Hubble Space Telescope

For more than 35 years, a telescope roughly the size of a large school bus has circled Earth and sent home some of the sharpest pictures of the universe ever taken. That telescope is Hubble, and it changed how we understand the cosmos.

Quick facts

• Launched: April 24, 1990, aboard Space Shuttle Discovery on mission STS-31; deployed into orbit the next day.
• Built by: NASA, with the European Space Agency (ESA) contributing roughly 15% of the project in exchange for a guaranteed share of observing time.
• Named for: astronomer Edwin Hubble.
• Primary mirror: 2.4 meters (7.8 feet) across.
• Size and mass: about 13.2 m (43 ft) long, 4.2 m wide, and roughly 12,200 kg (27,000 lb) at launch.
• Orbit: a few hundred kilometers up, circling Earth about every 95 minutes (around 15 laps a day) at roughly 27,000 km/h (17,000 mph).
• Pointing accuracy: about 0.007 arcseconds — steady enough to hold aim on a distant target without drifting.
• Output: over 1.7 million observations, feeding more than 22,000 peer-reviewed papers.

What it is

Hubble is a space-based observatory — a telescope that operates in orbit rather than on the ground. It was the first of NASA’s “Great Observatories,” a set of large space telescopes, and it remains the only telescope ever designed to be repaired and upgraded in orbit by astronauts. It observes in ultraviolet, visible, and near-infrared light. (Visible light is what human eyes see; ultraviolet and infrared are forms of light just beyond what we can see, with slightly shorter and longer wavelengths.)

How it works

Hubble is a Cassegrain reflecting telescope, meaning it gathers light with mirrors instead of lenses. Light enters the tube, strikes the 2.4 m concave primary mirror, bounces to a smaller secondary mirror, and reflects back through a hole in the primary to the science instruments waiting behind it. Those instruments include cameras and spectrographs — tools that split light into its colors to reveal what an object is made of, how hot it is, and how it is moving.

The key advantage is location. From the ground, starlight passes through Earth’s atmosphere, which makes stars “twinkle” and blocks much of the ultraviolet. That is like trying to read a sign at the bottom of a swimming pool while the water ripples. Above the atmosphere, Hubble sees clearly. Spinning gyroscopes and reaction wheels hold it steady to a fraction of an arcsecond, and data travels to the ground through NASA’s relay satellites at about 1 megabit per second.

Why it matters

Hubble reshaped cosmology, the study of the universe as a whole. By measuring certain reliable stars and stellar explosions, it helped pin down how fast the universe is expanding and narrowed its age to about 13.7 billion years. Its supernova surveys helped reveal that this expansion is speeding up, evidence for a mysterious force called dark energy that contributed to the 2011 Nobel Prize in Physics. Hubble also gathered early measurements of planets orbiting other stars and showed that giant black holes sit at the centers of most large galaxies. Its famous images made astronomy feel accessible to everyone, and its design laid the groundwork for later observatories like the James Webb Space Telescope.

Notable moments

• The flaw and the fix: Hubble’s mirror was ground to slightly the wrong shape (a defect called spherical aberration), blurring its first images. In December 1993, Servicing Mission 1 (STS-61) installed corrective optics and a new camera during a record five back-to-back spacewalks totaling 35 hours and 28 minutes.
• Five servicing missions flew between 1993 and 2009. The final visit, STS-125 in 2009, added Wide Field Camera 3 and the Cosmic Origins Spectrograph.
• Deep Field images (1995 and 2004) used long exposures to reveal thousands of distant, ancient galaxies.
• Exoplanet HD 209458b became, in 2001, the first planet beyond our solar system to have its atmosphere measured — using Hubble.

Because Hubble orbits close to Earth, its path slowly decays from faint atmospheric drag, and no crewed servicing has been possible since the Space Shuttle retired in 2011. It now leans on its remaining gyroscopes, and since 2024 it has mostly run in a reduced one-gyro mode — yet it is still observing as of 2026.