Welcome to the final installment of my guest blog series on the countdown, ascent, and landing of the space shuttle (here are the first and second parts). On Thursday, July 21 at 5:56AM EDT (9:56 UTC), the space shuttle program will come to an end when Atlantis returns to the tarmac for the final time at the Kennedy Space Center Shuttle Landing Facility.
The Sun won't rise until 6:38AM EDT, so there won't be much for viewers to see until a grainy image of Atlantis appears on tracking cameras a few minutes before landing. Touchdown will be a little easier to spot; the shuttle landing facility has a giant set of spotlights to illuminate the orbiter as it cruises across the runway threshold.
Just like the launch coverage, NASA TV will be your best viewing medium for technical explanations of what's happening. If your cable provider gets the NASA channel, great – and if not, you can still tune in live on the Internet. If you live in the U.S., it's going to be a little early in the morning, so you may want to set your DVR.
Either way, for the best coverage, plan to start watching a little more than an hour and a half before landing (3:36 AM EDT). Let's bring Atlantis home!
1 hour, 30 minutes until touchdown
By now, the shuttle will have fired its reaction control system (RCS) thrusters, orienting the vehicle so that it is travelling upside-down and tail-first. At mission control, the weather has been checked and verified safe for a landing at Kennedy Space Center. Alternatively, the shuttle can land at Edwards Air Force Base, California, but this requires an expensive, cross-country piggyback ride on NASA's specially modified Boeing 747 to bring the shuttle home after it lands. NASA will always try for Florida first, and depending on the situation the space agency can wait for another window of opportunity before giving up and sending the shuttle to the desert.
When everything is ready, mission control gives the traditional "go for de-orbit burn" call to shuttle commander Chris Ferguson. The de-orbit burn fires the rockets in the orbital maneuvering system against the direction of motion, slowing the vehicle enough to allow it to be overcome by the force of gravity, which slowly begins to pull it into the atmosphere.
1 hour until touchdown
The de-orbit burn lasts for two and a half minutes and slows the orbiter by more than 76 meters per second (250 feet per second). The shuttle's RCS thrusters then rotate the shuttle approximately 180 degrees clockwise so it approaches the atmosphere belly down, heat shield first, with the nose pitched upwards at 40 degrees. At this point there is no turning back; the shuttle is now essentially a flying brick with thrusters, at the mercy of the Earth's gravity, and it's going to land, one way or another.
30 minutes until touchdown
The shuttle starts to feel the effects of the atmosphere. It is still 8,000 kilometers (5,000 miles) away from the landing site, travelling at a blistering pace of 28,000 kilometers per hour (17,500 miles per hour). As the ship is jostled by the forces of re-entry, the RCS thrusters keep it on course. However, as air pressure increases, the flight control surfaces (ailerons and elevons) start to operate, and the RCS system is deactivated.
The heat from re-entry is building. The shuttle's fast dive through the atmosphere massively pressurizes the air in front of it, and Boyle's Law tells you that when a gas's pressure goes up, so must its temperature. So the pressurization creates a superheated shock wave in front of the vehicle. Between the shock wave and the shuttle's skin is a layer of ionized gas called plasma. The plasma transfers heat from the shock wave to the skin, increasing the temperature to 1650 degrees Celsius (3000 Fahrenheit) in some spots. This is where the thermal protection system kicks in. The shuttle's skin contains more than 27,000 tiles made of ceramic and other materials to dissipate the heat so that the shuttle's frame (not to mention the astronauts inside it) receives a much milder toasting.
To use up excess energy, the shuttle begins a series of long, graceful 80 degree banks called "S turns," named for what the vehicle's path resembles.
20 minutes until touchdown
Atlantis is now in the most dangerous part of its descent. The plasma shroud around the vehicle is intense, and would be strong enough to black out radio contact with the ground on traditional space capsules. For this reason, the shuttle has a small, open window near its tail through which it can send signals backwards into space, which are captured by the Tracking and Data Relay Satellite System and relayed back to Earth.
4 minutes until touchdown
The shuttle crosses the Florida panhandle and emits two, thunderous sonic booms that reverberate throughout the region, signaling its arrival. (Why two? Read the answer here.) Commander Chris Ferguson takes the helm and aligns the shuttle into a descent path more than 19 degrees below horizontal -- seven times steeper than a commercial airliner.
15-30 seconds from touchdown
Atlantis is on final approach, losing altitude over the Florida swampland at 20 times the rate of a standard passenger jet. At last, the shuttle assumes a more graceful landing orientation, lifting up its nose and deploying its landing gear. The vehicle is still traveling at 350 kilometers per hour (218 miles per hour) when the rear wheels make contact with the runway.
Shortly after its rear wheels touch the landing strip, Atlantis' parachute is deployed. The nose gear hovers gently off the runway for a few seconds longer, and eventually eases onto the ground. Wheel braking is progressively applied, the parachute is cut loose, and the vehicle finally rolls to a stop. The final confirmation that Atlantis has stopped will come from Commander Chris Ferguson over the radio: "wheels stopped!"
A massive contingent of service trucks and technicians approach the darkened shuttle. A flight surgeon boards Atlantis and performs preliminary medical checks on the four crew members. Meanwhile, toxic chemicals are being drained and purged from the orbiter.
Take a good look at Atlantis as it sits silently in the pre-dawn Cape Canaveral light -- it's the last time you'll see it genuinely intact. Soon, it will be towed back to the Orbiter Processing Facility to begin its decommissioning process. When Atlantis, Discovery, and Endeavour finally arrive at their new museum homes, they will look like the real thing, but large portions of the vehicles will be nothing more than hollow replicas. A chilling reminder that the shuttle program is over was seen last week, when Discovery left the confines of its facility to move over to the Vehicle Assembly Building, making room for the inbound Atlantis. The proud shuttle was seen missing engines, windows, and part of its nose.