Space Telescopes

1. Hubble Space Telescope - broadest transformation of astronomy

Hubble transformed almost every branch of astronomy: cosmology, galaxy evolution, black holes, star formation, planetary science, and exoplanets. Its importance comes not from one single discovery but from its combination of longevity, exceptionally sharp observations, astronaut servicing missions, and an enormous public and scientific legacy.

It has studied planets and moons in our Solar System, protoplanetary disks, stellar nurseries, dying stars, supernovae, gravitational lenses, galaxies across cosmic history, supermassive black holes, and exoplanet atmospheres.

Credit: Interactive visualization courtesy of NASA/JPL-Caltech.

Major discoveries and contributions

1. Helped establish and precisely study the accelerating expansion of the universe. Hubble observations of distant supernovae strengthened evidence for dark energy and later traced how cosmic expansion changed over billions of years.
2. Greatly improved measurement of the Hubble constant. Cepheid-variable and supernova observations reduced uncertainty in the expansion rate to around one percent while highlighting the ongoing Hubble tension.
3. Hubble Deep Field and Ultra Deep Field. Long exposures of apparently empty sky revealed thousands of distant galaxies and reshaped models of galaxy formation and evolution.
4. Showed that supermassive black holes are common in galactic centres. Hubble measured rapidly moving gas and stars near galactic nuclei and helped establish black-hole and host-galaxy relationships.
5. Pioneered exoplanet atmosphere studies. Hubble made the first direct measurement of an exoplanet atmosphere and later detected water vapour, hydrogen, carbon, and other constituents.

It also obtained the first clear images of protoplanetary disks, often called proplyds, around young stars in the Orion Nebula.

Further reading: NASA telescope resource and NASA Hubble mission page.

2. Cosmic Background Explorer - COBE - foundational evidence for the hot Big Bang and cosmic structure

COBE provided the first decisive satellite measurements confirming that the universe began in an extremely hot, dense state. Its discoveries established the observational foundation on which WMAP, Planck, and modern precision cosmology were built.

COBE mapped microwave and infrared radiation across the entire sky, including the cosmic microwave background, interstellar dust, and the diffuse infrared glow produced by early stars and galaxies. John Mather and George Smoot received the 2006 Nobel Prize in Physics for this work.

Major discoveries

1. Measured the CMB's nearly perfect blackbody spectrum. COBE showed that the microwave background has the thermal spectrum expected from a hot early universe.
2. First detection of primordial CMB temperature fluctuations. These tiny differences seeded galaxies, clusters, and the later cosmic web.
3. Detected the cosmic infrared background. This diffuse glow records accumulated radiation from generations of stars and dust-obscured galaxies.

COBE changed the Big Bang from a broadly supported model into a theory backed by extraordinarily strong quantitative evidence.

Further reading: NASA COBE resource.

3. Chandra X-ray Observatory - transformed X-ray astronomy and black-hole and cluster physics

Chandra did for the high-energy universe what Hubble did for visible-light astronomy. Its angular resolution made it possible to obtain sharp X-ray images of black holes, neutron stars, supernova remnants, colliding galaxies, and million-degree gas inside galaxy clusters.

It detects matter heated to millions of degrees, gas falling toward black holes, shock waves from explosions, neutron-star magnetic fields, jets from active galactic nuclei, and the hot gas that fills galaxy clusters.

Credit: Interactive visualization courtesy of NASA/JPL-Caltech.

Major discoveries and contributions

1. The Bullet Cluster and direct evidence for dark matter. Chandra mapped the hot ordinary matter in colliding clusters while lensing showed most of the mass elsewhere.
2. Resolved most of the cosmic X-ray background into individual sources. Chandra Deep Fields showed that much of this glow comes from actively feeding supermassive black holes and other compact objects.
3. Independent dark-energy constraints from galaxy clusters. Chandra measurements of hot-gas and total-mass fractions provided an independent way to constrain the universe's matter and dark-energy content.

Chandra also mapped black-hole feedback, revealing jets, cavities, and shock waves that regulate galaxy and cluster evolution.

Further reading: NASA Chandra resource and Bullet Cluster example.

4. Kepler Space Telescope - demonstrated that planets are ubiquitous

Before Kepler, astronomers knew exoplanets existed but did not know how common they were. Kepler established that planets are ordinary products of star formation and that the Milky Way probably contains more planets than stars.

Kepler continuously measured tiny changes in stellar brightness to search for planetary transits, while also enabling research on stellar oscillations, rotating stars, flares, eclipsing binaries, supernovae, and variable stars.

Credit: Interactive visualization courtesy of NASA/JPL-Caltech.

Major discoveries

1. Showed that planets are extremely common. Kepler observed more than half a million stars and enabled the discovery of thousands of planets and candidates.
2. Kepler-186f. It discovered the first validated Earth-size planet orbiting within the habitable zone of another star.
3. Revealed a dominant class of planets absent from our own Solar System. Super-Earths and sub-Neptunes turned out to be common.
4. Demonstrated the prevalence of compact multiplanet systems. Systems such as Kepler-11 showed that several planets can orbit extremely close to their star in stable configurations.
5. Placed statistical limits on potentially habitable planets. Kepler data suggest that a substantial fraction of Sun-like stars may host roughly Earth-size planets receiving moderate stellar energy.

Kepler changed the central question from whether other stars have planets to which kinds of planets and systems are most common.

Further reading: NASA Kepler resource and Kepler-186f example.

5. James Webb Space Telescope - already revolutionary, but still early in its scientific lifetime

Webb is arguably the most capable astronomical space observatory ever constructed, and it may eventually rank second only to Hubble or even challenge it. This ranking, however, counts realised historical impact, and Webb is still early in its scientific lifetime.

Webb observes early galaxies, early black holes, star and planet formation inside dusty clouds, exoplanet atmospheres, directly imaged giant planets, outer Solar System bodies, and the chemistry of interstellar material.

Major discoveries and achievements so far

1. Revealed unexpectedly developed galaxies in the early universe. Webb has detected and spectroscopically examined galaxies from the first few hundred million years after the Big Bang.
2. Detected very early star clusters. Webb found massive young clusters in a galaxy seen when the universe was about 460 million years old.
3. Revolutionised exoplanet atmospheric spectroscopy. Webb obtained the first clear detection of carbon dioxide in an exoplanet atmosphere and the first detection of sulfur dioxide produced through photochemistry in WASP-39 b.
4. Confirmed the earliest supernova detected to date. The star exploded when the universe was only about 730 million years old.
5. Exposed populations of early black holes. Webb is detecting both relatively small and massive black holes much earlier in cosmic history than previous observatories could.

Webb has also directly imaged giant exoplanets at several infrared wavelengths and confirmed an approximately Earth-size rocky planet, though it has not detected life or an accepted biosignature.

Source: https://eyes.nasa.gov/apps/solar-system/#/sc_jwst. Credit: Interactive visualization courtesy of NASA/JPL-Caltech.

Further reading: NASA Webb resource and Webb's first images.

6. Gaia - reconstructed the structure and history of the Milky Way

Gaia is less famous publicly because it does not primarily produce Hubble-style photographs. Its importance lies in measurements: it created the most accurate multidimensional map of the Milky Way ever attempted.

Gaia made more than three trillion observations of about two billion stars and other objects. It measured stellar positions, distances, motions, brightness, temperature, and chemical properties, and also observed asteroids, exoplanet systems, quasars, gravitational lenses, binary stars, and compact objects.

Major discoveries and contributions

1. Reconstructed a major ancient galactic collision. Gaia data identified stars belonging to Gaia-Sausage-Enceladus, a smaller galaxy that merged with the early Milky Way about 10 billion years ago.
2. Discovered dormant black-hole systems. Gaia identified nearby black holes through the motion of visible companion stars, including Gaia BH3.
3. Revealed the Milky Way's detailed structure and history. It mapped stellar streams, a warped galactic disk, ancient central populations, and motions created by past mergers.
4. Transformed asteroid tracking. Its precision improved positions and orbital calculations for more than 150,000 Solar System objects.
5. Created a fundamental celestial reference frame. Distant quasars allow Gaia's catalogue to serve as a very precise coordinate system for astronomy and spacecraft navigation.

Gaia's full legacy will continue to grow as later data releases are completed and analysed.

Further reading: ESA Gaia overview image and Gaia mission in numbers.

7. Spitzer Space Telescope - transformed infrared and exoplanet astronomy

Spitzer opened a sensitive infrared window onto objects obscured by dust or too cool to emit much visible light. It connected studies of early galaxies, star formation, exoplanets, and the outer Solar System.

Spitzer studied dusty stellar nurseries, cool stars, brown dwarfs, exoplanets, distant infrared galaxies, active galactic nuclei, comets, asteroids, and planetary rings.

Major discoveries

1. First direct detection of light from an exoplanet. Spitzer measured infrared emission from hot Jupiters and opened the direct study of exoplanet temperatures and atmospheres.
2. The seven-planet TRAPPIST-1 system. Spitzer and ground-based observations established that the system contains seven approximately Earth-size planets.
3. Discovered Saturn's enormous Phoebe ring. This very diffuse infrared ring extends far beyond Saturn's bright visible rings.
4. Produced the first exoplanet temperature and weather maps. Changes in infrared brightness showed how heat moves around tidally locked giant planets.
5. Detected very distant, dust-obscured galaxies and black holes. Spitzer's infrared sensitivity uncovered objects largely invisible to optical observatories.

Its archive remains scientifically valuable and has kept producing discoveries after the spacecraft itself was retired.

Further reading: NASA Spitzer resource and TRAPPIST-1 example.

8. Wilkinson Microwave Anisotropy Probe - WMAP - established precision cosmology

COBE discovered primordial fluctuations; WMAP measured them precisely enough to establish the basic parameters of the universe. It helped turn cosmology into a precision observational science.

WMAP mapped tiny temperature and polarisation variations in the cosmic microwave background across the entire sky.

Major discoveries and contributions

1. Precisely measured the age of the universe. WMAP data placed it at about 13.7 to 13.8 billion years.
2. Established that the universe is spatially very close to flat. Its measurements constrained cosmic curvature to within roughly 0.4 percent of Euclidean flatness.
3. Measured the universe's basic composition. WMAP established a universe dominated by dark energy and dark matter, with ordinary atomic matter making up only a small minority.
4. Measured primordial fluctuations that became cosmic structure. Their statistical properties strongly supported an early period of cosmic inflation.
5. Used CMB polarisation to study reionisation. This helped constrain when the first generations of stars began ionising intergalactic gas.

WMAP established the broad modern Lambda-CDM model that Planck later tested at even higher precision.

Further reading: NASA WMAP resource and NASA universe overview.

9. Planck Space Observatory - produced the most precise CMB cosmology to date

WMAP established precision cosmology; Planck produced the most detailed full-sky CMB measurements yet obtained and subjected the standard cosmological model to extremely demanding tests.

Planck observed CMB temperature and polarisation, galactic dust, synchrotron radiation, cold molecular clouds, galaxy clusters, and extragalactic microwave sources.

Major discoveries and contributions

1. Produced the most precise full-sky map of the CMB. The map resolved primordial variations from which later cosmic structures developed.
2. Measured basic cosmological parameters at sub-percent precision. The six-parameter Lambda-CDM model provided an excellent fit to more than a billion map pixels.
3. Placed strong limits on inflationary models. Planck constrained primordial gravitational waves, non-Gaussianity, and departures from simple inflation scenarios.
4. Mapped polarised Galactic dust. Planck showed that dust could imitate a primordial gravitational-wave signal and helped correct the earlier BICEP2 interpretation.
5. Sharpened the Hubble tension. Early-universe data favoured a lower expansion rate than many local-universe measurements.

Planck also produced catalogues of galaxy clusters and cold Galactic objects and mapped dust polarisation related to the Milky Way's magnetic field.

Further reading: ESA Planck overview image and Planck CMB results.

10. Fermi Gamma-ray Space Telescope - transformed the gamma-ray and time-domain universe

Fermi created the most comprehensive continuing survey of the gamma-ray sky. It observes extreme phenomena that may be faint or invisible at ordinary wavelengths and scans almost the entire sky about every three hours.

Fermi observes pulsars, magnetars, blazars, supernova remnants, gamma-ray bursts, solar flares, terrestrial gamma-ray flashes, cosmic-ray interactions, and possible dark-matter signals.

Major discoveries and contributions

1. Discovered the Fermi bubbles. These two giant gamma-ray structures extend above and below the Milky Way and probably record past activity near the central supermassive black hole.
2. Expanded the gamma-ray pulsar population dramatically. Fermi has identified roughly 300 gamma-ray pulsars, including the first one found outside the Milky Way.
3. Helped inaugurate gravitational-wave multi-messenger astronomy. Fermi detected the short gamma-ray burst associated with the 2017 neutron-star merger GW170817 only seconds after the gravitational waves arrived.
4. Identified supernova remnants as sources of cosmic-ray particles. Fermi measurements showed accelerated particles colliding with surrounding matter and producing characteristic gamma rays.
5. Discovered new classes and behaviours of gamma-ray sources. These include gamma-ray-emitting novae, extremely energetic gamma-ray bursts, and high-energy phenomena in Earth's thunderstorms.

Fermi remains one of the most important observatories for rapidly changing events because it continuously watches such a large portion of the sky.

Further reading: NASA Fermi resource and Fermi gamma-ray sky map.