Planning Hubble’s Day

he Hubble Space Telescope whirls around Earth at 5 miles per second. If a car could travel that fast, cross-country traveling would be a breeze. A 2,451- mile road trip from Los Angeles to New York would take just 10 minutes.

The telescope is so fast that it completes an Earth orbit in 97 minutes. For half of each orbit, the telescope passes through Earth’s shadow, where temperatures plunge to 250 degrees below zero. When Hubble swings back into sunlight, outside temperatures skyrocket to above boiling. The extreme temperature shift is almost like traveling from Antarctica to the Sahara Desert within minutes.

Hubble is protected from these temperature extremes by an exterior thermal blanket, which allows the telescope to maintain a constant temperature. But a steady temperature isn’t the only requirement the telescope needs to function properly. Hubble’s computers, pointing systems, and imaging instruments need electricity to operate. The telescope receives its power from a pair of rectangular 40-foot-long solar arrays. Each array is an 8-foot-wide blanket of solar cells. The flexible, lightweight arrays collect sunlight and convert it into 2,400 watts of electricity - enough to power two dozen household light bulbs.

Because Hubble is not always in direct communication with ground stations, operators cannot control the telescope every minute, as people do radio-controlled airplanes. Instead, every second of Hubble’s activities in space must be planned in detail. Imagine if you had to plan every minute of every day. Get out of bed, put on a pair of slippers, walk to the bedroom door, walk down the hall to the bathroom, turn on the light, pick up the toothbrush, take the cap off the toothpaste, etc. It would take a whole day to plan a whole day. Planning Hubble’s day is an even longer and more complicated task for experts at the Space Telescope Science Institute. Their detailed instructions for Hubble are converted into a code the spacecraft’s main computer can understand. Several times a day operators at Goddard Space Flight Center radio these instructions to Hubble’s electronic "brain."

The telescope’s pointing systems then find and lock onto distant planets, stars, and galaxies. This task requires the same precision as pointing and holding a laser on a dime 400 miles away.

To make an observation, Hubble must find a pair of bright stars called guide stars near each target planet, star, or galaxy. These bright stars are Hubble’s anchors. Guide stars allow Hubble to maintain its steadiness on a target, just as anchors keep a ship from drifting. To find guide stars for nearly every object in the sky, mission planners use an immense catalog containing the sky addresses of 15 million stars.

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