Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A Space Shuttle abort is an emergency procedure due to equipment failure on NASA''s Space Shuttle, most commonly during ascent. A main engine failure is a typical abort scenario. There are fewer abort options during reentry and descent. For example, the Columbia disaster happened during reentry, and there were no alternatives in that portion of flight. Later in descent certain failures are survivable, although not usually classified as an abort. For example, a flight control system problem or multiple auxiliary power unit failure could make reaching a landing site impossible, thus requiring astronauts to bail out.