On a sunny afternoon in May, 2009, seven astronauts strapped themselves into the space shuttle Atlantis and rocketed toward the heavens. They had a relatively simple, but absolutely vital mission: replacing a camera and other key components of the Hubble Space Telescope.

The telescope, orbiting some 350 miles up, was slowly going blind. The men and women of STS-125, also known as Hubble Space Telescope Servicing Mission 4, were to give it new eyes. It was in some ways a routine mission, but nevertheless nerve-wracking. Memories of the Columbia disaster were foremost in NASA’s mind, and this mission had been delayed, canceled and reborn following a lot of hand-wringing and a few rounds of musical chairs by NASA administrators.

Much was riding on what would become the fifth and final shuttle mission to the telescope, which had been plagued by problems even as it provided invaluable new insights into the cosmos.

“This telescope has been able to observe all the way back to some of the very first galaxies in the universe,” John Grunsfeld, a mission specialist on that flight, says, reciting some of Hubble’s contributions to science. “The impacts of asteroids on Jupiter. Proving the existence of black holes. The discovery of dark energy, which nobody knew about prior to Hubble at all.”

The mission was, of course, a success, and Hubble still peers into the furthest reaches of space. To commemorate the upcoming 25th anniversary of the telescope’s launch, six of the seven astronauts who made that last mission gathered at at the Intrepid, Sea, Air & Space Museum in New York City to tell the tale.

Hubble’s Troubles and Triumphs

The Hubble Space Telescope is a Cassegrain telescope with a primary mirror 8 feet in diameter. Everything about it is huge—the orbiting observatory is the size of a school bus and weighs more than 12 tons. And it’s packed with sophisticated equipment, imaging sensors and cameras that, among other things, record visible, ultraviolet, and infrared light.

In that way, Hubble is not so much a telescope as a time machine. The light it sees was, in many cases, emitted billions of years ago—literally a long time ago in a galaxy far, far away. Hubble sees the distant past, providing key insights into the very origin of the cosmos.

It’s also the Ship of Theseus. Since its launch on April 24, 1990, all of its imaging instruments have been replaced during a series of repairs and upgrades. The revisions started almost immediately; Hubble was initially deemed a $1.5 billion disaster because it produced blurry images, the result of its mirror being ground just a bit too flat. “It was about a 50th of the width of a hair off,” says STS-125 pilot Greg C. Johnson. “That was just enough to make it out of focus.”

The Hubble Space Telescope is 43 feet long and orbits the Earth once every 90 minutes or so. Intrepid Sea, Air & Space Museum

That was corrected with COSTAR, which could be called the world’s largest contact lens. It was installed in December, 1993, along with Wide Field Camera 2, which could see ultraviolet, infrared, and visible light. Years later, NASA associate administrator Ed Weiler would call it “the camera that saved Hubble.” But all cameras, even state of the art models orbiting Earth, eventually become obsolete. After more than a decade in space, the camera, and much of the other specialized equipment aboard the Hubble, was in dire need of an upgrade.

Wide Field Camera 2 “was based on a digital camera that you would get back in 1993,” Grunsfeld says. “Now if you were to go to Wal-Mart and try to get a digital camera back in 1993, they didn’t exist. So this was a pretty basic camera.”

By 2009, it was well past time to replace the Wide Field Camera 2 with the Wide Field Camera 3.

Prepare for Launch Delays

Getting the mission off the ground was no easy feat. The Columbia disaster prompted NASA to reexamine and reconsider every mission that had been approved before that fateful day in 2003. For a while, it seemed STS-125—which had originally been slated for 2004, with Columbia—wouldn’t happen.

At first it was cancelled. Then it was proposed as a robotic unmanned mission. Then it was shelved. When Michael Griffin took over at NASA in 2005, he put the mission on the docket for late 2008.

Shuttle flights resumed in 2005, with the requirement that every shuttle have its heat shields checked at the International Space Station before reentry. But that wasn’t an option for STS-125. The orbit needed to rendezvous with Hubble wouldn’t allow a visit to the space station.

Instead, Endeavour was placed on standby, ready to launch should *Atlantis *be unable to return. And the crew would have to inspect its own ship. NASA had a plan for that, too—the Orbiter Boom Sensor System, a 100-foot arm fitted with a camera, a laser and other equipment. It was first deployed on STS-114, the inaugural flight after Columbia. Once STS-125 was in orbit, mission commander Scott Altman would deploy it to check Atlantis for launch-related damage to the heat shields.

“We were the only mission that used it to completely inspect the shuttle,” he says.

Everything checked out, and the crew set to work. The earliest portions of the missions went according to plan. But once the business of servicing Hubble started, a problem arose.

A bolt wouldn’t come out.

”Don’t Break the Hubble”

Going to space is expensive, so NASA maximizes the amount of time astronauts spend working. The crew of STS-125 had a long list of things to do during five days of spacewalks. The tasks included installing two new cameras and repairing two others. They also were to replace Hubble’s six gyroscopes, install a new guidance sensor and other equipment, and of course freshen up its battery packs.

Doing that much work in zero gravity creates many opportunities to screw things up, so Grunsfeld made a list of the 10 biggest things to keep in mind. “Number one was, ‘Be safe’,” he says. “Number two was, ‘Don’t break the Hubble.’”

They almost broke the Hubble.

The first job was supposed to be the easiest: Remove Wide Field Camera 2 and install its 980-pound replacement. That required removing a few bolts using a wrench fitted with a torque limiter to prevent snapping them. Snapping a bolt on Earth is a pain. In space, it can be a disaster, because you can't easily replace it.

Mission specialist Drew Feustel put the wrench on the bolt and twisted. He heard the clicking of the torque limiter, but the bolt wasn’t moving. He increased the torque–risky, because that could snap the bolt. Still, it wouldn’t budge.

“We didn’t have any WD-40, either,” jokes mission specialist Mike Good.

Feustel grabbed another wrench, one without a torque limiter. It was all or nothing at this point. Feustel yanked, hard. Slowly, the bolt turned.

“There was this release of tension, literally, in the bolt,” Grunsfeld recalls. “And it spun a little bit and I still wasn’t happy because two things could have happened.”

It might have turned loose. Or it might have snapped. “I heard Drew say, ‘Oh, it broke’,” Altman says. Then he completed the sentence with, “It broke free!”

The rest was as easy as any maintenance performed at an altitude of 350 miles can be. The new camera was a major upgrade. It has been essential to the study of dark energy, which Grunsfeld calls the “mysterious force causing the universe to accelerate its expansion.” The crew installed the Cosmic Origins Spectrograph, which has provided key insights into the birth and distribution of planets, stars, and galaxies. The team also repaired the Advanced Camera for Surveys, which was for a time Hubble’s primary camera until a glitch shut it down a few years before.

The EVA (Extra-Vehicular Activity) checklist from STS-125 and other Hubble artifacts are at the Intrepid Museum in New York through September 2015. Intrepid Sea, Air & Space Museum

Extending Hubble’s Warranty

All this work was done in five days. Each day’s outing involved two crew making spacewalks of six to eight hours apiece, for a total of nearly 37 hours outside Atlantis. Three of the first four spacewalks ran long, prompting concern at Mission Control. “Your day is regimented, and you look at the setup of the day,” says Altman. “(It) leaves you eight hours of sleep.” A delay “pushes everything back. You’re working well into your sleep period.”

That didn’t please the brass. “No more. You’re gonna have to end on time,” was the word from Mission Control, Good says. “We thought, ‘OK, we’ll start early.’

The crew had to work a lot of overtime and get up early for its final spacewalk, but it got everything done. Hubble was once again in tip-top condition, better than it ever was—or will be ever again. The “little telescope that could,” as Grunsfeld calls it, was designed to operate through 2013. But it’s still going, and should remain in orbit at least another decade. How productive it is in that time will depend upon how long the instruments keep working.

“The gyros are very special little machines with parts spinning at 70,000 RPM. They do wear out,” says Grunsfeld. “We’ve already had one set wear out, so how long those last will determine how well we can point the observatory.”

Even if the equipment lasts, Hubble may not. Its orbit is slowly decaying, and solar activity—flares and so forth—can influence the rate of decay. “It depends on how active the sun is, something we can’t control,” Grunsfeld says. “We thought that by the early 2020s, Hubble would be too low in altitude to keep observing. But as many of you know, the sun has been extraordinarily quiet. So there’s some thought that the telescope itself will stay up until the 2030s.”

A "rose" made of galaxies, captured by Hubble's Wide Field Camera 3 on its 21st anniversary in 2011. Intrepid Sea, Air & Space Museum

The Future of Hubble and Space Telescopes

Eventually, Hubble will fall to earth. And we’ve got its replacement ready. The James Webb Space Telescope is scheduled to launch in 2018. It’s bigger and more powerful than Hubble, and will peer at the furthest corners of the cosmos from 1 million miles beyond Earth. Once it’s launched in October, 2018, if all goes according to plan, that’ll be it. The Webb will be too far away to fix should anything go wrong.

“The Hubble was an experiment, if you will, on how to build a telescope that can be fixed by astronauts in space,” says Grunfeld. “The James Webb telescope is an experiment building something as complex as Hubble, but it was designed so delicate that astronauts can’t service it. So it has to work.”

As the Webb settles into its groove, the Hubble probably will be falling to earth. Unless you’re at sea, you won’t need to worry about it hitting you. One of the STS-125 crew’s tasks was to put a handle on Hubble so that it can be captured by a rocket once it’s ready to tumble.

“We won’t be able to predict where it goes, and so on [the] mission we actually put a fixture on the bottom of the telescope called a soft capture mechanism,” says Grunfeld. “Sometime, right now we’re looking at the late 2020s, we’ll have to send up an unmanned rocket to latch onto the bottom of Hubble so that at the end of its life we can burn it and send it into the ocean.”

The astronauts of STS-125 back in the day. From left to right: Massimino, Good, Johnson, Altman, McArthur, Grunsfeld, and Feustel. NASA

The Intrepid Sea, Air & Space Museum in New York City has an ongoing “Hubble@25” exhibit that runs through September 2015. It showcases photography, artifacts, and multimedia presentations that celebrate the Hubble’s legacy.