Space-based telescopes

Since the 1960s, robotic observatories placed in space have become a key part of the overall effort by astronomers to understand the solar system and universe. Able to view the universe without the interfering effects of Earth’s atmosphere, these space telescopes have revealed sights and wonders that were previously unimagined.

Overview

With the exception of relatively few objects within the solar system, objects of study outside our planet are too far away to visit and sample. We cannot simply travel to distant stars, nebulae, and galaxies to learn about them. Thus, if humans wish to study these distant objects, they must find another way to do so.

Electromagnetic Radiation. The primary method of learning about distant objects is observing various electromagnetic radiation. One must bear in mind that since light travels at a finite speed (300,000 kilometers per second, or 186,000 miles per second), objects can be studied only as they were when they emitted the light we observe. Thus, the telescope can be considered a time machine in space or on Earth. For example, suppose an object is determined to be ten billion light-years away (a light-year is the distance light travels in a vacuum in one year, or 9.467 trillion kilometers). In that case, the telescope provides a view of what that object looked like ten billion years ago. For example, considering the presently accepted value for the universe's age, a galaxy twelve billion light-years away is observed as it was only slightly less than two billion years after the Big Bang.

Sir Isaac Newton demonstrated that visible white light is composed of an infinite sequence of wavelengths from red to violet in color. If one disperses white light with a prism or diffraction grating, a rainbow band of colors called a spectrum is seen. Visible light is only a very small part of the total electromagnetic spectrum; the rest of it is made of other radiation, “invisible” light. If humans could see the other parts of the spectrum, they would find, in turn, beyond the violet end of the spectrum, at ever shorter wavelengths, ultraviolet, X-ray, and gamma-ray radiation. Beyond the red end, at ever longer wavelengths, are infrared, microwave, and radio radiation. Radio radiation includes frequencies used for broadcast, radar, and television signals. All these kinds of “light” are electromagnetic waves, or electromagnetic radiation, with specific frequencies and wavelengths. Electromagnetic radiation is such that all waves share the property that the product of their wavelength and frequency is the speed of light, approximately 300,000,000 meters per second (yielding the per-year results that define the light-year described previously). These waves of spatially and temporally oscillating crossed electric and magnetic fields (or streams of photons) comprise the electromagnetic spectrum.

The instrument commonly used to collect light is the telescope. The telescope can be defined as a “light bucket,” which gathers as much light as possible from a given region of the sky and directs it to a focal point. Modern telescopes study every part of the electromagnetic spectrum. The first telescopes were optical telescopes that collected light only in the visible region. These are still the most common types of modern telescopes, especially on Earth’s surface.

The opening in the telescope permitting light entry is called the aperture, and its size serves two functions. The larger the aperture, the more light can enter, enabling one to see fainter objects. Larger apertures also result in greater resolution. “Resolution” pertains to the ability to “see” something in sharp focus. The smaller an object, the more resolution one needs to see it clearly. However, if greater resolution were the sole requirement for viewing faint objects, making larger and larger telescopes to see them would simply be necessary. The biggest problem facing astronomers, however, is Earth’s atmosphere. Most electromagnetic frequencies are blocked because of their chemical and physical composition. Only specific frequencies can penetrate the atmosphere to the Earth’s surface. One obvious frequency band is visible light. Another is radio waves. Other frequencies are absorbed or reflected by the atoms and molecules in the atmosphere, water vapor, or natural or industrial dust and pollution in the air. If it were possible to see in any of these absorbed frequencies, such as ultraviolet light or X-rays, the air would be opaque, like a wall. However, when instruments can be positioned beyond the atmosphere, rising above all the components blocking invisible light, they can “see” in these invisible frequencies.

For these reasons, telescopes have historically been built in high places, such as mountaintops. Telescopes have also been sent aloft in airplanes, balloons, and rockets for short-term observations. However, it was not until the later half of the twentieth century, with the advent of the space age, that scientists were able to add telescopes and other detectors of electromagnetic radiation to orbiting spacecraft and interplanetary space probes to capture imaging data beyond Earth’s atmosphere for long periods. As a result, several areas of astronomy—infrared, X-ray, and gamma-ray—blossomed. For example, infrared measurements from space are important to astronomers because infrared radiation is related to heat, and star-forming regions of the universe release large amounts of infrared radiation that is not detectable from Earth’s surface. Similarly, high-energy astrophysics is studied by X-ray and gamma-ray observatories placed in Earth orbit.

Orbiting Solar Observatories. The idea of placing telescopes into orbit is an old one. The early space theorist Hermann Oberth wrote about this in the 1920s. In 1946, the astronomer Lyman Spitzer, Jr., composed a report titled Astronomical Advantages of an Extra-Terrestrial Observatory. Under the direction of its first chief astronomer, Nancy Roman, the National Aeronautics and Space Administration (NASA) prepared for the development of space-based observatories, an idea that was not initially welcomed by large portions of the astronomical community for fear that money would be diverted from the construction of ever larger ground-based observatories. NASA launched eight Orbiting Solar Observatories (OSOs) between 1962 and 1971, designed specifically to make observations of the Sun in visible, ultraviolet, and X-ray wavelengths. Seven of the eight OSOs were successful and provided data that added to our understanding of the eleven-year sunspot cycle and characterizing the solar corona. An OSO produced the first full-duration image of the solar corona. Some OSO instruments were test beds for later space-based X-ray telescopes, such as those incorporated into the Skylab space station’s Apollo Telescope Mount. OSO studies also investigated cosmic rays, neutron emissions, and extrasolar X-ray sources.

Orbiting Astronomical Observatories. The OSOs were followed by a sequence of Orbiting Astronomical Observatories (OAOs). The first OAO was launched on April 8, 1966. However, its pointing system failed when activated and caused the onboard batteries to explode. A successful OAO-2 was launched in December 1968, making observations for four and a half years in the infrared, ultraviolet, X-ray, and gamma-ray portions of the electromagnetic spectrum. This advancement was followed by several other very successful orbiting observatories, including OAO-3 in 1972, the International Ultraviolet Explorer (IUE) in 1978, and the European Space Agency’s (ESA’s) Infrared Astronomical Satellite (IRAS) in 1983.

Granat. The Russians operated an X-ray/gamma-ray observatory named Granat from December 1989 to November 1998. It probed the center of the Milky Way, recorded spectral and temporal variability in black holes, and detected 511-kilo electron volt (keV) X-rays produced by electron-positron annihilation.

Extreme Ultraviolet Explorer. NASA’s Extreme Ultraviolet Explorer (EUVE) was launched on June 7, 1992. This observatory was designed to detect and record radiation ranging in wavelength from seven to seventy-six nanometers, a portion of the electromagnetic spectrum that had not been investigated by many space-based instruments previously. The telescope was outfitted with an imaging microchannel plate detector and three spectrometers available at its focal plane. EUVE’s mission continued through January 31, 2001.

The Great Observatories. In the 1980s, NASA announced its Great Observatories Program, which aimed to place in orbit a series of telescopes to observe across the electromagnetic spectrum. Observatories under this program include the Hubble Space Telescope (HST), which was deployed in orbit from the space shuttleDiscovery (mission STS-31) on April 25, 1990; the Compton Gamma Ray Observatory (GRO), which was deployed in orbit from the space shuttle Atlantis (mission STS-37) on April 8, 1991; the Advanced X-Ray Astrophysics Facility (AXAF), which was deployed from the space shuttle Columbia (mission STS-93) on July 23, 1999; and the Space Infrared Telescope Facility (SIRTF), launched on August 25, 2003, by an expendable Delta II booster. After deployment, GRO was commissioned as the Compton Gamma Ray Observatory in honor of the American physicist Arthur Compton; AXAF was commissioned as the Chandra X-Ray Observatory (CXO) in honor of Indian astrophysicist Subrahmanyan Chandrasekhar; and SIRTF was commissioned as the Spitzer Space Telescope (SST) in honor of the American astronomer Lyman Spitzer, Jr.

Many of the observatories described thus far represented revolutionary steps in expanding humanity’s investigations of the universe. Many opened up entirely new windows on the universe since the types of radiation they detected could not penetrate Earth’s atmosphere to reach ground-based telescopes. To gain a clear picture of the physical processes in stars, galaxies, and exotic objects, such as quasars, active galactic nuclei, gamma-ray bursters, and black holes, it is necessary to examine those objects simultaneously across the different regions of the electromagnetic spectrum. That was the overriding aim of NASA’s Great Observatories Program: to construct several cutting-edge telescopes that, once in space, would provide means to observe across the spectrum, from gamma rays to infrared radiation. No radio telescope in space was included in this project since most of the radio spectrum is observable from ground-based facilities. Nevertheless, for reasons of having an electromagnetic quiet zone away from earthly sources, many astronomers have long dreamed of a radio telescope on the far side of the Moon.

Hubble Space Telescope. The crown jewel of all these Great Observatories is certainly the Hubble Space Telescope (HST); it has had the greatest appeal to the public. Hubble was launched aboard the space shuttle Discovery into the highest attainable space shuttle orbit, at 615 kilometers above Earth’s surface, with an orbital inclination of 28.5°. After requiring a heroic on-orbit repair by space shuttle mission STS-61 astronauts, Hubble began producing a steady stream of impressive images that fascinated the public and rewrote astronomy textbooks. For example, by early 1997, Hubble had taken more than 100,000 images of various objects in the solar system and universe, amazing scientists and the public alike with the incredible sights it had captured.

The size of its aperture controls a telescope’s ability to see, but it is also controlled by the medium through which the telescope must look (or what the radiation must travel through) to capture its images. Even though visible light can penetrate the air, the atmosphere has several detrimental effects on a telescope’s ability to see. These effects include twinkling, caused by movement in the air, weather, air humidity, dust, and air pollution, the natural glowing of the atmosphere (airglow), and light pollution caused by city lights. Because of these factors, observatories are built on remote mountaintops. The largest optical telescopes in the world are the two ten-meter (393.7-inch) Keck telescopes on Mauna Kea in Hawaii and the eleven-meter (433-inch) Hobby-Ebberly telescope at the McDonald Observatory on Mount Locke, Texas. Even these instruments, however, contend with atmospheric effects. By placing a telescope in orbit, these atmospheric factors are eliminated, allowing for clearer images. The Hubble Space Telescope has a primary mirror of only 2.4 meters (94.5 inches), yet it can see much better than either of these two earthbound telescopes.

Hubble was designed to provide the clearest view of the cosmos, providing astronomers with vision ten times clearer and fifty times more sensitive than the best ground-based telescopes. This provides astronomers with the largest boost in viewing capability since Galileo first used a telescope to view the sky in 1610. Hubble is 13.3 meters long, 4.3 meters in diameter, and twelve meters across with its solar array deployed. It weighs 11,200 kilograms (12.3 tons). The 2.4-meter primary mirror is a Cassegrain mirror—the light reflected by the primary mirror is reflected by a secondary mirror, after which the light travels through a hole in the primary mirror to the instruments positioned behind it.

The primary mirror was manufactured with greater precision than anything previously. Yet, shortly after the telescope was deployed in 1990, a problem became apparent. Although the mirror was extremely smooth, it was not shaped correctly, exhibiting what is known as a spherical aberration—the curvature of the mirror was slightly rounder than the parabolic shape required. As a result, the images were blurry. However, this problem was rectified by means of correcting lenses and mirrors known as the Corrective Optics Space Telescope Axial Replacement (COSTAR). COSTAR, along with several new gyroscopes and instruments, was installed by the STS-61 space shuttle servicing mission in December 1993. A second servicing mission (STS-82) occurred in February 1997. Additional servicing missions were conducted in late 1999 (STS-103), 2002 (STS-109), and 2009 (STS-125). These servicing missions allowed new instruments to be placed within the telescope and accomplished maintenance and repair work to keep Hubble operational.

Compton Gamma Ray Observatory. The Compton Gamma Ray Observatory (CGRO) was deployed in a high orbit (450 kilometers above the Earth’s surface), yet still underneath the Van Allen radiation belts to preclude interference with the observatory’s detectors. Compton was outfitted with four instruments: the Burst and Transient Source Experiment (BATSE), the Oriented Scintillation Spectrometer Experiment (OSSE), the Imaging Compton Telescope (COMPTEL), and the Energetic Gamma Ray Experiment Telescope (EGRET). Together, these scintillation detectors spanned six orders of magnitude of wavelengths in the gamma-ray portion of the electromagnetic spectrum, specifically from 20 keV to 30 gigaelectron volts (GeV) in energy. When Compton was on the verge of losing attitude control, NASA decided to deorbit the heavy observatory in a controlled fashion so that any debris, such as portions of its massive detectors, would not impact populated areas. Compton was deorbited on June 4, 2000, and safely rained debris over open regions of the Pacific Ocean.

Infrared Space Observatory. ESA followed up its highly successful IRAS cataloging mission with the Infrared Space Observatory (ISO). After launch on November 17, 1995, ISO produced more than twenty-six thousand infrared observations before its liquid helium cryogenic coolant supply ran out after a bit more than twenty-eight months. ISO was outfitted with a high-resolution infrared camera (ISOCAM), an infrared photo-polarimeter (ISOPHOT), a shortwave spectrometer (SWS), and a long-wave spectrometer (LWS) to observe wavelengths between 2.4 and 240 microns. Among ISO’s achievements were the finding of planet formation around old stars nearing their end of life, the detection of dust between galaxies, finding several chemical compounds in interstellar gas clouds, locating protoplanetary disks around stars in early stages of formation, and determining chemical abundances in planets within this solar system.

Far Ultraviolet Spectroscopic Explorer. NASA launched the Far Ultraviolet Spectroscopic Explorer (FUSE), an Explorer-class satellite (specifically, Explorer 77), on June 24, 1999, atop a Delta II booster. FUSE, an observatory in the agency’s Origins program, was placed in a low Earth orbit. The spacecraft was designed to last at least three years. Scientists at the Johns Hopkins University used the spacecraft’s Wolter-type grazing incidence telescope and its spectrograph to observe far-ultraviolet emissions ranging from 90.5 to 119.5 nanometers. FUSE was designed specifically to determine the distribution of deuterium (the isotope of hydrogen having a nucleus consisting of a proton and a neutron) in the aftermath of the Big Bang. FUSE covered a region of the electromagnetic spectrum not detected by any of the members of the Great Observatory program and investigated astrophysical and cosmological questions that added to and complemented discoveries made by Hubble, Chandra, and other space-based telescopes.

FUSE suffered failures in its pointing system and, for a time, was on the verge of being shut down, despite its science hardware functioning perfectly. Engineers developed an alternate means of pointing the telescope, and FUSE got a reprieve for more science. However, on July 12, 2007, the spacecraft lost its last reaction wheel. Efforts to restore FUSE to science operations failed, and two months later, the telescope was abandoned.

Chandra X-Ray Observatory. Because X-rays cannot be focused in the same manner as visible light, the design of the Chandra X-Ray Observatory required a quartet of nested pairs of mirrors in the shape of paraboloids and hyperboloids. At normal incidence (perpendicular to the mirror's surface), X-rays are much more likely to be absorbed than reflected off a mirror; hence, the design for any X-ray telescope must allow for low grazing angles. Chandra’s X-ray resolution was over a thousand times greater than any previous space-based X-ray detector. In a highly elliptical orbit ranging from 10,000 to 140,000 kilometers, Chandra can collect data during as much as 85 percent of its sixty-five-hour orbital period. Chandra is outfitted with four instruments: an imaging spectrometer, a high-resolution camera, and high-energy and low-energy high-resolution transmission grating spectrometers. Launched in July 1999, the observatory was predicted to have a five-year life, but in the early 2020s, the mission remained ongoing.

Spitzer Space Telescope. The Spitzer Space Telescope was placed in an Earth-trailing heliocentric orbit on August 25, 2003. The telescope was designed to be cooled to 5.5 kelvins and last a minimum of 2.5 years. Outfitted with an infrared camera (the Infrared Array Camera), an Infrared spectrometer (the Infrared Spectrograph), and far-infrared detection arrays (the Multiband Imaging Photometer for Spitzer), Spitzer was designed to cover infrared emissions from 3 to 180 microns. Spitzer’s supply of liquid helium cryogenic coolant lasted longer than a minimal mission, and, in late 2008, it was calculated that Spitzer could last between another six months and a year, but the telescope's utility lasted until January 2020, when it was decommissioned.

Swift Gamma-Ray Burst Mission. The Swift Gamma-Ray Burst Mission, usually referred to simply as Swift, was launched on November 20, 2004, to study and identify the origin of gamma-ray bursts. Aptly named, Swift could reorient itself quickly to place an incoming gamma-ray flux centrally within the observatory’s field of view and thereby alert the astronomical community rapidly so that other telescopes and detectors could record the afterglow of a gamma-ray burster (GRB), even short-period ones.

Gamma-Ray Large Area Space Telescope. The Gamma-Ray Large Area Space Telescope (GLAST) was launched on June 11, 2008. It was the next step in Gamma-ray astronomy after the loss of the Compton Observatory. GLAST was designed to investigate black holes, the high-speed jets of gas and energetic particles emitted from black holes, dark matter physics, solar flares, pulsars, cosmic-ray production, and gamma-ray bursts.

Kepler Observatory. The Kepler Observatory was designed to survey more than 100,000 stars in four years to detect transits of exoplanets across those stars’ photospheres. Such a crossing would result in a slight diminishment of the star’s light curve and additions to the stellar spectrum of absorption lines from the planet’s atmosphere if it were to have one. Kepler’s primary objective was to detect terrestrial planets within habitable zones around various stars. A star’s spectral class and luminosity determine the location of habitable zones. For a G2-class star, such as the Sun, the habitability zone—where life as humanity knows it is permissible due to the possibility of water existing in liquid form—extends from 0.72 astronomical units (AU) out to 1.5 AU, essentially from Venus to Mars. However, for a red dwarf star, the habitability zone would be closer. The chance of detecting an Earth-class planet transiting a star is a complicated combination of star spectral class, the possibility of planets forming about that particular star, and the planet’s mean distance from the star.

For example, for the star Tau Ceti (spectral class G8), if a terrestrial planet similar to Earth were located one AU away from the star, the possibility of a transit being observed is approximately 1 in 210. By the sheer number of survey stars Kepler would examine, the chances of finding evidence of terrestrial planets would be high. It was estimated that if Kepler were able to examine its planned 100,000 stars, then 480 or more terrestrial planets should be observed by this transit method.

James E. Webb Space Telescope. Originally called the Next Generation Space Telescope (NGST), the James E. Webb Space Telescope (JWST, named for NASA administrator James E. Webb) has been described as the replacement for the Hubble Space Telescope. This is not strictly accurate, as JWST is designed as an exclusively infrared observatory to be placed in a position where the Sun’s gravitational attraction on the telescope is balanced precisely by the Earth's. This location is called the L2 Lagrangian point. It is located 1.5 million kilometers from Earth.

JWST comprises eighteen hexagonal mirror segments that deploy from a folded configuration at launch. The JWST has a light-collecting area six times greater than Hubble’s. Instruments incorporated into JWST’s design include a near-infrared camera, a near-infrared spectrograph, a mid-infrared instrument, and fine guidance sensors. These provide coverage of the infrared between wavelengths of 0.6 to 28 microns. The launch of JWST was planned for sometime in the 2010s but was launched in December 2021 and began operation in July 2022. Hubble still operated during the launch, allowing coordination between the two instruments. Among JWST’s primary objectives are an investigation of the earliest epoch of star and galaxy formation, observation of evolutionary processes in galaxies, examination of star-forming regions and developing planetary systems, and searches for planetary systems that may have materials and conditions necessary for the evolution of life.

Knowledge Gained

Findings from OAOs. Among the achievements of the Orbiting Astronomical Observatories were the finding that halos of hydrogen surround comets, the discovery that novae may increase in ultraviolet brightness while diminishing in visible luminosity, the discovery of long-period pulsars, and the collection of several hundred high-resolution spectra of stars in X-ray and ultraviolet wavelengths.

Findings from IRAS. The Infrared Astronomical Satellite was the first fully dedicated infrared observatory in space. Its greatest achievement was the location of an enormous number of infrared sources to be studied in detail by later generations of infrared detection systems placed in space, such as ISO and Spitzer.

Findings from IUE. Among the scientific discoveries of the International Ultraviolet Explorer were auroras in Jupiter’s atmosphere, sulfur in comets, stellar spots, the progenitor star for Supernova 1987A, the extent of active regions in Seyfert galaxy nuclei, and galactic halos. IUE produced the first light curves collected undisturbed for more than twenty-four hours, and it detected the first emissions of wavelengths less than fifty nanometers. IUE made more than 104,000 observations before its decommissioning.

Findings from Hubble. One of the primary goals of Hubble was to measure the brightness of certain kinds of stars, called Cepheid variables, in several distant galaxies, making it possible to determine how far away these galaxies are. The importance of this is related to a discovery by the American astronomer Edwin Hubble, the man after whom the Space Telescope is named. Hubble discovered that the universe is expanding and that the distance to an object is proportional to just how far away the object is. This proportionality is known as Hubble’s constant. If one finds a reliable value for Hubble’s constant, it is possible to determine how far away a galaxy is. It is also possible to determine how old the universe is. The problem is finding a reliable value. Astronomers could determine a definitive value for this constant using measurements made with the Hubble after installing COSTAR. In 2008, the accepted value for the age of the universe was determined to be 13.7 billion years, and for the Hubble constant, seventy-seven kilometers per second per megaparsec.

Hubble was not necessarily expected to make significant discoveries because it observes in the same visible light in which observations from the Earth’s surface are made. Instead, it was expected to see the objects previously observed with unequaled clarity and resolution. Yet, it has succeeded in making previously unsuspected discoveries, mainly because it can see much fainter objects than could ever be seen using ground-based telescopes. One such discovery occurred during a ten-day exposure of a section of the sky that was previously thought to be empty and was found by Hubble to be filled with distant galaxies. Two such Hubble deep field surveys were eventually made. The other Great Observatories in orbit made radical discoveries from the beginning because they observe in light frequencies, which humans were previously unable to observe due to atmospheric influences.

While it is the mirror that gathers the light, it is the instruments on the Hubble Space Telescope that provide its tremendously exciting scientific advances. One of these instruments is the Wide Field/Planetary Camera-2 (WF/PC-2). WF/PC-2 serves as the primary imaging camera. This instrument has provided us with startling images of towers of interstellar matter and other phenomena that have become so familiar.

Another instrument is the Space Telescope Imaging Spectrograph (STIS), which gives the Hubble unique and powerful spectroscopic capabilities. A spectrograph separates light gathered by the telescope into its spectral components so that astronomical objects' composition, temperature, motion, and other chemical and physical properties can be analyzed. STIS’s two-dimensional detectors allow the instrument to gather thirty times more spectral data and five hundred times more spatial data than existing spectrographs on the Hubble that observe one location at a time. The STIS is a particularly powerful tool for studying supermassive black holes. STIS searches for massive black holes by studying the star and gas dynamics around galactic centers. It also measures the distribution of matter in the universe by studying quasar absorption lines, using its high sensitivity and spatial resolution to examine star formation in distant galaxies and performing spectroscopic mapping of solar-system objects.

Another instrument is the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), which promises to gather valuable new information on the dusty centers of galaxies and the formation of stars and planets. Consisting of three cameras, NICMOS can perform infrared imaging and spectroscopic observations of astronomical targets. Because these detectors perform more efficiently than previous infrared detectors, NICMOS has given astronomers their first clear view of the universe at near-infrared wavelengths between 0.8 and 2.5 micrometers. These views in the infrared are essential because the universe's expansion has shifted the light from very distant objects toward longer red and infrared wavelengths. Hence, NICMOS’s near-infrared capabilities provide views of objects too distant for research by Hubble’s optical and ultraviolet instruments.

Astronauts installed the Advanced Camera for Surveys (ACS) inside Hubble in 2002, replacing the Faint Object Camera. ACS was designed to observe from far-ultraviolet to visible wavelengths and collect images that shed light on some of the earliest galaxies. ACS possesses a field of view twice that of the Wide Field Planetary Camera 2, allowing it to accomplish extensive surveys of the early universe. ACS lost performance in its charge-coupled devices in early 2007 and was slated for repair during the STS-125 final Hubble servicing mission.

Included as a part of a final planned shuttle service mission to the Hubble (the shuttle flight designated STS-125) was the installation of the Cosmic Origins Spectrograph (COS), designed to perform spectroscopy between 115 and 320 nanometers in ultraviolet wavelength. According to the Space Telescope Science Institute, this instrument’s principal research would center on investigating the universe's large-scale structure, the early formation of galaxies and their subsequent evolution, stellar and planetary systems, and the interstellar medium. Several spacewalks would be required on STS-125 to achieve a life extension for Hubble. The mission intended to install six new gyrodynes, six new solar batteries, the COS and the Wide Field Camera 3, a cooling system, and a replacement Fine Guidance Sensor. The astronauts would also repair the thermal insulation and STIS, leaving Hubble in an almost new configuration that would last at least five years.

Among the major discoveries from ACS research was the collection of gravitational lensing data sufficient to determine the mass of seventy distant galaxies. Gravitational lensing is the process whereby massive objects bend and focus light passing near them so that a telescope will see a ring of identical images, referred to as an Einstein ring. The amount of lensing present in an image is determined by the mass of the lensing objects, such as these seventy galaxies, which divert the light and form the Einstein ring.

Findings from Compton. Among the Compton Gamma Ray Observatory’s impressive achievements over its nine years in orbit was the amassing of an all-sky survey of emissions over 100 megaelectron volts (MeV). Compton identified 271 individual sources, compiling an all-sky map of gamma-ray emissions from decays of the radioactive isotope Al²⁶ of aluminum. Compton also detected gamma-ray burster 990123, the brightest object recorded to that time, along with gamma-ray emissions from the tops of terrestrial thunderstorms.

Findings from EUVE. The Extreme Ultraviolet Explorer’s mission consisted of two phases. During the first, which lasted only half a year, the telescope was used in imaging mode to generate a full-sky atlas of sources of extreme ultraviolet radiation. The telescope’s second phase of investigation involved pointed studies of individual sources using the spectroscopic capabilities of EUVE. Among EUVE’s discoveries and observations concerning the solar system were extreme ultraviolet/X-ray emissions in Comet P/Encke, changes in interplanetary helium wind, day glow at Venus, mechanisms whereby solar flares promote coronal heating, and images of the full Moon in extreme ultraviolet wavelengths.

Findings from Chandra. The Chandra X-Ray Observatory’s contributions to X-ray astronomy include the first detection of X-ray emissions from the Milky Way’s supermassive black hole located at the galactic center, the detection of a mid-mass-range black hole in galaxy M82, providing data that may be evidence of a star composed of matter collapsed to quarks (a quark star), demonstrating that virtually all main sequence stars also emit somewhat in the X-ray region of the electromagnetic spectrum, helping determine the Hubble constant (and hence the age of the universe), and collecting evidence of dark matter involved in supercluster collisions.

In 2008, Chandra and the Very Large Array of Earth-based radio telescopes located near Socorro, New Mexico, collaborated to observe the supernova in the Milky Way. This supernova had occurred only 140 years earlier. Before this discovery, Cassiopeia A was the most recently noted supernova in the galaxy. The data on this young supernova allowed astronomers to investigate the stellar explosion process and the creation of a central neutron star or black hole. This discovery indicated how valuable the coordination of observations in X-ray and radio emissions and the coordination of space-based and Earth-based telescope facilities can be.

Another unique way Chandra was used involved developing a new method for determining the mass of supermassive black holes, such as those found at the centers of many galaxies. Looking for X-ray emissions from hot gas in the central region of the elliptical galaxy NGC 4649, Chandra was able to determine the peak temperature of that hot gas. The temperature of the hot gas is determined by the gravitational compression of the gas from the black hole. That, in turn, is dependent on the black hole’s mass. This method was checked against earlier methods of “weighing” black holes. The results from Chandra for NGC 4649 agreed nicely.

Findings from Spitzer. The Spitzer Space Telescope greatly expanded on previous infrared research produced by IRAS and ISO and collaborated with NASA’s other Great Observatories for coordinated studies of essential objects and deep-sky field surveys. Among Spitzer’s significant accomplishments are the first direct detection of light from the hot-Jupiter extrasolar planets HD 209458b and TrES-1; the detection of the youngest star ever found; the determination that the Milky Way galaxy’s core has a bar structure; capturing the glow from stars formed only 100 million years or so after the Big Bang; and the first determination of an extrasolar planet’s atmospheric temperature (that of HD 189733b).

Spitzer was used to study Messier 101, the Pinwheel galaxy, detailing the infrared signature of the galaxy’s spiral arms and central region. Data revealed polycyclic aromatic hydrocarbons throughout the galaxy but disappeared in the outer region. This suggested that hydrocarbons, and hence organic materials, had a threshold as one looked far away from the central region of this galaxy. Spitzer observed a pair of young stars, DR Tau and AS 205A, located 457 and 391 light-years from Earth, respectively, concentrating on their protoplanetary disks using the telescope’s spectrographs. Both stars’ disks displayed emission lines of water vapor within their innermost portions. Although the amount of water found was still considerably less than in Earth’s oceans, this discovery indicated that water could be available and abundant in the inner regions of forming solar systems.

Findings from Swift. The Swift Gamma-Ray Burst Mission located a gamma-ray burster (GRB) with a burst duration of only 0.05 seconds. Swift also found the brightest object ever seen, GRB 080319B, located 7.5 billion light-years from Earth with a luminosity 2.5 million times that of the most brilliant supernova seen previously.

Findings from European and Russian Observatories. Although this essay has primarily concentrated on NASA observatories in space, the Europeans and Russians have placed many important and productive space-based telescopes in orbit, including the previously discussed IRAS and ISO. Also important are XMM-Newton, an X-ray observatory; the Salyut 6 space station’s KRT-10 radio telescope; and COROT. These are only a few of the international astrophysical facilities in space.

One of the many discoveries made by the European Space Agency’s XMM-Newton X-ray observatory was the finding of an exploding star in the Milky Way that was previously missed. This object was once so bright the naked eye could have seen it during its initial explosion and subsequent nova phase. XMM-Newton accidentally found this nova on October 9, 2007, as it slewed from one planned target of opportunity to the next planned observation. The discovery reminded astronomers that serendipity in space-based astrophysics is just as important as it was during earthbound astronomical observations.

COROT was launched in December 2006 to survey stars and search for extrasolar planets using a transit method. In this type of investigation, light from a star diminishes by a small but measurable amount when a planet passes in front of that star. The absorption of light by the planet’s atmosphere reveals something about the nature of that atmosphere. By mid-2008, COROT had surveyed more than fifty thousand stars. A discovery in 2008 was particularly intriguing. COROT detected the presence of an extrasolar planet the size of Jupiter orbiting a star similar in mass to the Sun. The Jupiter-class exoplanet is close to this star and completes an orbit in 9.2 days. The star rotates at the same rate, thereby synchronizing the planet and star. COROT was decommissioned in 2013.

Context

Earthbound observatories have not been made obsolete by space-based telescopes. The former continues to be needed to perform some of the most important investigations in contemporary astrophysics. Space-based telescopes, however, have opened the eyes of scientists to a surprising, wondrous, energetic, and violent universe that could not have been envisioned based solely on earthbound observations. Our view of the solar system has been dramatically changed by widening the portion of the electromagnetic spectrum to which astronomers gained access once observatories were operational in space. One is reminded of the poetic verse of T. S. Eliot: “We shall never cease from exploration, and the end of all our exploring will be to arrive where we started and know the place for the first time.”

The Hubble Space Telescope is among the best examples of space-based telescopes that have altered the public’s view of the universe and the utility of studying it. Hubble’s dramatic images of towering columns of interstellar matter and other phenomena have replaced the common notion of a sterile and relatively empty universe with a new understanding of the endless variety and dynamism of the cosmos. Besides adding to our rapidly advancing scientific knowledge of the universe, Hubble has directly contributed to the health, safety, and quality of people’s lives through various technological spin-offs. For example, a nonsurgical breast biopsy technique using a device initially developed for Hubble’s Imaging Spectrograph saves women pain, scarring, radiation exposure, time, and money. This technique, called stereotactic automated large-core needle biopsy, enables a doctor to locate a suspicious lump precisely and use a needle instead of incisional surgery to remove tissue for pathology. This precise procedure has been rendered possible because of a critical improvement in digital imaging technology known as the charge-coupled device, or CCD. Additionally, perhaps Hubble’s most significant contribution has been in education, where it has been highly successful in generating enthusiasm for astronomy among students and the public.

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