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"What if" is not a question most people would want to ask themselves the day before a mission as complicated and risky as a shuttle launch, but for NASA safety officers, it's their job. And this time, they have a real answer. Once in orbit, if Discovery is deemed unsafe for manned re-entry, shuttle managers have the option to initiate an audacious plan for rescuing the $2 billion craft: Fly her home by remote control.

Three years go, when mission control first questioned the integrity of Columbia's thermal protection system (TPS) after a block of foam damaged her left wing, they didn't have much of a choice. Columbia's orbital inclination made the International Space Station (ISS)—the only safe haven for a marooned Space Shuttle crew—unreachable. Ground controllers could either strand the shuttle and her crew in orbit, or, as they did, attempt a risky—and, ultimately, tragic—re-entry. It's a decision NASA does not want to face again. As a result, since Columbia, the Space Shuttle's missions have been restricted to trips to the ISS, greatly increasing options for repair or rescue.

During the first four days in orbit, astronauts will perform a thorough inspection of Discovery to determine the orbiter's condition. That will include a nine-minute long, 360-degree backflip, 600 feet below the ISS, so that station crew can inspect the orbiter's TPS. The crew will also perform space walks to evaluate the shuttle (and, if necessary, perform rudimentary repairs) as ground controllers analyze imagery of the vehicle's approach to the International Space Station to determine:

Is Discovery safe for manned re-entry?

YES: The mission continues as planned, with additional safety checks on flight days 10 and 11.

The astronauts will dock their vehicle at the ISS, using the station to provide extra power and oxygen, while the crew remains on the orbiter. Discovery has enough supplies for the crew to sit for up to 81 days in orbit while connected to the ISS. If necessary, the crew can transfer to the ISS, although most of the crew lacks station-specific training. In the meantime, ground control determines:

Can Space Shuttle Atlantis launch a rescue mission?

YES: After 51 days of preparation, Atlantis and her crew of four launches on a trajectory to rendezvous with the ISS, and rescue the stranded astronauts. In order for Discovery's crew to safely board Atlantis, Discovery will have to be undocked from the ISS and "parked" in orbit, where it can remain on station for up to 17 days.

If the same malfunction that prevents Discovery from returning to Earth prevents Atlantis from launching into space, ground controllers will likely turn to talks with Russia to organize the transport of the astronauts back home aboard Soyuz, although that "is beyond what we normally plan for" and no official agreement is in place, according to NASA mission management team chairman John Shannon. Once the crew is rescued, there's one big question remaining:

Although the risk is too high to carry humans, can Discovery survive re-entry unmanned?

YES: Hook up the remote control. Although most of the re-entry process is automated anyway, the shuttle still requires some manual input from the commander and pilot. NASA has developed a cable that hooks into Space Shuttle cockpit controls, and allows Ground Control in Houston to perform these tasks remotely.

The 28-foot-long braided cable is composed of approximately 800 feet of wiring, and will be stowed in a bag on Discovery. The crew will perform an in-flight maintenance procedure to connect the octopus-like branches of the cable into various systems in the interior of the shuttle—there is no spacewalk required for the procedure. The cable gives Houston control systems such as landing gear, drag chutes and auxiliary power units, as well as down-linking information from air probes that collect real-time atmospheric data during descent.

NASA spokesman Kyle Herring says that it's more than likely that the cable will never be used, but shuttle managers wanted the option. "Choosing to use this would be a fairly major decision for the mission management team of the shuttle program," Herring tells PM, adding that the scenarios under which the space agency would even attempt a remote-controlled landing would be extremely limited.

The RC cable technology was developed in approximately 10 months, and has been fully certified and tested in the Shuttle Avionics Integration Laboratory (SAIL) at Johnson Space Center. If the cable is used, a landing site will be chosen that's both remote, and doesn't require the Discovery to fly over densely populated areas. Two possible landing locations are NASA's facility in White Sands, NM, and Vandenberg AFB, CA.

NO: Place the unmanned, $2 billion shuttle on a re-entry trajectory that terminates in the open ocean—NASA says there is no way they're going to leave a dead shuttle in orbit. —Erin Scottberg

NEXT: DISCOVERY LAUNCH DAY LIVE BLOGEarlier: NASA Says Shuttle Risk Overstated; Yet Some Risk Unavoidable, Images from Discovery's Launch to Aid Future Redesigns, The Clock is Running: Discovery Set for Saturday Launch, NASA's "Go" for Launch, But Is the Space Shuttle?, The Great Space Shuttle Debate: Is it Safe? A Special Report.

Roll Control: Space Shuttle Discovery will perform a backflip on day three of the mission, allowing the ISS crew to photograph her underside and inspect for damage. Discovery performed a similar maneuver last year (pictured). NASA

Star Rotation: Discovery in mid-flip. NASA

360: Discovery continues her backflip. NASA

Shield Survey: A closeup of Discovery's thermal protection system, photographed from the ISS. NASA

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