Of course, China doesn't need fancy robot spaceplanes to destroy enemy satellites. In 2007, China became only the second nation (after the United States) to shoot down a satellite when it destroyed one of its own weather satellites using a modified version of the DF-21 ballistic missile. Needless to say, the United States and several other nations condemned the test, saying the debris created by the shot posed a serious risk to other nations' satellites, spacecraft, and space stations. The incident also alarmed U.S. defense officials, who saw this development as evidence that Chinese military planners are preparing to knock out a major U.S. advantage in the event of war: its network of spy, communications, and navigation satellites. This worried some in the U.S. military so much that the Pentagon has begun working on terrestrial and airborne backups to its space systems, and the Air Force has even begun practicing operations without relying on satellites under the theme "a day without space."