Los space telescopes They have become one of the best tools we have for spying on the universe from outside Earth's atmosphere. By placing them in orbit or at strategic points like Lagrange points, we avoid problems such as... air turbulence, light pollution or the absorption of certain wavelengths, and that allows us to see the cosmos with a clarity that, from the ground, is simply impossible.
Over the past few decades, a diverse fleet of space observatories has been deployed covering the entire electromagnetic spectrumFrom the most energetic gamma rays to radio waves, including X-rays, ultraviolet, visible light, infrared, and microwaves. Missions have also been launched to detect particles such as cosmic rays, and even prototypes of gravitational wave telescopes have been developed. We will explore, calmly and in considerable detail, the main types of space telescopes, their most representative missions, and the major projects that are on the horizon.
What is a space telescope and why is it so important?
A space telescope is, in essence, a astronomical Observatory Mounted on a spacecraft or satellite that operates above the atmosphere. Unlike ground-based telescopes, these platforms can observe regions of the spectrum (such as X-rays, gamma rays, or extreme ultraviolet) that the atmosphere almost completely blocks, and they also avoid the distortions that blur optical images seen from ground-based observatories.
Depending on the type of radiation they study, space telescopes are classified into gamma rays, X-rays, ultraviolet, optical rays, infrared rays, microwaves, and radio wavesIn addition, there are missions dedicated to high-energy particles (cosmic rays) and nascent projects for detecting gravitational waves from space. Each of these bands reveals a different universe: from black holes and gamma-ray bursts to the faint glow of the cosmic microwave background or the distribution of dark matter.
Gamma-ray space telescopes: the most extreme universe
Gamma-ray telescopes measure photons of extremely high energy originating from violent astrophysical phenomena. This radiation is absorbed by the Earth's atmosphere, so we can only study it from stratospheric balloons or, even better, from orbiting satellites or probes in deep space.
Typical sources of gamma rays are supernovae, neutron stars, pulsars, and black holes in binary systems or active galactic nuclei. In addition, there are the enigmatic gamma-ray bursts, extremely brief but tremendously energetic bursts whose nature has been studied for decades.
Numerous gamma-ray observatories have been launched over time. Among the pioneers were the Soviet probes. Proton-1, Proton-2 and Proton-4all in low Earth orbit in the 60s. They were followed by missions such as the SAS 2 NASA's Small Astronomy Satellite 2 Cos-B from the ESA, or the HEAO 3 American, who combined instruments for high energies.
During the 1980s and 1990s, key projects such as Garnet (Franco-Soviet collaboration), the satellite Gamma and, above all, the Compton Gamma Ray Observatory (CGRO) From NASA, part of the Great Observatories series. CGRO observed the sky between 1991 and 2000 in low Earth orbit, mapping hundreds of gamma-ray sources and helping to classify gamma-ray bursts into different types.
Later came specialized missions such as the LEGRI (Low Energy Gamma Ray Imager) Spanish, the HETE 2 focused on transient outbursts, the European observatory INTEGRAL or the satellite Swiftcapable of rapidly detecting gamma-ray bursts and pointing its instruments to track the evolution of the phenomenon. In recent years, the following have stood out: AGILE, the Fermi Gamma Ray Space Telescope and the experiment GAP, mounted on a JAXA mission in heliocentric orbit, which studies the polarization of gamma bursts.
X-ray telescopes: X-ray of the cosmos
X-ray telescopes focus on photons of high energy but less extreme than gamma raysThe atmosphere also blocks this radiation, so these observations are only possible from high-altitude balloons or in orbit. X-rays are emitted from galaxy clusters and active galactic nuclei to supernova remnants, X-ray binaries with white dwarfs, neutron stars, and black holes, as well as some sources in our own Solar System, such as the Moon, although in this case much of the brightness comes from reflected solar X-rays.
Among the first X observatories, the following stand out: Uhuru (1970), the first satellite dedicated exclusively to this band. It was followed by missions such as the ANS (Astronomical Netherlands Satellite), Ariel Vthe Indian aryabhata, the SAS-C from NASA or high-energy observatories HEAO-1 and HEAO-2 (the latter known as Einstein Observatory), which drastically improved the catalogs of X-ray sources.
Japan played a key role with satellites such as Hakucho (CORSA-b), tenma, Ginga, ASCA or later, Suzuki y HitomiThe European was also important. EXOSAT and Russian Astron, which combined ultraviolet and X-ray observations in a highly elliptical orbit.
In the 90s and 2000s, missions arrived that are now true benchmarks. ROSAT He conducted an in-depth census of soft X-ray sources; BeppoSAX It played a fundamental role in locating gamma-ray bursts thanks to its X-ray tracking capabilities; and the Rossi X-ray Timing Explorer (RXTE) It allowed for the study, in unprecedented detail, of the variability of systems with black holes and neutron stars.
Those still active include Chandra X-ray Observatory (NASA) and XMM Newtons (ESA), both in highly elliptical orbits that allow for long continuous observations. More recent are Nustar, specializing in hard X-rays, the Indian observatory Astrosatthe Chinese telescope HXMT, the Russian-German Spektr-RG and missions focused on polarimetry such as IXPE, and XRISM o XPoSat and Einstein Probe, which expand capabilities in spectroscopy and X-ray variability.
Ultraviolet telescopes: looking beyond violet
Ultraviolet telescopes specialize in wavelengths between approximately 10 and 320 nanometersThis radiation is largely absorbed by the atmosphere, so we can only study it from the upper atmosphere, the lunar surface, or space. The Sun, numerous hot stars, and many galaxies emit large amounts of UV light, which is key to analyzing star formation processes and chemical composition.
Among the first UV missions are OAO-2 (Stargazer) y OAO-3 Copernicus NASA's telescopes Orion 1 and Orion 2 mounted on Soviet space stations. A unique case was the Far Ultraviolet Camera/Spectrograph installed by the Apollo 16 astronauts on the surface of the Moon, which allowed UV observations to be made from an environment without an atmosphere.
The satelite ANS It also had UV instruments, but the big leap was made by International Ultraviolet Explorer (IUE)The ESA, NASA, and UK joint mission operated for nearly two decades in a highly elliptical orbit, becoming a true workhorse for the spectroscopic study of ultraviolet light. The USSR contributed the telescope. Astron, also sensitive to this band.
El Hubble Space TelescopeAlthough famous for its visible-light images, it possesses very powerful instruments in the near-ultraviolet, which have allowed it to examine stellar atmospheres, star-forming regions, and young clusters. It was followed by missions such as... EUVE (Extreme Ultraviolet Explorer), the observatory Astro 1 and Astro 2, or the FUSE (Far Ultraviolet Spectroscopic Explorer), focused on the far ultraviolet.
Already in the 21st century, projects such as CRISPS, The mission GALEX To study the evolution of galaxies in UV, the Korean satellite Kaistsat 4and more recent missions such as IRIS, oriented towards the solar transition region, the Japanese observatory Hisakisuborbital experiments such as Venus Spectral Rocket Experiment, or telescopes mounted on the Moon like the Lunar-based Ultraviolet Telescope (LUT). Astrosat It also combines UV instruments, and solar missions such as Aditya-L1 These include observations in this range from the Lagrange point L1.
Space optical telescopes: visible light of unsurpassed quality
Optical astronomy is the most classical: it focuses on wavelengths between about 400 and 700 nanometersPlacing an optical telescope in space eliminates atmospheric turbulence and most absorption, resulting in extremely high-resolution images. These instruments are used to observe planets, stars, nebulae, galaxiesprotoplanetary disks and virtually any object that shines in visible light.
One of the first major milestones was Hipparcos (ESA), dedicated to precision astrometry: measuring the positions and parallaxes of stars to determine their distances. In the late 80s and early 1990s, it revolutionized star catalogs. Shortly after, in 90, the Hubble Space Telescope, a joint project of NASA and ESA that is still operational today in a low orbit around the Earth.
Hubble observes primarily in visible and near-ultraviolet light, although after a servicing mission it was also given additional capabilities. near infraredThanks to its stability and sharpness, it has provided some of the most iconic images of the universe, allowed for highly precise measurements of the Hubble constant, and revealed details of distant galaxies, globular clusters, planet-forming disks, and much more.
Other orbiting optical observatories have included the small Canadian telescope MOST, the French-European COROTdedicated to exoplanets and stellar oscillations, or the constellation of nanosatellites BRITEMissions such as SwiftAlthough they were created to study gamma-ray bursts, they also include optical instruments to track the evolution of these phenomena.
In the field of exoplanets, the satellite Kepler It marked a turning point by detecting thousands of worlds using the transit technique from a heliocentric orbit. It was followed by the observatory TESS from NASA and the European mission CHEOPS, aimed at characterizing already known exoplanets from a synchronous orbit with the Sun. Astrosat It also incorporates optical instruments, and projects such as GaiaLocated at the L2 Lagrange point, they have further refined astrometry, generating the most accurate three-dimensional map of our galaxy.
Infrared telescopes: unveiling the cold, dark universe
Infrared light has lower energy than visible light It is ideal for studying cold or very distant objects whose brightness has been redshifted by the expansion of the universe. In the infrared, we observe cool stars (including brown dwarfs), star-forming dust clouds, protoplanetary disks, and very distant galaxies.
Among the first major projects is IRASwhich produced the first complete infrared map of the sky and discovered dust disks around stars such as Fomalhaut, Beta Pictoris, and Vega. Then came the Japanese telescope Infrared Telescope in Spaceand the European Observatory ISO (Infrared Space Observatory), which explored the sky in a wide infrared range from a highly elliptical orbit.
The military-scientific mission MSX It also provided infrared data, while the satellite SWAS It focused on submillimeter wavelengths, key to studying molecules in interstellar clouds. The mission WIREUnfortunately, it failed to achieve its goal after an early failure.
El Spitzer Space TelescopeThe Space Telescope, part of NASA's Great Observatories, studied the mid- and far-infrared from a solar-draw orbit, producing spectacular results on star formation, infrared galaxies, and exoplanets. The Japanese mission Akari expanded these studies, while the observatory Herschel The ESA/NASA telescope, located at the L2 Lagrange point, was the largest infrared telescope launched until it ran out of helium in 2013.
The satelite WISE It mapped the sky across the entire mid-infrared, detecting everything from nearby asteroids to very distant galaxies. And the current star is the James Webb Space Telescope (JWST)Also at L2, it is designed to observe primarily in infrared. Its enormous 6,5-meter segmented mirror and cryogenic instruments allow it to study the first galaxies, star and planet formation, and exoplanet atmospheres with unprecedented detail. The mission will also work in near-infrared and visible light. Euclid from the ESA, focused on dark matter and dark energy from L2.
Microwave telescopes: the echo of the Big Bang
Microwave space telescopes have been used primarily to measure with great precision the microwave cosmic backgroundthe fossil glow of the Big Bang. From these observations, key cosmological parameters are determined, such as the age of the universe, its content of dark matter and dark energy, and its large-scale geometry.
The satellite was a pioneer in this band. COBE NASA's Cosmic Background Explorer, which first measured the tiny temperature anisotropies of the cosmic microwave background. Later, the Swedish observatory Odin It combined microwave and submillimeter studies in low Earth orbit.
The next big leap was the mission WMAP NASA's Wilkinson Microwave Anisotropy Probe, located at the L2 Lagrange point, dramatically refined COBE's measurements and established the so-called "standard cosmological model." The ESA subsequently launched the satellite PlanckAlso at L2, it obtained the most accurate map to date of the cosmic background, before being retired to a safe heliocentric orbit after the mission ended.
Space radio telescopes: interferometry on a planetary scale
Although the atmosphere is relatively transparent to radio waves, placing antennas in space allows us to... very long baseline interferometry by combining an orbiting radio telescope with antennas on the Earth's surface. By correlating the signals, an angular resolution equivalent to a telescope the size of the distance between them is achieved, which is ideal for studying extremely compact structures.
A key mission in this area was HALCA (VSOP), launched by the Japanese agency ISAS. It orbited Earth in a highly elliptical orbit, providing a baseline of up to tens of thousands of kilometers. It observed supernova remnants, masers, gravitational lenses, and active galactic nuclei with extraordinary resolution.
More recently, the Russian project Spektr-R (RadioAstron) It further expanded these possibilities with an extremely elongated orbit (from 10,000 to almost 390,000 km), forming, together with ground-based radio telescopes, one of the largest interferometry systems ever built.
Particle and cosmic ray detectors in space
In addition to photons, many space missions include instruments capable of detecting cosmic rays and energetic particles originating from the Sun, our galaxy, or extragalactic sources. Some of these cosmic rays reach extremely high energies, associated with processes such as relativistic jets from active galactic nuclei.
Among the first missions with particle detectors were the Soviet ones Proton-1 and Proton-2, which measured protons and electrons in low Earth orbit. The satellite HEAO 3 It also incorporated instruments for studying cosmic nuclei.
It was launched in the 90s SAMPEX (NASA/DE), focused on energetic particles in Earth's magnetosphere. The experiment AMS-01 He flew briefly on a space shuttle mission to test the alpha magnetic spectrometer, precursor of AMS-02, permanently installed on the International Space Station to search for antimatter and clues of dark matter.
The mission PAMELAA collaboration between European and Russian agencies studied the flow of high-energy particles in low Earth orbit. Meanwhile, IBEX NASA examines neutral energetic atoms to map the interaction between the solar wind and the interstellar medium, and satellites such as DAMPE (China) are investigating high-energy electrons, positrons and gamma rays in search of indirect signals of dark matter.
Gravitational wave space telescopes
Gravitational waves are ripples in space-time These signals are produced by events such as the merger of black holes or neutron stars. On Earth, detectors like LIGO and Virgo have already measured these signals, but the next major frontier is to take gravitational interferometry into space, where much longer arms, sensitive to lower frequencies, can be built.
The first technological step was Lisa pathfinder (ESA), a demonstrator mission that tested the trial mass control and laser interferometry systems in a heliocentric orbit. Its success paved the way for the future project LISA (Laser Interferometer Space Antenna), planned for the 2030s, which will consist of three satellites separated by millions of kilometers forming a triangle and capable of tracking gravitational waves from massive sources on cosmological scales.
Major observatories and flagship missions
Within its fleet of space telescopes, NASA promoted a series of Great Observatorieseach one focused on a part of the spectrum. The aforementioned Hubble It covers the visible and near-ultraviolet (with some infrared), the CGRO He specialized in gamma rays, the Chandra X-ray Observatory explores soft X-rays and the Spitzer Space Telescope He dedicated himself to infrared.
In addition, there are a number of missions that, while not formally being Large Observatories, have had a huge impact: IRAS as the first infrared sky tracker; Astron y Garnet in the Soviet sphere; the ISO European; the exoplanetary COROT; the IUE in ultraviolet; the solar observatory business center; the Canadian satellite SCISAT-1 to study the Earth's atmosphere; the pioneers of X-rays Uhuru, HEAO; the astrometric HipparcosThe compact Canadian telescope MOSTor Japanese ASTRO-F (Akari), among many others.
In the cosmological field, missions such as WMAP y Planck have allowed for the precise determination of the parameters of the standard cosmological model. At high energies, observatories such as INTEGRAL y Swift They continue to detect transient phenomena, while projects like INTEGRAL, WMAP, Spektr-R o Odin They have provided a more complete view of energetic radiation and the large-scale structure of the universe.
The New Giants: James Webb, Roman, Euclid and Beyond
El James Webb Space Telescope It has become the leading observatory of the current decade. Jointly operated by NASA, ESA, and CSA from the L2 Lagrange point, it is designed to study all phases of the universe's history: from the first galaxies to the formation of planetary systems and the analysis of exoplanet atmospheres. Its infrared images have allowed comparisons, for example, between observations of galaxies like NGC 628 and those taken by Hubble, revealing previously unseen details in dust and gas.
Thanks to Webb, candidates have been identified to extremely ancient galaxiesIt provides stunningly clear images of supernova remnants and detailed views of planets in the Solar System. Its success is built on four decades of experience with previous infrared telescopes such as IRAS, ISO, Spitzer, and Akari, which laid the technological and scientific groundwork.
Looking to the near future, NASA is preparing the Roman Space Telescope (formerly WFIRST), also at L2, designed to study dark energy, large-scale structure, and the exoplanet population with a very wide field of view. In the field of exoplanets, ESA will develop PLATO, which will focus on the search and characterization of habitable exoplanets around stars similar to the Sun.
Among the most ambitious projects, the following stands out: Habitable Worlds Observatorydesigned to study in detail Earth-sized planets in habitable zones and search biosignatures in their atmospheres. To do this, it will use techniques such as coronagraphs or possibly external sails (starshades) capable of blocking the star's light and revealing the faint signal of the planet.
X-ray telescope ATHENA The Advanced Telescope for High Energy Astrophysics (ATE), a collaboration between ESA, NASA, and JAXA, is designed to study supermassive black holes, galaxy clusters, and the hot gas that fills the universe on a large scale. In the realm of gravitational waves, the mission LISA It will be the great space observatory for tracking collisions of massive black holes and other compact systems.
There are also numerous concepts of the future under the umbrella of Great Observatory Technology Maturation Program (GOMAP) and the so-called New Great Observatories, which look beyond 2040 and seek to develop the technology needed to build even larger and more precise telescopes, both in optical and infrared as well as in high energies.
Other projects and missions in development
Alongside the big names, there are a whole host of projects that will populate the next generation of space telescopes. NASA is working on TOLIMANfocused on studying the Alpha Centauri system in search of potentially habitable planets using high-precision astrometry. China, for its part, is preparing the telescope Xuntian, an optical observatory that can be attached to the Chinese space station for maintenance and will offer a very wide field of view.
Other missions on the horizon include the variable object monitor Space Variable Objects Monitor, the spectroscopic observatory SPHEREx, the AstroSat-2 Indian as a replacement for Astrosat, or the European telescope ARIEL, specializing in analyzing exoplanet atmospheres from L2. All of them will join the current fleet to cover different energy ranges and scientific objectives.
New solar observatories and missions dedicated to studying our star better are also being developed. Understanding the solar storms and coronal mass ejections It is essential for protecting satellites, power grids, and communications systems on a planet increasingly dependent on technology. Missions such as business center o PROBA-3These veteran instruments have paved the way for a new generation of instrumentation both in Earth orbit and at specific points in the Sun-Earth system.
Looking at the big picture, from Galileo pointing a modest telescope at the Sun in the 17th century to colossal observatories at L2 capable of seeing infant galaxies, it becomes clear that each new generation of space telescopes It expands our boundaries: we detect more distant galaxies, track supermassive black holes, analyze the chemical composition of exoplanetary atmospheres, and refine cosmological parameters. All indications are that the upcoming observatories—Webb, Roman, Euclid, PLATO, ARIEL, LISA, Habitable Worlds Observatory, and others—will not only help us answer classic questions about the origin and evolution of the universe, but will also pose new enigmas we hadn't even imagined.
