Was Hubble's Mirror Ever Replaced? How NASA Fixed It in 1993
How Did NASA Fix Hubble Without Replacing Its Mirror?
I first heard the name Hubble in middle school. Not the astronomer, Edwin Hubble — the telescope. For a while, that name was almost the entire universe to me: nebulas in a magazine, a narrator's voice in a documentary, a photo pinned to the corner of a textbook page. All of it traced back to the same machine.
One question stuck, though: did astronauts actually replace Hubble's mirror? The version I picked up was short. The telescope launched nearsighted, astronauts flew up, and they fixed it. Said often enough, that summary started to feel like the whole story. Looking it up recently, I found the answer was no, and that the real story cost NASA something the short version leaves out.
Young boy discovering the universe through Hubble Space Telescope images in a science magazine
Why Hubble's First Images Came Back Blurry in 1990
In the spring of 1990, the first pictures arrived from a telescope that had cost more than a billion dollars to build and launch. The stars in them looked smeared, like someone had breathed on the glass.
Astronauts never replaced Hubble's primary mirror. During a 1993 servicing mission, they installed a new camera carrying its own corrective optics, plus a separate device called COSTAR, short for Corrective Optics Space Telescope Axial Replacement. It used small mirrors to redirect light into three other science instruments, compensating for the flaw rather than removing it.
The fix is difficult to picture. Space Exploration Stack Exchange even has a thread asking what it actually looked like.
NASA's account of what went wrong is specific. The outer edge of the primary mirror had been ground too flat, so light bouncing off the rim never came to the same focus as light from the middle. Opticians call that spherical aberration. It isn't a crack or a scratch. It's a shape, and a wrong one, which is exactly what made a fix possible later. What NASA's own pages don't quite agree on is how wrong.
Hubble Space Telescope primary mirror showing spherical aberration caused by a microscopic optical error
How Big Was Hubble's Mirror Flaw? Two NASA Numbers Disagree
NASA's servicing-mission overview puts the edge error at 2.2 micrometers, roughly a fiftieth the width of a human hair. The agency's 30th-anniversary account of STS-61 gives 0.003 millimeters instead, which works out to 3 micrometers. Neither page explains the gap, and I didn't find one that reconciles them.
Hubble's primary mirror is 2.4 meters across. The flaw that held it out of focus for more than three years measured just two or three millionths of a meter.
Either way, the effect on the light was the same. A star that should have been a sharp point showed up as a faint halo with a bright core buried somewhere inside it.
It's tempting to read the mismatched figures as a sign that nobody pinned the flaw down, but that reads it backward. The correction NASA built worked, and a correction like that only works when the error has been measured precisely. Whatever caused the gap, it seems to lie in how the number was reported later, not in how well engineers knew it.
Measuring the flaw turned out to be the manageable part. Reaching the telescope 360 miles up, and working on it in a pressurized suit, was not.
Space Shuttle Endeavour approaching and servicing the Hubble Space Telescope during STS-61 in 1993
STS-61, December 1993: Five Spacewalks and a Runaway Solar Panel
Endeavour launched on December 2, 1993, carrying seven astronauts and a repair plan rehearsed underwater for the better part of a year. Once the crew had captured Hubble, two pairs of spacewalkers alternated shifts outside the telescope for five straight days, logging 35 hours and 28 minutes in vacuum between them.
The first two spacewalks dealt with problems that had nothing to do with the mirror. On the first, Story Musgrave and Jeffrey Hoffman replaced failing gyroscopes and electronics. On the second, Kathryn Thornton and Thomas Akers were supposed to swap out the old solar arrays. The day before, ground controllers had commanded both arrays to retract, and one of them wouldn't cooperate.
A bent support rod had left it stuck partway open. Thornton, anchored to the end of the shuttle's robotic arm, released it by hand while Claude Nicollier swung the arm clear. When Kenneth Bowersox fired the thrusters to back away, the exhaust caught the loose panel and set it flapping as it drifted off into the dark.
The optical work fell to the third and fourth spacewalks. On the third, Hoffman and Musgrave pulled the original Wide Field/Planetary Camera and slid its replacement, WFPC2, into the same slot. WFPC2 needed no outside help; it carried corrective optics inside the camera itself. On the fourth, Akers and Thornton opened a different hatch, removed the High Speed Photometer, and put COSTAR in its place. The photometer still worked, but NASA's Hubble team picked it because it contributed a smaller share of the science than the other instruments.
That was the cost. To restore three instruments, NASA had to give up a fourth for good.
COSTAR corrective optics using precision mirrors to compensate for Hubble's flawed primary mirror
What COSTAR Actually Did Inside Hubble
I'd always pictured COSTAR as something bolted onto the front of the telescope, a lens cap with better manners. That's not what it was. Sitting in the photometer's old bay, COSTAR unfolded a set of arms that positioned five pairs of mirrors, ten in total, directly in the light paths of three original instruments: the Faint Object Camera and two spectrographs.
NASA describes COSTAR as about the size of a telephone booth, yet the smallest of its mirrors were no bigger than a nickel.
Sidestepping an optical flaw, instead of trying to perfect the part that causes it, has a long history in telescope design. Isaac Newton's reflecting telescope dodged a different one, the color fringing that plagued the lens telescopes of his day, by bouncing light off a mirror rather than passing it through glass.
Eyeglasses correct one person's vision. COSTAR corrected three instruments at once. So here's the plain version. NASA built two separate paths around the flaw: one designed into a replacement camera, one installed where the photometer used to be. Neither ground so much as a micrometer off the glass everyone blamed. It's hard not to prefer that to the version where NASA simply mailed up a pair of glasses.
Astronauts replacing Hubble scientific instruments during the STS-61 spacewalking mission
Did the Repair Work? WFPC2, COSTAR, and What Came After
Less than a month after the crew landed, fresh images confirmed that the optical repair had worked. That beat NASA's schedule: a pre-launch mission report had expected the first corrected images about seven weeks after Hubble's release and allowed up to sixteen weeks to verify every optical path.
WFPC2 stayed in service until 2009 and took more than 135,000 images before astronauts swapped it for Wide Field Camera 3. On that same flight, Servicing Mission 4, the crew pulled COSTAR and fitted the Cosmic Origins Spectrograph into the space it left. Both came home and now belong to the Smithsonian's National Air and Space Museum. Maybe that's the truest measure of the fix: NASA retired COSTAR not because it failed, but because nothing on board needed it anymore.
Since 1993, every new instrument sent up to Hubble has had its correction built in from the start. What began as a rescue became standard practice.
| WFPC2 | COSTAR | |
|---|---|---|
| Replaced | Original Wide Field/Planetary Camera | High Speed Photometer |
| How it corrected the flaw | Built-in corrective optics | External mirrors redirecting light to three instruments |
| Installed | Spacewalk 3 (Hoffman & Musgrave) | Spacewalk 4 (Akers & Thornton) |
| Retired | 2009, after 135,000+ images | 2009, replaced by the Cosmic Origins Spectrograph |
Hubble has been circling the Earth for more than thirty-six years now. The spherical aberration found just after its 1990 launch is still the crisis most tied to its name, and one it survived. The crew that flew up in 1993 never touched the flawed mirror. They built around it, and what they built held. Through four more servicing missions, Hubble kept sending back the images that changed how people picture the universe.
These days the telescope is slowly losing altitude, pulled down by the thin outer atmosphere it has been flying through since 1990. NASA doesn't expect Hubble to come down before the mid-2030s. The agency lays out two possible endings, both of which depend on fitting the telescope with a propulsion module: steer it down to a controlled reentry over the South Pacific, or push it into a far higher orbit where it could stay up for decades.
That timeline depends heavily on solar activity, which changes how much drag the upper atmosphere creates. In February 2026, The Register, drawing on an altitude plot shared by astronomer Jonathan McDowell, warned that Hubble could come down as early as 2028 without a reboost. That is an outside reading of the tracking data, not a change to NASA's forecast. NASA's FAQ, last updated in June 2026, still says no human or robotic servicing mission is planned, because the shuttle's retirement left no way to service the telescope.
Hubble was never just a demonstration of what American engineering could do. It was closer to a shared window, the telescope that let people who would never leave Earth feel the real size of what is out there. I don't know exactly when that window closes. But it's hard to think about the day it does without feeling something close to loss. All that time, we thought we were looking at the universe through Hubble. Maybe Hubble was the mirror, showing us how badly we wanted to understand what we were seeing.
Restored Hubble Space Telescope observing the universe above Earth after its 1993 servicing mission
Frequently asked questions
Why didn't NASA just replace Hubble's flawed mirror?
The primary mirror is buried inside the telescope's tube and was never one of the parts designed to be swapped in orbit. Bringing Hubble back to Earth wasn't the plan either: in 1983, NASA had dropped its return-and-relaunch approach in favor of servicing by spacewalking astronauts. So engineers corrected the light after it left the mirror, with fixes built into a new camera and into COSTAR.
Did Hubble return any useful science before the 1993 repair?
Yes. The flaw badly degraded several instruments, but NASA notes that some of Hubble's pre-repair images were still good. Its pre-launch report framed the 1993 mission as restoring the science Hubble had been built to do.
Who built COSTAR?
According to NASA, the agency developed COSTAR together with Ball Aerospace of Boulder, Colorado, which built it. The corrective mirrors inside came from Tinsley Laboratories of Berkeley, California.
Sources & References
- NASA, "Servicing Mission 1," science.nasa.gov
- NASA, "30 Years Ago: STS-61, The First Hubble Servicing Mission," nasa.gov
- NASA, STS-61 pre-launch mission document, ntrs.nasa.gov (PDF)
- NASA, "Hubble FAQs," science.nasa.gov
- NASA Engineering and Safety Center, "Hubble Space Telescope and Swift Observatory Orbit Decay Study" (Nov. 2025) — ntrs.nasa.gov
- The Register, "Hubble could re-enter atmosphere as early as 2028" (Feb. 2026; independent orbit-tracking report, cited for the earlier-reentry scenario only) — theregister.com
- Space Exploration Stack Exchange, "What did the Hubble fix look like?" (cited only as evidence of how the public asks this question, not as a factual source) — space.stackexchange.com
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