Two spacecraft have taken a giant leap towards explaining the Sun’s engimatic heat.

The collaboration between NASA and the European Space Agency (ESA) explored an enduring mystery. In theory, the sun’s atmosphere — the corona — should be cooler than its surface. This is because the Sun’s energy comes from the nuclear furnace in its core. As the corona is further away from this heat source, logic should dictate that it’s cooler.

In reality, that’s not the case. The sun’s surface is “only” around 6,000 degrees Celcius. The corona, meanwhile, is a whopping 1 million degrees — over 150 times hotter. 

To explain the divergence, scientists point to an electrically charged gas known as plasma, which comprises the corona. They suspect that turbulence in the solar atmosphere is heating the plasma, but have struggled to prove the theory.

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To gather further evidence, two spacecraft were needed. One would conduct remote sensing at a certain distance away from the target. It would then use cameras to observe the Sun and its atmosphere at different wavelengths.

The other spacecraft would fly through the region. As it moved, the satellite would take measurements of particles and magnetic fields in the area.

Alone, each spacecraft could unearth useful clues. But together, they could paint a fuller picture of what’s happening in the plasma.

NASA and the ESA had the ideal candidates for the mission: the Solar Orbiter and the Parker Solar Probe.

The Solar Orbiter approaching the Sun