Space History: The Lunar Orbiters

Post by Nic Quattromani:

The Apollo missions, as intrepid as they were, did not venture into wholly uncharted territory. By the time Neil Armstrong famously planted his boots in the lunar soil, a whole fleet of US spacecraft had already explored and mapped out the globe of the Moon in meticulous detail. There were the nine Ranger probes, launched on lunar impact trajectories between 1961 and 1965, and there were the seven Surveyor landers, which embarked on much less violent lunar impact trajectories between 1966 and 1968. What perhaps contributed the most to our knowledge of lunar geography, though, was the often overlooked Lunar Orbiter program. Five probes, from 1965 through 1967, took more than three thousand images of the Moon’s surface, enabling NASA mission planners to select landing sites. These humble spacecraft very much did lay the groundwork for the astronauts who followed.

Lunar_orbiter_1_(large) — The probe design. Image is from NASA.

Each of the Lunar Orbiters massed 388 kilograms, and comprised a roughly conical bus, four solar panels, and a communications system. A deck on the bottom of the craft held the extensive camera subsystem as well as computational and navigational systems, while the middle deck carried the propulsion system, and the upper deck (also the “back” of the spacecraft) was a square heat shield with a rocket nozzle in the middle. The camera was the central feature, designed by Kodak and derived from similar equipment that flew on the U-2 and SR-71 reconnaissance aircraft.

Delivering the Lunar Orbiters to the moon took five Atlas-Agena launches (the Atlas-Agena was one of the most prolific boosters of the decade). The launch vehicle sent each probe on a ninety-hour course to the Moon, and then the onboard engine braked it into a lunar parking orbit, from which science could begin properly.

In those days NASA was not yet confident in its ability to operate probes beyond Earth orbit. The late 1950s and early 1960s had seen many attempted lunar or interplanetary flights, most of which failed—the Ranger program in particular suffered six consecutive failures before its first success, earning it the nickname of “shoot and hope.” So NASA had a backup proposal in case the Lunar Orbiter program didn’t pan out. The agency would have instead reconnoitered the Moon with a manned flight, using the Lunar Mapping and Survey System (LM&SS). This would have substituted a derivative of the KH-7 spy satellite for the Lunar Module in the Saturn V launch stack. The Apollo CSM was to fly around the Moon docked to it, as shown below, and astronauts would have operated its camera systems on-site.

lmss__1 — Old NASA illustration of the LM&SS.

In the event, the LM&SS never advanced past the drawing board: the Lunar Orbiters proved to be a huge success. All five of them operated without a hitch, the first three focusing on Apollo landing site selection and the last two undertaking broader surveys of the lunar environment. Lunar Orbiter 4 mapped all of the near side and about a tenth of the far side, while Lunar Orbiter 5 mapped the rest of the far side, in places with a resolution as low as two meters. NASA was able to stitch together a complete lunar atlas from countless grainy, black-and-white images like these:

2049_med — Image captured by Lunar Orbiter 2, taken from the Lunar Orbiter Photo Gallery at the Lunar and Planetary Institute. Here one gets a sense of the scale and strangeness of the lunar surface, which really is a world unto itself, and I’m sure the sight was even more profound for the flight controllers who first received this picture in 1966.

First_View_of_Earth_from_Moon — History’s first photographed Earthrise, captured by Lunar Orbiter 1.

Still other scientific boons arose from the program. Radiation and micrometeorite experiments verified that the environment out there would be safe for astronauts, while careful tracking of the satellites’ orbits revealed that the Moon did not have a uniform internal structure—denser patches, dubbed “mascons” for “mass concentrations,” affected their flightpaths. Over a span of just a few years, our understanding of the Moon went from patchy to good enough to land people there.

The Lunar Orbiters are a relic of NASA’s heroic age. We’ve sent other orbiters to the Moon since then, granted, but it’ll never again be the first time, like it was in those halcyon days of 1966. I think the engineers and scientists of that era were truly lucky to witness it.

Sources:

https://space.skyrocket.de/doc_sdat/lmss.htm

https://en.wikipedia.org/wiki/Lunar_Orbiter_program

http://www.astronautix.com/l/lunarorbiter.html