NASA's Kennedy Space Center (KSC) in Florida and Edwards Air Force Base (EAFB) in California serve as primary landing sites for the Space Shuttle orbiter. Each site offers unique advantages.

Landing the orbiter at KSC, where it is also processed and launched, saves at least an estimated $1 million and five to seven days of processing time for its next mission. Also, a KSC landing eliminates the necessity of exposing the orbiter, a national resource, to the uncertainties of a cross-country ferry flight from the California landing site to KSC atop one of NASA's modified 747 Shuttle Carrier Aircraft.

Edwards AFB, with its diverse choice of concrete and spacious dry lakebed runways, is the preferred site when the returning orbiter is carrying a payload and is heavier than usual, is undergoing testing or there is a mission-specific reason for landing there. Located in California's high desert country, Edwards also offers more stable and predictable weather conditions than the Florida landing site. A switch in sites usually can be made up to 90 minutes prior to landing. Almost half the Shuttle orbiter landings in the future are expected to take place at KSC.

As originally envisioned, KSC's Shuttle Landing Facility (SLF) would be the primary landing site for the operational Space Shuttle era. The initial six Shuttle missions ended at EAFB so the crews and support teams could gain experience in landings. STS-7 in June 1983 was the first end-of-mission landing scheduled for KSC. The orbiter Challenger landed instead at EAFB, two orbits later than planned, because of marginal weather conditions at KSC.

The first landing at KSC was Mission 41-B on Feb. 11, 1984. KSC was the landing site for four of the next six missions. Extensive brake damage and a blown tire at the conclusion of the 51-D mission in April 1985 prompted officials to postpone further KSC landings until nose wheel steering and improved brakes were installed in the orbiters. Landings were scheduled to resume at KSC with Mission 61-C in January 1986, but that flight also was diverted to EAFB due to bad weather in Florida. The Space Shuttle Challenger accident less than two weeks later resulted in renewed concerns about safety, weather and runway conditions. KSC landings again were put on hold.

Planned end-of-mission landings at KSC resumed in 1991 after safety modifications and improvements were begun to the orbiters and KSC's landing strip.

The space planes have been outfitted with upgraded main landing gear and carbon brakes, and additional nose wheel steering capability. Another orbiter modification is improved tires. Drag chutes are being installed on the three older orbiters to help reduce rollout speed after touchdown. Endeavour, delivered to KSC in 1991, was the first to have this modification.

The original lateral cross grooves cut on the KSC runway to help prevent hydroplaning were ground down on the first 3,500 feet at both ends of the landing strip to reduce the friction and abrasion levels on the orbiter's tires at the time of touchdown. Smaller corduroy-like ridges run longitudinally in the nominal touchdown area. Other enhancements completed or planned to increase the runway safety margin include resurfacing the 1,000-foot overruns and rebuilding and strengthening the runway shoulders, and replacing runway edge lights.

A returning orbiter's glide to Kennedy Space Center begins on the opposite side of the planet. The deorbit burn which will bring the space plane back to Earth occurs about an hour before landing.

Approximately 30 minutes before touchdown, the space vehicle begins entering the atmosphere at an altitude of about 400,000 feet. At approximately 45,000 feet, the orbiter starts maneuvers enabling it to intercept the landing approach corridor at the desired altitude and velocity. As the orbiter nears the landing site, the commander takes manual control and the vehicle is steered into the nearest of two heading alignment cones (HACs) to line up the spacecraft with the center line of the runway.

Depending on the mission and its orbital parameters, the path the vehicle takes as it enters the atmosphere and lands can vary greatly.

The ground track is determined by the inclination of launch. Generally, re-entry will follow one of two general patterns, either from a low-inclination or a high-inclination orbit.

Space Shuttles carrying most communication satellites, for example, usually have a launch azimuth of about 90 degrees, which places the vehicle in an orbit that has a 28.5-degree inclination to the equator.

This means that as it circles the Earth, the orbiter's ground track ascends to approximately 28.5 degrees above the equator (28.5 degrees north latitude) and 28.5 degrees below the equator (28.5 degrees south latitude) - a relatively narrow band of the globe.

Typically, re-entry from this orbit begins with a deorbit burn over the Indian Ocean off the western coast of Australia. Usually, the flight path of the orbiter then proceeds across the Pacific Ocean to the Baja Peninsula, across Mexico and southern Texas, out over the Gulf, and on to the west coast of Florida.

Depending on the mission, the space plane passes over Florida's west coast somewhere between Sarasota and Yankeetown and proceeds across the central part of the state, with its tell-tale sonic booms heralding its arrival.

The final approach to the KSC landing strip takes the orbiter over the Titusville-Mims area, and out over the Atlantic Ocean where it circles for a landing approach from either the south (Runway 33) or the north (Runway 15), depending largely on wind direction and speed.

Shuttles launched into high-inclination orbits generally follow the second major re-entry pattern. Usually, these missions fly in an orbit with a 57-degree inclination to the equator.

The ground track on these missions covers a much broader section of the globe, as the orbiter reaches as far north as 57 degrees above the equator and 57 degrees below the equator. This type of orbit is well-suited to Earth observation missions.

The entry ground track of high-inclination orbits will vary. Depending on where re-entry occurs, landing on a descending portion of an orbit could take the orbiter across Canada and the eastern United States; or, from an ascending portion of an orbit, above the Southern Pacific and across South America.

On these re-entries, the space plane may parallel the northeast Florida coast after cutting across Georgia, or will fly over the Florida Everglades and up the southeast coast of the state.

The sonic booms as the orbiter slices through the atmosphere at velocities greater than the speed of sound may be heard across the width of Florida, depending on the flight path.

The sonic boom is really two distinct claps that occur a fraction of a second apart, and are audible to the human ear. It is the noise produced by an aircraft flying at supersonic speeds. The vehicle, in effect, compresses the air in front of the nose and the wing, creating shock waves that spread away from the aircraft.

Although the boom may rattle some windows, it has little or no effect on humans, wildlife or property. At peak intensity, the boom is about as loud as an automobile backfiring on the next block or the clap of thunder from a lightning strike about a half mile away. The pressure wave of the boom at its maximum intensity is equivalent to about half the force exerted on a person's ears when the door of a full-sized car is slammed with the windows shut.

The boom should be barely audible as the orbiter crosses the western part of the state. It will get louder as the space plane drops in altitude, although for much of Central Florida it may be at a level which goes unnoticed by persons indoors. The orbiter goes subsonic as it flies over the Indian River before circling to the north or the south for a final landing approach.

KSC's Shuttle Landing Facility (SLF), first opened for flights in 1976, was specially designed for returning Space Shuttle orbiters. The SLF is located approximately three miles northwest of the huge Vehicle Assembly Building, with the launch pads only an additional three to four miles to the east. The runway is longer and wider than found in most commercial airports, yet comparable in size to runways designed for research and development facilities.

The paved runway is 15,000 feet long and 300 feet wide, with a 1,000-foot overrun on each end. Orbiters can land from either the northwest on Runway 15 or from the southeast on Runway 33.

In comparison, Orlando International Airport's longest runway is 12,004 feet long and 200 feet wide. The John F. Kennedy International Airport in New York has a runway nearly as long, 14,572 feet but much narrower at 150 feet. O'Hare International Airport in Chicago has a runway 13,000 feet long and 200 feet wide; and Miami International Airport's longest runway is 13,002 feet long by 150 feet wide.

In contrast, the other prime end-of-mission landing site, Edwards Air Force Base in California, has several dry lakebed runways and one hard surface runway on which an orbiter can land. The longest strip, part of the 44-square-mile Rogers Dry Lake, is 7.5 statute miles long. Concrete runways are generally preferred for night landings so the dust from the lakebed does not obscure the lighting.

About the size of a DC-9 jetliner, a Space Shuttle orbiter does not require such a large runway for landing. However, EAFB offers an extra safety margin because of its choice and size of landing strips. The orbiter differs in at least one major aspect from conventional aircraft: it is unpowered during re-entry and landing so its high-speed glide must be perfectly executed the first time - there is no go-around capability. The landing speed is up to 226 miles per hour. One concern about landing at KSC has been the canals and marshes surrounding the runway.

Foreign object debris (FOD) is any material which does not belong on or over the surface of the runway environment, where it becomes a potential hazard to the returning orbiter. Workers check the runway for FOD up to about 15 minutes prior to landing.

Birds are a hazard to the orbiter, as well as other aircraft. This "airborne FOD" could damage the orbiter's delicate outer skin of thermal protection materials. Birds are of special concern at KSC because much of the space center is a national wildlife refuge which provides a home to more than 300 species of birds. SLF employees use special pyrotechnic and noise-making devices, as well as selective grass cutting, to discourage birds around the runway.

The KSC concrete runway is 16 inches thick in the center with 15 inches on the sides. The landing strip is not perfectly flat; it has a slope of 24 inches from the center line to the edge to facilitate drainage.

The Shuttle Landing Facility includes a 550-by-490-foot parking apron, or ramp, on the southeastern end of the runway. On the northeast corner of the ramp is the Mate/Demate Device (MDD) which attaches the orbiter to or lifts it from the Shuttle Carrier Aircraft during ferry operations. The MDD is 150 feet long, 93 feet wide and 105 feet high.

When an orbiter lands anywhere other than KSC, it must be ferried back to the Florida space center riding piggyback style atop the Shuttle Carrier Aircraft.

Whether an orbiter lands here on its own or atop the Shuttle Carrier Aircraft, it is towed by a diesel-driven tractor to processing facilities via a two-mile towway from the Shuttle Landing Facility.

Adjacent to the MDD is the Landing Aids Control Building, which houses equipment and the personnel who operate the Shuttle Landing Facility on a daily basis. Other aircraft operations at the SLF include the astronauts' T-38 trainers; the Shuttle Training Aircraft, NASA's flying orbiter simulators; military and civilian cargo; and helicopters.

At the midfield point is the convoy staging area for the recovery team, a control tower, a fire station, and a viewing area for press and guests.

An array of visual aids as well as sophisticated guidance equipment at the Shuttle Landing Facility help to guide the orbiter to a safe landing.

The Tactical Air Navigation (TACAN) system on the ground provides range and bearing measurements to the orbiter when it is at an altitude of up to 145,000 feet. More precise guidance signals on slant range, azimuth and elevation come from the Microwave Scanning Beam Landing System (MSBLS) when the orbiter gets closer - up to 18,000 to 20,000 feet. Both TACAN and MSBLS are automatic systems that update the orbiter's onboard navigation systems.

The MSBLS also provides an autoland capability that can electronically acquire and guide the space plane to a completely "hands off" landing. So far, Shuttle mission commanders have taken control of the orbiter for all final approach and landing maneuvers during subsonic flight, usually about 22 miles from the touchdown point.

The initial landing approach at a glide slope of 19 degrees is more than six times steeper than the 3-degree slope of a typical commercial jet airliner as it approaches landing.

The Precision Approach Path Indicator (PAPI) lights are an electronic visual system that shows pilots if they are on the correct outer glide slope. PAPI lights are used at airports all over the world, but these have been modified for the unique configuration of the orbiter. A set of PAPI lights are at 7,500 feet and at 6,500 feet to delineate an outer glide slope of between 17 and 19 degrees. White lights are seen by the crew if the vehicle is above the glide slope; red lights show if they are below the glide slope. If they are "on" the correct glide slope, both red and white lights can be seen.

The "Ball-Bar" light system is a visual reference to provide inner glide slope information. The bar lights are 24 red lamps in horizontal sets of four each. They are about 2,200 feet from the runway threshold, and 300 feet from the first nominal touchdown point. Five-hundred feet closer to the runway threshold are three white lights - the ball - at a higher elevation.

If the orbiter is above the glide slope of 1.5 degrees, the white lights will appear to be below the bar of red lights. If the vehicle is below the glide slope, the white lights will appear to be above the red lights. If the red and white lights are superimposed, the orbiter is on the correct glide slope.

Just before touchdown, flare or a pull-up maneuver brings the orbiter into its final landing configuration. Touchdown nominally is 2,500 to 2,700 feet beyond the runway threshold.

Distance markers show the crew the distance remaining to the end of the runway during landing and rollout.

For night landings, the SLF has 16 powerful xenon lights, each of which produces up to one billion candlepower. Flatbed trailers hold eight lights, in two groups of four, at each end of the runway. To avoid blinding the crew, only the xenon lights at the end of the runway which will be behind the orbiter at landing are turned on.

Weather plays a major role in determining whether an end-of-mission landing is at KSC or Edwards Air Force Base, or is postponed until a later orbit.

The following weather constraints apply to KSC:

At the time of the deorbit burn go/no-go decision, which occurs approximately 90 minutes prior to landing, observed cloud cover below 10,000 feet should not exceed 20 percent.

Also, observed visibility and the forecast for visibility should be five miles or greater. Crosswinds should not be greater than 12 knots; in more restrictive landings - because of weight, for example, - crosswinds should not exceed 10 knots. Thunderstorms within 30 nautical miles and/or rain within 10 nautical miles are also landing constraints.

Wind direction usually will be the key factor in determining the final approach to the runway. Under normal circumstances, the orbiter will land into the wind. If the wind direction is from the south, the final approach will be from the north; if the wind direction is from the north, the orbiter will approach from the south.

Pre-landing weather forecasts will be issued by the Spaceflight Meteorology Group at Johnson Space Center, Houston, Texas. The group is part of the National Weather Service and works closely in coordinating its forecasts with the Cape Canaveral Forecast Facility. Weather instrumentation at KSC and the adjacent Cape Canaveral Air Force Station provides some of the data which the Spaceflight Meteor-ology Group uses in preparing their landing forecast. Weather conditions are also evaluated by NASA astronauts piloting reconnaissance aircraft along the orbiter's landing approach before the space plane is committed to re-entry.

Although on call during an entire mission in case of an earlier-than-scheduled landing, the Orbiter Recovery Convoy normally begins recovery operations in earnest about two hours before the orbiter is scheduled to land.

The convoy consists of 20 to 30 specially designed vehicles or units and a team of some 150 trained personnel who "safe" the orbiter, prepare it for towing, assist the crew in leaving the orbiter and tow the vehicle to the processing facilities.The team which recovers the orbiter is primarily composed of KSC personnel, whether the landing takes place at KSC, at EAFB in California, or elsewhere.

The first staging position of the convoy after landing is 200 feet upwind of the orbiter. A safety assessment team dressed in protective garb and with breathing apparatus uses a high-range flammability vapor detector to obtain vapor level readings and to test for possible explosive or toxic gases such as hydrogen, monomethyl hydrazine, nitrogen tetroxide and hydrazine and ammonia.

The Vapor Dispersal Unit, a mobile wind machine, blows away the potentially dangerous gases if high levels are detected and winds are calm. The safety assessment team continues to determine whether hazardous gases are present. Purge and Coolant Umbilical Access Vehicles are moved into position behind the orbiter to get access to the liquid hydrogen umbilical. The ground half of the onboard hydrogen detection sample lines are connected to determine the hydrogen concentration.

If the hydrogen concentration is less than 3 percent, convoy operations continue. If it is greater than 3 percent, the crew is evacuated immediately, convoy personnel are cleared from the area and an emergency power-down of the orbiter is conducted. After the carrier plates for the hydrogen and oxygen umbilicals are installed, the flow of coolant and purge air through the umbilical lines begins. Purge air provides cool and humidified air conditioning to the payload bay and other cavities to remove any residual explosive or toxic fumes. The purge of the vehicle normally occurs within about 45 minutes after the orbiter comes to a full stop. Cooling transfer to ground services occurs at approximately the same time, allowing onboard cooling to be shut down.

When it is determined that the area in and around the orbiter is safe, the Crew Hatch Access Vehicle moves to the hatch side of the orbiter and a "white room" is mated to the orbiter hatch. The hatch is opened and a physician performs a brief preliminary medical examination of all the crew members before the astronauts depart. Crew egress generally occurs within an hour after landing.

Astronauts now can egress from the orbiter quicker and more comfortably by transferring from the white room directly into a new Crew Transport Vehicle (CTV), a modified "people mover" used at airports. The crew passes through a curtained ramp to the CTV so they no longer will be visible to people watching the landing.

It is only after the crew has left the orbiter and the cool-down completed that Johnson Space Center, Houston, Texas, "hands over" responsibility of the vehicle to Kennedy Space Center.

The flight crew is replaced aboard the orbiter by exchange crew members who prepare the orbiter for ground tow operations, install switch guards and remove data packages from any onboard experiments. After a 45-minute tire cool-down period, vehicle ground personnel make numerous preparations for the towing operation, including the installation of landing gear lock pins, disconnection of the nose landing gear drag link, positioning of the towing vehicle in front of the orbiter and connection of the tow bar. Towing normally begins within four hours after landing, and is completed within six hours.

In addition to convoy operations on the runway, a KSC engineering test team monitors data from the orbiter from its station in one of the Launch Control Center's firing rooms. After the orbiter "hand over" to KSC, this team is enabled to issue commands to the orbiter to configure specific orbiter systems for the tow to one of three high bays of the Orbiter Processing Facility at KSC for the beginning of the processing flow which will ready the vehicle for its next flight. If the orbiter lands at Edwards, the vehicle will be towed to the Mate/Demate Device there to be attached to the Shuttle Carrier Aircraft which will bring the space plane back to KSC.