A Rate Sensor Unit which contains two gyros.

Inside a gyro a wheel is spinning more than 19 000 times per minute and there are electronics to detect the very small deflections of the wheel's axis.

The gyros are the most accurate in the world and can keep Hubble pointing at the same target for long periods at a time.

Unfortunately some of Hubble's gyros have been hit by problems. Normally this is not a cause for concern (there are three spare gyros), but if four out of six gyros fail, Hubble can no longer observe. This happened a few weeks before Servicing Mission 3A (during which all six gyros were replaced). The source of the gyro problems is thought to be corrosion of some very thin wires inside the gyros.

	Solar Panels
	Electrical Systems