It's a scenario NASA hopes it never has to try out.
If Discovery is badly damaged during today's launch, the astronauts could take refuge in the international space station.
And NASA would try to fly the unmanned space shuttle home, landing it with a new remote-controlled cable system.
NASA would only take such action if the Discovery were so damaged that it couldn't be repaired while docked at the space station, and if it were unsafe for the astronauts to return in it. The astronauts could stay as long as 81 days at the space station until a rescue vehicle arrived.
"If we got into that scenario, we would really have to hustle," John Shannon, deputy manager of the space shuttle program, said Thursday.
In that case, space shuttle Atlantis would be launched to bring back Discovery's crew members. Discovery would have to be undocked from the orbiting space lab to make room for Atlantis.