Artist's concept of HESSI spacecraft in orbit, spinning around an axis pointing at the sun. Hi-res version. NASA picture

Once in orbit, the satellite comes under UC Berkeley's control, with commands uplinked and data downlinked through a 36-foot (11 meters) radio dish perched in the wooded hills above UC Berkeley. From mission control in the nearby Space Sciences Laboratory, HESSI mission operators will monitor the automatic pointing of the satellite toward the sun, deployment of the four solar panels, and the spin-up of the satellite to about 15 revolutions per minute.

Once power is being supplied from the solar panels to onboard batteries, the operators will start the onboard cooler to get the germanium detectors down to 75 Kelvin (324 degrees below zero Fahrenheit or 198 degrees below zero Celsius). This will take several days, but is necessary before any flare observations can be made. The nine germanium detectors represent the largest and most advanced array of germanium detectors ever flown in space.

Because hard X-rays and gamma rays cannot be focused like visible light — gamma rays can pass right through the spacecraft — HESSI uses a novel technique to produce pictures of flares. Pairs of grids, each etched from dense materials that block gamma rays and hard X-rays, are aligned to let radiation from only a small portion of the sun pass through to the detector. As the satellite spins along an axis pointing at the sun, nine of these grid pairs create a moving light show on the germanium detectors.

The information is sent back to Earth, where computers translate the data into pictures of the flare every second or so. It is also possible to obtain the energy spectrum of the hard X-rays and gamma-rays at each location in the pictures.

Information is stored on the spacecraft and sent to the ground whenever the satellite passes over Berkeley, about every 90 minutes for about a third of the day. The HESSI team has enlisted the help of radio antennas in Europe and the eastern United States to download data during very active periods when several large flares may fill the onboard memory before it can be emptied during the Berkeley passes. The team hopes to detect upwards of a hundred gamma-ray flares, more than a thousand X-ray flares, and perhaps 10,000 microflares during the two- to three-year duration of the mission.

Lin and his scientific team will compare the X-ray and gamma-ray images from HESSI with pictures from other solar observing satellites at different wavelengths to probe the mechanism of flare formation. The data will be made freely available online within hours of receipt at the ground station.

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The Berkeley Ground Station in the hills above the University of California campus. UC Berkeley photo