Hubble has a total of six gyroscopes grouped in pairs inside three Rate Sensor Units (RSUs). They are arranged in such a way that any three gyroscopes can keep Hubble operating with full accuracy. At the end of Servicing Mission 2, all six gyroscopes were working normally. In the fall of 2005, Hubble began operating in 2-gyro mode. According to senior Hubble scientist David Leckrone at Goddard, "Hubble science on two gyros is indistinguishable from the superb science we have become accustomed to over the years."