How the Hubble Space Telescope Works

01 Why Put a Telescope in Space

Earth's atmosphere is an astronomer's adversary. Turbulence blurs starlight into dancing speckles that limit even the largest ground-based mirrors to roughly one arcsecond of resolution. Atmospheric absorption blocks most ultraviolet and large swaths of infrared light entirely, rendering vast stretches of the electromagnetic spectrum invisible from the ground. In 1946, astrophysicist Lyman Spitzer wrote a paper titled "Astronomical Advantages of an Extra-Terrestrial Observatory," arguing that a telescope placed above the atmosphere would achieve diffraction-limited resolution and unobstructed spectral coverage. It took 44 years for that vision to become reality.

The Hubble Space Telescope orbits at approximately 540 kilometres altitude, well above the bulk of the atmosphere. At that height, starlight no longer twinkles, and the ultraviolet window from roughly 115 to 400 nanometres — completely absorbed by atmospheric ozone and nitrogen — opens for observation. Hubble also extends into the near-infrared to about 2500 nanometres, giving it a spectral reach of nearly two orders of magnitude broader than any ground telescope can achieve without adaptive optics correction. The result is a telescope that sees sharper, sees deeper, and sees wavelengths that ground-based instruments physically cannot detect.

02 Optics: The Mirror and Its Flaw

Hubble's optical heart is a single 2.4-metre primary mirror, a hyperbolic Ritchey–Chrétien design that trades a more complex figure for coma-free images across a wide field. The mirror was polished between 1979 and 1981 by Perkin-Elmer, using a custom null corrector — an optical device that verifies the mirror's shape by removing its inherent aberrations. The null corrector was misassembled by a fraction of a millimetre, and the mirror was polished to exquisite precision against the wrong shape. The result was spherical aberration: the outer rim of the mirror deviated from the intended curve by about 2.2 micrometres, causing light from the edges to focus 4 centimetres behind light from the centre.

The flaw was discovered weeks after launch, in June 1990, when the first images revealed a halo of unfocused light around every star. The telescope still produced useful ultraviolet data — because at short wavelengths the diffraction limit is smaller than the aberration — but resolution at visible wavelengths was crippled to roughly 0.5 arcseconds rather than the design target of 0.05. The crisis was public and intense. NASA's solution was elegantly simple: Hubble's modular instrument bay accepted refrigerator-sized units that could be swapped during Space Shuttle servicing missions. Engineers designed COSTAR, a "contact lens" of corrective optics that refocused the light before it entered the existing instruments. The flaw was corrected entirely in orbit during the first servicing mission in December 1993, and Hubble began producing the razor-sharp images that would define its legacy.

03 Instruments: Hubble's Eyes Over Thirty Years

Hubble carries instruments the way a surgeon carries different tools — each designed for a specific spectral range and purpose. The Wide Field and Planetary Camera 2 (WFPC2), installed during the 1993 repair mission, became the workhorse of Hubble's golden decade. It produced the iconic "Pillars of Creation" image of the Eagle Nebula in 1995. The Advanced Camera for Surveys (ACS), installed in 2002, expanded Hubble's field of view by a factor of roughly two and improved sensitivity at blue wavelengths by an order of magnitude. The Cosmic Origins Spectrograph (COS), installed in 2009, became Hubble's most sensitive ultraviolet spectrograph, probing the intergalactic medium and the composition of quasar absorption lines.

The Wide Field Camera 3 (WFC3), also installed in 2009, is arguably the most productive instrument Hubble has ever carried. Its ultraviolet-visible channel covers 200 to 1000 nanometres with a 16-megapixel detector, while its infrared channel covers 850 to 1700 nanometres. WFC3 was the instrument that made the Hubble Deep Field and Hubble Ultra Deep Field observations possible, collecting photons from galaxies so distant that their light left when the universe was less than 500 million years old. Together, the five servicing missions essentially replaced every original instrument, transforming Hubble into a telescope that bears little resemblance internally to the one launched in 1990.

04 Pointing and Stabilisation

A telescope in orbit is not a static mount. Hubble moves at 7.5 kilometres per second, circling the Earth every 95 minutes. To capture exposures lasting hours, the telescope must hold a target fixed in its focal plane to within 0.007 arcseconds — the width of a coin seen from 500 kilometres. This extraordinary stability is achieved by a suite of gyroscopes, reaction wheels, star trackers, and fine guidance sensors. The gyroscopes measure attitude changes; reaction wheels counter-rotate to cancel unwanted motion; and the Fine Guidance Sensors lock onto guide stars, generating error signals that drive the pointing system with sub-milliarcsecond precision.

The gyroscopes have been Hubble's most persistent hardware challenge. The telescope originally carried six rate-sensing units arranged in three pairs; failures have repeatedly degraded the system, forcing astronomers to operate with fewer gyros and reduced pointing flexibility. The 2009 servicing mission replaced all six units, but further failures since then have reduced the operational count. As of recent operations, Hubble has been running on a single gyro in certain configurations, with a backup held in reserve — a compromise that limits the sky accessible at any given time but extends the mission's scientific life. Even in reduced-gyro mode, Hubble continues to produce groundbreaking science.

05 The Servicing Missions

Hubble was designed from the outset to be serviced in orbit — a requirement driven by the Space Shuttle's payload bay dimensions and the telescope's modular architecture. Five Shuttle missions visited Hubble between 1993 and 2009. Each mission involved astronauts training for years in NASA's neutral-buoyancy underwater simulator, rehearsing spacewalks down to individual bolt turns. SM1 in December 1993 installed COSTAR and WFPC2, correcting the mirror flaw and restoring full scientific capability. SM2 in February 1997 added new spectrographs and upgraded the solar arrays. SM3A in December 1999 replaced failing gyroscopes and a fine guidance sensor after an urgent reprioritisation, since three of six gyros had failed and the telescope had entered a protective safe mode.

SM3B in March 2002 installed the Advanced Camera for Surveys, which immediately became Hubble's most-requested instrument. The final mission, SM4 in May 2009, was the most complex of all: two new instruments, all six gyroscopes replaced, batteries swapped, and the Science Instrument Command and Data Handling unit. After SM4, the Space Shuttle programme retired, and no further servicing missions are possible. The instruments installed during SM4 — WFC3 and COS — remain the backbone of Hubble operations today, more than fifteen years later.

06 Key Discoveries

Hubble's scientific output — more than 1.5 million observations feeding over 20,000 peer-reviewed papers — spans nearly every domain of astrophysics. In cosmology, Hubble's observations of Cepheid variable stars in distant galaxies refined the Hubble constant, narrowing the expansion rate of the universe to approximately 73 km/s/Mpc, though the tension between this method and Planck mission measurements remains one of the most consequential open problems in physics. In planetary science, Hubble discovered that Jupiter's aurora emits molecular hydrogen lines never before detected, imaged new moons of Pluto before the New Horizons flyby, and mapped seasonal changes on Mars.

The Hubble Deep Field observations — first conducted in December 1995 by pointing at an apparently empty patch of sky for ten consecutive days — revealed thousands of galaxies in a region smaller than a grain of sand held at arm's length. Each subsequent deep field, culminating in the eXtreme Deep Field in 2012, pushed further back in time, assembling a portrait of cosmic evolution from roughly 500 million years after the Big Bang to the present. Hubble also provided the first direct visible-light images of an exoplanet (Fomalhaut b), the first measurement of the atmospheric composition of an exoplanet, and the confirmation that nearly every large galaxy harbours a supermassive black hole, with the black hole's mass tightly correlated with the galaxy's bulge luminosity.