The Grand Spectacle of Orbiting Telescopes

Observing the Universe from Space

Space telescopes are observatories located beyond Earth's atmosphere, free from optical, thermal, and radio-frequency disturbances that affect ground-based instruments. Their purpose is to observe the cosmos across all wavelengths, from gamma radiation to radio waves, to explore the deep Universe, galaxy formation, and extreme energetic phenomena.

Advantages of Space Observation

Earth's atmosphere absorbs a large portion of the electromagnetic spectrum. By placing a telescope in space, we gain a complete view of the cosmos, free from turbulence or atmospheric absorption. This provides exceptional angular resolution and increased sensitivity, particularly in the infrared and ultraviolet.

Onboard Technology

Space telescopes use:

• Large segmented mirrors to collect maximum light.
• CCD or infrared detectors cooled to very low temperatures to minimize thermal noise.
• Gyroscopic stabilization systems to maintain precise pointing.
• High-speed data transmission to relay observations to ground-based science centers.

Note:
The mentions "Active" and the end dates reflect the status of missions as of the update date of this article (August 13, 2026) and are subject to change. Missions marked "scheduled" are under development and launch dates are indicative.
Missions added in 2026: Nancy Grace Roman, PLATO, SVOM, Xuntian. Future missions: ARIEL, NewAthena, LISA.

Major Historical Space Telescopes

Since the launch of Hubble in 1990, several space observatories have revolutionized our understanding of the cosmos, each exploring a different part of the electromagnetic spectrum. Beyond dates and raw numbers, each mission has its own story: a unique launch context, an unprecedented technical feat for its time, and often an unexpected discovery that surpassed initial objectives.

1970: Uhuru

The first space observatory dedicated exclusively to X-rays, Uhuru (Explorer 42, or SAS-1), was launched on December 12, 1970, from the Italian San Marco floating platform off the coast of Kenya; this date coincides with the seventh anniversary of Kenyan independence, in honor of which the satellite takes its name, meaning "freedom" in Swahili. With an instrument weighing only 65 kg, the satellite scanned about 95% of the sky and cataloged more than 300 X-ray sources, but its most remarkable discovery remains the rapid variability of Cygnus X-1, the first source considered a serious candidate for a stellar black hole — a major conceptual breakthrough for high-energy physics.

1990: Hubble Space Telescope (HST)

Launched by NASA and ESA, Hubble has observed the Universe in visible and ultraviolet light. Shortly after its commissioning, astronomers discovered a manufacturing defect in its primary mirror (spherical aberration) that blurred its images; the problem was corrected in 1993 with the installation of a corrective optical system (COSTAR) during the first servicing mission by the Space Shuttle Endeavour. Since then, its high-resolution images — including the famous Hubble Deep Field — have helped estimate the age of the Universe, study distant galaxies, and confirm the acceleration of cosmic expansion.

1991: Compton Gamma Ray Observatory

At 17 tons, Compton was the heaviest scientific satellite ever placed in orbit at the time of its launch. Its BATSE instrument recorded over 2,700 gamma-ray bursts and demonstrated, through their uniform distribution across the sky, that they originate from distant galaxies and not from our own Milky Way — a discovery that ended years of debate about their origin. In December 1999, the failure of one of its three gyroscopes led NASA, as a precaution, to voluntarily deorbit the observatory; its controlled re-entry into the atmosphere on June 4, 2000, remains the first maneuver of its kind ever performed on such a large satellite.

1999: Chandra X-ray Observatory

Named in honor of astrophysicist Subrahmanyan Chandrasekhar, Chandra observes the sky in the X-ray domain with an angular resolution of 0.5 arcseconds, unmatched to this day for this type of instrument. Its highly elliptical orbit, which takes it up to one-third of the Earth-Moon distance, keeps it far from the Van Allen radiation belts and allows for continuous observations lasting several tens of hours. It has revealed emissions from black holes, supernovas, and galaxy clusters — notably those of the Bullet Cluster, providing one of the most convincing observational proofs of the existence of dark matter.

2003: Spitzer Space Telescope

Designed for infrared observations, Spitzer detected forming stars and protoplanetary disks. After the expected depletion of its liquid coolant in 2009, it continued a "warm mission" for over a decade with two of its detectors remaining operational. It was during this phase that it made one of its most sensational discoveries: in 2016-2017, nearly 500 hours of nearly continuous observation of the dwarf star TRAPPIST-1 confirmed a system of seven Earth-sized planets, three of which are in the habitable zone — a record still standing for a single star.

2009: Herschel Space Observatory

Built by ESA, Herschel explored the far-infrared and submillimeter range using a 3.5-meter diameter mirror, the largest ever sent into space at the time — nearly one and a half times that of Hubble. Positioned at the L2 Lagrange point, it operated until its liquid helium coolant was exhausted in April 2013, revealing the filamentary structure of star-forming regions and the thermal evolution of galaxies throughout the Universe's history.

2009: Kepler

Designed to detect exoplanets using the transit method, Kepler fixed a single field of about 105 square degrees in the constellations Cygnus and Lyra for four years, continuously monitoring the brightness of over 150,000 stars. The loss of two of its four reaction wheels in 2012 and 2013 ended this fixed observation, but the team devised the extended K2 mission, using solar light pressure to stabilize pointing and explore new fields until 2018. In total, Kepler confirmed over 2,600 extrasolar worlds and revolutionized comparative planetology.

2013-2025: Gaia

The Gaia mission mapped over 1.8 billion stars in the Milky Way with unprecedented astrometric precision, allowing the study of galactic dynamics in 3D. Due to a lack of cold gas to stabilize its pointing, the satellite completed its scientific observations on January 15, 2025, before being placed on a retirement heliocentric orbit and permanently deactivated on March 27, 2025. However, data exploitation will continue for many years: the Data Release 4 is expected in 2026, and the final publication, Data Release 5, toward the end of the decade.

2018: TESS

The Transiting Exoplanet Survey Satellite searches for nearby and bright exoplanets. Unlike Kepler, which scrutinized a fixed and distant field, TESS scans nearly the entire sky in successive sectors, targeting nearby bright stars more suitable for follow-up observations. This complementary strategy has identified thousands of planetary candidates accessible to detailed characterization by other instruments, notably James Webb.

2021: James Webb Space Telescope (JWST)

The James Webb represents a major breakthrough. With its 6.5-meter segmented mirror, folded at launch and deployed in orbit like origami, and its infrared instruments passively cooled to about -233°C by a sunshield the size of a tennis court, it observes the first galaxies formed after the Big Bang, analyzes exoplanet atmospheres, and explores star formation processes.

Lifespan and End of Mission for Space Telescopes

Multiple Factors and a Notable Exception

The lifespan of a space telescope depends on many factors: availability of power, thermal stability, aging of sensors, etc. Unlike ground-based observatories, they generally cannot be repaired or resupplied once in orbit, with the notable exception of Hubble, which benefited from five servicing missions by the US Space Shuttle.

Operational Lifespan: Forecasts Often Exceeded

Missions are designed with a nominal operational lifespan, often 3 to 10 years, but many instruments far exceed these projections thanks to the robustness of their systems. For example, Spitzer operated for nearly 17 years instead of the planned 5, while Chandra and Hubble are still active more than two decades after their launch.

Main Causes of Mission End

Several factors lead to the end of a mission:

• Fuel depletion needed for orbital corrections or Lagrange point maintenance. This factor ended Gaia's observations in January 2025, as the satellite exhausted its cold stabilization gas.
• Thermal degradation of electronic and optical components due to extreme temperature cycles.
• Aging of detectors (CCDs, bolometers, photodiodes) affecting spectral sensitivity.
• Communication loss or failure of transmission antennas.
• Orbital constraint: for telescopes in low Earth orbit, atmospheric drag causes gradual deorbitation. This is how NEOWISE, lacking propulsion to maintain its altitude, disintegrated in Earth's atmosphere on November 1, 2024, after being decommissioned on August 8, 2024.

Fate of Telescopes: Deorbitation or Graveyard Orbit

At the end of their operational life, telescopes are either deorbited for controlled re-entry into Earth's atmosphere (like Compton in 2000 and, more recently, NEOWISE in 2024), or left on a stable, distant "graveyard orbit" to avoid contamination of active orbits. Observatories located at the L2 Lagrange point, such as James Webb or Euclid, will follow the latter procedure. Gaia, also positioned at L2, was moved in 2025 to a retirement heliocentric orbit before its final deactivation.

The Decommissioning Phase: A Programmed End of Cycle

Engineers plan a progressive decommissioning phase from the design stage to optimize the use of residual energy and secure the shutdown. This step marks the end of a technological cycle but prepares the advent of a new generation of more powerful observatories.

Future Prospects: From Design to Adoption

Several major space telescope projects, long relegated to the conceptual stage, have recently crossed decisive milestones toward their concrete realization.

Habitable Worlds Observatory (HWO)

The competing concepts LUVOIR and HabEx, once separately considered by NASA, were merged following the recommendation of the decadal survey (Astro2020 Decadal Survey) published in late 2021. The agency is now developing a single project, the Habitable Worlds Observatory, a telescope with a segmented mirror at least 6 meters in diameter observing in ultraviolet, visible, and near-infrared. Designed as a "super-Hubble" repairable in orbit, it will aim for direct detection and characterization of Earth-sized planets in the habitable zone around nearby stars, with a launch envisioned in the 2040s.

NewAthena

The European project ATHENA, selected by ESA in 2014, was resized in 2023 to account for budget constraints and renamed NewAthena. This X-ray observatory, intended to study the growth of supermassive black holes and the structure of the hot and energetic Universe, is to be formally adopted by ESA in 2027, with a launch planned for the late 2030s.

LISA, the First Space Ear for Gravitational Waves

After several decades of study, the LISA mission (Laser Interferometer Space Antenna) was officially adopted by ESA on January 25, 2024, in partnership with NASA. Composed of three satellites forming an equilateral triangle with 2.5 million kilometers on each side, it will detect low-frequency gravitational waves emitted notably by the merger of supermassive black holes. Its launch is planned for around 2035.