Apollo Telescope Mount

• 1. Introduction

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  On the day Skylab launched, the solar arrays of the Apollo Telescope Mount were the only ones to survive the journey intact. A problem during liftoff destroyed one of the station's main workshop panels. The second failed to deploy automatically. Those two failures left the station critically short of power before any crew had arrived. The ATM's windmill-like arrays, tucked inside the launch shroud, had come through undamaged. They supplied enough power for crewed operations until astronauts on the first mission could free the remaining workshop panel by hand.

  Skylab, America's first space station, carried the Apollo Telescope Mount as a crewed solar observatory built to study the Sun from orbit. Its name held a history. The ATM had begun life as a project tied to the Apollo spacecraft, not to the space station it eventually joined. How it arrived there, what its instruments were built to see, and what became of its data are the questions this story answers.

• 2. From Apollo to Skylab

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  Extended lunar stays and a permanent Moon base were among the ideas studied by NASA's Apollo Applications Program in the late 1960s. The program explored a wide variety of uses for the infrastructure developed for the Moon landings. The ATM was one of the few proposals from that effort to make it to orbit.

  Its earliest design called for solar instrumentation mounted on a deployable unit attached to the Apollo Service Module. That concept gave way to a more ambitious one. A modified Apollo Lunar Module would house the controls, observation instruments, and recording systems. The lunar descent stage would be replaced by a large solar telescope and the panels to power it. Once in orbit, a three-person Apollo crew would operate the observatory and return home with the data.

  Over time, most Apollo Applications proposals were dropped. Only the space station and the ATM remained on the planning books. Plans then called for the ATM to launch separately and connect to Skylab in orbit. That changed when cancelled Apollo landing missions freed up a Saturn V rocket. That rocket was powerful enough to carry the station and the ATM as a single payload. The combined launch allowed engineers to replace the planned wet workshop concept with a fully outfitted, dry station sent up ready to use. NASA's Marshall Space Flight Center oversaw the ATM's design and construction through every version of those plans.

• 3. Eight Instruments, One Star

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  Eight major instruments were mounted on the ATM's frame. Together they could observe the Sun across wavelengths spanning from 2 to 7000 Angstroms. That range covered soft X-rays at one end and visible light at the other, with ultraviolet filling the gap between.

  Two X-ray telescopes anchored the hard-radiation end of the suite. A third instrument, designated S-020, operated as both an X-ray and extreme ultraviolet camera. On the ultraviolet side, three instruments each handled a different technique. The S-082A was an extreme ultraviolet spectroheliograph, the S-082B a spectroheliometer, and the S-055 a spectrograph. Two Hydrogen Alpha telescopes focused on the specific red wavelength associated with the Sun's chromosphere. A coronagraph, designated S-052, was built to block the Sun's disk and reveal the faint structure of the solar corona beyond it.

  The ATM was actively cooled to hold all its instruments within a stable temperature range. Pointing commands reached the observatory through the Skylab computer, issued either by astronauts inside the station or sent via a communication link from Earth. One additional experiment, S149, was fixed to one of the ATM's solar panels rather than the central instrument frame. Those panels provided around 30% of Skylab's total electrical power, feeding the station as well as the observatory itself.

• 4. Film and Archive

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  Six of the ATM's instruments recorded their observations on film. Over the Skylab missions from 1973 to 1974, those instruments accumulated more than 150,000 successful exposures.

  Film magazines could not be swapped from inside the station. Astronauts had to conduct spacewalks to reach the instruments and retrieve the canisters by hand. Almost 30 canisters were loaded and used across the full run of missions. The heaviest weighed 40 kilograms, around 88 pounds, and each could hold up to 16,000 frames. At the end of each mission, the canisters traveled home inside the Apollo capsule. They ranked among the heaviest single items the crews returned to Earth.

  Among the first Polaroid photographs taken in space were images an astronaut shot of a Skylab CRT video screen. That screen was displaying the Sun as captured by an ATM instrument. Polaroid cameras produced an instant hard copy on the spot, giving crews a quick way to document what the observatory's video channels were showing. Some of the ATM's instruments had live video feeds that astronauts could watch from inside the station.

  As of 2006, the original film exposures were on file at the Naval Research Laboratory in Washington, D.C., accessible to interested researchers. A backup ATM spare, with instruments still mounted to its frame, was restored in 2015 and placed on public display at the Steven F. Udvar-Hazy Center in Chantilly, Virginia. The restoration required repairing Kapton layers that had degraded over four decades.