The Solar Probe Plus: A Mission to Solve the Mystery of the Sun's Corona

Why is our star's atmosphere thousands of times hotter than its surface?

Here is something strange about the sun that you have probably never considered, as I heard it from Justin Kasper, an astrophysicist who studies our star at the Harvard-Smithsonian Center for Astrophysics.

The sun is powered by fusion reactions at its core in which hydrogen atoms fuse together into helium, releasing tremendous energy in the process. That energy moves out from the core to the surface of the sun, called the photosphere. The temperature there is about 10,000 degrees Fahrenheit.

As you continue to move out from the sun, we cross upwards into the corona, where a very strange thing happens: the temperature jumps to a million degrees. That is to say, the atmosphere of the sun is thousands of times hotter than its surface.

To consider how weird this is, imagine you light your stove and the area farther away from the heat source is actually hotter than the regions closer in. That's not normally how heat flows. And yet, on the sun, that is exactly how it works, and scientists just aren't sure why.

Kasper called it "the mystery of the sun's corona" here at the Aspen Ideas Festival in making his pitch for the importance of a mission to fly an instrument into the sun's corona to take direct measurements. They call it the Solar Probe Plus, and he hopes it will launch in 2018. The mission would follow the about-to-launch NASA satellite IRIS, which will be studying the interface between the surface and corona from the safety of earth orbit. 

The Solar Probe Plus will be a much more daring mission, flying closer to the sun's surface than any mission before it. The goal is literally to enter the sun's corona and sample the particles there. It'll have a thick shield of carbon foam that will protect its instruments from the sun's energy.

Astronomers have come a long, long way in studying the corona. The stargazers of yore had to wait for the moon to block out the bulk of the sun's light -- that is to say, solar eclipses -- to catch any glimpse of the less bright corona. Now, astrophysicists will actually be able to sample the corona directly.

Below, you can see NASA's lead scientist for its Living With a Star initiative, Madhulika Guhathakurta, describe the importance of the craft, which will carry five different instruments to conduct different experiments, including Kasper's SWEAP.

This story has been corrected to include the correct spelling of Justin Kasper's last name.