First spacecraft to fly through the atmosphere of a star

On 28/04/2021, NASA’s Parker Solar Probe became the first spacecraft to fly through the atmosphere of a star when it crossed into the Sun’s corona during its eighth close flyby. The spacecraft passed through the Alfvén critical surface, the boundary where solar material transitions from being magnetically bound to the Sun to flowing outward as solar wind. This crossing marked the first time a human-made object directly sampled particles and magnetic fields inside the Sun’s upper atmosphere. NASA publicly announced the milestone on 14/12/2021 during the American Geophysical Union Fall Meeting after analysis confirmed the event. During the encounter, Parker Solar Probe moved in and out of the corona multiple times and spent several hours within the solar atmosphere. The spacecraft collected in situ measurements of plasma density, magnetic fields, and energetic particles, helping scientists study the origin of the solar wind and investigate why the corona reaches temperatures of millions of degrees Celsius, far hotter than the Sun’s visible surface. Data from the flyby showed that the coronal boundary is not smooth but contains irregular structures, described as wrinkles and valleys, shaped by solar magnetic activity. Observations also helped trace the origin of magnetic switchbacks, sudden reversals in the solar wind’s magnetic field, to processes occurring near the solar surface. To survive the extreme environment, Parker Solar Probe uses a 4.5 inch thick carbon composite heat shield known as the Thermal Protection System. The shield is designed to withstand external temperatures of about 1,377 degrees Celsius while maintaining near room temperature conditions for the spacecraft’s instruments. The probe also travels at record breaking speeds during its close solar approaches. During a December 2024 perihelion, the spacecraft reached roughly 430,000 mph, about 191 kilometers per second, making it the fastest human made object. These high speed passes allow Parker Solar Probe to repeatedly enter deeper regions of the corona during successive orbits. The measurements from the April 2021 crossing provided the first direct sampling of the Sun’s outer atmosphere and offered new constraints on models of solar wind acceleration and coronal heating. The mission continues to perform progressively closer flybys, gathering additional data on plasma behavior, magnetic structures, and energy transfer near the Sun.