The Hubble Space Telescope

Why put a telescope in space

Earth's air is always moving. Pockets of warm and cool air bend starlight slightly, which is why stars twinkle and why images from the ground smear out. The atmosphere also blocks most ultraviolet light.

A telescope above the air avoids both problems. The idea was suggested in the 1940s by astronomer Lyman Spitzer, and after decades of planning Hubble was carried into orbit by Space Shuttle Discovery on April 24, 1990. It is named after Edwin Hubble, who showed in the 1920s that the universe is expanding.

A flawed mirror and a famous repair

The first images were a shock: sharper than ground pictures, but fuzzy around the edges. The 7.9-foot main mirror had been polished to a slightly wrong curve. The error was about one fiftieth of the width of a human hair, yet that was enough to spread light into a blurry halo. Optical engineers call this spherical aberration.

Because the mistake was so precise, it could be corrected like eyeglasses for a nearsighted eye. In December 1993, astronauts on Space Shuttle Endeavour spent five spacewalks installing a corrective package and a new camera with its own built-in fix. The next images were crisp, and Hubble's reputation was rescued.

Four more servicing missions followed, in 1997, 1999, 2002 and 2009. Each replaced worn parts and added better instruments, so the telescope of today is far more capable than the one launched in 1990.

What Hubble discovered

By measuring distances to pulsating stars in other galaxies, Hubble helped narrow the age of the universe to about 13.8 billion years. In the late 1990s, Hubble observations of distant exploding stars supported a surprise: the expansion of the universe is speeding up, a finding that earned the 2011 Nobel Prize in Physics.

The Hubble Deep Field images pointed the telescope at seemingly empty patches of sky for days. They revealed thousands of galaxies, some seen as they were billions of years ago. Hubble also found that nearly every large galaxy hosts a supermassive black hole at its center.

Closer to home, it watched fragments of Comet Shoemaker-Levy 9 crash into Jupiter in 1994, discovered small moons of Pluto, and made the first measurement of the chemical makeup of an exoplanet's atmosphere. Its pictures of nebulas, such as the Pillars of Creation, became some of the best-known images in science.

How Hubble works

Hubble is a reflecting telescope: light bounces off the large primary mirror onto a smaller secondary mirror, then passes through a hole in the center of the main mirror to the instruments. These include cameras and spectrographs, which split light into its colors to reveal temperature, motion and chemistry.

It circles Earth about every 95 minutes, roughly 320 miles up. To stay locked onto a faint target, it uses gyroscopes and very precise guidance sensors. Solar panels supply power, and data is relayed to the ground through NASA communication satellites.

Status as of 2026

Hubble has now worked for more than 36 years. Its gyroscopes, which sense how the telescope is turning, have been wearing out. Since 2024 NASA has run it with just one gyroscope at a time, keeping a spare in reserve. This slows down how fast it can switch targets and limits some observations, but most science continues.

Air drag at that height is very thin but not zero, so the orbit slowly sinks. Without a boost, Hubble is expected to reenter the atmosphere sometime in the 2030s. NASA has looked at ideas for a reboost but has no servicing visit planned. Meanwhile, Hubble works alongside the James Webb Space Telescope: Hubble covers ultraviolet and visible light, while Webb sees deeper in the infrared.

Can you see Hubble from the ground?

Yes, sometimes. Like the space station, Hubble shines by reflecting sunlight and appears as a steady point gliding across the sky shortly after sunset or before sunrise. It does not blink.

Because its orbit is tilted about 28.5 degrees to the equator, it is only visible from places closer to the equator than roughly 30 degrees of latitude. Observers farther north or south never see it rise high enough. Satellite-tracking apps and websites list upcoming passes for your location.