The collapse of the Soviet Union in 1991 allowed the United States and Russia to enter a new era of space collaboration. NASA made a financial commitment of $400 million to the Russian space program to continue the expeditions to the Mir space station. The space shuttle Atlantis was fitted with a special adaptor to allow it to dock with Mir. There were eleven space shuttle flights to Mir between February 1994 and June 1998. In a new spirit of cooperation, Russian cosmonauts began to fly on the shuttle, while American astronauts began to spend time on Mir conducting experiments with their Russian counterparts. The experiences gained by the U.S. astronauts on Mir were beneficial in planning for the International Space Station. A total of seven U.S. astronauts spent 980 cumulative days aboard Mir.