With a little help from a modified Boeing 747 carrier aircraft, the space shuttle Endeavour takes off from Edwards Air Force Base on Dec. 10, 2008 on its way back to the Kennedy Space Center in Florida. (Tom Tschida/NASA)On April 14, 1981, the first NASA space shuttle mission came to a successful end when the space shuttle Colombia returned to Earth following a two-day test flight.

On Friday, July 31, 2009, the crew of mission STS-127 is set to touch down at Florida's Kennedy Space Center on board the space shuttle Endeavour. Its safe return would mark the 125th time NASA has successfully completed a shuttle mission. Putting a damper on the celebrations, however, is the forecast.

Bad weather plagued the shuttle's launch, and if the storm clouds remain, as the forecast predicts, there's a chance it may also delay the shuttle's return. Because the space shuttle glides back to Earth during its descent, NASA has strict criteria in place to dictate when and where it can land. Conditions that can hinder the shuttle's return include cloud cover, wind velocity, turbulence, precipitation, lightning and thunderstorms near the landing site.

The space shuttle Discovery approaches landing on Runway 15 to end the STS-124 mission, a 14-day flight to the International Space Station, on June 14, 2008. (Kim Shiflett/NASA)

The Kennedy Space Center is NASA's preferred location for a shuttle landing, since it has the staff and equipment to handle the landing and is only a tow-ride away from the shuttle's storage facility. NASA can keep the shuttle in orbit, supplies permitting, if the bad weather in Florida persists, but by Saturday it may have to look at diverting the landing to another location. In this case, the shuttle will most likely touch down at Edwards Air Force Base in California.

The second most commonly used site, Edwards has handled more than 50 shuttle landings. While it offers more stable and predictable weather conditions, it lacks Kennedy's staff, meaning NASA would have to fly people there from Florida. Its location also means that the shuttle would have to be piggybacked by a modified Boeing 747 for its return to Florida. Landing at Edwards would delay the shuttle's return to Florida by a week.

NASA also has a network of national and international landing sites should it need to divert a space shuttle landing in an emergency. Such sites are ideally military air bases rather than commercial airports, have a runway at least 3,050 metres long and have access to personnel and equipment capable of responding to the landing.

So while there are dozens of potential sites around the world, only a select few are uniquely qualified. After the Kennedy Space Center and Edwards Air Force Base, here's a look at the next four sites most capable of handling a mission-ending landing.

White Sands Space Harbor, New Mexico

White Sands has only ever been used for a shuttle landing once, when mission STS-3 landed there in March 1982. White Sands lacks the equipment needed to transport the shuttle back to Florida. Landing at White Sands would mean a month's delay in the shuttle's return to the Kennedy Space Center.

The space shuttle Endeavour hitches a ride over California's Mojave Desert, mounted atop a modified Boeing 747 carrier aircraft on Dec. 10, 2008. The shuttle was being ferried back to the Kennedy Space Center in Florida after touching down at Edwards Air Force Base to conclude the STS-126 mission. (Carla Thomas/NASA)

Istres Air Base, France

One of three European air bases designated as Transoceanic Abort Landing, or TAL, sites. The primary purpose of these sites is to provide an emergency landing for the shuttle on launch day, but the bases' unique training in this area makes them better prepared than other international air bases to handle a potential shuttle landing. Shuttle-unique landing aids consist of visual landing guides, tactical air navigation and microwave scanning systems, and a remote weather tower.

Morón Air Base, Spain

A joint-use U.S. and Spanish air force base, Morón was designated a TAL site in 1984. Its 3,600-metre runway is equipped with shuttle-unique visual landing aids. Despite its proximity to the Sierra de Ronda mountain range, most of the surrounding countryside is flat.

Zaragoza Air Base, Spain

Formerly a joint U.S.-Spanish air force base, Zaragoza has retained its status as a TAL site. It has two runways, a civilian runway under 3,050 metres and an air force runway for the shuttle that is more than 3,650 metres long.