The mission insignia for STS-125 - the fourth Hubble servicing mission. Image: NASA

It's a mission to once more push the boundaries of how deep in space and far back in time humanity can see. It's a flight to again upgrade what already may be the most significant satellite ever launched.

And, for the space shuttle, it's a final visit to a dear, old friend.

The STS-125 mission will return the space shuttle to the Hubble Space Telescope for one last visit before the shuttle fleet retires in 2010. Over 11 days and five spacewalks, the shuttle Atlantis’ crew will make repairs and upgrades to the telescope, leaving it better than ever and ready for another five years – or more – of research.

The shuttle Discovery launched Hubble in 1990, and released it into an orbit 304 nautical miles above the Earth. Since then it’s circled Earth more than 97,000 times and provided more than 4,000 astronomers access to the stars not possible from inside Earth’s atmosphere. Hubble has helped answer some of science’s key questions and provided images that have awed and inspired the world.

“We’ve actually seen an object that emitted its light about 13 billion years ago,” said Hubble senior scientist Dave Leckrone. “Since the universe is 13.7 billion years old, that’s its infancy, the nursery. From the nearest parts of our solar system to further back in time than anyone has ever looked before, we’ve taken ordinary citizens on a voyage through the universe.”

But Hubble has not done it alone.

Atlantis’ crew – Commander Scott Altman, Pilot Gregory C. Johnson and Mission Specialists Andrew Feustel, Michael Good, John Grunsfeld, Mike Massimino and Megan McArthur – will be the fifth shuttle crew to fly to the telescope. Their predecessors have replaced and repaired failed and faulty components and added new and improved cameras and scientific equipment, and the STS-125 crew will be no different.

In fact, just 17 days before they were originally scheduled to launch to the satellite, one side of the computer that sends commands to Hubble’s science instruments and formats science data for transmission to the ground – called the Science Instrument Command and Data Handling Unit, or SIC&DH – malfunctioned. Mission managers were able to postpone the visit to allow time for a spare unit to be tested and to give the astronauts time to learn to install it, further extending Hubble’s life.

And it will be a more robust life, thanks to the new scientific instruments Atlantis’ spacewalkers will install. The Cosmic Origins Spectrograph, for instance, will observe the light put out by extremely faint, far-away quasars and see how that light changes as it passes through the intervening gas between distant galaxies. In this way scientists will learn what that gas is made of, how it’s changed over time and how it affects the galaxies around it.

The Hubble Space Telescope is in the grip of space shuttle Columbia's robotic arm in March 2002 at the beginning of STS-109, the third Hubble servicing mission. Image: NASA

“It’s an important player in the story of how galaxies are formed and how the chemical makeup of the universe has changed over time,” Leckrone said.

And the new Wide Field Camera 3 will allow Hubble to take large-scale, extremely clear and detailed pictures over a very wide range of colors. At ultraviolet and infrared wavelengths the WFC3 represents a dramatic improvement in capability over all previous Hubble cameras. It is also a very capable visible light camera, though by design not quite as capable at visible wavelengths as Hubble’s Advanced Camera for Surveys. The WFC3 and ACS are designed to work together in a complementary fashion.

“If I want a complete family album of the universe, I need to look at it in all these different wavelengths,” Leckrone said. “This will be the first time we’ve had an opportunity to take all these different images together, to have a comparable quality of pictures across this whole wavelength band.”

Before those much anticipated views are seen, though, the equipment has to be installed – a process that will be exciting in its own right. The spacewalks necessary to outfit Hubble will be very different from the spacewalks conducted at the International Space Station.

“It’s more like brain surgery than construction,” Lead Flight Director Tony Ceccacci said. “On station spacewalks, you’re installing large pieces of equipment – trusses, modules, etc. – and putting it together like an erector set. You can’t do that with Hubble. Hubble spacewalks are comparable to standing at an operating table, doing very dexterous work.”

Although the installation of the new equipment and the replacement of some old items – gyroscopes, batteries and a fine guidance sensor, in addition to the SIC&DH – will be challenging, it’s the repairs the astronauts plan that will be the most complicated.

The new camera and spectrograph are designed to complement the scientific instruments already on the telescope – specifically the Advanced Camera for Surveys and the Space Telescope Imaging Spectrograph. But pieces of those instruments have failed in past years – not the entire instrument, but specific pieces inside of them. The crew will replace only the pieces that have failed.

But those instruments were never designed to be repaired in space. In fact, they were specifically designed not to come apart.

“When we first looked at it, we were going ‘well, maybe, maybe not,’” Ceccacci said.

The Hubble Space Telescope is seen in March 2002 with its new solar arrays after the completion of STS-109, the third Hubble servicing mission. Image: NASA

Since then, the team has come up with a plan for the work that Ceccacci believes will be very successful. But it won’t be easy – the repair of the spectrograph, for instance, requires the spacewalkers to remove more than 100 screws to access a computer card they will pull out and replace.

Still, the mission’s commander pointed out that it’s good practice for the future.

“I think it’s a step that we need to take to make us better able to go to places like Mars,” Altman said. “You don’t want to drag a whole spare giant box along – you’d like to be able to have the one little transistor you need to plug in when that fails. Being able to demonstrate this in space is a key element of us growing as a space-faring people.”

The Hubble spacewalks won’t be the only things that differ from missions to the space station. Confined to just the shuttle, the quarters will be tighter; with five back-to-back spacewalks, the pace will be faster.

Without the station crew to give the shuttle a once over and photograph its heat shield , the customary survey of the heat shield done the day after launch will be much more intensive. The crew will use the shuttle robotic arm and its 50-foot boom extension and sensor systems to perform not only the standard nose cap and wing leading edges inspection, but also a survey of the upper crew cabin and the entire underside.

In the unlikely event that irreparable damage is found, the crew also won’t be able to get to the space station to wait for a ride home – Atlantis can't reach the station from Hubble’s orbit. Because the crew won't have access to the station and the support it could provide in an emergency, the mission to Hubble requires some changes on the ground.

For every shuttle mission since Columbia, there has been a contingency plan in place to allow another shuttle to be launched if needed to rescue a stranded shuttle crew. On station missions, that stranded crew can wait longer at the station than would be the case for Atlantis. So, for 125, another shuttle will be standing ready on Kennedy Space Center’s Launch Pad 39-B. If needed, space shuttle Endeavour, manned by the flight deck crew of mission STS-123 which flew in March, will be ready to fly to Hubble and retrieve Atlantis’ crew within days.

What puts Altman’s mind at ease, however, are the changes NASA has made to keep damage from occurring in the first place.

“I feel pretty good that we’ve made incredible improvement in the external tank,” he said. “That’s the root cause. But if something does happen, I think we have the tools to find it, see where it is, evaluate how serious it is and fix it. And then on that one-way-down-at-the-edge-of-the-probability-level chance that you could have damage such that you wouldn’t want to come home on it, we have the capability to stay up there – extend our time and have another shuttle come get us.”

The risks, he believes, are relatively small, and the payoff is huge.

“Hubble puts cutting edge science together with a visual image that grabs the public’s imagination,” Altman said. “I think that’s the first step in exploration. Because Hubble takes light that’s been traveling for billions of years, sucks it in and shows it to us. It’s like taking you on a journey 13 and a half billion light years away while you sit there at home and look out at the universe.”