each of the eight times it was tried on Apollo missions. [Note 2]

Atmospheric entry Passage of an object through the gases of an atmosphere from outer space

Atmospheric entry is the movement of an object from outer space into and through the gases of an atmosphere of a planet, dwarf planet, or natural satellite. There are two main types of atmospheric entry: uncontrolled entry, such as the entry of astronomical objects, space debris, or bolides; and controlled entry of a spacecraft capable of being navigated or following a predetermined course. Technologies and procedures allowing the controlled atmospheric entry, descent, and landing of spacecraft are collectively termed as EDL.

Project Gemini NASAs second human spaceflight program

Project Gemini was NASA's second human spaceflight program. Conducted between projects Mercury and Apollo, Gemini started in 1961 and concluded in 1966. The Gemini spacecraft carried a two-astronaut crew. Ten Gemini crews flew low Earth orbit (LEO) missions during 1965 and 1966, putting the United States in the lead during the Cold War Space Race against the Soviet Union.

Apollo Mission mode selection

Apollo 11 Lunar Module rendezvousing with Command Module in lunar orbit Apollo 11 lunar module.jpg
Apollo 11 Lunar Module rendezvousing with Command Module in lunar orbit

When the Apollo Moon landing program was started in 1961, it was assumed that the three-man Command and Service Module combination (CSM) would be used for takeoff from the lunar surface, and return to Earth. It would therefore have to be landed on the Moon by a larger rocket stage with landing gear legs, resulting in a very large spacecraft (in excess of 100,000 pounds (45,000 kg)) to be sent to the Moon.

If this were done by direct ascent (on a single launch vehicle), the rocket required would have to be extremely large, in the Nova class. The alternative to this would have been Earth orbit rendezvous, in which two or more rockets in the Saturn class would launch parts of the complete spacecraft, which would rendezvous in Earth orbit before departing for the Moon. This would possibly include a separately launched Earth departure stage, or require on-orbit refueling of the empty departure stage.

Tom Dolan [5] proposed the alternative of lunar orbit rendezvous, which had been studied and promoted by Jim Chamberlin and Owen Maynard at the Space Task Group in 1960 early Apollo feasibility studies. [6] This mode allowed a single Saturn V to launch the CSM to the Moon with a smaller Lunar Excursion Module (LEM). [Note 3] When the combined spacecraft reaches lunar orbit, one of the three astronauts remains with the CSM, while the other two enter the LEM, undock and descend to the surface of the Moon. They then use the ascent stage of the LEM to rejoin the CSM in lunar orbit, then discard the LEM and use the CSM for the return to Earth. This method was brought to the attention of NASA Associate Administrator Robert Seamans by Langley Research Center engineer John C. Houbolt, who led a team to develop it.

Besides requiring less payload, the ability to use a lunar lander designed just for that purpose was another advantage of the LOR approach. The LEM's design gave the astronauts a clear view of their landing site through observation windows approximately 4.6 metres (15 ft) above the surface, as opposed to being on their backs in a Command Module lander, at least 40 or 50 feet (12 or 15 m) above the surface, able to see it only through a television screen.

Developing the LEM as a second crewed vehicle provided the further advantage of redundant critical systems (electrical power, life support, and propulsion), which enabled it to be used as a "lifeboat" to keep the astronauts alive and get them home safely in the event of a critical CSM system failure. This was envisioned as a contingency, but not made a part of the LEM specifications. As it turned out, this capability proved invaluable in 1970, when just such a critical failure occurred on the Apollo 13 mission when an oxygen tank failure disabled the Service Module.


John Houbolt explains Lunar orbit rendezvous John C. Houbolt - GPN-2000-001274.jpg
John Houbolt explains Lunar orbit rendezvous

Dr. John Houbolt would not let the advantages of LOR be ignored. As a member of Lunar Mission Steering Group, Houbolt had been studying various technical aspects of space rendezvous since 1959 and was convinced, like several others at Langley Research Center, that LOR was not only the most feasible way to make it to the Moon before the decade was out, it was the only way. He had reported his findings to NASA on various occasions but felt strongly that the internal task forces (to which he made presentations) were following arbitrarily established "ground rules." According to Houbolt, these ground rules were constraining NASA's thinking about the lunar mission—and causing LOR to be ruled out before it was fairly considered. [8]

In November 1961, Houbolt took the bold step of skipping proper channels and writing a private letter, nine pages long, directly to Robert C. Seamans, the associate administrator. "Somewhat as a voice in the wilderness," Houbolt protested LOR's exclusion. "Do we want to go to the Moon or not?" the Langley engineer asked. "Why is Nova, with its ponderous size simply just accepted, and why is a much less grandiose scheme involving rendezvous ostracized or put on the defensive? I fully realize that contacting you in this manner is somewhat unorthodox," Houbolt admitted, "but the issues at stake are crucial enough to us all that an unusual course is warranted." [9] [10]

It took two weeks for Seamans to reply to Houbolt's extraordinary letter. The associate administrator agreed that "it would be extremely harmful to our organization and to the country if our qualified staff were unduly limited by restrictive guidelines." He assured Houbolt that NASA would in the future be paying more attention to LOR than it had up to this time.

In the following months, NASA did just that, and to the surprise of many both inside and outside the agency, the dark horse candidate, LOR, quickly became the front runner. Several factors decided the issue in its favor. First, there was growing disenchantment with the idea of direct ascent due to the time and money it was going to take to develop a 15-metre (50 ft) diameter Nova rocket, compared to the 10-metre (33 ft) diameter Saturn V. Second, there was increasing technical apprehension over how the relatively large spacecraft demanded even by Earth-orbit rendezvous would be able to maneuver to a soft landing on the Moon. As one NASA engineer who changed his mind explained:

The business of eyeballing that thing down to the Moon really didn't have a satisfactory answer. The best thing about LOR was that it allowed us to build a separate vehicle for landing.

The first major group to break camp in favor of LOR was Robert Gilruth's Space Task Group, which was still located at Langley but was soon to move to Houston. The second to come over was the Von Braun team at the Marshall Space Flight Center in Huntsville, Alabama. Then these two powerful groups of converts, along with the original true believers at Langley, persuaded key officials at NASA Headquarters, notably Administrator James Webb, who had been holding out for direct ascent, that LOR was the only way to land on the Moon by 1969. With the key players inside NASA lined up behind the concept, Webb approved LOR in July 1962. [11] The decision was officially announced at a press conference on July 11, 1962. [12] President Kennedy's science adviser, Jerome Wiesner, remained firmly opposed to LOR. [13] [8]

Other plans using LOR

Episode 5 of the television miniseries From the Earth to the Moon , "Spider", dramatizes John Houbolt's first attempt to convince NASA to adopt LOR for the Apollo Program in 1961, and traces the development of the LM up to its first crewed test flight, Apollo 9, in 1969. The episode is named after the Apollo 9 Lunar Module.


  1. Gemini 6A, Gemini 8, Gemini 9A, Gemini 10, Gemini 11, and Gemini 12
  2. Apollo 9 in Earth orbit; in lunar orbit on Apollo 10, Apollo 11, Apollo 12, Apollo 14, Apollo 15, Apollo 16, and Apollo 17.
  3. This was shortened to "Lunar Module" (LM) in June 1966. [7]

Related Research Articles

Apollo program Crewed U.S. lunar missions from 1966–1972

The Apollo program, also known as Project Apollo, was the third United States human spaceflight program carried out by the National Aeronautics and Space Administration (NASA), which succeeded in landing the first humans on the Moon from 1969 to 1972. First conceived during Dwight D. Eisenhower's administration as a three-person spacecraft to follow the one-person Project Mercury which put the first Americans in space, Apollo was later dedicated to the national goal set by President John F. Kennedy of "landing a man on the Moon by the end of this decade and returning him safely to the Earth" in an address to Congress on May 25, 1961. It was the third US human spaceflight program to fly, preceded by the two-person Project Gemini conceived in 1961 to extend spaceflight capability in support of Apollo.

Apollo 9 3rd crewed mission of the Apollo space program

Apollo 9 was a March 1969 human spaceflight, the third crewed mission in the United States Apollo program. Flown in Low Earth Orbit, it was the second crewed Apollo mission to be launched by a Saturn V rocket, and the first flight of the full Apollo spacecraft: the command and service module (CSM) with the Lunar Module (LM). The mission was flown to qualify the LM for lunar orbit operations by demonstrating its descent and ascent propulsion systems, showing that its crew could fly it independently, then rendezvous and dock with the CSM again, as would be required for the first crewed lunar landing. Other objectives of the flight included firing the LM descent engine to propel the spacecraft stack as a backup mode, and use of the Portable Life Support System backpack outside of the LM cabin.

Constellation program manned spaceflight program developed by NASA from 2005 to 2009

The Constellation Program is a cancelled crewed spaceflight program developed by NASA, the space agency of the United States, from 2005 to 2009. The major goals of the program were "completion of the International Space Station" and a "return to the Moon no later than 2020" with a crewed flight to the planet Mars as the ultimate goal. The program's logo reflected the three stages of the program: the Earth (ISS), the Moon, and finally Mars—while the Mars goal also found expression in the name given to the program's booster rockets: Ares. The technological aims of the program included the regaining of significant astronaut experience beyond low Earth orbit and the development of technologies necessary to enable sustained human presence on other planetary bodies.

Apollo (spacecraft) American spacecraft

The Apollo spacecraft was composed of three parts designed to accomplish the American Apollo program's goal of landing astronauts on the Moon by the end of the 1960s and returning them safely to Earth. The expendable (single-use) spacecraft consisted of a combined command and service module (CSM) and an Apollo Lunar Module (LM). Two additional components complemented the spacecraft stack for space vehicle assembly: a spacecraft–LM adapter (SLA) designed to shield the LM from the aerodynamic stress of launch and to connect the CSM to the Saturn launch vehicle; and a launch escape system (LES) to carry the crew in the command module safely away from the launch vehicle in the event of a launch emergency.

Apollo command and service module spacecraft

The Apollo command and service module (CSM) was one of two principal components of the United States Apollo spacecraft, used for the Apollo program, which landed astronauts on the Moon between 1969 and 1972. The CSM functioned as a mother ship, which carried a crew of three astronauts and the second Apollo spacecraft, the lunar module, to lunar orbit, and brought the astronauts back to Earth. It consisted of two parts: the conical command module, a cabin that housed the crew and carried equipment needed for atmospheric reentry and splashdown; and the cylindrical service module which provided propulsion, electrical power and storage for various consumables required during a mission. An umbilical connection transferred power and consumables between the two modules. Just before reentry of the command module on the return home, the umbilical connection was severed and the service module was cast off and allowed to burn up in the atmosphere.

Space capsule type of spacecraft

A space capsule is an often crewed blunt-body spacecraft that reenters the Earth's atmosphere without wings. Capsules are distinguished from satellites primarily by the ability to survive reentry and return a payload to the Earth's surface from orbit. Capsules make up the majority of crewed spacecraft designs, although one crewed spaceplane has launched to orbit.

The Apollo Applications Program (AAP) was established by NASA headquarters in 1968 to develop science-based manned space missions using hardware developed for the Apollo program. AAP was the ultimate development of a number of official and unofficial Apollo follow-on projects studied at various NASA labs.

Earth orbit rendezvous

Earth orbit rendezvous (EOR) is a potential methodology for conducting round trip human flights to the Moon, involving the use of space rendezvous to assemble, and possibly fuel, components of a translunar vehicle in low Earth orbit. It was considered and ultimately rejected in favor of lunar orbit rendezvous (LOR) for NASA's Apollo Program of the 1960s and 1970s, mainly because LOR does not require a spacecraft big enough to both make the return trip from earth orbit to splashdown in the ocean, and a soft landing on the lunar surface. Three decades later, it was planned to be used for Project Constellation, until that program's cancellation in October 2010.

Direct ascent is a method of landing a spacecraft on the Moon or another planet directly, without first assembling the vehicle in Earth orbit, or carrying a separate landing vehicle into orbit around the target body. It was proposed as the first method to achieve a crewed lunar landing in the United States Apollo program, but was rejected because it would have required developing a prohibitively large launch vehicle. It was used to launch uncrewed lunar landers in the United States Surveyor program and the Soviet Luna program.

Moon landing Arrival of a spacecraft on the surface of the Moon

A Moon landing is the arrival of a spacecraft on the surface of the Moon. This includes both crewed and uncrewed (robotic) missions. The first human-made object to reach the surface of the Moon was the Soviet Union's Luna 2 mission, on 13 September 1959.

Altair (spacecraft) Planned lander spacecraft component of NASAs cancelled Project Constellation

The Altair spacecraft, previously known as the Lunar Surface Access Module or LSAM, was the planned lander spacecraft component of NASA's cancelled Constellation program. Astronauts would have used the spacecraft for landings on the Moon, which was intended to begin around 2019. The Altair spacecraft was planned to be used both for lunar sortie and lunar outpost missions. On February 1, 2010, U.S. President Barack Obama announced a proposal to cancel the Constellation program, to be replaced with a re-scoped program, effective with the U.S. 2011 fiscal year budget.

Thomas Dolan was an American engineer who proposed the first fully developed concept of Lunar orbit rendezvous for the Apollo program while working at Vought Astronautics.

John Houbolt American aerospace engineer

John Cornelius Houbolt was an aerospace engineer credited with leading the team behind the lunar orbit rendezvous (LOR) mission mode, a concept that was used to successfully land humans on the Moon and return them to Earth. This flight path was first endorsed by Wernher von Braun in June 1961 and was chosen for Apollo program in early 1962. The critical decision to use LOR was viewed as vital to ensuring that Man reached the Moon by the end of the decade as proposed by President John F. Kennedy. In the process, LOR saved time and billions of dollars by efficiently using existing rocket technology.

Lunar escape systems series of proposed emergency spacecraft for the Apollo Program

Lunar escape systems (LESS) were a series of emergency vehicles designed for never-flown long-duration Apollo missions. Because these missions were even more hypothetical than the planned cancelled Apollo missions, the designs were never constructed. This concept was an outgrowth of the Lunar Flying Vehicle designed by Bell Aerospace.

Saturn C-3

The Saturn C-3 was the third rocket in the Saturn C series studied from 1959 to 1962. The design was for a three-stage launch vehicle that could launch 45,000 kilograms (99,000 lb) to low Earth orbit and send 18,000 kilograms (40,000 lb) to the Moon via trans-lunar injection.

Advanced Gemini is a number of proposals that would have extended the Gemini program by the addition of various missions, including manned low Earth orbit, circumlunar and lunar landing missions. Gemini was the second manned spaceflight program operated by NASA, and consisted of a two-seat spacecraft capable of maneuvering in orbit, docking with unmanned spacecraft such as Agena Target Vehicles, and allowing the crew to perform tethered extra-vehicular activities.

Apollo spacecraft feasibility study

The Apollo spacecraft feasibility study was conducted by NASA from July 1960 through May 1961 to investigate preliminary designs for a post-Project Mercury multi-crewed spacecraft to be used for possible space station, circum-lunar, lunar orbital, or crewed lunar landing missions. Six-month, $250,000 study contracts were awarded to General Dynamics/Convair, General Electric, and the Glenn L. Martin Company. Meanwhile, NASA conducted its own inhouse design study led by Maxime Faget, intended as a gauge of the competitors' entries. The three companies spent varying amounts of their own money in excess of the $250,000 to produce designs which included a re-entry module separate from the mission module cabin, and a propulsion and equipment module.


PD-icon.svg This article incorporates  public domain material from websites or documents ofthe National Aeronautics and Space Administration .


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  3. Wilford (1969), p. 41-48.
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  6. Gainor (2001), p. 62-66.
  7. Scheer, Julian W. (Assistant Administrator for Public Affairs, NASA). Memorandum from Project Designation Committee, June 9, 1966.
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  10. Hansen (1995).
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  14. Laxman (2012).