Despite the recent rash of space-debris problems, the risk that the space shuttle mission to service the Hubble Space Telescope will have a catastrophic collision with space junk and micrometeoroids won't exceed NASA guidelines.

NASA said Thursday the new orbital debris risk for STS-125 had fallen to 1 in 221. A couple of precautionary maneuvers -- in particular coming into a lower, less crowded orbit on the 10th day of the mission and using Hubble as a shield -- reduced the spaceship's chance of getting hit with a stray paint chip or metal bolt.

New, more precise observations of the orbital debris encircling Earth also showed the amount of junk was slightly lower than had previously been calculated. The new information means the shuttle mission to the Hubble can fly without any safety reservations.

"It's an improvement," said Steve Stich, the head of the orbiter project office at Johnson Space Center.

An initial risk assessment in October calculated a 1 in 185 risk that a piece of space debris or a micrometeorite would hit the shuttle and cause a catastrophic loss of the crew and vehicle. Only the Hubble service mission STS-61 in December 1993 faced a higher calculated risk of a catastrophic hit by space junk, at 1 in 150. Risk higher than 1 in 200 requires a special waiver to fly. The absolute cut-off for a shuttle mission is 1 in 60.

The Hubble is located in a debris-ridden area that's three times riskier than the orbit the International Space Station flies in. Given the difficult environment, it seemed the final Hubble service mission would be the first in 58 shuttle launches to exceed the 1 in 200 risk threshold.

Then, in February, an American communications satellite collided with a Russian Kosmos satellite, spreading debris around the Earth and prompting fears that the risk to the Hubble service mission would would be even higher. Orbital debris specialist Mark Matney of Johnson Space Center told Nature the mission was "uncomfortably close to unacceptable levels" and that the satellite collision was "only going to add on to that."

But NASA officials have managed to re-engineer the flight plan to not just keep the risk level from rising, but to reduce it. They came up with ways to carry out the mission's important scientific mission while protecting the vulnerable areas on the spacecraft, namely the nose cap and the leading edge of the wings. And, in what has become standard procedure, the astronauts will give the Shuttle a thorough inspection on the next-to-last day of the mission before attempting to re-enter Earth's atmosphere.

Despite the good news for this mission, the overall orbital debris trend remains disturbing. The satellite collision alone increased the risk to the shuttle mission by 8 percent. The risk of a micrometeroid or orbital debris (MMOD) hit is, in fact, very real.

"MMOD is the highest risk for the shuttle program," Stich said. "That's why we take such extra special precautions."

If during inspections, the astronauts find that their shuttle has been hit, they'll have options to save themselves.

"We have a couple mitigations there. One is that we could effect a repair using a material called NOAX," Stich said, alluding to a putty-like material akin to space-strength Bondo. Astronauts would spacewalk out to the problem and patch up the craft.

In the case of a more severe hit, NASA would fly the shuttle Endeavor up to the crew. Because they won't be able to wait in the International Space Station, which would be normal protocol for a shuttle docked there, they've brought extra food and water. Stich said they'd have 18 days to retrieve the crew from a damaged vehicle.

Impacts occur regularly on shuttle flights. Wired Science obtained the Hypervelocity Impact Database, which revealed that in the 54 missions from STS-50 through STS-114, space junk and meteoroids hit shuttle windows 1,634 times necessitating 92 window replacements. In addition, the shuttle's radiator was hit 317 times, actually causing holes in the radiator's facesheet 53 times.