Mysteries of the sun: Solar Probe aims to solve them

LAUREL, MD.

So how do you send a spacecraft to the sun without broiling it to a cinder?

"We go at night," said Andrew Dantzler, project manager for Solar Probe, a new space science mission announced this month by Johns Hopkins University's Applied Physics Lab (APL).

That's been the running joke around the lab, Dantzler said. But APL scientists and engineers are perfectly serious about the $750 million NASA-funded mission -- the first to fly scientific instruments through the sun's million-degree corona and to within 4.1 million miles of its roiling surface.

Launch is planned for 2015, with the craft's first solar flyby just three months later -- thanks to a boost from the sun's gravity.

If it succeeds, Solar Probe will fulfill a dream that solar scientists have struggled with for a half-century, and could solve critical mysteries about how the sun works and how that affects the Earth and everything on it.

"Fifty years ago it was brought up as one of the most important things you could do in heliophysics," Dantzler said. Understanding how the sun affects the Earth without understanding the sun's corona is like trying to predict hurricanes without factoring in the ocean. "A hurricane model without the ocean is almost useless," he said.

Solar physicists want this mission to help them answer two of their most vexing questions:

• How temperatures in the corona -- the sun's outermost atmosphere -- can reach more than a million degrees centigrade while those down on the surface are only about 6,000 degrees.

• How the solar wind, which is not very strong near the sun's surface, is accelerated to several million miles per hour as it moves out of the corona and across the solar system.

"The only way we can try to answer this is to send a spacecraft into the environment which will actually measure those particles and nail it," said Madhulika Guhathakurta, program scientist for NASA's Living With a Star program.

We need to know, she said, "because we live in the outer atmosphere of the sun, and this environment couples with the Earth's magnetosphere and ionosphere to create space weather, and that is becoming very important."

Solar storms unleash radiation, electrical and magnetic energy on and around the Earth that can knock out satellites, threaten humans in space or flying polar air routes, and affect utility grids on the ground.

Solar Probe's instruments will measure high-energy protons and electrons in the sun's atmosphere, as well as the strength and direction of its magnetic field, and the amount of dust swirling around it.

Three decades of earlier studies reached the same basic conclusion. Sending a probe into the solar corona -- that shimmering halo of glowing gas that becomes visible around the sun during total eclipses -- was technically too difficult, too expensive, or both.

In 2005, APL tried again, but its proposal to NASA contained two deal-killers.

It would have required a "radioisotope thermoelectric generator," RTG, to supply electrical power. An RTG's energy comes from heat produced by a decaying bit of plutonium 238. On past missions, that design has drawn protests from people who feared a launch accident.

The mission also carried a prohibitive price tag of $1.2 billion.

Worse, from a scientific standpoint, it would have given scientists just one or two swings through the corona. "That's really putting all your eggs in one basket, and that's a lot of eggs," Dantzler said. "So we took another look at the study."

First, APL ditched the plutonium-powered generator. Should a mission to the sun not use solar energy for electrical power?

The answer isn't simple. In fact, "It's counterintuitive," Dantzler said.

It turns out that to reach the sun, the original mission would swing by Jupiter first -- too far from the sun for solar panels to do the job.

That route seems even more counterintuitive. What could be easier than flying -- falling, really -- into the sun's gravitational pull?

"It's not that easy," Dantzler said.

In order to get close to the sun and fly over its poles, where the solar physicists want to be, the craft would need an enormous amount of energy, he said. First it would have to shed the momentum it inherited at launch from the Earth's speed around the sun. It would need more to climb above the plane of the Earth's orbit, and up, over the sun's pole.

The best place to get all that energy free of charge is to swing by Jupiter and gather momentum from its gravitational pull. But with the plutonium generator and solar panels both ruled out, APL decided to abandon its plan for a polar solar orbit.

Instead, "we would go around the equator," Dantzler said.

That had its own benefits -- giving scientists more opportunities -- 24 in all -- to fly through the corona at speeds up to 125 miles per second.

And by flying "downhill" -- almost straight down the sun's gravity "well" -- the spacecraft makes its first approach to the sun just three months after launch. That change also shortened the mission from 10 years to seven, further cutting total costs.

After rounding the sun, Solar Probe will soar back out as far as Venus's orbit, and use seven such Venus flybys to shrink its orbit and edge closer to the sun.

But buzzing the sun repeatedly demands a tough spacecraft. Solar Probe -- 9 feet long and 1,000 pounds -- will need to shield its instruments and communications gear from temperatures up to 2,600 degrees Fahrenheit.

APL's designers are working on a 6-inch-thick solar shield made of advanced carbon-carbon foam. It must always be pointed toward the sun, keeping the craft's camera and sensors at room temperature.