Re-Purposing Mercury: Recoverable Space Observatory (1964)

Launch of astronaut John Glenn on board Friendship 7. Image credit: NASA

Hermann Potočnik, an Austrian Army officer writing under the pseudonym Hermann Noordung, described the benefits of telescopes in space in his seminal 1929 book Das Problem der Befahrung des Weltraums: der Raketen-Motor. The 1995 NASA-sponsored English translation of Noordung's work includes a brief section titled "Unlimited Visibility." It describes how,

beyond Earth’s blanket of air, nothing weakens the luminosity of the stars; the fixed stars no longer flicker; and the blue of the sky no longer interferes with the observations. At any time, the same favorable, almost unlimited possibilities exist, [and] telescopes of any arbitrary size, even very large ones, could be used. . .

In 1946, Princeton University astronomer Lyman Spitzer also wrote about the possibilities of space-based astronomy, and it was with him that U.S. efforts to place telescopes into space originated. In 1960, NASA Goddard Space Flight Center (GSFC) in Greenbelt, Maryland, began work on the Orbiting Astronomical Observatory (OAO) series of space telescopes. The Grumman-built satellites would image the cosmos in wavelengths that could not easily penetrate Earth's atmosphere and radio the images they captured to receiving stations on Earth.

Astronomers eagerly anticipated the OAOs, but for the general public NASA in 1960 was all about Project Mercury. The first manned Mercury orbital flight, designated MA-6, took place on 20 February 1962. A modified Atlas missile propelled astronaut John Glenn into space on board the Friendship 7 spacecraft. Glenn orbited Earth three times and, despite a sensor fault which made it appear that his spacecraft's heat shield had come loose in orbit, splashed down safely in the Atlantic Ocean a little less than five hours after launch.

Three more astronauts rode Mercury capsules into orbit. The last Mercury mission, MA-9, saw Gordon Cooper orbit Earth 22.5 times in the Faith 7 capsule. His 34-hour mission spanned 15-16 May 1963.

Final Mercury: Technicians hoist Gordon Cooper's Faith 7 Mercury spacecraft on Launch Complex 14 at Cape Canaveral, Florida. Image credit: NASA

If Windsor Sherman, an engineer at NASA's Langley Research Center (LaRC) in Hampton, Virginia, had had his way, then Mercury would have found a new role as part of NASA's space astronomy program. In a NASA Technical Note published a year and a half after MA-9, Sherman proposed that NASA modify manned Mercury capsules to serve as recoverable unmanned Earth-orbiting observatories.

Sherman's Mercury-derived observatory would weigh more than the manned Mercury (2150 kilograms versus 1660 kilograms) and would require a higher orbit (at least 500 kilometers) to ensure that it would operate above Earth's atmospheric "airglow." The manned Mercury's Atlas booster would not be up to the task, so the recoverable observatory would launch on an Atlas with an Agena B upper stage. A similar booster-upper stage combination launched Ranger robot explorers to the moon.

Cutaway drawing of Mercury-derived recoverable space observatory. Image credit: NASA

Upon attaining orbit, the Mercury-derived observatory's nose would split down the middle and hinge open like a clam-shell to clear the light path for a reflecting telescope with a curved primary mirror 76 centimeters in diameter. The telescope would occupy an inverted mushroom-shaped volume one meter wide at its widest point and 2.81 meters long. It would collect the light of astronomical targets such as comets, stars, nebulae, and galaxies, and, using a rotating tilted mirror, direct it by turns to six cameras mounted between the underside of the primary mirror and the Mercury heat shield. The cameras would record images in the gamma-ray, infrared, visible, and ultraviolet (UV) parts of the spectrum on up to 6000 frames (1000 frames per camera) of 70-millimeter photographic film.

Sherman called photographic film "one of the best information storage devices yet devised." A good photographic image of a celestial object would, he wrote, contain 10 times as much information as a good television image of the same object. On the down side, photographic film would require shielding against space radiation lest it become clouded and its information storage capacity degraded.

He acknowledged that, as an alternative to film recovery, exposed film might be developed in space automatically and scanned using a television camera. This technique would be used on board the automated Lunar Orbiter spacecraft. Sherman noted, however, that scanning an photographic image, transmitting it to Earth, and reassembling it would inevitably cause data loss. He estimated that images from scanning would contain half as much information as the exposed film the Mercury-derived observatory would return to Earth.

Sherman estimated that, unless GSFC and Grumman upgraded its systems, OAO would need about 860 days to transmit to Earth the 6000 image frames his Mercury-derived recoverable observatory could collect and return to Earth in 200 days. Upgrades to improve image transmission rate would increase OAO complexity, power consumption, and mass, so that the non-recoverable observatory could not be launched as planned on an Atlas rocket with an Agena upper stage.

As Sherman's Mercury-derived recoverable observatory orbited the Earth, it would rely for stability and pointing on a modified OAO guidance system. Sherman expected, however, that it would be unable to track astronomical targets with sufficient precision for film photography. He offered a preliminary design for a "fine-image stabilization system" meant to compensate for image smear by automatically adjusting the focus of the six cameras. He acknowledged, however, that designing a sufficiently stable pointing system for the recoverable observatory remained an important "problem area."

Sherman only briefly discussed the Mercury observatory's electrical power needs. He noted that non-rechargeable batteries sufficient to power the spacecraft for 200 days could not fit within the tight confines of the Mercury capsule, and would in any case be far too heavy. The LaRC engineer suggested that a deployable solar array might instead be used to recharge batteries, but gave no hint as to its likely dimensions, design, or location.

The Langley engineer also did not contend with the thorny issues of the Mercury spacecraft's demonstrated poor longevity. By the time Cooper manually guided Faith 7 to a splashdown in the Pacific Ocean, all of his spacecraft's automatic piloting systems had failed. He was reduced to timing his retrorocket burn using his wristwatch. Similar malfunctions would doom the wholly automated Mercury observatory.

Assuming that its endurance could be extended, at the end of its 200-day mission the Mercury-derived observatory would close its clam-shell nose, orient itself with its broad heat shield pointed approximately in the direction of its orbital motion, ignite its solid-propellant retrorocket pack, and reenter Earth's atmosphere. The Mercury-derived observatory's bifurcated nose would mean that it would deploy two separate main parachutes, each smaller than manned Mercury’s single parachute. Splashdown and recovery would otherwise occur as in manned Mercury missions.

In addition to its superior information capture potential, advantages of the recoverable Mercury-derived observatory would include cost-saving reuse of instruments and spacecraft components during subsequent missions. The Mercury observatory would also permit an ancillary scientific/engineering experiment; because it would return to Earth, any signs of long-term exposure to the space environment that it carried (for example, micrometeoroid pitting) could be subjected to analysis.

Sherman's plan for giving Mercury a new lease on life generated scant enthusiasm. OAO-1 reached orbit on an Atlas-Agena D rocket on 8 April 1966, 15 months after Sherman completed his paper. It carried UV, X-ray, and gamma-ray instruments. Unfortunately, its electrical system overheated, developed arcing, and failed, so that OAO-1's mission ended after only three days. The satellite returned no astronomical data.

Pre-flight artist concept of OAO-1 in Earth orbit. Image credit: Grumman/NASA

OAO-2, with a suite of 11 UV astronomy instruments, abandoned the Atlas-Agena rocket. It reached orbit atop a more powerful Atlas-Centaur on 7 December 1968. OAO-2 operated for a little more than four years. It revealed, among other things, that enormous haloes of hydrogen gas surround comets and that young stars burn very hot.

The third OAO, launched on 3 November 1970 and retroactively dubbed OAO-B, included a 38-inch UV telescope. Unfortunately, the Centaur upper stage meant to push the satellite into orbit malfunctioned, so that it crashed into the Atlantic minutes after launch.

OAO-3, the last in the series, bore the name "Copernicus" to commemorate the 500th anniversary of the birth of the great Polish natural philosopher. Launched on 21 August 1972, it carried the heaviest NASA scientific payload up to that time (2220 kilograms). This included a Princeton University-built UV telescope and a British X-ray telescope. The non-recoverable observatory explored the cosmos until February 1981.

Sources

Conversion of a Spacecraft Designed for Manned Space Flight to a Recoverable Orbiting Astronomical Observatory, NASA Technical Note D-2535, Windsor L. Sherman, NASA Langley Research Center, December 1964

The Problem of Space Travel: The Rocket Motor, Hermann Noordung, NASA SP-4026, 1995

Encyclopedia of Satellites and Sounding Rockets of Goddard Space Flight Center 1959-1969, NASA Goddard Space Flight Center, no date (1970?)

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