section_path: Soviet circumlunar loop flights (1967–1970)
section_name: Soviet circumlunar loop flights (1967–1970)
paragraph_text: Soviet circumlunar loop flights (1967–1970)
  
  It is possible to aim a spacecraft from Earth so it will loop around the Moon and return to Earth without entering lunar orbit, following the so-called free return trajectory. Such circumlunar loop missions are simpler than lunar orbit missions because rockets for lunar orbit braking and Earth return are not required. However, a crewed circumlunar loop trip poses significant challenges beyond those found in a crewed low-Earth-orbit mission, offering valuable lessons in preparation for a crewed Moon landing. Foremost among these are mastering the demands of re-entering the Earth's atmosphere upon returning from the Moon.
  
  Inhabited Earth-orbiting vehicles such as the Space Shuttle return to Earth from speeds of around 7,500 m/s (27,000 km/h). Due to the effects of gravity, a vehicle returning from the Moon hits Earth's atmosphere at a much higher speed of around 11,000 m/s (40,000 km/h). The g-loading on astronauts during the resulting deceleration can be at the limits of human endurance even during a nominal reentry. Slight variations in the vehicle flight path and reentry angle during a return from the Moon can easily result in fatal levels of deceleration force.
  
  Achieving a crewed circumlunar loop flight prior to a crewed lunar landing became a primary goal of the Soviets with their Zond spacecraft program. The first three Zonds were robotic planetary probes; after that, the Zond name was transferred to a completely separate human spaceflight program. The initial focus of these later Zonds was extensive testing of required high-speed reentry techniques. This focus was not shared by the U.S., who chose instead to bypass the stepping stone of a crewed circumlunar loop mission and never developed a separate spacecraft for this purpose.
  
  Initial crewed spaceflights in the early 1960s placed a single person in low Earth orbit during the Soviet Vostok and U.S. Mercury programs. A two-flight extension of the Vostok program known as Voskhod effectively used Vostok capsules with their ejection seats removed to achieve Soviet space firsts of multiple person crews in 1964 and spacewalks in early 1965. These capabilities were later demonstrated by the U.S. in ten Gemini low Earth orbit missions throughout 1965 and 1966, using a totally new second-generation spacecraft design that had little in common with the earlier Mercury. These Gemini missions went on to prove techniques for orbital rendezvous and docking crucial to a crewed lunar landing mission profile.
  
  After the end of the Gemini program, the Soviet Union began flying their second-generation Zond crewed spacecraft in 1967 with the ultimate goal of looping a cosmonaut around the Moon and returning him or her immediately to Earth. The Zond spacecraft was launched with the simpler and already operational Proton launch rocket, unlike the parallel Soviet human Moon landing effort also underway at the time based on third-generation Soyuz spacecraft requiring development of the advanced N-1 booster. The Soviets thus believed they could achieve a crewed Zond circumlunar flight years before a U.S. human lunar landing and so score a propaganda victory. However, significant development problems delayed the Zond program and the success of the U.S. Apollo lunar landing program led to the eventual termination of the Zond effort.
  
  Like Zond, Apollo flights were generally launched on a free return trajectory that would return them to Earth via a circumlunar loop if a service module malfunction failed to place them in lunar orbit. This option was implemented after an explosion aboard the Apollo 13 mission in 1970, which is the only crewed circumlunar loop mission flown to date.
  
  Zond 5 was the first spacecraft to carry life from Earth to the vicinity of the Moon and return, initiating the final lap of the Space Race with its payload of tortoises, insects, plants, and bacteria. Despite the failure suffered in its final moments, the Zond 6 mission was reported by Soviet media as being a success as well. Although hailed worldwide as remarkable achievements, both these Zond missions flew off-nominal reentry trajectories resulting in deceleration forces that would have been fatal to humans.
  
  As a result, the Soviets secretly planned to continue uncrewed Zond tests until their reliability to support human flight had been demonstrated. However, due to NASA's continuing problems with the lunar module, and because of CIA reports of a potential Soviet crewed circumlunar flight in late 1968, NASA fatefully changed the flight plan of Apollo 8 from an Earth-orbit lunar module test to a lunar orbit mission scheduled for late December 1968.
  
  In early December 1968 the launch window to the Moon opened for the Soviet launch site in Baikonur, giving the USSR their final chance to beat the US to the Moon. Cosmonauts went on alert and asked to fly the Zond spacecraft then in final countdown at Baikonur on the first human trip to the Moon. Ultimately, however, the Soviet Politburo decided the risk of crew death was unacceptable given the combined poor performance to that point of Zond/Proton and so scrubbed the launch of a crewed Soviet lunar mission. Their decision proved to be a wise one, since this unnumbered Zond mission was destroyed in another uncrewed test when it was finally launched several weeks later.
  
  By this time flights of the third generation U.S. Apollo spacecraft had begun. Far more capable than the Zond, the Apollo spacecraft had the necessary rocket power to slip into and out of lunar orbit and to make course adjustments required for a safe reentry during the return to Earth. The Apollo 8 mission carried out the first human trip to the Moon on 24 December 1968, certifying the Saturn V booster for crewed use and flying not a circumlunar loop but instead a full ten orbits around the Moon before returning safely to Earth. Apollo 10 then performed a full dress rehearsal of a crewed Moon landing in May 1969. This mission orbited within 14.4 kilometres (47,400 ft) of the lunar surface, performing necessary low-altitude mapping of trajectory-altering mascons using a factory prototype lunar module too heavy to land. With the failure of the robotic Soviet sample return Moon landing attempt Luna 15 in July 1969, the stage was set for Apollo 11.