The National Aeronautics and Space Administration said it is considering a plan to modify space shuttles so they can remain in orbit for as long as 16 days. The longest shuttle mission to date has been 10 days. The plan was outlined in a report requested by Congress that examines key aspects of an extended-duration shuttle. NASA said in a statement that if a decision were made to proceed, a modification kit to provide extra fuel, oxygen and other supplies could be ready in about 45 months. It estimated the development cost at $126 million.