Hubble was designed to use three of six onboard gyroscopes to meet its very precise pointing requirements, with the other three held as spares. Gyros have limited lifetimes, and three of the six (all installed in late-1999) are currently working. Following a thorough analysis and testing by engineers, it was determined that Hubble could operate productively on two gyros. After the implementation of three new control modes in Hubble's main computer, and major changes to Hubble's planning and scheduling system at the Space Telescope Science Institute, two-gyro operations began in 2005. By operating on two gyros, with the other gyro turned off (until needed), it is expected that Hubble can continue science operations through the end of 2008. With SM4 scheduled for mid-2008, a fresh set of six new gyros is needed to make the most of Hubble's new science instruments and lifetime peak performance through 2013. Each Rate Sensing Unit (RSU) contains two gyroscopes. Astronauts will replace all three RSUs giving Hubble a full compliment of six new gyroscopes.