In an unprecedented exchange, a Soviet cosmonaut may fly aboard a U.S. shuttle and a NASA astronaut may spend up to six months on the Soviet Mir space station, if a superpower summit agreement can be reached, sources said.

The Bush administration and Soviet officials hope to reach an agreement on a historic astronaut-cosmonaut exchange during a proposed summit in Moscow, possibly in May, the sources said Thursday. The summit has not yet been formally scheduled.But NASA officials confirm that some sort of joint activity may be in the offing.

"I do know there is some discussion going on," William Lenoir, NASA's associate administrator for space flight, said at the Kennedy Space Center. "Obviously, we have submitted some things that we think could be done.

"I wouldn't go so far as to call them recommendations, but we've laid out some options that could be accommodated if that's what we ought to do."