The U.S. space agency NASA is taking no chances that anything will strike the shuttle Discovery when it launches on its mission to the space station. It has worked three years to minimize the amount of hard insulating foam that breaks off the external fuel tank during liftoff, the cause of the fatal damage to the orbiter Columbia in 2003. NASA is even trying to eliminate the threat of birds.

Ever since a suitcase-sized piece of foam punctured Columbia's wing, NASA has been extra vigilant about protecting the fleet's fragile skin. It is a layer of ceramic tile and reinforced carbon that protects against the searing heat of re-entry through the atmosphere.

The agency's focus since the Columbia disaster has been on preventing as much external tank foam as possible from hitting the surface during liftoff to avoid another shuttle loss. But when Discovery collided with a large vulture during its ascent last year, shuttle managers realized the vehicle also faces a different threat.

"It's an impact and debris kind of issue for us," said shuttle launch director Mike Leinbach. "Prior to that event, frankly we didn't give the birds in the area a heck of a lot of attention. We hit it just slightly after three seconds into flight, and so we hit it very, very slowly. It hit on the opposite side of the external tank from the orbiter, so it really wasn't an issue for us, but it obviously heightened our awareness of the issue of hitting birds."

NASA's shuttle launch pad is next to a national wildlife refuge with vultures and hundreds of other bird species. It is also a habitat for many types of mammals, reptiles, amphibians, and fish -- animals on which vultures feast.

Many of these creatures die on roads surrounding the launch pad, so NASA has instituted a road kill roundup. It encourages employees to report the location of dead animals so workers can remove them to discourage vultures.

A vulture sits by the roadside at NASA's Kennedy Space Center in Florida, which shares a boundary with the Merritt Island National Wildlife Refuge

With the help of government wildlife experts, it is also testing a baited trap where vultures can be caught and released after launches. Another measure is to blare various sounds from loudspeakers to scare them from the launch pad. NASA is also using its technical prowess to test sophisticated bird tracking devices, a radar system and computer software based on camera images.

Leinbach says the vulture containment methods are humane. "We live on a national wildlife refuge out here, so we are not free to mess with the birds in a terminal fashion, let me put it that way. So we're doing all we can to mitigate the risk, and I feel we have a very good plan in place."

Deputy shuttle program manager John Shannon says it is no laughing matter. "We talk of a quarter pound [113 gram] of foam being catastrophic to the vehicle. These are four to six-plus pound [two to three kilogram] birds. We laugh about it, but it is a serious risk," he said.

Shannon says if birds get near the shuttle at launch time, the countdown will stop until they clear the area.