NASA Readies a Satellite to Probe the Sun--Inside and Out

The Solar Dynamics Observatory will provide heliophysicists with a flood of data about the sun


NASA's Solar Dynamics Observatory is what might be called a satellite for the information age. It is designed to provide scientists who study the sun with a torrent of data—the space agency says the observatory will return 150 million bits of data about Earth's host star per second, or about 1.5 terabytes per day.

The spacecraft, known as SDO, is scheduled to launch into orbit at 10:26 A.M. Wednesday from Cape Canaveral Air Force Station in Florida, weather permitting. Its data stream is so broad that the observatory will have a dedicated pair of 18-meter radio dishes near Las Cruces, N.M., at its disposal for downlink; the satellite's geosynchronous orbit will keep it within radio range of New Mexico at all times.

The data SDO gathers should help hone forecasts of solar activity and the space weather it creates, which can wreak havoc on and around Earth with storms of charged particles and radiation. With advance warning about an impending solar storm, astronauts in orbit could seek refuge in shielded compartments, utilities could brace the power grid for disturbances, communications firms and the military could place their satellites in safe mode, and airplanes could change course to avoid the most dangerous altitudes and routes. (Earth's magnetic shielding is weakest at the poles, for instance, so transpolar flights would be at greater risk from solar storms.)

"Our big goal is to learn how to predict what the sun is going to do," says Dean Pesnell, SDO project scientist at NASA Goddard Space Flight Center in Greenbelt, Md. Pilots and astronauts, he says, would like to know solar forecasts hours or even days in advance, whereas the U.S. Air Force would like weeks of notice for mission-planning purposes. "And I'm interested, scientifically, in predicting what's going to happen a year or 10 years from now," Pesnell says.

To that end, SDO carries three scientific instruments, including the Atmospheric Imaging Assembly, a set of four telescopes that return eight images every 10 seconds in a variety of light bands, from the visible to the shorter-wavelength extreme ultraviolet. Those wavelengths reveal the roiling activity, such as explosive solar flares and looping prominences, on and above the sun's surface. Each image packs enough pixels—more than 4,000 a side—to fill 15 high-definition television screens.

A second instrument, the Extreme Ultraviolet Variability Experiment, uses spectrographs to break down the sun's extreme UV light into its component wavelengths with unprecedented resolution. The goal is to track the sun's shifting irradiance in the extreme UV, where high-energy photons pack a dangerous punch, potentially allowing heliophysicists to link variations in the extreme UV to other solar phenomena.

But Pesnell is most excited about the third instrument. "The one I think is the coolest is the Helioseismic and Magnetic Imager," he says. The instrument monitors magnetic flux and takes helioseismology readings, tracking the propagation of sound waves across the sun's surface. "It looks amazingly like ocean waves, if you can imagine looking down on an ocean and seeing waves just going in all directions," Pesnell says. "Well, if you study those waves you can infer what's going on inside the sun."

A recent study led by National Oceanic and Atmospheric Administration research scientist Alysha Reinard showed how helioseismology can be used to track swirling plasma flows in the sun and predict solar flares with unprecedented accuracy. But that forecasting approach is only as good as its data; at present the false alarm rate is above 50 percent. Helioseismology data from by SDO should lead to more reliable flare forecasting, which would be a boon to monitors of space weather.

SDO is the first mission to reach fruition from NASA's Living with a Star program, which seeks to better characterize how solar activity affects life on Earth. The mission will cost an estimated $850 million, including five years of operations, according to NASA. But with nearly 1.5 metric tons of maneuvering fuel onboard, SDO could well operate 10 years or longer.

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