'Frankenstein' fix lets asteroid mission cheat death - space - 20 November 2009 - New Scientist
The Hayabusa asteroid probe is back on track to Earth after losing three of its four engines. With only one working engine, the craft seemed doomed to remain in the asteroid belt, but Japanese scientists managed to produce the equivalent of one working engine using the three broken ones. Specifically, one of the ion engines could only generate positive ions, but too much of this would have resulted in a dangerous buildup of charge so they used one of the other engines that were still able to generate negative ions to counter the charge buildup. If all goes well, the craft will return to Earth in June 2010.