Had the cruelty of fate not intervened, a half-century ago, this month, the fifth and sixth humans ever to set foot on another world twice have walked on the dusty surface of the Moon. Following their launch aboard Apollo 13 and a voyage across 240,000 miles (370,000 km) of cislunar space, on 16 April 1970 Commander Jim Lovell and Lunar Module Pilot (LMP) Fred Haise would have boarded the Lunar Module (LM) Aquarius and planted it on the rugged terrain of our nearest celestial neighbor. And if near-disaster had no so radically altered their mission, Lovell and Haise would have become the first Apollo astronauts to explore a hilly upland site on the Moon. It would have been a quite different lunar landing in scope and complexity even from those of its predecessors, Apollo 11 and 12.
“It was driven by confidence in the LM capability and steerage,” Haise told the NASA Oral History Project of the site selection process, “but also, if you’re going to properly sample the Moon…you had to become more diverse in…where you went to get a proper sampling.”
And Apollo 13’s destination, the Fra Mauro foothills, was nothing if not diverse. The site was named in honor of the 15th-century Venetian cartographer-monk Fra Mauro, who created one of the earliest (and relatively accurate) maps of the Old World. His lunar namesake differed markedly from the relatively flat, open plains (or mare) explored by the Apollo 11 and Apollo 12 astronauts and was considerably more rugged, resembling a low “island” in the Moon’s Ocean of Storms. In the late 1960s, many geologists suspected that the lunar highlands had remained virtually unchanged, geochemically and morphologically, since the Moon formed, around 4.5 billion years ago. By exploring into the older and more heavily cratered lunar highlands, it was hoped that Lovell and Haise would identify some of the oldest rocks on the surface.
Fra Mauro had been extensively photographed from lunar orbit by the Apollo 12 crew in November 1969, and samples returned from the Sea of Tranquility and the Ocean of Storms differed markedly in composition from “ordinary” mare materials, to such an extent that they were believed to have been violently ejected over long distances by vast impacts in the lunar highlands. One obvious example of such an impact was the object which created the 750-mile-wide (1,200 km) Mare Imbrium basin, whose southern rim lay 300 miles (480 km) to the north of Apollo 13’s selected landing site at Fra Mauro. In fact, much of Fra Mauro was thought to be composed of ejecta from this ancient cataclysm. By sampling these foothills, Lovell and Haise might shed significant new light on the composition of the pre-Imbrium lunar crust and help to establish an absolute date for when the impact took place.
Of central importance in the so-called Fra Mauro Formation was Cone Crater, a yawning bowl, spanning 1,000 feet (300 meters), whose impact was believed to have dug deeply into a ridge of Imbrium ejecta. Imagery from NASA’s unmanned Lunar Orbiters had shown its rim to be littered with boulders drawn from deep within the blanket of Imbrium material, and Cone was one of Apollo 13’s key sampling locations. “By strategically sampling up toward the crater, you would be sampling material that at the…outside ray of the crater would be the deepest material,” Haise explained to the NASA Oral History Project. “If it’s due to an impact facet, it “inverts”—it’s an inverted flap—so if you’re sampling up a ray, the farther-out stuff is the deepest stuff within the crater. And as you get up near the edge of the crater, you’re sampling literally at the surface.”
Reaching Fra Mauro and Cone Crater involved a novel propellant-conservation plan. Previous Apollo missions had entered near-circular orbits at an altitude of about 70 miles (110 km), after which the LM undocked from the Command and Service Module (CSM) to commence its Powered Descent to the surface. However, on Apollo 13, the spacecraft would enter an elliptical orbit with a high point of 70 miles (110 km) and a low point of only 9.3 miles (15 km). The result was that LM Aquarius would be effectively relieved of the need to perform a Descent Orbit Insertion (DOI) maneuver, which gave Lovell an extra 15 seconds of hovering time to pick an appropriate landing spot. During his approach, he would clear the ridge into which Cone was embedded and find a safe patch, somewhere between two groups of craters, nicknamed “Doublet” and “Triplet”.
Had the crippling explosion in one of two oxygen tanks aboard Apollo 13 on the evening of 13 April 1970 not occurred, and had the mission proceeded as intended, the crew would have entered lunar orbit at 7:38 p.m. EDT on 14 April 1970, about 77.5 hours after launch. Almost a full day later, at 5:29 p.m. EDT on the 15th, during Apollo 13’s 12th orbit of the Moon, Lovell and Haise would have undocked LM Aquarius from CSM Odyssey, leaving Command Module Pilot (CMP) Jack Swigert alone in orbit. “A radially-downward Service Module Reaction Control System (RCS) burn of 1 fps (0.3 meters/sec),” it was noted in the Apollo 13 pre-flight press kit, “will place the CSM on an equiperiod orbit with a maximum separation of 2.5 nautical miles (4.6 km).” An hour later, Swigert would have executed a Circularization Burn to put Odyssey into an orbit of 52 x 62 nautical miles (96.3 km x 114.8 km). Due to perturbations of the lunar gravitational potential, this orbit would virtually circularize by the time of rendezvous with Aquarius’ returning ascent stage, two days later.
In the meantime, Lovell and Haise would have commenced their Powered Descent during the 14th orbit, braking the LM out of the descent orbit by means of Aquarius’ Descent Engine. “Spacecraft attitude will be windows-up from the Powered Descent Initiation to the end of the braking phase,” it was explained in the press kit, “so that the LM landing radar data can be integrated continually by the LM guidance computer and better communications can be maintained.” About 7,400 feet (2,250 meters) above the Moon, the braking phase would end and Aquarius would be rotated to an “upright”, windows-forward, attitude, thereby permitting the crew a view of the landing site.
Progressing through the upper (“High Gate”) and lower (“Low Gate”) stages of approach regime, Aquarius would initiate a final vertical descent at an altitude of about 100 feet (30 meters), by which point all forward velocity would have been nulled out. According to the Apollo 13 press kit, touchdown at Fra Mauro was intended to occur at 9:55 p.m. EDT on 15 April, about 103 hours and 42 minutes after departing Earth. The predicted landing spot was situated 30 miles (48 km) north of the Fra Mauro crater, kicking off an estimated 33.5 hours on the surface and two sessions of Extravehicular Activity (EVA).
The second part of this article will appear tomorrow.
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