ll telescopes have optical systems, and some even have specialized instruments. Hubble has additional requirements because it operates in space — the telescope is actually "flown" as a spacecraft. Therefore, several spacecraft support systems are in place to keep Hubble functioning smoothly in space. These systems girdle the body of the telescope.

COMMUNICATIONS ANTENNAE

Much like a robot or computer in space, Hubble performs only in response to detailed instructions from the people on the ground. Hubble has communications antennae so that astronomers and technicians can communicate with the telescope — telling it what to do and when to do it. Four antennae send and receive information between the telescope and the Flight Operations Team at the Space Telescope Science Institute.

Scientists communicate with the telescope via the Tracking and Data Relay Satellite (TDRS) system. There are currently five TDRS satellites located at various locations in the sky.

In order for this system to work, at least one of the five satellites must be visible from the spacecraft's line of sight. Scientists can interact directly with the telescope during times of satellite visibility, allowing them to make small changes in the spacecraft pointing to fine-tune their observations.

Satellite visibility does not affect a planned observation because the commanding is done well in advance. When none of the satellites are visible from the spacecraft, a special data recorder stores the observation. The data are stored and then transmitted during periods of satellite visibility.

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SOLAR ARRAYS FOR POWER

Hubble needs electricity to operate. Since it can't be plugged in to a ground-based power source, it runs on sunlight, making it the ultimate cordless power tool. Flanking the telescope's tube are two thin, blue solar arrays. Each wing-like array has a solar cell "blanket" that converts the Sun's energy into 2,800 watts of electricity. The solar arrays convert sunlight directly into electricity to run the telescope's scientific instruments, computers, and radio transmitters.

Some of the energy generated is stored in onboard batteries so the telescope can operate while it's in Earth's shadow (which is about 36 minutes out of each 97-minute orbit). Fully charged, each battery contains enough energy to sustain the telescope in normal science operations mode for 7.5 hours, or five orbits.

The solar arrays are designed for replacement by visiting astronauts. They can be folded for shuttle trips to and from Hubble.

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HUBBLE'S COMPUTERS AND AUTOMATION

In order to run all the many subsystems onboard the Hubble, several computers and microprocessors reside in the Hubble body and in each science instrument. Two main computers, which girdle Hubble's "waist," direct the show. One talks to the instruments, receives their data and telemetry, sends the data off to interface units for transmission to the ground, and sends commands and timing information to the instruments. The other main computer handles the gyroscopes, the pointing control subsystem, and other system-wide functions. Special backup computers keep Hubble safe in the event of a problem.

Each instrument itself also houses small computers and microprocessors, which direct their activities. These computers direct the rotation of filter wheels, open and close exposure shutters, maintain the temperature of the instruments, collect data, and talk to the main computers.

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HOUSING
HUBBLE'S FLESH AND BONES

Designers of the Hubble Space Telescope had to take into account the conditions in which it was to operate. Hubble would be subject to the rigors of zero gravity and temperature extremes — fluctuations of more than 100 degrees Fahrenheit during each trip around Earth.

To accommodate this less-than-hospitable operating environment, Hubble was given a "skin," or blanket, of multilayered insulation (MLI), which protects the telescope from temperature extremes. Beneath the MLI is a lightweight aluminum shell, which provides an external structure to the spacecraft and houses its optical system and science instruments.

Hubble's optical system is held together by a truss (supporting "skeleton") measuring 210 in (5.3 m) in length and 115 in (2.9 m) in diameter. The 252 lb (114 kg) truss is made of graphite epoxy — the same material used in many golf clubs, tennis racquets, and bicycles. Graphite epoxy is a stiff, strong, and lightweight material that resists expanding and contracting in extremes of temperature.

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