Mission Control is currently targeting an 5:53 p.m. PDT landing of Space Shuttle Discovery at Edwards Air Force Base here in California. In the last approximately 15 minutes prior to the landing there should be the Shuttle’s signature double sonic boom.
The rapid fire, double sonic boom is particularly noticeable because of the size of the shuttle. The first one is the nose, and the second one, less than 1/2 a second later, is caused by the tail (some 122 feet behind the nose of the shuttle).
More on sonic boom from NASA at:
And, here’s a rare picture of the Shuttle’s sonic boom on take off: