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Finally, a device that can touch the sun

By Carolyn Y. Johnson
Globe Staff / November 28, 2011
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CAMBRIDGE - Astrophysicist Justin Kasper’s hand can easily hold the shallow cup - a solid, heavy hunk of gray metal with a grand purpose: to touch the sun.

The cup, being built at an office park near the Alewife T station in Cambridge, is a prototype of the instrument NASA plans to launch in 2018 aboard the Solar Probe Plus, the first mission intended to provide an extreme close-up of our sun. The cup will scoop the superheated, turbulent solar atmosphere, measuring the direction, density, charge, and speed of the particles flowing away from the sun.

Some of the biggest remaining mysteries in the solar system regard the sun. Though astronauts have walked on the moon, rovers have traversed Mars, and spacecraft are now nearing the brink of our solar system, the inferno at the center has simply been too darn hot to visit.

Or, as Kasper puts it, “the equipment will react negatively - by melting.’’

Kasper, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics who leads a team building the solar probe cup and another instrument that will ride on the solar mission, said sending a mission to the sun has been discussed since the 1950s.

“It’s just the technology has had to catch up with the desire,’’ he said.

Now, advances in heat-shielding materials and an innovative use of the planet Venus’s gravity to slow the spacecraft down and get it to the sun more quickly have enabled a mission that can withstand repeated encounters with the sun, coming closer with each orbit.

Among the mysteries scientists would like to solve are why the sun’s atmosphere is many times hotter than its surface. How exactly does the solar wind of hot, charged particles flowing from the sun accelerate to faster than the speed of sound and then blow outward, forming a vast protective bubble that surrounds the solar system? Is it possible to predict and understand the activity on the sun, including solar flares and eruptions that can knock out communications satellites, endanger astronauts, and even wreak havoc on power grids on the earth?

Such questions can be answered only with an up-close view. Trying to make observations of some properties of the sun from afar is something Marco Velli, a senior research scientist at NASA’s Jet Propulsion Laboratory, likens to “looking through fog - you can’t tell what’s coming from where.’’

Over the decades, the ideas for a solar probe mission have evolved. One early version included an enormous heat shield that would burn up faster than it would conduct heat, Kasper said. But the most recent plans, reviewed by NASA last month, include a new heat shield that uses a carbon foam and liquid-cooled radiator to shield four sets of instruments built by different teams. Two instruments will peek out from behind the main heat shield - including the small cup being designed in Cambridge.

The questions the spacecraft will answer date back at least a century, when it first became apparent to scientists that something weird was happening at the sun. One observation led scientists to think that a previously unknown form of matter had been discovered - one a thousand times lighter than hydrogen, called “coronium.’’

Eventually it became clear that the surface was about 10,000 degrees, but the corona, the crown of heat and light that is visible during an eclipse, was hundreds of times hotter.

The idea of a supersonic solar wind made up of hot charged particles was put forward in the late 1950s by Eugene Parker, now an 84-year-old emeritus professor of physics at the University of Chicago.

Parker pays close attention to the new work unfolding. Solar Probe Plus, he said, will answer important basic questions about the sun, including practical problems such as understanding solar activity that can endanger astronauts and communications satellites. But perhaps more exciting, he said, it is an opportunity to visit and understand a star.

“The sun controls space all the way out through the solar system with the solar wind . . . and what’s more, it’s the only star in the sky where you can measure that kind of activity,’’ Parker said.

To do that, teams are working on an array of instruments. In Cambridge, in a caged-in work area protected from static that could harm the electronics, Kasper and his team are working on the cup design and electronics that will measure the sun’s wind.

To ensure that the equipment can withstand the extreme conditions, they are testing equipment at a “solar furnace’’ in France that focuses sunlight onto a chamber, heating it up to 5,000 degrees - twice as hot as the researchers anticipate needing. They are also working with a team at NASA’s Marshall Space Flight Center to recreate in a particle accelerator the conditions the probe will encounter.

Once the instruments are built, they will be shipped to Johns Hopkins University Applied Physics Laboratory, the primary contractor for the mission, to be mounted on the spacecraft. After launch in 2018, the spacecraft will swing around Venus, using the gravity of the planet to slow it down and head toward the sun, passing closer and closer to the sun with each orbit, until it gets within 3.7 million miles of the surface - well into the solar corona and nearly seven times closer than any previous mission.

“This is really a mission of discovery. We don’t know how the sun generates the solar wind’’ and the protective bubble that surrounds the solar system, Velli said. “We just don’t know how, and there’s stuff that could be happening we have no clue about.’’

Carolyn Y. Johnson can be reached at cjohnson@globe.com. Follow her on Twitter @carolynyjohnson.

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