A team led by Anita Sengupta, engineer and member of JPLs advanced propulsion technology group, has kept the Deep Space 1 flight spare ion engine running for 24,750 hours. If it had been an automobile engine instead of an ion engine, and it was driven for 24,750 hours at 80.5 kilometers per hour (50 mph), it would have traveled 1.93 million kilometers (1.2 million miles) without an oil change or tune up. The purpose of the test is to understand how an ion engine wears, and what phenomena contribute to its eventual end of life. With this information, NASA scientists can plan missions better and design improved ion engines.
http://www.spaceref.com/news/viewpr.html?pid=9186
