Inner Workings: Spacecraft dare to fly close to the sun.

A wide swath of North America was reminded during last summer’s eclipse that you can’t stare at the sun without protection. But to study the sun, space probes have to do much more than stare. They have to get extremely close to the sun’s unforgiving environs. Next year, two solar missions will do exactly that— traveling tens of millions of miles to study the sun’s outer atmosphere, its corona, in hopes of understanding why it’s dramatically hotter than the sun’s surface. Normally, the temperature drops as you move farther from a source of heat. Not so near the sun. Although the sun’s temperature falls from ∼15 million degrees Celsius at its core to ∼6,000 °C at the surface, the corona heats up again to more than one million degrees Celsius, thus creating an aura of ionized gas, or plasma, around the sun. The corona accelerates ionized particles near the sun to nearly 1,000 times the speed of sound—creating a solar wind. Over the last few decades spacecraft, from NASA’s Pioneer explorations in the 1960s to the latest Solar and Heliospheric Observatory (SOHO), have ventured closer and closer to peek at this hightemperature plasma (1). SOHO, which will continue to make precise measurements of the solar wind at least until the end of 2018, is the same distance from the sun as Earth and follows the same orbit. Scientists have argued for decades that the best way to study the corona is to send probes closer—much closer (2).