Moon Landing - Soviet Circumlunar Loop Flights (1967–1970)

Soviet Circumlunar Loop Flights (1967–1970)

It is possible to aim a spacecraft from Earth so that it will loop around the Moon and return to Earth without actually entering lunar orbit, following the so-called free return trajectory. Such circumlunar loop missions are simpler than actual lunar orbit missions because rockets for lunar orbit braking and Earth return are not required. However, a manned circumlunar loop trip poses significant challenges above and beyond those found in a manned low-Earth-orbit mission, offering valuable lessons in preparation for a manned Moon landing. Foremost among these are mastering the demands of re-entering the Earth's atmosphere upon returning from the Moon.

Manned Earth-orbiting vehicles such as the Space Shuttle return to Earth from speeds of around 17,000 miles per hour (27,400 km/h, 7,600 m/s). Due to the effects of gravity, a vehicle returning from the Moon hits Earth's atmosphere at a much higher speed of around 25,000 miles per hour (40,200 km/h, 11,200 m/s). 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 manned circumlunar loop flight prior to a manned lunar landing became a primary goal of the Soviets with their Zond spacecraft program. The first three Zonds were unmanned planetary probes; after that, the Zond name was transferred to a completely separate manned program. The initial focus of these later Zonds was extensive testing of required high-speed reentry techniques. This focus was not shared by the Americans, who chose instead to bypass the stepping stone of a manned circumlunar loop mission and never developed a separate spacecraft for this purpose.

Initial manned spaceflights in the early 1960s placed a single person in low Earth orbit during the Soviet Vostok and American 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 Americans 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 critical techniques for orbital rendezvous and docking that were crucial to a manned lunar landing mission profile.

After the end of the Gemini program, the Soviets Union began flying their second-generation Zond manned spacecraft in 1967 with the ultimate goal of looping a cosmonaut around the Moon and returning him immediately to Earth. The Zond spacecraft was launched with the simpler and already operational Proton launch rocket, unlike the parallel Soviet manned 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 manned Zond circumlunar flight years before an American manned lunar landing and so score a propaganda victory. However, significant development problems delayed the Zond program and the success of the American Apollo lunar landing program led to the eventual termination of the Zond effort.

Like Zond, Apollo Moon flights were generally launched on a free return trajectory that would return them to Earth via a circumlunar loop in the event that a Service Module malfunction failed to place them in lunar orbit as planned. This option was implemented after an explosion aboard the Apollo 13 mission in 1970, which is the only manned 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 turtles, 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 of these Zond missions actually flew off-nominal reentry trajectories resulting in deceleration forces that would have been fatal to human crewmembers had they been aboard.

As a result, the Soviets secretly planned to continue unmanned Zond tests until their reliability to support manned flight had been demonstrated. However, due to NASA's continuing problems with the lunar module, and because of CIA reports of a potential Soviet manned 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 Baikonour on the first manned 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 manned Soviet lunar mission. Their decision proved to be a wise one, since this unnumbered Zond mission was destroyed in another unmanned test when it was finally launched several weeks later.

By this time flights of the third generation American 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 manned trip to the Moon on 24 December 1968, certifying the Saturn V booster for manned 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 manned Moon landing in May 1969. This mission stopped short at ten miles (16 km) altitude above the lunar surface, performing necessary low-altitude mapping of trajectory-altering mascons using a factory prototype lunar module that was too overweight to allow a successful landing. With the failure of the unmanned Soviet sample return Moon landing attempt Luna 15 in July 1969, the stage was set for Apollo 11.