A guest blog by Benjamin Palmer.
The Orbiting Astronomical Observatory satellites were a series of four American space observatories launched by NASA between 1966 and 1972. // WikiMedia Commons
What makes an astrophysical event timeless? Time itself. A concept ideally illustrated by the atmosphere at Cape Canaveral on April 8, 1966. Immortality was absent from Launch Complex 12 when an Atlas-Agena D roared to life. Tracing a brilliant arc through the Floridian sky, the fiery parabola soared to its apex, vaulting hopes, dreams, and a 3,800lb octahedron into Earth’s exosphere. Its unassuming name veiled a revolutionary purpose. The Orbiting Astronomical Observatory (OAO) would turn the tide of modern science, and 50 years later, the celebration continues.
Creating a Legend
Recall astronomy before Hubble (HST): Vacant “Pillars of Creation.” No lens of “Einstein’s Cross.” Ultra Deep Field galaxies? Nonexistent. Atmospherically locked for millennia---until a prophetic NASA telegram changed the course of cosmology.
Authored by Dr. Lloyd Berkner, this 1958 bulletin issued a challenge. Soliciting experiment ideas, Berkner’s communiqué angled for a neoteric spacecraft, sustaining a 50kg payload capacity and poised to soar within two years. The audacious agenda was clear: Slip a functioning telescope above our terrestrial sphere. Two hundred proposals answered the call, and OAO was born.
Talk about big picture thinking. One year after Sputnik 1, the “beep heard round the world” still galvanized the astronomical mindset. Theories turned to other suns, exospheric observing, and Ultraviolet (UV) spectrums. Waves of giant astrophysical proportion were brewing, and NASA was seizing a monumental surfboard.
But a space-based astrophysical eye? Could such dramatic innovation be cast? A cadre of Ames, Grumman, and Goddard scientists was tasked with engineering the unknown. Piece by piece, an orbiting observatory, capable of “night sky anomaly” targeting took shape. Quite literally, a reach for the stars.
A futuristic Four Seasons of the skies, OAO’s intended quartet promised to be an astonishing array. On lead vocals, OAO-1, primed to guide a novel series of successive satellites.
And what satellites they were! Boasting enough technology to turn Vanguard 1 green, OAO became a tool-bristling cosmic voyager of the highest degree.
Though individually modified, all OAO platforms convoked a common structural DNA. A 10ft by 7ft cylinder formed OAO’s spine, octagonal in architecture and cloaked in aluminum. Joined by two solar sheet panels, this arrangement amplified solar flux, while aesthetically evoking a dragonfly-esque appearance. At the helm a harmonic maze of gyros, pressure jets, and star trackers guided OAO through the heavens.
Inside, a concentric cylinder core featured coplanar experiment mounting lugs. Like a high-tech paper towel tube, this cylinder could remain open at both ends, aperture coverage achieved via sunshade lids. This igneous, flexible design supported a wide variety of customizable instrument packages.
Tucked within, OAO’s experiment clusters itself. Scientific sagas were written here, scripted in sensors and etched on mirrors. From an extensive initial list, four primary programs (Goddard, Princeton, Smithsonian, and Wisconsin) made the cut, each displaying creative parameters and groundbreaking innovation. As we’ll soon discover, these instruments carved the veritable future of spaceborne observing, charting numerous “firsts” along the way.
By early spring of ’66, everything came together. Rocket, satellite, and payload traversed the launch pad. All systems go.
Or so it seemed. Like HST to follow, OAO was destined for an inauspicious start. With Hubble, it was eyesight, with OAO-1, it was heartbeat. Days after launch, severe tumbling hindered OAO-1’s solar panel deployment. The throbbing batteries flickered, then died. Mission failure.
But as in life and politics, tenacity challenged budgetary. Tenacity won. With backup components available, NASA breathed life into cosmic chariot #2. Little did anyone realize it was the end of the beginning…
The Dawn of an Era
Teamed with a next-gen Atlas-Centaur rocket, OAO-2 blazed into space on December 7, 1968. Unlike her ill-fated sister, she sported an evocative new nickname: Stargazer.
The latest and greatest sailed with her. Flying in diametric experiment mode, Stargazer showcased the finest two instruments in aerospace, and neither was lacking in ambition. Leading off, the Smithsonian Astrophysical Observatory a.k.a Celescope. Optics in quarto, Celescope engaged four 12” Schwarzchild reflectors feeding Uvicon photo-electron imaging tubes. With minute and extended exposures, Celescope aimed for an arduous goal: chart entire regions of the nighttime sky.
Her partner in crime did Celescope three components better. Enter the breathtaking Wisconsin Experimental Package (WEP), a seven instrument symphony of photometers, spectrometers, and reflectors. Anchoring the WEP cluster, four broad-angstrom (A) stellar photometers, fed by 8” parabolic mirrors. Working in tandem, WEP’s nebular photometer, sporting a hefty 16” reflector and a six-shooter filter wheel. Two facile objective grating spectrometers capped off this striking assortment. WEP’s intention, you ask? Better make that plural. Many items filled the manifest, including WEP’s bold principle doctrine: resolve variance of stellar energy distributions in the 3000A-800A range.
It was a match made in heaven. Census taker vs. investigative reporter, the Celescope/WEP counterbalance could decipher hordes of astronomical objects. But what exactly would they find? There was no time to lose. Swiftly oriented in a 480mi orbit, Stargazer opened for business. The hunt was on.
Celescope jumped into action, initializing survey procedures. Raking in 700 stars per day, astronomy’s inaugural UV map rapidly took shape, painting an animated new picture on stellar dispersion.
Meanwhile, WEP was busy snapping UV spectrums, en route to over 1400 celestial portraits. And when this stellar portfolio was analyzed, the spectra exhibited tantalizing cosmic clues.
OAO-3 in the clean room // WikiMedia Commons
A salient OAO-2 enterprise involved brilliant blue/white stars, among cosmology’s most luminous nuclear engines. Scouring various type O/B spectra, OAO-2’s came to a startling revelation: Astronomy’s hottest stars were far hotter than predicted. Exposing frenetic hydrogen burn flows, OAO-2 determined these energy behemoths were aging at double the suspected rate, a riveting stellar diagnosis.
From the young and restless, to the evolved and expansive, giant and supergiant spectra joined the research loop. Dissecting type Gs, Ks and Ms, OAO-2 found magnesium emissions nestled within UV spectra. It was an epic discovery. Vivid gauges of chromosphere activity, these magnesium emissions offered Stargazer a profound glance into stellar fundamentals. Piggybacking on this finding, OAO-2 could illuminate the nature and life cycles of endless chromospheres, performing under various gravity and temperature settings.
Peering between the stars, OAO-2 perused the Interstellar Medium (ISM), tracing Lyman Alpha absorptions through ISM dust. Closer to home, the solar system provided fertile ground for fresh discoveries. OAO-2 plotted solar hydrogen distributions, and shocked science, revealing UV emissions from Uranus. Earth’s upper atmosphere also fell into Stargazer’s crosshairs, imagery documenting starlight scattering in our pale blue dot’s outer sheathing. And when Comet Tago-Sato-Kosaka swung into town, OAO-2 put those frequent flier miles to good use. Unmasking a million-mile hydrogen halo around the nucleus, Stargazer broke new cometary ground, creating astronomy’s first UV comet portrait.
Reaching a thrilling climax, stellar fireworks arrived in the form of novae and supernovae. Once again, OAO-2’s spectra told the story.
In February of 1970, a flickering Mag 7.0 orb blinked into existence, rooted deep in the constellation Serpentis. The culprit was FH Serpentis, a brightening nova ideally situated for UV inquiry. In a carpe diem spectra, OAO-2 turned to FH Serpentis as the magnitude shifted, and espied a breath-taking connection. Noticing UV luster increased as visual glow faded, OAO-2 deduced UV/visible brightness inversely varies in certain novae, an enticing new component in novae mechanics. And as OAO-2’s journey drew to a close, a visual phenomenon awaited. On May 13, 1972, a type 1a supernova (SN1972E) burst forth in NGC 5253. No orbital satellite had ever observed such an event. Could Stargazer take a final stab at immortality? She could. With delicate precision, OAO-2 captured SN1972E near/ peak magnitude, her UV profile briefly outshining her host galaxy.
When the stardust settled, OAO-2 had blitzed through 3,000 star fields, accumulating 10,000 observations. On a daily basis, OAO-2 assimilated more data than in 15 years of previous flights combined. NASA had scored a remarkable triumph. But all good things must come to and end. Signing off in January 1973, Stargazer passed the astrophysical baton to a new realm of explorers.
With the bar set high, eyes turned to OAO-2’s immediate successor, OAO-B. Outfitted with an extremely potent 38” UV primary, OAO-B promised to harness fainter, nuanced spectra at levels beyond OAO-2’s capacity. She was primed to join her sibling on November 30, 1970. But in an eerie OAO-1 throwback, the nose cone shroud on OAO-B’s Atlas-Centaur failed to separate. Plunging into the atmosphere, it was a bitter loss for astrophysics. But one silver bullet lingered, the expanded OAO-C orbiter. Could the planned fearsome four become the dynamic duo?
NASA saved the best for last. More enterprising and intelligent than her previous partners, OAO-C offered nothing short than the evolving future of UV astronomy. Her name firmly underscored status as a craft of destiny: Copernicus, in stirring tribute to the astronomer’s 500th birthday.
Taking a “teamwork is dream work” approach, Copernicus spawned a US/UK coalition, both nations providing capital experiments for the mission ahead. America’s contribution was the Princeton UV Telescope, more commonly identified as the Princeton Experimental Package (PEP). Questing to understand stellar origins, PEP aimed for narrow UV type O/B spectra, while inspecting gaseous interstellar clouds to outline composition and physical structure. To accomplish this objective, Princeton required a stalwart instrument, well adjusted to rigorous observing. That desire was amply fulfilled. Centered on a massive 32” reflector (F/3.4), Princeton’s UV spectrometer could push the boundaries of contemporary cosmology.
Princeton’s British counterpart was the University College London X-ray Experiment (UCLXE). Venturing beyond the UV realm, UCLEX sought X-ray observations in the 0.5-10 Kelvin energy range, conducive for hunting pulsars and X-ray binaries. To detect these and other X-ray sources, UCLXE utilized a trio of Wolter type 0 grazing incidences telescopes. With multiple proportional counters (1A-18A) and a channel photomultiplier at the foci, a modifiable FOV (1-12 arcminutes) made UCLXE a robust platform for X-ray analysis. To accommodate these cardinal tools, Copernicus received a 13.1 ft length extension for the upcoming flight. Launch was slated for Aug. 21, 1972.
With bated breath, the Copernicus team watched as OAO #4 raced skyward, hoping to put a magnificent stamp of finality on the OAO narrative. They needn’t have worried. Upon orbital insertion, Copernicus commenced an inquisitive sojourn that reverberates to this day.
After low-resolution UV spectrophotometry of OAO-2, the high-resolution imagery of Copernicus came as an exhalation of wonder. Hundreds of different spectrums were immediately generated, including superstar type Bs Rigel (β Orionis), Alniyat (τ Scorpii), and Algenib (γ Pegasi).
Picking up where WEP left off, Princeton relentlessly scrutinized ISM gas, charting molecular hydrogen, oxygen, carbon, and silicon absorptions. Yielding overwhelming acumen of these and additional interstellar elements, Copernicus helped determine intrinsic ISM gas cloud structure. In addition, Copernicus vastly exceeded UV stellar expectations, exposing novel information related to hot stellar mass outflow.
As for UCLXE? Her scientists were busy shaping the young domain of X-ray astronomy. Exhaustive observations returned seductive results. Dashing through the heavens, UCLXE accrued numerous X-ray datasets, proving enormously proficient at long term energy source observation. But new detections placed UCLXE permanently on the radar. Interaction with Cygnus X-1 revealed never-before seen absorption dips, while a glance at starburst galaxy Centaurus A (NGC 5128) unlocked rapid X-ray variability, shedding light on Cen A’s powerful relativistic jet. Yet one discovery reigns supreme. Gazing at certain pulsars, UCLXE encountered bizarre long period energy outputs well in excess of second or less rotations. What was causing these somewhat leisurely rates? As it turned out, UCLXE had uncovered more examples from a breed of long-period pulsars, punctuated by distinctive multi-second rotations.
The sophisticated tech on Copernicus forged a unique lineage all its own. The largest scope ever orbited at the time, PEP’s 32” reflector awakened feasibility of the Large Space Telescope, eventual precursor to HST. UCLXE would lead to Chandra and NuStar kicking off a golden epoch of X-ray astronomy
Copernicus would report live from the scene for nearly a decade. When the finale arrived in February 1981, the road to tomorrow was firmly ensconced. Bringing the OAO program to a resplendent conclusion, Copernicus paved the way for a generation of complex probes to follow in her wake.
Before Hubble, FUSE, GALEX, and Webb, OAO stood alone. There was no “orbital telescope” guidebook, few handy “go to” manuals for spaceflight control. Yet OAO showed a watching world how it’s done.
Casting dreamlike ripples on the ocean of aerospace, OAO left behind a bright universe, filled with mystery, marvels, and the promise of discovery. Echoing across time, it’s a message embraced by every OAO descendant. Just see for yourself.
On the next starry night, step outside and be greeted by radiant flashes above. One can easily imagine ghostly orbits joining the sea of research satellites swimming beyond. Winking and flaring, they embody OAO’s living legacy, a captivating reminder that the future always lingers an idea away.