Spacecraft Systems

As mentioned above, the HST is also a spacecraft. It must have power, communicate with the ground and be able to change its attitude (orientation). Let's take a look at these systems.

Power
All of the instruments and computers on board the HST need electrical power. This electrical power is supplied by two large solar panels, each panel measuring 40 feet (12.2 m). The solar panels provide 2,400 watts of electricity, which is equal to the electricity used by sixty 40-watt light bulbs. When the HST is in the Earth's shadow, electrical power is provided by six nickel-hydrogen batteries, which provide the same storage as 20 car batteries. The batteries are re-charged by the solar panels when the HST comes around to sunlight again.

Communications
The HST must be able to talk with controllers on the ground to relay data from its observations and receive commands for its next targets. To communicate, the HST uses a series of relay satellites called the Tracking and Data Relay Satellite System (TDRSS), which is the same system used by the International Space Station.

Photo courtesy NASA / STScI Communications system used by the Hubble Space Telescope.

Incoming light from an object gets received by the HST (2) and converted to digital data. The data is then sent to the TDRSS in orbit (3), which then transmits it to the Ground Receiving Station at White Sands, N.M. (4). The White Sands Facility transmits the data to NASA's Goddard Spaceflight Control Center (5), where HST operations are centered. The data are then analyzed by scientists at the nearby Space Telescope Science Institute in Baltimore, MD. (6). Most of the time, commands are relayed to the HST in advance of a planned observing run; however, real-time commands are possible when necessary.

Computing
The HST has two main computers that fit around the telescope's tube above the scientific instrument bays. One computer talks to the ground to transmit data and receive commands. The other computer is responsible for steering the HST, as well as various housekeeping functions. There are also backup computers in the event of an emergency.

Each instrument on board the HST also has microprocessors built in to move filter wheels, control the shutters, collect data and talk to the main computers.

Structure
The HST has a skeleton to hold the optics, instruments and spacecraft systems in place. To hold the optics, the HST has a truss system, which is made of graphite epoxy resin like tennis racquets and golf clubs. The truss is 17.5 ft (5.3 m) long, 9.6 ft (2.9 m) wide and weighs 252 lbs (114 kg). The tube that holds the optics and scientific instruments is made of aluminum surrounded by many layers of insulation. The insulation shields the telescope from extreme changes in temperature between sunlight and shadow.