The space shuttle Atlantis is less than a week away from blasting off to overhaul the Hubble Space Telescope for the fifth and final time in its nearly 20-year lifetime, but this last flight stands out from the pack in more ways than one.

Atlantis and a crew of seven astronauts are slated to launch toward Hubble on May 11 at 2:01 p.m. EDT (1801 GMT) from NASA's Kennedy Space Center in Cape Canaveral, Fla. Their mission is packed with five back-to-back spacewalks to repair vital systems and boost Hubble's vision into the universe.

NASA has even primed a second space shuttle to serve as a rescue ship in the unlikely event that Atlantis suffers critical damage space junk and strands its crew in orbit.

Here are reader questions asked in a recent Live Forum hosted by SPACE.com reporters, along with a few others on the nuts and bolts of the mission:

Why does Hubble need this last overhaul flight?

It's been seven years since NASA's last Hubble servicing mission (STS-109 in 2002) and the space telescope was designed to typically go only three years between overhauls. Because of the long gap, several systems have experiences some failures and need repair. NASA had already built new cameras, instruments and maintenance gear to support this flight, too.

Why is this NASA's last mission to Hubble?

Under NASA's current plan, the agency will retire its aging three space shuttles in 2010 after completing construction of the International Space Station. There will then no vehicle large enough to carry new instruments to Hubble or return broken components to Earth.

The shuttles typically carry seven people on two-week flights. Each has a cavernous cargo bay 15 feet (4.5-meters) wide and 60 feet (18 meters) long. NASA's shuttle replacement, the Orion Crew Exploration Vehicle, is a smaller, capsule-based spacecraft currently designed to launch four astronauts to the space station or on lunar missions.

Didn't NASA cancel this final shuttle flight to Hubble? What happened?

Yes. NASA canceled Atlantis' STS-125 mission to extend Hubble's operational life in January 2004 after a review board found it too risky in the wake of the 2003 Columbia tragedy that killed seven astronauts. The board directed NASA to fly only missions that kept shuttle astronauts within reach of the International Space Station, where they could take refuge if their spacecraft was damaged.

But the cancellation was met by fierce opposition from the public and science community. In 2005, NASA considered sending a robot to perform the Hubble repairs and upgrades, but ultimately found it infeasible.

By September 2006, NASA had successfully resumed space shuttle launches and flown the first two missions since the Columbia accident. That success, coupled with new heat shield and repair tools, led NASA to reconsider — and later approve — the final Hubble servicing mission.

If it was approved, why is there a rescue shuttle for the Hubble servicing mission?

While NASA is confident Atlantis' mission to Hubble will go well, the agency is also prepared in case it doesn't. Because the shuttle must fly higher and in a different inclination than the space station to reach Hubble, it does not have enough fuel to reach both the space telescope and orbiting lab.

Atlantis pilot Gregory C. Johnson has said the shuttle will spend nearly half its propellant just to reach Hubble, leaving it very little margin to return home. That is a standard feature of all Hubble repair missions because of the telescope's high orbit (300 miles up, whereas the space station is around 220 miles above the planet).

So, the shuttle Endeavour is already poised on a second launch pad. NASA plans to have Endeavour and a small crew of four astronauts ready to launch within seven days of any possible declared emergency.

What new science will the upgrades allow for Hubble?

Hubble can currently see distant galaxies and stars that were forming when the universe was just 700 million years old. The universe is currently 13.7 billion years old, and with the upgrade during STS-125, astronomers hope Hubble will be able to see the universe at time about 500 million years after the theoretical Big Bang that started it all. Astronomers are hopeful the new gear will allow Hubble to see infant galaxies among other new findings.

When would the first images from Hubble be released?

If all goes well with the Hubble repair mission, the first new image is expected to be released around September. The telescope's control center at NASA's Goddard Space Flight Center in Greenbelt, Md., plans about 10 weeks of tests and checkouts to make sure the telescope is fully operationally.

What are the main goals for the Atlantis' STS-125 Hubble Servicing Mission?

The following is a basic rundown of work to be performed during five spacewalks.

New Instruments and Gear:

Installing the Wide Field Camera 3 (WFC3): a new and more powerful main camera that bests its predecessors by seeing in both ultraviolet and near infrared as well as visible light. Hubble would be able to see 90 times more objects than it did at launch in April 1990. This instrument will replace Hubble's current Wide Field Camera 2, which will be brought back to Earth.

Adding the Cosmic Origins Spectrograph (COS): This instrument uses the ultraviolet range to find out the temperature, density, chemical composition and velocity of intergalactic gas and galaxies, with ten times the sensitivity of current Hubble instruments. This device will replace Hubble's corrective lens — COSTAR — which fixed the blurry vision caused by the telescope's flawed main mirror. The corrective optics are no longer required because all of Hubble's new instruments automatically compensate for the mirror's flaw.

Attaching the Soft Capture and Rendezvous System: This docking port will allow a future spacecraft, most likely robotic, to latch onto Hubble in order to guide it to a fiery demise in Earth's atmosphere or up to the relative safety of a graveyard — or 'museum' — orbit.

Maintenance work:

Replacing six Rate Sensor Units (RSU): The gyroscopes help keep Hubble pointed precisely at distant stars and galaxies for hours at a time. Hubble can technically limp by on two or even one gyroscope, but the fresh exchange ensures that the science keeps flowing.

Replace one of three Fine Guidance Sensors: Hubble has three Fine Guidance Sensors used to keep the telescope stead for long-duration observations. They are also used for astrometry, which studies the precise position and motion of stars.

Replacing six Nickel Hydrogen Batteries: The suitcase -sized batteries get swapped out for the first time in 16 years, giving Hubble an extra lease on life for the next 5 to 10 years. The batteries keep Hubble humming during the night portion of its orbit.

Replacing Thermal Insulation: The multilayer insulation on Hubble has become torn and broken by the harsh environment of space. The new thermal blankets protect the damaged insulation and helps maintain a steady temperature for Hubble.

Vital Repairs:

Repairing the Space Telescope Imaging Spectrometer (STIS): A two-sided instrument that uniquely scans across all light wavelengths of objects such as planets, comets, stars and galaxies. Spacewalkers will replace a failed power converter to restore one side of the damaged device.

Replacing the Fine Guidance Sensor (FGS): One of three optical sensors that help Hubble lock onto targets with a system of mirrors and lenses. The old FGS returns home after being removed from Hubble during an earlier servicing mission.

Fixing the Advance Camera for Surveys (ACS): A highly efficient survey tool with a wide field of view that became damaged. Astronauts hope to repair some of its capabilities since it failed last year.

Repairing the Science Instrument Command & Data Handling Unit: This repair will restore full redundancy to Hubble's vital data handling unit, which serves as the central relay that beams telescope imagery and data back to Earth.

How many days will the shuttle be docked with Hubble during the mission/repair?

Atlantis will arrive at Hubble on the third day of the 11-day mission and cast off on Flight Day 9, so it will be attached for about a week. Five of those consecutive days include spacewalks.

How would the rescue mission work, if needed, and how long would it last?

A rescue mission to Atlantis — called STS-400 — would last about a week. Endeavour would launch from Pad 39B at the Kennedy Space Center with shuttle commander Chris Ferguson, pilot Eric Boe and spacewalkers Shane Kimbrough and Stephen Bowen. The rescue crew consists of the same four astronauts who last launched on Endeavour's flight deck during its most recent flight, STS-126 in November 2008.

The astronauts would fly Endeavour to Atlantis, and grapple the stricken ship with Endeavour's robotic arm. They would then perform a complicated series of three spacewalks to move the seven astronauts from the damaged Atlantis into Endeavour, with Atlantis commander Scott Altman the last to leave.

After a day to rest, the astronauts would perform their standard prelanding heat shield inspection and return to Earth a day or so later.

NASA considers the need for such a rescue mission as extremely unlikely, but wanted to have an option just in case.

What is the greatest risk to Atlantis' crew?

NASA officials have repeatedly said that the STS-125 mission to Hubble faces a higher risk of severe damage from space junk and micrometeorites. Initially, that risk was about a 1-in-185 chance, which exceeded NASA's safety rules that call for a maximum risk of a 1-in-200 chance.

Since then, NASA has found ways to reduce the risk further by flying Atlantis in a tail-first orientation with its payload bay facing the Earth as much as possible. The shuttle will also fire its engines to move to a safer orbit just after releasing Hubble near the end of its mission.

Altogether, NASA's management efforts now set the risk at about a 1-in-229 chance of critical damage from space junk, despite recent spikes in debris levels from a Feb. 10 satellite collision and China's 2007 anti-satellite test. The current 1-in-229 chance puts the risk on par with other risks associated with human spaceflight, NASA officials have said.

How does the Delta V profile for this mission differ from a "normal" ISS flight?

Atlantis will launch due east from NASA's Kennedy Space Center in Central Florida. It is destined for an orbit of about 300 miles (482 km), or about 100 miles higher than the International Space Station, which has an inclination — or tilt with respect to the Earth's equator — of about 28.5 degrees. On space station missions, a space shuttle launches to a high inclination of 51.6 along a northeasterly track to reach the outpost's 220-mile (354-km) orbit.

How much longer will Hubble last? It seems like it's been up there forever!

It's certainly has been up there for awhile! The mission engineers say that if everything goes according to plan and they get the necessary repairs done, Hubble will last for at least another five years, or through at least 2014 — possibly longer. Hubble launched aboard the space shuttle Discovery on April 24, 1990 and celebrated its 19th birthday in orbit last month.

What could spell the death of Hubble?

Without the upcoming STS-125 mission, Hubble is one failure away (the data handling unit they revived last fall) from being space junk. With it, it has a good chance of going at least through 2014, but some of the repaired gear will be single string only. So that can fail. Even a wayward space rock or orbital trash could pose a real, and unexpected threat down the road.

Is this really NASA's final service call to Hubble or could a future spacecraft like Orion attempt some type of flight?

From what the mission scientists said, this really is the last call. Orion won't be in place until no earlier than 2015, and Hubble is meant to be serviced every three years (this is the longest it has gone — seven years — without servicing). They also haven't commissioned any new parts and those would take awhile to build if they did want to send anything new up there.

Looking to the future, how will Hubble be decommissioned? Will they capture it and return it to Earth when it is finally ready to be taken offline? Or, is it a situation where as long as it's healthy, it will stay in service like the Mars rovers?

NASA originally discussed plans to return Hubble to Earth at the end of its mission, but with the shuttle fleet retiring that's not in the cards now. Lead STS-125 spacewalker John Grunsfeld has said even if NASA wanted to, Hubble may now be too big and heavy (with all the added hardware over the last 19 years) to fit in a shuttle payload bay.

During the STS-125 mission, spacewalkers will attach a soft docking mechanism, a docking port for a future robotic spacecraft that — at the end of Hubble's mission — could latch on and then steer the school bus-sized space telescope into a controlled re-entry. A sort of space cremation, most likely over the Pacific Ocean.

Grunsfeld said the jury is still out on this. NASA could also attach a motor and boost it into a graveyard, or museum orbit.

NASA hopes Hubble will last well beyond 2014, but the STS-125 mission should extend its mission at least that much.

Will the Atlantis crew return any keepsakes of Hubble for the Smithsonian Air and Space Museum?

On this mission, the astronauts are actually going to bring back the Wide Field Planetary Camera 2 and NASA scientists are hoping it'll be up in the National Air & Space Museum before too long.

Among the other items returning to Earth aboard Atlantis is the COSTAR, the corrective optics that served as Hubble's first contact lens to correct its blurry vision after its initial launch. All of Hubble's new instruments automatically compensate for the mirror defect that caused the blurry vision.

In terms of the decommissioning, isn't it true that the robotic vehicle that would use the soft-docking mechanism for Hubble is not even built yet?

Yes. Right now, NASA's plan is to use a generic soft-docking mechanism, one it has developed for spacecraft like Orion and others at the International Space Station. The robotic craft envisioned to steer Hubble down to its death or a graveyard orbit is not yet built.

How much will the mission to Hubble cost?

According to Ed Weiler, NASA's science mission directorate chief, this last Hubble servicing mission is expected to cost about $1.1 billion. That is an increase from the estimated $900 million price tag NASA initially announced in 2006 when the agency committed to the STS-125 mission. The cost has swelled due to the last few years of delays.

Can NASA save money and risk by building a new space telescope to replace Hubble instead of flying the servicing mission?

Weiler has said that while the cost of the mission is one that can be criticized, even attempting to build a new space telescope would face the same scrutiny later on because cost overages would most assuredly follow then too.

Hubble was initially slated to cost about $400 million, but by the time of its 1990 launch it had increased to several times that. To date, NASA has spent about $10 billion to build Hubble and keep it operating in space for the last 19 years.

However, a future space telescope — like a daughter of Hubble — would cost much more, be larger and require a new launch vehicle (like NASA's heavy-lift Ares V) that does not yet exist.