Hubble Turns 36 — and Astronomers Still Can't Agree on How Fast the Universe Is Expanding

Thirty-six years ago today, NASA astronauts lifted a 12-ton mirror into orbit aboard the space shuttle Discovery. The Hubble Space Telescope — named for Edwin Hubble, who in 1929 had proved the universe was expanding — was supposed to give astronomers their first clear view of the cosmos, unfiltered by Earth's atmosphere.

Its first images came back blurry. A tiny flaw in the primary mirror, just 2.2 micrometers off from specification, had slipped past quality checks on the ground. For three years, Hubble was a punchline.

Then in December 1993, a seven-astronaut crew aboard Endeavour fitted the telescope with corrective optics — essentially a pair of contact lenses the size of a phone booth. Hubble went from embarrassment to revelation in a single servicing mission.

Why It Still Matters in 2026

Hubble was never supposed to be flying in 2026. Its original design life was 15 years. After five servicing missions — the last in 2009 — and the retirement of the space shuttle fleet, there is no way to send astronauts to it again. Every additional year of science data is borrowed time.

Yet the telescope is arguably more scientifically useful now than it was at launch. Paired with the James Webb Space Telescope, which sees in infrared, Hubble's visible and ultraviolet view has become an indispensable complement. When Webb spots something unusual, astronomers often ask Hubble to look at the same target in optical light to understand what is happening.

The Hubble Tension — the Discovery Hubble Did Not Want to Make

The telescope's namesake measurement is the Hubble constant — the rate at which the universe is expanding. In 2001, a team using Hubble data nailed this value to about 72 km/s per megaparsec, with an uncertainty of roughly 10 percent.

That was supposed to be the end of the story. It was not.

Over the following two decades, astronomers refined the local measurement using Hubble's observations of Cepheid variable stars and Type Ia supernovae. By 2024 the local value had settled near 73 km/s/Mpc, with uncertainty under 2 percent. Meanwhile, the Planck satellite, measuring the same expansion rate from the cosmic microwave background — the universe's oldest light — got an answer of 67.4 km/s/Mpc.

The two numbers should agree. They do not. The gap, known among cosmologists as the Hubble tension, suggests our standard model of the universe is missing something — possibly new physics, possibly early dark energy, possibly something nobody has thought of yet. Hubble's original job was to measure how fast the universe is expanding. Its enduring legacy may be to have shown that we still do not understand why.

What You Can Still See Tonight

Most of what Hubble has changed about the night sky is invisible to a backyard observer. The Hubble Deep Field, taken in 1995, pointed the telescope at a patch of "empty" sky the size of a grain of rice held at arm's length. It found 3,000 galaxies. That single image rewrote what astronomers thought was out there.

For anyone who wants to celebrate Hubble's anniversary tonight, look east after sunset for the planet Venus, currently the brightest object in the evening sky. Hubble has watched Venus's cloud patterns change across decades. Or find a dark spot and look up — Hubble's Cepheid measurements used to calibrate the cosmic distance ladder came from stars in the Virgo and Coma galaxy clusters, both overhead in April.

The Countdown That Hasn't Started

NASA has not announced a decommissioning date. Hubble's orbit decays by a few kilometers a year; without a reboost, it will eventually re-enter the atmosphere, probably in the late 2030s. The agency is considering private-sector options — SpaceX has discussed a Dragon-based boost mission, though nothing has been approved.

For now, Hubble keeps taking data. On April 24, 2026, the telescope observed a distant galaxy cluster as part of a survey on dark matter distribution. It was scheduled for 43 minutes. The target, a galaxy called Abell 1689, had been imaged by Hubble first in 2002, before most of today's PhD astronomers had written their first line of code.

The telescope is old. The universe is older. They have 36 years of work together — and, with a little luck, a few more years to go.