At 6:28 am EDT on August 21, 1972, NASA’s Copernicus satellite, the heaviest and most complex space telescope of its time, lit up the sky as it lifted into orbit from Launch Complex 36B at what is now the Cape Canaveral Space Station, Florida.
Originally known as the Orbiting Astronomical Observatory (OAO) C, it became OAO 3 once in orbit in the fashion of the time. But it was also renamed to commemorate the 500th anniversary of the birth of Nicolaus Copernicus (1473–1543). The Polish astronomer formulated a model of the solar system with the Sun at the center instead of the Earth, breaking 1,300 years of tradition and sparking 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 dedicated multiwavelength astronomical observatory. This makes it an ancestor of operational satellites such as NASA’s Neil Gehrels Swift Observatory, which monitors the sky in visible, ultraviolet and X-ray light.
“The two spacecraft also share institutional connections,” notes Swift principal investigator S. Bradley Cenko at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Goddard managed both missions, and the Copernicus X-ray experiment was provided by the Mullard Space Science Laboratory at University College London, which also contributed to Swift’s ultraviolet/optical telescope.”
Learning to steer and keep a telescope orbiting a star large enough for detectors to capture its light turned out to be much more difficult than expected. Satellites designed to study the Sun at the time had a built-in advantage – they aimed at the brightest object in the solar system. Copernicus flew with a new inertial reference unit (IRU) developed by the Massachusetts Institute of Technology. Gyroscopes on the IRU sped up the target acquisition process, while other systems kept the satellite locked. In a study of the first 500 days of the mission, one engineer summed it up by noting that the IRU had made the Copernicus flight “a boring business”.
In the early days of NASA, astronomers stressed 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 the four satellites launched, one failed after three days in space and another failed to reach orbit at all. Launched in 1968 and named Stargazer, OAO 2 provided years of observations, including low-resolution stellar spectra that spread out wavelengths like a rainbow to reveal the UV fingerprints of specific molecules and atoms. Copernicus went even deeper, capturing spectra with up to 200 times better detail at certain wavelengths.
Orbital Astronomical Observatory C – named Copernicus in orbit – sits in the Hangar AE clean room at Cape Canaveral Air Force Station, Florida, after its fixed solar panels have been installed. Copernicus was the only member of the series to carry the large cylindrical structures on top of the spacecraft, which prevented stray light from reaching the instruments. Credit: NASA
“This mission acquired high-resolution spectra of many stars in the ultraviolet 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 in Washington and the Copernicus program scientist. During the mission, Roman became one of the driving forces behind the Large Space Telescope project, now known as NASA’s Hubble Space Telescope. It is also the namesake of NASA’s Roman Space Telescope, which is expected to fly in a few years.
The main instrument on Copernicus was the Princeton Orbiter Experiment, which captured ultraviolet light using a 32-inch (0.8-meter) mirror about one-third the size of Hubble. Led by Lyman Spitzer Jr. at Princeton University in New Jersey, the instrument produced a wealth of information about the interstellar gas and ionized outflows of hot stars. Its first target, a star named Zeta Ophiuchi that is partially obscured by an interstellar cloud, showed strong absorption by hydrogen molecules. Measurements of dozens of other stars confirmed a theory predicting that most of the hydrogen in gas clouds existed in this form.
In 1946, Spitzer began speculating about the kinds of science that might be possible with a large telescope in orbit, and later became the catalyst for the development of Hubble. NASA’s Spitzer Space Telescope, which operated from 2003 to 2020 and explored, among other sources, the cold clouds where stars are born, is 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 research team led by Riccardo Giacconi at American Science and Engineering Inc., then in Cambridge, Massachusetts, discovered the first X-ray source beyond the solar system, called Scorpius X-1. Additional flybys revealed more cosmic sources, including Cygnus X-1, long suspected and now known to host a stellar-mass black hole.
With this discovery, Giaconni proposed the first satellite dedicated to mapping the X-ray sky. Launched in 1970 and operating for three years, NASA’s Uhuru satellite mapped more than 300 sources, showing that many are neutron stars or black holes that powered by gas flowing from stellar companions and detected X-rays from hot gas in galaxy clusters. Giaconi would go on to propose more powerful X-ray satellites—NASA’s Einstein Observatory, which operated from 1978 to 198, and NASA’s current flagship, the Chandra X-ray Observatory, launched in 1999.
The Copernicus X-ray experiment was led by Robert Boyd at University College London, and the three X-ray telescopes faced significant challenges. The longer wavelength detectors were overwhelmed by an unexpectedly high level of background radiation. It turned out to come from a huge comet-shaped cloud of hydrogen atoms surrounding the Earth, called the geocorona, which scatters the sun’s ultraviolet light. Later instruments added a filter tuned to absorb UV radiation but let X-rays through.
In June 1973, scientists at Goddard noticed a problem with a shutter in the X-ray telescopes. The device was used to periodically block X-rays from reaching the detector so that scientists could observe the changing background radiation from charged particles at different parts of the track. Now his operation had become hesitant. Concerned that the shutter might remain permanently in the closed position, the instrumentation team decided to stop using it. But one last command got through — and the sticky shutter stuck shut, blinding the instruments.
A fourth probe that was not attached to a telescope continued to operate during 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, about 40 times the apparent area of a full moon.
The X-ray experiment discovered several long-period pulsars, including X Persei. Pulsars—typically, rotating neutron stars—had been discovered five years before Copernicus’ launch. These objects swing a beam of radiation in our direction every time they rotate, usually tens to thousands of times per second. Amazingly, the pulsar X Persei lasts 14 minutes per revolution.
Copernicus conducted long-term monitoring of pulsars and other light sources and observed Nova Cygni 1975, a white dwarf explosion in a close binary system. The experiment discovered strange dips in X-ray absorption in Cygnus X-1, likely caused by cold, dense clumps in the gas flowing away from the star. And the satellite captured a variety of X-rays from the Centaurus A galaxy moving with the black hole, located about 12 million light-years away.
Copernicus returned ultraviolet and X-ray observations for 8.5 years before being retired in 1981, and is still orbiting Earth today. It left the center stage of space astronomy as more advanced observatories appeared, most notably Einstein and the International Ultraviolet Survey, which launched in 1978 and operated for nearly 19 years. Copernicus’ observations appear in more than 650 scientific papers. Its instruments studied about 450 unique objects targeted by more than 160 researchers in the United States and 13 other countries.
Hubble Gazes at a Starry Sky Provided by NASA’s Goddard Space Flight Center
Reference: 50 years ago, NASA’s Copernicus set the bar for space astronomy (2022, August 20) Retrieved August 20, 2022 by
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