The guest speaker at the May monthly meeting of Keighley Astronomical society was Mr Peter Rea (FRAS) from Cleethorpes and district astronomical society. The title of his stunning presentation was ‘Apollo 16, into the Highlands at Descartes – Cayley’.
The lunar mission personnel were:- Commander John Young,
Command Module Pilot Thomas “Ken” Mattingly,
and Lunar Module Pilot Charles Duke
The Saturn five rocket was launched on 16th April 1972, and successfully completed the fifth human landing on the Moon. Following a 6-hour delay caused by unexpected oscillations in the control system for the service module’s main engine, Young and Duke landed the lunar module (LM), Orion, less than 0.3 km from the planned landing location between North Ray Crater and South Ray Crater in the Descartes highlands northwest of Mare Nectaris. This site was certified as safe for landing based on photographs obtained on Apollo 14.
Three of the first four Apollo Moon landings were in mare regions and the fourth was in ejecta from the Imbrium impact. When selecting the Apollo 16 landing site, the highest priority was given to landing at a site in the lunar highlands, which occupy more than five times the surface area occupied by mare units.
The objectives were to sample two geologic units, the Descartes Formation and the Cayley Formation. Based on the interpretations of telescopic and orbital imagery, it was thought that both units were volcanic in origin. The Cayley Formation was thought to be relatively smooth and formed from a low viscosity lava or volcanic ash flow; in actuality, they turned out to be rather hummocky. From the density of impact craters, the Cayley Formation was thought to be older than the lunar mare and comparable in age to the Imbrium impact.
The Descartes Formation is much rougher and somewhat older than the Cayley Formation. It was thought to be formed of magmas that were more viscous than mare lavas. This could be due to a higher abundance of silicon, as in rhyolite volcanic domes on Earth. Samples obtained by Apollo 16 proved that both units are actually breccias produced by impacts rather than volcanic features.
The pre-mission geologic studies suggested that these two formations covered about 11% of the lunar nearside, making them important for the overall understanding of the Moon’s history. Also, the large distance between the Descartes site and previous landing sites was helpful for the network of geophysical instruments created by the Apollo 12 through 16 missions.
The principal alternative landing site that was considered was inside Alphonsus Crater, to sample both old lunar crust in the crater wall and possible young volcanism on the crater floor. Once Descartes was selected for Apollo 16, Alphonsus became a prime contender for the Apollo 17 landing site.
Young and Duke spent a total of 71 hours on the Moon, performing three extra-vehicular activities (EVA, or “moonwalks”) totaling 20 hours and 14 minutes. At the start of the first EVA, the crew deployed both the lunar rover and a set of experiments, including passive and active seismometers, a heat flow experiment, a cosmic ray detector, and a solar wind composition experiment.
Modifications to the lunar drill made it easier than on Apollo 15 to drill the holes for the Heat Flow Experiment, although Young tripped on a cable, breaking it and preventing use of the experiment. The crew also set up the Far Ultraviolet Camera/Spectrograph near the LM, a small telescope that was used to make observations of selected astronomical targets. Film from the telescope was returned to Earth for science analysis.
The crew then drove the rover west of the landing site, collecting Cayley Formation samples at Flag Crater and Spook Crater and making measurements with a portable magnetometer to determine how the Moon’s magnetic field varied with location around the landing site. A total of five such measurements were made during the mission. The EVA lasted 7 hours and 11 minutes. By the end of EVA 1, crew descriptions of the collected samples had already made it clear that there was little or no volcanic material at the Apollo 16 landing site.
On the second EVA, lasting 7 hours and 23 minutes, Young and Duke drove south of the landing site, making several stops on the lower slope of Stone Mountain and reaching a maximum height of 160 meters above the base of the mountain. Prior to the mission, Stone Mountain was believed to be a volcano, and it was intended to be the primary sampling location for material from the Descartes Formation. Several additional stops on the return to the LM were made in an ejecta ray from South Ray Crater, a 2-million-year-old, 680-meter diameter crater about 4 km away. These stops were planned to ensure that material from South Ray Crater was included in the sample collection.
Because of the delayed landing, the third EVA was shortened to 5 hours and 40 minutes. Young and Duke drove north of the landing site to the southeastern rim of North Ray Crater. Most of the pre-planned sampling stops were eliminated, allowing the crew to spend two hours working on the rim of North Ray Crater. The crater is 1 km across and 230 meters deep, acting as a natural drill hole and ejecting material from deep below the lunar surface in boulders that are now located along and outside the crater rim for sampling by the astronauts.
The largest boulder, nicknamed House Rock, is 20 meters long and 12 meters high and likely represents material that was ejected from 200 meters below the surface. An additional stop about 0.8 km south of North Ray Crater allowed sampling of the outer edge of North Ray’s ejecta blanket.
During the three EVAs combined, Young and Duke collected 95.7 kilograms of lunar samples. They drove the lunar rover for a total distance of 26.9 km and reached a maximum distance of 4.6 km from the landing site.
During the moon landing, Mattingly remained in lunar orbit in the Command Module, Casper, operating set of orbital experiments similar to those on Apollo 15 and observing the lunar surface through binoculars. Apollo 16 spent a total of 5 days and 6 hours in lunar orbit, circling the Moon 64 times.
The crew departed for Earth one day earlier than planned due to the earlier problems with the service module engine. While on the return voyage to Earth, Mattingly made a 1 hour, 23-minute spacewalk to collect the film cassettes from the mapping cameras in the Service Module. The crew landed in the Pacific Ocean on 27th April after a flight of 11 days and 2 hours.
Mr Peter Rea will be returning to Keighley Astronomical society next year on 23rd June 2023 with a presentation titled ‘How it Began – The Origins of Lunar Exploration 1958 to 1976’.