Astronauts Find 21-Inch Scratch on Shuttle

Astronauts onboard Atlantis have discovered a 21-inch scratch on the starboard side of the shuttle, the result of debris that hit the ship 104 seconds after launch Monday.

They uncovered the damage during a thorough examination of Atlantis' thermal-protection system that surveyed the shuttle's wings, nose and underbelly.

The area in question is the point where the wing meets the underbelly on the orbiter's right side. The scratch stretches across four thermal tiles.

At a news conference this afternoon, lead flight director Tony Ceccacci said initial reports are that the scratches do not appear to be significant.

VIDEO: NASA flight director talks about tile damage to space shuttle Atlantis.Play

"To my untrained eye, I would think they are minor, but again, let those folks look at it," he said, referring to tile experts.

Dan Burbank, the spacecraft communicator (CAPCOM), told the crew, "At this point, the preliminary assessment is it doesn't look very serious. Those tiles look pretty thick, the tiles are small. It is probably too early to say whether we are looking at a focused inspection, but we are, at this point, discussing getting more information on the debris."

Imagery will be downlinked tonight and NASA will have a full report Wednesday. The astronauts initially had difficulty downloading the best photos from the external tank camera.

VIDEO: Atlantis Astronauts Have the Right StuffPlay

Atlantis Launched Monday to Repair Space Telescope

From the imagery, NASA will have to determine the depth of the scratch and the amount of heat it expects the area to receive when the shuttle re-enters Earth's atmosphere.

This evening the astronauts were told mission control would not ask them to do a "focused inspection," in which they record more detailed images of the broken streak. Cheers could be heard in response over the communications link.

In August 2007, the skin of the shuttle Endeavour had a 6-inch gash on its underside after launch, and it landed safely.

VIDEO: The Last Mission of AtlantisPlay

Atlantis thundered into a hazy blue sky Monday at Florida's Kennedy Space Center, on its way to rescue the Hubble Space Telescope.

Seven astronauts are on oard the shuttle, led by Cdr. Scott Altman. If all goes well, they will catch up with the Hubble, 350 miles out in orbit, Wednesday.

Then they will begin five days of spacewalks to repair the telescope.

This mission, designated STS 125 by NASA, is the last chance to save the Hubble telescope, whose batteries, cameras and gyroscopes are badly in need of replacement.

VIDEO: How did astronauts prepare for space flightPlay
Hubble Telescope Rescue: How Do Astronauts Prep for Space Flight?

As the shuttle approaches Hubble, it is undergoing a tedious but necessary inspection.

When orbiters travel to the International Space Station, the shuttles do a flip so that the space station crew can inspect the belly for damage. Since Hubble, obviously, doesn't have a crew to help them out, Atlantis will have to inspect itself for the first time.

The Risks Facing Shuttle Atlantis

"We are going to inspect the vehicle to the same standards as a [space] station mission," Altman told ABC News before the launch. "We are doing that by using the boom out there with the sensors on it."

A boom attached to the end of the shuttle's robot arm was moved slowly over Atlantis' wings and underside Tuesday, with a camera and laser rangedfinder to look for potantial damage.

VIDEO: Shuttle Atlantis launches on mission to repair the Hubble telescope.Play

Such work makes the flight a bit longer, Altman said.

This is the fifth and final space shuttle mission to the Hubble. It has been seven years since a shuttle last visited the telescope, and it is in need of a service call.

On this mission, astronauts will install a new wide-field camera and cosmic origins spectrograph, two instruments NASA said should significantly increase Hubble's potential.

Full-Scale Rescue Mission Ready If Needed

But the mission is also the riskiest one attempted by a shuttle since the Columbia accident in 2003.

Not only could debris hit the orbiter during launch, which was what happened to Columbia, orbiting debris could hit the shuttle while the astronauts fix Hubble.

Hubble orbits about 350 miles above Earth, in an area with a higher density of debris. Two satellites collided over Siberia earlier this year, which has increased the risk even more, as junk from that collision drifts lower.

As soon as Atlantis fixes Hubble, and releases it back into orbit, it will immediately maneuver to a lower altitude to reduce the chances of getting hit by space junk.

The most dramatic step NASA has taken to reduce risk is the preparation of a full-scale rescue mission.

In the event that Atlantis sustains damages the crew cannot repair, a second space shuttle, Endeavour, is standing by on another launch pad at the Kennedy Space Center to rescue crew members.

STS 400 can be ready in three days if a rescue is necessary.