Hubble Space Telescope

The space telescope that changed how humanity sees the universe — launched with a flawed mirror, fixed by astronauts on a spacewalk, and subsequently responsible for some of the most consequential observations in the history of astronomy. Joint NASA/ESA mission, operating since 1990. Named after Edwin Hubble.

Key Properties

The Flaw and the Fix

When Hubble's first images arrived in 1990, they were blurry. Investigation revealed the primary mirror had been ground to the wrong shape — off by just 2.2 micrometres, less than one-fiftieth the width of a human hair, but enough to cause spherical aberration.

The telescope became a public symbol of NASA failure. The fix required the ingenuity of the corrective optics instrument COSTAR (Corrective Optics Space Telescope Axial Replacement), designed to cancel the mirror's error with an equal and opposite distortion. Astronauts installed it during Servicing Mission 1 (STS-61, December 1993) — five spacewalks over ten days that became one of NASA's most celebrated human spaceflight achievements. When the corrected images came back, Hubble's reputation was restored overnight.

The Deep Fields

The most consequential single decision in Hubble's programme: point the telescope at an apparently empty patch of sky and hold it there for days.

Hubble Deep Field (1995) — 10 days of exposure on a ~0.04 square arcminute patch of sky near the Big Dipper. The result: ~3,000 galaxies, most never previously seen, spanning the observable universe. It became one of the most studied images in astronomy.

Hubble Ultra Deep Field (2004) — ~11 days, a smaller patch, ~10,000 galaxies, reaching back to ~800 million years after the Big Bang.

Hubble eXtreme Deep Field (2012) — composite of 10 years of observations, ~5,500 galaxies, lookback to ~600 million years after the Big Bang.

The deep fields demonstrated that "empty" sky is never empty — the universe is filled with galaxies to its observable edge.

Dark Energy Discovery

In 1998, two independent teams using Hubble to measure distances to Type Ia supernovae found that distant supernovae were dimmer than expected — meaning they were farther away than a decelerating universe would predict. The universe was not slowing down. It was accelerating.

This confirmed the existence of dark-energy — the cosmological constant, or something like it — driving the expansion to accelerate. The discovery won the 2011 Nobel Prize in Physics (Riess, Schmidt, Perlmutter). Hubble's distance measurements were central to the result.

The Hubble Constant Tension

Hubble's precise measurements of Cepheid variable star distances anchor one of modern cosmology's deepest problems: the Hubble tension. The expansion rate measured via Cepheids (~73 km/s/Mpc) disagrees at ~5σ significance with the rate inferred from the cosmic-microwave-background (~67 km/s/Mpc). This discrepancy has survived every systematic check and may indicate new physics. JWST is now independently measuring Cepheid distances to test whether Hubble's calibration is the source of the tension.

Major Instruments (Current)

• WFC3 (Wide Field Camera 3) — workhorse imager, UV through near-IR
• ACS (Advanced Camera for Surveys) — wide-field optical imaging
• COS (Cosmic Origins Spectrograph) — UV spectroscopy of faint objects
• STIS (Space Telescope Imaging Spectrograph) — spectroscopy across UV/optical/near-IR
• FGS (Fine Guidance Sensors) — astrometry; also used for precise distance measurements

Selected Discoveries

• Protoplanetary disks (proplyds) in the Orion Nebula — direct imaging of planetary systems forming
• Supermassive black holes in most large galactic nuclei; correlation of black hole mass with bulge mass
• Atmospheric transmission spectra of exoplanets — precursor to JWST atmospheric science
• Discovery of Pluto's small moons (Nix, Hydra, Styx, Kerberos) — see pluto-dwarf-planet
• Jupiter's UV auroras; atmospheric dynamics on outer planets
• Age of the universe constrained to 13.7–13.8 Gyr via globular cluster dating

Longevity

Hubble has far outlasted its original 15-year design life. Servicing Mission 4 (2009, STS-125) — the last shuttle servicing mission — installed WFC3 and COS and repaired ACS and STIS. No further servicing is possible after the shuttle retired. Hubble's orbit slowly decays; without a reboost it will re-enter the atmosphere in the 2030s–2040s. NASA is studying a commercial reboost option to extend operations.