Orbiting Astronomical Observatory 2

• 1. Stargazer

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  On the 7th of December 1968, an Atlas-Centaur rocket carried a machine nicknamed Stargazer into a nearly circular orbit roughly 750 kilometers above the Earth. Its full name was the Orbiting Astronomical Observatory 2, or OAO-2. It was the first space telescope that actually worked. The one before it, OAO-1, had failed to operate once it reached orbit. Inside an octagonal frame, two sets of instruments faced in opposite directions, both straining to catch a kind of light the human eye cannot see. What can a telescope find when it looks at the universe in ultraviolet, from a vantage point no ground-based instrument can reach? And why would a comet, of all things, turn out to be one of its most revealing targets?

• 2. Celescope's Electric Eye

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  Four telescopes, each 12 inches across, or 30.5 centimeters, formed the heart of one of the two instrument packages aboard OAO-2. This was the Smithsonian Astrophysical Observatory experiment, also called Celescope. Each of these Schwarzschild telescopes fed light into a device called a Uvicon. The Uvicon was an ultraviolet detector built on the Westinghouse Vidicon. Its job was a small act of translation. Ultraviolet light struck the detector and was converted into electrons. Those electrons, in turn, hit the detection area of the tube and became a voltage that could be recorded. Filters, photocathodes, and electronics let Celescope gather light across several distinct ultraviolet passbands. The detectors did not last. They suffered a gradual loss of sensitivity, and in April 1970 the experiment was switched off for good. By then Celescope had surveyed only about 10 percent of the sky. That fraction was enough to assemble a catalog of 5,068 ultraviolet stars. One of those Uvicons did not vanish into history. A Uvicon has sat in the collection of the Smithsonian Institution since 1973.

• 3. The Wisconsin Package

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  Seven telescopes made up the second instrument set, the Wisconsin Experiment Package. Each was tuned for ultraviolet observation. Among them was a nebular photoelectric photometer fed by a 16-inch telescope, 40.64 centimeters across, fitted with a six-position filter wheel. That particular photometer did not survive long. It failed only a few weeks after launch. Arthur Code of the University of Wisconsin-Madison supervised the construction of this package. Despite the early loss of one instrument, the Wisconsin equipment kept working far longer than its counterpart. It observed more than 1200 targets in ultraviolet light before the mission came to an end in early 1973.

• 4. Hydrogen Around A Comet

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  Comet Tago-Sato-Kosaka became one of the spacecraft's defining discoveries. The Wisconsin Experiment Package observed it and found the comet wrapped in a vast cloud of hydrogen. That cloud carried a clear message. It confirmed that the comet was made largely of water. The same observations turned up something stranger and still unresolved. OAO-2 detected the 2175-angstrom bump, a rise in ultraviolet absorption at that specific wavelength. Decades on, that bump is still not fully explained. Comets were not the only place where halos of hydrogen gas appeared. The observatory found large hydrogen halos around comets more broadly, and it also turned its instruments on Nova Serpentis, a nova discovered in 1970.

• 5. The Body In Orbit

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  An octagonal prism held all of this together. The OAO-2 observatory was built in that shape, measuring about 10 by 7 feet. It weighed 4,400 pounds. Within that frame, the data it gathered spanned an unusual range of subjects for a single mission. Its ultraviolet measurements reached comets, planets, and galaxies alike. The catalog of 5,068 ultraviolet stars and the more than 1200 targets logged by Wisconsin both flowed from this one octagonal body, circling the Earth at 750 kilometers until its instruments fell silent in early 1973.