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Into the Fire

By Carl Zimmer
Dec 1, 1994 6:00 AMNov 12, 2019 4:09 AM


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In 2001, NASA plans to launch a spacecraft that will fly within less than 2 million miles of the sun--and do so without melting.

What we know of the sun we have learned from a safe distance. The Ulysses probe that flew under the south solar pole last June, for instance, did so at a distance of more than 200 million miles. The closest we’ve come to the inferno was 26 million miles, with a 1974 satellite named Helios. As a result there are a lot of things we don’t understand about the sun--why small flares on its surface can produce huge magnetic storms on Earth, for example, or why big flares sometimes barely cause a stir. Ever since Helios NASA researchers have been scheming to send a probe 15 times closer to the sun, and now they finally have a design that may fly. NASA has penciled in a launch date of 2001.

When the NASA group started working on their idea in 1975, a lot of people were skeptical. The chuckle factor was very high, says James Randolph, who has managed the program since its inception. A probe that came within 1.7 million miles of the sun, everyone realized, would get rather warm. Imagine standing outdoors on a summer day under a sky filled with 3,000 suns. That is roughly how the solar probe would feel.

Yet Randolph’s team managed to design a shade that would protect the probe’s sensors. It was to be a cone woven of carbon fibers, pointy as a witch’s hat. While the sunward side of the cone was warming to 4000 degrees Fahrenheit, the steep flanks would radiate heat so efficiently that the sensors would stay at room temperature. In 1982 Randolph and his colleagues proposed a two-ton, $1 billion design--$2 billion in today’s dollars--and got nowhere. NASA wasn’t willing to spend that much on a solar probe.

For the next decade the team tried to bring the price down. But it was only last year that Randolph realized the answer was floating overhead. His inspiration was a new type of antenna on communications satellites. On old-fashioned satellites, the transmitter is centered directly in front of a parabolic dish that reflects and focuses the signal. Lately, though, engineers have figured out how to tilt the dish and move the transmitter off center so that it no longer blocks any of its own reflected signal.

When Randolph heard about the new satellites, it came to him: Why not replace the cone on his probe with a parabolic dish that would serve as both a solar shield and a radio reflector? To deflect sunlight without melting, the dish would have to be tilted at a steep angle toward the sun, just like the flanks of the cone. With the new kind of off-center transmitter, that became possible: the transmitter and all the probe’s sensors could be nestled safely in the cool shadow of the dish without blocking the probe’s radio signal back to Earth. Getting rid of the separate dish would make the probe lighter and free room for other equipment. And since the new radio dish would be four times as large as the old one, the transmitter could be less powerful.

The probe that is now on Randolph’s drawing boards weighs 440 pounds, uses a tenth the power required by the 1982 probe, and costs only $400 million. If all goes according to plan--a big if, of course--the probe will be launched in 2001 on a Delta rocket. It won’t go directly to its destination; to cancel the momentum imparted by Earth’s orbit and fall in toward the sun it will have to loop around Jupiter first. After that the sun will be in total control, says Randolph.

The flyby will take place in 2005. Within 5 million miles of the sun, the probe will enter exotic territory, where the solar wind of charged particles is mysteriously accelerated to 450 miles a second. It may pass by glowing dust rings from exploded comets; it may dive through arches of magnetic flux millions of miles high. It will never look straight at the sun, though, because its parabolic shield will be in the way. It will be looking up from the sun’s surface, watching streamers of plasma rise overhead and form structures you couldn’t imagine in your wildest dreams, says Randolph.

Streaking through space at 675,000 miles per hour--the fastest speed ever reached by a spacecraft--the probe will take just 14 hours to go from the north solar pole to the south. Then it will either hurtle away into deep space, its batteries spent and its sensors useless, or it will expire more dramatically. We may run through a solar flare, which is why we’ll always keep a communication link open, says Randolph. That last millisecond of data before we die will be a delight.

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