The space shuttle Discovery may not be able to get as close to the Russian space station Mir as planned. Shortly after Discovery reached orbit this morning, two of its steering thrusters failed; one of the thrusters is leaking and may compromise the crew's ability to steer the shuttle at close range to the station. The eight-day mission is a rehearsal for a June mission during which the shuttle Atlantis is scheduled to become the first to dock at Mir. At Discovery's controls on this mission: Captain Eileen Collins, the first woman to pilot a U.S. spacecraft. Nineteen-year veteran Russian cosmonaut Vladimir Titov is also aboard. On a Soviet flight in 1983, he survived an explosion shortly after launch by catapaulting to the ground.