Astronauts Joseph Tanner – on the robotic arm – and Gregory Harbaugh replace the Hubble Space Telescope’s Fine Guidance Sensors (FGS) in 1997. The FGS are used to locate and lock onto a target star while science instruments make observations. They can also perform measurements on the positions and motions of stars. Each sensor is more than five feet (1.5 m) wide and three feet (0.9 m) long, and weighs 485 pounds (219 kg). The telescope’s pointing accuracy and stability depend heavily on the Fine Guidance Sensors.