At 6:28 a.m. EDT, August 21, 1972, NASA’s Copernicus satellitethe heaviest and most complex space telescope of its time, lit up the sky as it climbed into orbit from Launch Complex 36B to what is now Cape Canaveral Space Force StationFlorida.
Originally known as Astronomical observatory in orbit (OAO) VSit has become AO 3 once in orbit in the fashion of the day. But it was also renamed to honor the 500th anniversary of the birth of Nicolaus Copernicus (1473-1543). The Polish astronomer formulated a model of the solar system with the Sun in the central position instead of the Earth, breaking with 1,300 years of tradition and triggering a scientific revolution.
Equipped with the largest ultraviolet telescope ever in orbit at the time as well as four co-aligned X-ray instruments, Copernicus was arguably NASA’s first observatory dedicated to multi-wavelength astronomy. This makes it an ancestor of satellites in operation such as NASA’s Neil Gehrels Swift Observatorywhich observes the sky in visible, ultraviolet and X-ray light. (Download HD video and hi-res images from NASA’s Science Visualization Studio)
“The two spacecraft also share institutional tiessaid Swift’s lead researcher, S. Bradley Cenko at NASA Goddard Space Flight Center in Greenbelt, Maryland. “Goddard managed both missions, and the X-ray experiment on Copernicus was provided by the Mullard Space Science Laboratory at University College London, which also contributed Swift’s Ultraviolet/Optical Telescope.”
Learning to point and keep a telescope orbiting a star long enough for detectors to pick up its light has proven to be much more difficult than expected. Satellites designed to study the Sun at the time had an intrinsic advantage: they targeted the brightest object in the solar system. Copernicus flew with a new Inertial Reference Unit (IRU) developed by the Massachusetts Institute of Technology. The IRU’s gyroscopes accelerated the target acquisition process, while other systems kept the satellite locked. In a study of the first 500 days of the mission, an engineer summed up the situation by noting that the IRU had Copernic flown “a boring operation.”
Early NASA astronomers emphasized the need for ultraviolet (UV) studies, which could not be done from the ground, and this became the main focus of the OAO program. Of four satellites launched, one failed after three days in space and another never reached orbit at all. OAO 2, launched in 1968 and named stargazer, has provided years of observations, including low-resolution stellar spectra, which spread out wavelengths much like a rainbow to reveal the UV fingerprints of specific molecules and atoms. Copernicus went even further, capturing spectra with up to 200 times more detail at certain wavelengths.
“This mission obtained high-resolution spectra of many stars in the UV and provided information at the shortest wavelengths reached in many years.,” wrote Nancy Grace Roman, the first chief of astronomy in the Office of Space Science at NASA Headquarters, Washington, and Copernicus Program Scientist. During the mission, Roman became one of the driving forces behind the Large Space Telescope project, now known as the NASA’s Hubble Space Telescope. She is also the namesake of NASA Roman space telescopewhich should take off in a few years.
The main instrument aboard Copernicus was the Princeton Experiment Package, which captured UV light using a 32-inch (0.8 meter) mirror about a third the size of Hubble. Directed by Lyman Spitzer Jr. at Princeton University in New Jersey, the instrument has produced a wealth of information about interstellar gas and ionized outflows from hot stars. Its first target, a star named Zeta Ophiuchi which is partly veiled by an interstellar cloud, showed a strong absorption of hydrogen molecules. Measurements of dozens of other stars have confirmed a theory predicting that most of the hydrogen in gas clouds exists in this form.
In 1946, Spitzer began to speculate on the kinds of science that might be possible with a large orbiting telescope, later becoming the catalyst for the development of Hubble. from NASA Spitzer Space Telescopewhich operated from 2003 to 2020 and explored, among other sources, the cold clouds where stars are born, was named in his honor.
At the time NASA was considering instrument proposals for Copernicus, only one celestial object, the Sun, was known to emit X-rays. That changed in 1962. Flying new X-ray detectors on a suborbital rocket, a team research led by Ricardo Giacconi at American Science and Engineering Inc., then to Cambridge, Massachusetts, discovered the first X-ray source beyond the solar system, named Scorpius X-1. Additional flights have discovered other cosmic sources, including Cygnus X-1, long suspected and now known to harbor a stellar-mass black hole.
With this breakthrough, Giaconni offers the first satellite dedicated to X-ray sky mapping. Launched in 1970 and in operation for three years, NASA’s Uhuru satellite mapped more than 300 sources, showed that many are neutron stars or black holes fueled by the flow of gas from stellar companions, and discovered X-rays of hot gas in galaxy clusters. Giaconni will then propose more powerful X-ray satellites – NASA’s Einstein Observatorywhich operated from 1978 to 1981, and NASA’s current flagship, the Chandra X-ray Observatorylaunched in 1999.
The X-ray experiment aboard Copernicus was led by Robert Boyd of University College London, and all three telescopes faced significant challenges. The longer wavelength detectors were overwhelmed by a surprisingly high level of background radiation. It turned out to come from a vast, comet-shaped cloud of hydrogen atoms surrounding the Earth, called the geocorona, which scatters far-ultraviolet light. Later instruments added a filter set to absorb UV but pass X-rays.
In June 1973, Goddard scientists noticed a problem with a shutter in X-ray telescopes. The device was used to periodically block X-rays from reaching the detector so scientists could track the progress of the radiation background of charged particles in different parts of the orbit. Now its functioning had become hesitant. Fearing that the shutter would remain permanently in the closed position, the team of the instrument had decided not to use it any longer. But one last command went through – and the sticky shutter jammed, blinding the instruments.
A fourth detector not attached to a telescope continued to operate for the duration of the mission. This X-ray counter measured radiation from 1 to 3 angstroms over a wide field of view – 2.5 by 3.5 degrees, or about 40 times the apparent area of a full moon.
The X-ray experiment discovered several long-period pulsars, including X Persei. Pulsars – usually, rotating neutron stars – swinging a beam of radiation in our direction with each rotation, usually tens to thousands of times per second. Curiously, the X Persei pulsar takes a leisurely 14 minutes per turn.
Copernicus conducted long-term monitoring of pulsars and other light sources, and observed Nova Cygni 1975, an outburst on the white dwarf in a nearby binary system. The experiment found curious dips in X-ray absorption at Cygnus X-1, likely caused by cold, dense clumps in the gas moving away from the star. And the satellite recorded variable X-rays from the black hole-powered galaxy Centaurus Alocated about 12 million light-years away.
Copernicus returned UV and X-ray observations for 8.5 years before it retired in 1981, and it still orbits Earth today. It left center stage in space astronomy with the appearance of more advanced observatories, including Einstein and the International ultraviolet explorer, which was launched in 1978 and operated for almost 19 years. Copernicus observations appear in more than 650 scientific papers. Its instruments have studied some 450 unique objects targeted by more than 160 investigators in the United States and 13 other countries.
Author of the article: Francois Reddy, NASA Goddard Space Flight CenterGreenbelt, Maryland.