Part of an old European rocket is expected to pass within two miles of space shuttle Discovery and the international space station late Friday morning.
NASA officials say that should be a safe distance. Flight controllers are confident they won't need to change the course of the linked spacecraft.
The debris is believed to have an area surface of 200 square feet.
Astronauts Danny Olivas and Christer Fuglesang will install a new ammonia tank outside the space station. It will be the second spacewalk of the mission.