NASA’s Parker Solar Probe has reached out to our star and touched its atmosphere. On 28 April, it became the first spacecraft to enter the upper atmosphere of the sun, called the corona. This milestone was announced on 14 December at a meeting of the American Geophysical Union in New Orleans.
The edge of the sun’s corona is the area of space just far enough from the sun’s centre that its gravity and magnetic field are no longer dominant and cannot trap material on the star. That boundary is called the Alfvén critical surface, which is what the Parker Solar Probe crossed in April to dip into the corona.
“For centuries, humanity has only been able to observe this atmosphere from afar. Now… we have finally arrived,” said Nicola Fox, director of NASA’s Heliophysics Division, in a press conference. “Humanity has touched the sun.”
The spacecraft entered the corona on its eighth close pass of the sun, when it was only about 13 million kilometres from the centre of the star. The boundary was wiggly, though, and the spacecraft exited after about five hours, only to enter and exit the corona twice more before continuing to a more distant part of its orbit. It may have passed through again in August, but that data hasn’t yet been fully analysed.
Until the probe entered the region, researchers weren’t sure exactly how far from the sun the Alfvén critical surface would be or what it would be like, but they knew that its presence could be measured by changes in the magnetic field and a slowing of the solar wind below the surface. The Parker Solar Probe’s measurements confirmed this, and demonstrated that the critical surface wasn’t a smooth bubble around the sun, but rather a wrinkled edge.
Studying this surface could help us understand how the sun spits out charged particles that can pose issues for satellites and space explorers, and maybe even predict those outbursts. It is also a step towards understanding other stars beyond our solar system. “These are stellar phenomena, not just solar phenomena,” said Parker Solar Probe team member Kelly Korreck during the press conference.
The probe is planned to continue circling ever closer to the sun well into 2025, repeatedly breaking its own records for the fastest-moving spacecraft and the closest spacecraft to the sun. Now that we have touched the star, researchers will keep working to unravel its many mysteries.
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