Solar probe may swoop toward the sun in 2015, with help from NASA Glenn

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on August 15, 2008 at 8:24 PM, updated August 15, 2008 at 8:27 PM

Geoff Landis, a physicist at the NASA Glenn Research Center, talks about the Solar Probe Plus.

NASA story on the probe.

The lesson of Icarus' melted wings hasn't been lost on NASA.

For half a century, scientists have longed to catapult a probe near the sun to pry loose its most tightly held secrets -- the source of its fierce atmospheric heat and its mighty solar winds.

But the hazards of swooping in close to the star, not to mention the staggering cost of such a mission, have kept the space agency away. The nearest pass was Helios 1's 29 million-mile flyby in 1975. Though scientists have learned a lot about the sun, they've had to do it by long-distance sleuthing.

Until now.

With a major assist from Cleveland's NASA Glenn Research Center, researchers believe they've overcome the technical challenges of a solar close encounter. They're planning a small, hardy spacecraft protected by the world's most impenetrable umbrella, and aiming for a 2015 launch.

If all goes well, Solar Probe Plus will slingshot around Venus and dip a scant 3.7 million miles above the sun's roiling surface.

"It's the hottest, coolest mission I've ever worked on," said project manager Andy Dantzler of Johns Hopkins University's Applied Physics Laboratory, which is designing and building the satellite for NASA.

Solar research has come a long way since 1862, when the esteemed British physicist Lord Kelvin pronounced that the sun probably had formed less than 100 million years earlier from a collision of meteors and that it was a slowly cooling glob of liquid.

Observations using Earth-based telescopes and several generations of satellites have cleared up those early misconceptions, enabling scientists to compile a surprisingly detailed dossier on our nearest star.

They've figured out its age (4.6 billion years), mass (more than 320,000 Earths), composition (mostly hydrogen and helium) and core temperature (about 28 million degrees Farenheit).

They know that it formed when gravity compacted clouds of hydrogen atoms and that the squeezing together of those atoms in a process called nuclear fusion unleashes the energy that lights our sky and makes life on Earth possible.

Probes also have revealed the writhing magnetic field that drives the sun's behavior. Magnetic lines of force loop and twist across the solar surface like a cowboy's lariat. They dictate the location of sunspots, which wax and wane in an 11-year cycle. The lines whipsaw into the sun's atmosphere, sparking violent outbursts of energy.

Researchers have even learned enough to confidently predict the sun's demise. In about 5 billion years, when it has exhausted the last of its hydrogen fuel and begins desperately consuming its remaining elements, the sun will swell into a "red giant" 250 times its present girth. Its expansion will parboil Earth. Eventually, the convulsing, dying star will shed its outer skin and cool into a cinder called a white dwarf.

For all this remotely gained knowledge, though, scientists still can't explain two fundamental mysteries about the sun:

• Why its upper atmosphere, or corona, at nearly 2 million degrees, is hundreds of times hotter than the surface below.

• What propels the solar winds, the streams of high-energy particles originating in the corona that constantly billow out at a million miles an hour to bathe Earth and the other planets.

Researchers believe the sun's restless magnetic field is a culprit in both phenomena, but they lack physical evidence. A better understanding of the dynamics might eventually lead to improved predictions of the solar flare-ups that threaten communication and power grids on Earth, and astronauts and satellites in orbit.

Right now, "we're not very good at space weather forecasting," said Geoff Landis, a NASA Glenn physicist on the Solar Probe Plus planning team.

Shooting a probe studded with sensors through the solar corona has been a high scientific priority for decades, but the technology hasn't been there until recently.

In 2005, Hopkins' Advanced Physics Lab, or APL, working with an independent team of experts, proposed an ambitious solar flyby mission with the unambitious name of Solar Probe.

The spacecraft, its instruments powered by a nugget of plutonium, would head out to Jupiter and use the massive planet's gravity to slingshot it back toward the sun. The momentum would allow two close-in passes, less than 2 million miles from the star, with five years in between.

NASA liked the plan, with a few minor caveats: cut the $1.1 billion cost by at least one-third; scrap the scarce, costly plutonium power source; and trim the time between solar passes.

"It was tough criteria. We weren't even sure it wasn't impossible criteria," Landis said.

Added APL's Dantzler: "It was sort of like saying, 'Hey, you've designed the perfect car, but can you do it without the engine?'"

APL went back to the drawing board. The planners focused on the Jupiter slingshot maneuver. Its huge gravity boost enabled the spacecraft to make two close passes by the sun, but going out to the solar system's far suburbs and back twice added years to the mission. It also required nuclear power rather than solar, since at Jupiter's half-a-billion-mile distance, the sun is just a speck in the darkness of space.

But what if the spacecraft cracked the whip around Venus instead of using Jupiter?

The smaller planet wouldn't provide as powerful a push, meaning the probe's solar flybys couldn't be as close. However, Venus was on the way to the sun, not in the opposite direction. That would shorten the round-trip time, allowing many more data-gathering dips into the sun's corona -- 24, compared with the original two. And the spacecraft could harness the sun as a power source.

For that, APL sought the Glenn center's expertise in space power generation. Landis and several colleagues formed a team to study the options.

Solar panels were an obvious candidate. They were lightweight and had no moving parts -- big advantages for a long-distance, long-duration mission. But without special precautions, a close solar flyby might fry the panels.

The Glenn team's solution? Use two sets of arrays: a big one when the spacecraft was far from the sun, and a smaller, liquid-cooled one for the close passes.

The big panels would fold up when the spacecraft got within 23 million miles of the sun, hiding under a protective 6-inch-thick carbon-composite umbrella that keeps most of the probe in shadow. A much smaller array would take over, poking just far enough out from the shade to gather light, and gradually retracting as the probe got closer to the sun. Glenn researchers are testing the solar cells now to see how much heat they can take without damage.

With all the changes, the Solar Probe Plus planners were able to shave more than a quarter-billion dollars off the mission's price tag and cut the time between solar flybys from nearly five years to no more than 150 days.

NASA tentatively plans to launch in May 2015. That would allow the probe to sample the corona and solar winds as the sun's cycle intensifies over the next seven years, putting it in the eye of the storm.