“It feels like the surface of the Sun!” That’s a common way of expressing that something is very hot … but not the hottest part of our solar system. As you might expect, the center of the Sun is hotter, at 27 million degrees Fahrenheit. Move from the center out to the surface of the Sun and temperatures drop to a relatively balmy 10,000 degrees. But now for the surprise: keep moving outward into the Sun’s atmosphere, called the corona, and they don’t keep dropping. In fact, about 1,000 miles above the surface, temperatures spike up as high as 2 million degrees Fahrenheit.
Ever since scientists began studying coronal temperatures back in the mid-1800’s, they have been puzzled by this phenomenon, which they call the coronal heating problem. They’ve come up with several theories, but the only way to know for sure is to go to the Sun and investigate — and that’s about to happen.
NASA’s Parker Solar Probe is tentatively scheduled to launch this Saturday, August 11. This unmanned probe will journey to within 3.9 million miles of the Sun, far closer than any spacecraft has gone before. (For reference, the planet Mercury orbits the Sun at a distance of about 38 million miles.)
How can Parker do that without melting? First, it benefits from the difference between temperature and heat. Temperature measures how fast particles are moving, while heat measures how much energy they transfer. The Sun’s corona has very low density … so even though the particles there are moving very fast (high temperature), there are relatively few of them to strike the probe and transfer energy to it (heat). To understand the difference between temperature and heat, think about a hot oven versus a pot of boiling water. (Please — just think about this, don’t try it!) If you heat an oven to 212 degrees, you could hold your hand in there for a short time. But plunge that hand into the 212 degree water? No way. That’s because the air in the oven is far less dense than the water in the pot. More particles in the water mean more heat energy transferred into your hand.
The actual heat that Parker will encounter in its journey is estimated at 2,500 degrees — still hotter than molten lava. Its high-tech 4.5 inch-thick carbon composite heat shield should keep the temperatures inside the probe a comfortable 85 degrees. The video below shows a demonstration of the shield material.
Parker will spend about 7 years orbiting the sun and reporting back, so we can all look forward to a better understanding of our neighborhood star in the next decade.