The Hubble Space Telescope just barely fits into the cargo bay of the space shuttle. It is about the size of a large school bus, and its cameras and spectrographs are about the size of phone booths. Its total mass is about thirteen tons. The primary mirror of HST is 94 inches (2.4 meters) across—that is relatively small compared to modern ground-based telescopes, but very large for a telescope that has been launched into space. While HST is in operation, it has two large solar panels that generate electricity to power the telescope’s many systems.