Today, astronomers are able to study objects in our Universe that are over thirteen billion light-years from Earth. In fact, the farthest object studied is a galaxy known as GN-z11, which exists at a distance of 13.39 billion light-years from our Solar System.
But since we live in the relativistic universe, where time and space are similar expressions of the same reality, looking deep into space means also looking deep into the past. Ergo, looking at an object that is over 13 billion light-years away means seeing it as it appeared over 13 billion years ago.
This allows astronomers to see back to some of the earliest times in the Universe, which is estimated to be 13.8 billion years old. And in the future, next-generation instruments will allow them to see even farther, to when the first stars and galaxies formed - a time that is commonly referred to as "Cosmic Dawn."
Much of the credit for this progress goes to space telescopes, which have been studying the deep Universe from orbit for decades. The most well-known of these is Hubble, which has set the precedent for space-based observatories.
Since it was launched in 1990, the vital data Hubble has collected has led to many scientific breakthroughs. Today, it is still in service and will mark its 30th anniversary on May 20th, 2020. However, it's important to note that Hubble was by no means the first space telescope.
Decades prior to it making its historic launch, NASA, Roscosmos, and other space agencies were sending observatories to space to conduct vital research. And in the near future, a number of cutting-edge telescopes will be sent to space to build on the foundation established Hubble and others.
The case for space telescopes
The idea of placing an observatory in space can be traced back to the 19th century and the German astronomers Wilhelm Beer and Johann Heinrich Mädler. In 1837, they discussed the advantages of building an observatory on the Moon, where Earth's atmosphere would not be a source of interference.
However, it was not until the 20th century that a detailed proposal was first made. This happed in 1946 when American theoretical physicist Lyman Spitzer proposed sending a large telescope to space. Here too, Spitzer emphasized how a space telescope would not be hindered by Earth's atmosphere.
Essentially, ground-based observatories are limited by the filtering and distortion our atmosphere has on electromagnetic radiation. This is what causes stars to "twinkle" and for celestial objects like the Moon and the Solar Planets to glow and appear larger than they are.
Another major impediment is "light pollution", where light from urban sources can make it harder to detect light coming from space. Ordinarily, ground-based telescopes overcome this by being built in high-altitude, remote regions where light pollution is minimal and the atmosphere is thinner.
Adaptative optics is another method that is commonly used, where deforming mirrors correct for atmospheric distortion. Space telescopes get around all of this by being positioned outside of Earth's atmosphere where neither light pollution nor distortions are an issue.
Space-based observatories are even more important when it comes to frequency ranges beyond the visible wavelengths. Infrared and ultraviolet radiation are largely blocked by Earth's atmosphere, whereas X-ray and Gamma-ray astronomy are virtually impossible on Earth.
Throughout the 1960s and 1970s, Spitzer lobbied US Congress for such a system to be built. While his vision would not come to full fruition until the 1990s (with the Hubble Space Telescope), many space observatories would be sent to space in the meantime.
During the late 1950s, the race to conquer space between the Soviet Union and the United States began. These efforts began in earnest with the deployment of the first satellites and then became largely focused on sending the first astronauts into space.
However, efforts were also made to send the observatories into space for the first time. Here, "space telescopes" would be able to conduct astronomical observations that were free of atmospheric interference, which was especially important where high-energy physics was concerned.
As always, these efforts were tied to military advancements during the Cold War. Whereas the development of Intercontinental Ballistic Missiles (ICBMs) led to the creation of space launch vehicles, the development of spy satellites led to advances in space telescopes.
In all cases, the Soviets took an early lead. After sending the first artificial object (Sputnik 1) and the first man (Yuri Gagarin and the Vostok 1 mission) into orbit in 1957 and 1961, they also sent the first space telescopes to space between 1965 and 1968.
These were launched as part of the Soviet Proton program, which sent four gamma-ray telescopes to space (Proton-1 through -4). While each satellite was short-lived compared to modern space telescopes, they did conduct vital research of the high-energy spectrum and cosmic rays.
NASA followed suit with the launch of the four Orbiting Astronomical Observatory (OAO) satellites between 1968 and 1972. These provided the first high-quality observations of celestial objects in ultraviolet light.
In 1972, the Apollo 16 astronauts also left behind the Far Ultraviolet Camera/Spectrograph (UVC) experiment on the Moon. This telescope and camera took several images and obtained spectra of astronomical objects in the far-UV spectrum.
The post-Apollo era
The 1970s and 1980s proved to a lucrative time for space-based observatories. With the Apollo Era finished, the focus on human spaceflight began to shift to other avenues - such as space research. More nations began to join in as well, including India, China, and various European space agencies.
Between 1970 and 1975, NASA also launched three telescopes as part of their Small Astronomy Satellite (SAS) program, which conducted X-ray, gamma-ray, UV, and other high-energy observations. The Soviets also sent three Orion space telescopes to space to conduct ultraviolet observations of stars.
The ESA and European space agencies also launched their first space telescopes by the 1970s. The first was the joint British-NASA telescope named Ariel 5, which launched in 1974 to observe the sky in the X-ray band. The same year, the Astronomical Netherlands Satellite (ANS) was launched to conduct UV and X-ray astronomy.
In 1975, India sent its first satellite to space - Aryabata - to study the Universe in the X-ray spectrum. In that same year, the ESA sent the COS-B mission to space to study gamma-ray sources. Japan also sent its first observatory to space in 1979, known as the Hakucho X-ray satellite.
Between 1977 and 1979, NASA also deployed a series of X-ray, gamma-ray, and cosmic-ray telescopes as part of the High Energy Astronomy Observatory Program (HEAO). In 1978, NASA, the UK Science Research Council (SERC) and the ESA collaborated to launch the International Ultraviolet Explorer (IUE).
Before the 1980s were out, the ESA, Japan, and the Soviets would contribute several more missions, like the European X-ray Observatory Satellite (EXOSAT), the Hinotori and Tenma X-ray satellites, and the Astron ultraviolet telescope.
Rounding out the decade, the ESA and NASA sent their Hipparcos and Cosmic Background Explorer (COBE) in 1989. Hipparcos was the first space experiment dedicated to measuring the proper motions, velocities, and positions of stars, a process known as astrometry.
Meanwhile, COBE provided the first accurate measurements of the Cosmic Microwave Background (CMB) - the diffuse background radiation permeating the observable Universe. These measurements provided some of the most compelling evidence for the Big Bang theory.
In 1989, a collaboration between the Soviets, France, Denmark, and Bulgaria led to the deployment of the International Astrophysical Observatory (aka. GRANAT). The mission spent the next nine years observing the Universe from the X-ray to the gamma-ray parts of the spectrum.
Hubble (HST) goes to space
After many decades, Spitzer finally saw his dream of a dedicated space observatory come true with the Hubble Space Telescope (HST). This observatory was developed by NASA and the ESA and launched on April 24th, 1990, aboard the Space Shuttle Discovery (STS-31), commencing operations by May 20th.
This telescope takes its name from the famed American astronomer Edwin Hubble (1889 - 1953), who is considered by many to be one of the most important astronomers in history.
In addition to discovering that there are galaxies beyond the Milky Way, he also offered definitive proof that the Universe is in a state of expansion. In his honor, this scientific fact is known as the Hubble-Lemaître Law, and the rate at which it is expanding is known as the Hubble Constant.
Hubble is equipped with a primary mirror that measures 2.4-meters (7.8-feet) in diameter and a secondary mirror of 30.5 cm (12 inches). Both mirrors are made from a special type of glass that is coated with aluminum and a compound that reflects ultraviolet light.
With its suite of five scientific instruments, Hubble is able to observe the Universe in the ultraviolet, visible, and near-infrared wavelengths. These instruments include the following:
Wide Field Planetary Camera: a high-resolution imaging device primarily intended for optical observations. Its most recent iteration - the Wide Field Camera 3 (WFC3) - is capable of making observations in the ultraviolet, visible and infrared wavelengths. This camera has captured images of everything from bodies in the Solar System and nearby star systems to galaxies in the very distant universe.
Cosmic Origins Spectrograph (COS): an instrument that breaks ultraviolet radiation into components that can be studied in detail. It has been used to study the evolution of galaxies, active galactic nuclei (aka. quasars), the formation of planets, and the distribution of elements associated with life.
Advanced Camera for Surveys (ACS): a visible-light camera that combines a wide field of view with sharp image quality and high sensitivity. It has been responsible for many of Hubble’s most impressive images of deep space, has located massive extrasolar planets, helped map the distribution of dark matter, and detected the most distant objects in the Universe.
Space Telescope Imaging Spectrograph (STIS): a camera combined with a spectrograph that is sensitive to a wide range of wavelengths (from optical and UV to the near-infrared). The STIS is used to study black holes, monster stars, the intergalactic medium, and the atmospheres of worlds around other stars.
Near-Infrared Camera and Multi-Object Spectrometer (NICMOS): a spectrometer that is sensitive to infrared light, which revealed details about distant galaxies, stars, and planetary systems that are otherwise obscured by visible light by interstellar dust. This instrument ceased operations in 2008.
The "Great Observatories" and more!
Between 1990 and 2003, NASA sent three more telescopes to space that (together with Hubble) became known as the Great Observatories. These included the Compton Gamma Ray Observatory (1991), the Chandra X-ray Observatory (1999), the Spitzer Infrared Space Telescope (2003).
In 1999, the ESA sent the X-ray multi-Mirror Newton (XMM-Newton) observatory to space, named in honor of Sir Isaac Newton. In 2001, they sent the Wilkinson Microwave Anisotropy Probe (WMAP) to space, which succeeded COBE by making more accurate measurements of the CMB.
In 2004, NASA launched the Swift Gamma Ray Burst Explorer (aka. the Neil Gehrels Swift Observatory). This was followed in 2006 by the ESA's Convection, Rotation and planetary Transits (COROT) mission to study exoplanets.
2009 was a bumper year for space telescopes. In this one year, the Herschel Space Observatory, the Wide-field Infrared Telescope (WISE), the Planck observatory, and the Kepler Space Telescope. Whereas Herschel and WISE were dedicated to infrared astronomy, Planck picked up where left off by studying the CMB.
The purpose of Kepler was to advance the study of extrasolar planets (i.e. planets that orbit stars beyond the Solar System). Through a method known as transit photometry, Kepler spotted planets as they passed in front of their stars (aka. transited), resulting in an observable dip in brightness.
The extent of these dips and the period with which they occur allows astronomers to determine a planet's size and orbital period. Thanks to Kepler, the number of known exoplanets has grown exponentially.
Today, there have been over 4000 confirmed discoveries (and 4900 awaiting confirmation), of which Kepler is responsible for discovering almost 2800 (with another 2420 awaiting confirmation).
In 2013, the ESA launched the Gaia mission, an astrometry observatory and the successor to the Hipparcos mission. This mission has been gathering data on over 1 billion objects (stars, planets, comets, asteroids, and galaxies) to create the largest and most precise 3D space catalog ever made.
In 2015, the ESA also launched the Laser Interferometer Space Antenna Pathfinder (LISA Pathfinder), the first-ever observatory dedicated to measuring gravitational waves from space. And in 2018, NASA sent the Transiting Exoplanet Survey Satellite (TESS) - Kepler's successor - to space to search for more exoplanets.
Future space telescopes
In the coming decades, the space agencies of the world plan to launch even more sophisticated space telescopes with even higher-resolution. These instruments will allow astronomers to gaze back to the earliest periods of the Universe, study extrasolar planets in detail, and observe the role Dark Matter and Dark Energy played in the evolution of our Universe.
James Webb Space Telescope (JWST), an infrared telescope built with generous support provided by the ESA and the Canadian Space Agency (CSA). This observatory, the spiritual successor to Hubble and Spitzer, will be the largest and most complex space telescope to date.
Unlike its precessors, the JWST will observe the Universe in the visible light to mid-infrared wavelengths, giving it the ability to observe objects that are too old and too distant for its predecessors to observe.
This will allow astronomers to see far enough through space (and back in time) to observe the first light after the Big Bang and the formation of the first stars, galaxies, and solar systems.
There's also the ESA's Euclid mission, which is scheduled for launch in 2022. This space telescope will be optimized for cosmology and exploring the "dark Universe." To this end, it will map the distribution of up to two billion galaxies and associated Dark Matter across 10 billion light-years.
This data will be used to create a 3D map of the local Universe that will provide astronomers with vital information about the nature of Dark Matter and Dark Energy. It will also provide accurate measurements of both the accelerated expansion of the Universe and strength of gravity on cosmological scales.
By 2025, NASA will be launching the Wide-Field Infrared Space Telescope (WFIRST), a next-generation infrared telescope dedicated to exoplanet detection and Dark Energy research. It's advanced optics and suite of instruments will reportedly give it several hundred times the efficiency of Hubble (in the near-IR wavelength).
Once deployed, WFIRST will observe the earliest periods of cosmic history, study Dark Energy, and measure the rate at which cosmic expansion is accelerating. It will also build on the foundation built by Kepler by conducting direct-imaging studies and characterization of exoplanets.
The launch of the ESA's PLAnetary Transits and Oscillations of stars (PLATO) will follow in 2026. Using a series of small, optically fast, wide-field telescopes, PLATO will search for exoplanets and characterize their atmospheres to determine if they could be habitable.
Looking even farther ahead, a number of interesting things are predicted for space-based astronomy. Already, there are proposals in place for next-next-generation telescopes that will offer even greater observational power and capabilities.
During the recent 2020 Decadal Survey for Astrophysics hosted by NASA's Science Mission Directorate (SMD), four flagship mission concepts were considered to build on the legacy established by Hubble, Kepler, Spitzer, and Chandra.
NASA and other space agencies are also working towards the realization of in-space assembly (ISA) with space telescopes, where individual components will be sent to orbit and assembled there. This process will remove the need for especially heavy launch vehicles capable of sending massive observatories to space - a process that is very expensive and risky.
There's also the concept of observatories made up of swarms of smaller telescope mirrors ("swarm telescopes"). Much like large-scale arrays here on Earth - like the Very Long Baseline Interferometer (VLBI) and the Event Horizon Telescope (EHT) - this concept comes down to combing the imaging power of multiple observatories.
Then there's the idea of sending up space telescopes that are capable of assembling themselves. This idea, as proposed by Prof. Dmitri Savransky of Cornell University, would involve a ~30 meter (100 ft) telescope made up of modules that would assemble themselves autonomously.
This latter concept was also proposed during the 2020 Decadal Survey and was selected for Phase I development as part of the 2018 NASA Innovative Advanced Concepts (NIAC) program.
Space-based astronomy is a relatively new phenomenon whose history is inextricably linked to the history of space exploration. The first space telescopes followed the development of the first rockets and satellites.
As NASA and Roscosmos achieved expertise in space, space-based observatories increased in number and diversity. And as more and more nations joined the Space Age, more space agencies began conducting astronomical observations from space.
Today, the field has benefitted from the rise of interferometry, miniaturization, autonomous robotic systems, analytic software, predictive algorithms, high-speed data transfer, and improved optics.
At this rate, it is only a matter of time before astronomers see the Universe in the earliest stages of formation, unlock the mysteries of Dark Matter and Dark Energy, locate habitable worlds, and discover life beyond Earth and the Solar System. And it wouldn't be surprising if it all happens simultaneously!