Florida ? NASA and its international partners agreed Thursday to a new construction schedule for the space station, with 16 shuttle flights needed to finish the job in the next four years.

Under the revised schedule, U.S. orbiters would carry Japanese and European science modules to the station earlier than planned to ensure they are in place before the shuttles stop flying for good in 2010.

Leaders of the U.S., Russian, European, Canadian and Japanese space agencies said the schedule allowed room for delays. NASA also has the potential to launch two additional flights if all goes well.

For now, NASA is working toward a May launch of shuttle Discovery on a 12-day mission to take supplies and a third crew member to the outpost. But the launch date could slip to July or later if the agency needs extra time to test changes made to the shuttle’s external tank.