Launch of Apollo 13 (image above), the planned third Apollo moon landing, was just two months in the future when NASA Manned Spacecraft Center engineer Rocky Duncan proposed an alternate plan for the mission. He noted that Apollo 13 would mark the first flight of the Hycon Lunar Topographic Camera (LTC), a modified U.S. Air Force KA-74 aerial reconnaissance camera, which would be mounted in the Command and Service Module (CSM) crew hatch window for high-resolution overlapping photography of candidate future Apollo landing sites. In Apollo Program parlance, this was dubbed "bootstrap photography."
He also noted that previous Apollo lunar missions had followed a "free-return" path that would enable them to loop behind the moon and fall back to Earth if their CSM's Service Propulsion System (SPS) main engine failed. The Apollo 13 CSM, by contrast, would fire its engine midway to the moon to leave the free-return trajectory. This was necessary so that the mission's Lunar Module (LM) could reach its target landing site at Fra Mauro.
Duncan then described a scenario in which the Apollo 13 LM was judged to be "NO-GO" soon after Earth-orbit departure. Apollo 13 would then become a "CSM-only lunar alternate photographic mission." The mission's CSM would remain on a free-return path until it reached the moon, then its crew would perform a standard two-impulse lunar orbit insertion (LOI); that is, they would fire the SPS to slow their CSM so that the moon's gravity could capture it into an elliptical lunar orbit, then fire the SPS again at periapsis (orbit low point) to circularize their CSM's orbit.
Duncan noted that some "desirable photographic orbits with high inclinations. . .require a three-impulse LOI." He argued, however, that "since the crew has not been trained for this type of LOI. . .this type of profile [should] not be flown."
In Duncan's alternate mission, Apollo 13 would enter a lunar orbit that would take it over craters Censorinus and Mösting C beginning on its third revolution about the moon. He noted that these were, respectively, ranked first and eleventh in priority on the Apollo 13 list of targets for lunar-orbital photography. Censorinus was in fact the leading candidate for the Apollo 15 mission, which at the time was planned as a "walking" mission similar to Apollo 13 (that is, it would not include a Lunar Roving Vehicle). Duncan advocated delaying the crew's scheduled sleep period by two lunar revolutions to enable them to photograph the two sites.
The alternate Apollo 13 photographic program would begin during Revolution 3 with vertical stereo photography using window-mounted Hasselblad cameras. Revolution 4 would see the first high-resolution vertical Hycon LTC photography, then the astronauts would conduct high-resolution oblique (side-looking) LTC photography during Revolution 5. They would perform "landmark tracking" using the CSM's wide-field scanning telescope (a part of its navigation system) during Revolutions 6 and 7, then would begin their delayed sleep period.
The Apollo 13 crew would awaken during Revolution 12 and fire the SPS to change their CSM's orbital plane (that is, the angle at which it crossed the moon's equator). They would do this so that, beginning with Revolution 14, they could pass over Descartes, a suspected volcanic site in the lunar Highlands, and Davy Rille, a string of small craters of suspected volcanic origin. The astronauts would then repeat the five-revolution photographic sequence they had used to image Censorinus and Mösting C. Duncan noted that Descartes ranked second on the Apollo 13 list of photographic targets, while Davy was fourth.
Duncan briefly considered a scenario in which the Apollo 13 LM was judged to be incapable of landing yet had a working descent engine which the crew could use to perform plane-change maneuevers in lunar orbit. He noted that the LM would block some CSM windows while it was docked. The astronauts might undock the CSM from the LM for photography and redock for additional plane changes, or they might discard the LM after only a single plane change. Duncan favored a simpler approach; jettison the LM as soon as it was judged to be incapable of landing whether its descent engine worked or not.
The astronauts would perform "target of opportunity" photography during Revolutions 18 and 19, then would sleep. They would wake during Revolution 24 and perform a plane-change maneuver using the SPS during Revolution 25 so that they could fly over Alphonsus crater, Gassendi West, and Gassendi East beginning with Revolution 27. They would again carry out the five-revolution photo sequence. Alphonus was ranked ninth on the target list, Duncan reported, while Gassendi East and West were thirteenth and fourteenth, respectively.
Duncan estimated that, by the time the astronauts finished photographing the Alphonsus and Gassendi crater candidate landing sites, Apollo 13's cameras would likely have run out of film. He recommended that the crew fire the SPS to leave lunar orbit and return to Earth during Revolution 32 or two revolutions after the film ran out, whichever came first.
Apollo 13 left Earth on 11 April 1970. The LM Aquarius checked out as "GO" for landing on the moon, and the crew performed the SPS burn to leave the free-return trajectory. The next day, CSM Odyssey suffered an oxygen tank explosion in its Service Module (SM). Because the extent of the damage to the SM was unknown, NASA wrote off Odyssey's SPS. Astronauts James Lovell, Jack Swigert, and Fred Haise used* Aquarius*'s descent engine to get back on a free-return trajectory. During their lunar flyby, the crew photographed the moon through Aquarius's windows using hand-held cameras. Odyssey blocked part of their field of view.
The Hycon camera was not used for photography, though it did provide a hose for the improvised adapter the crew made so that Odyssey's carbon-dioxide removal canisters could be used in Aquarius. With help from the hose and the world-wide Apollo mission team, the crew splashed down safely in the Pacific Ocean in Odyssey's Command Module on 17 April.
Apollo 14 (31 January-9 February 1971) became the first (and last) lunar mission to use the Hycon LTC. By the time Apollo 14 flew, NASA had canceled Apollo 15 and 19 and renumbered the remaining flights so that Apollo lunar missions would end with Apollo 17. The camera's chief mission was to photograph Descartes, a lunar Highlands site that had moved to the top spot among Apollo 16 landing site candidates. The camera captured 192 images, but malfunctioned while imaging the surface about 70 kilometers east of Descartes and returned no images of the proposed landing site.
References:
Memorandum with attachment, FM5/Lunar Mission Analysis Branch to various, "Lunar alternate missions for Apollo 13 (Mission H-2)," Rocky Duncan, 13 February 1970.
Scientific Rationale Summaries for Apollo Candidate Lunar Exploration Landing Sites - case 340, J. Head, Bellcomm, Inc., 11 March 1970.
"Significant Results from Apollo 14 Lunar Orbital Photography," F. El-Baz and S. Roosa, Proceedings of the 1972 Lunar Science Conference, Vol. 2, pp. 63-83, 1972.
