Fastest Spacecraft Ever Made Did (and Didn’t) Touch the Sun, Here’s Why It’s Complicated

Last week, media went ablaze with news of a human-made spaceship touching the Sun. Some (not few) were quick to jump to the conclusion that somehow a piece of hardware landed on the surface of the Sun (guess what, there is no surface), somewhat confused by NASA's choice of a title when it made the announcement: "Parker Solar Probe Touches the Sun."

As some of you might already know, the Parker Solar Probe launched in August 2018 with the stated goal of approaching the Sun like no other human-made machine ever did, in the hopes we Earthlings will get a better understanding of the star that makes life here possible, but that also threatens us with the space weather events it is responsible for. The machine, sturdily-made to survive the incredible heat emanating from the star, will eventually get within 3.8 million miles (6.1 million km) from the Sun, reaching speeds of 430,000 miles per hour (692,000 kph), and earning it the title of fastest spacecraft ever made on Earth. Back in November, Parker was moving at 364,660 miles per hour (586,864 kph), while it was at a distance of 5.3 million miles (8.5 million km) from its target. As it moved closer since then, it prompted NASA into proclaiming it finally touched the Sun. But what does that mean? Like probably all stars out there, the Sun has no solid surface one can land on, assuming one could survive that. Instead, the flaming ball of gases is comprised of seven layers.
Deep down we have the core, the hottest, densest, and most hellish place in the solar system. Then come the radiative and convection zones at 86,000 miles (138,000 km) from the core, then the photosphere (which is considered the solar surface), the chromosphere, and the transition region. Last, but not least, comes the corona, the outermost layer of the Sun which starts at about 1,300 miles (2,100 km) above the photosphere. Temperatures there are of at least 900,000 degrees Fahrenheit (500,000 degrees Celsius), the place is invisible with the naked eye and, most importantly, it does not have an upper limit. And it is this very hard-to-define region that the Parker Solar Probe actually traveled through, at a distance of great many miles from the so-called surface, the photosphere, so those arguing it didn't actually touch the Sun do have a point. For comparison, it’s like a spacecraft passing through the tail of a comet and saying it reached it, or skimming through the upper atmosphere of a planet and claiming the same. So, in a sense, the Parker Solar Probe did not touch the Sun, it only grazed its fancy clothing.
But, in another sense, it did touch it. You see, the corona has something called the Alfvén critical surface. It is the place that “marks the end of the solar atmosphere and beginning of the solar wind,” and even if it is as elusive as everything else about the Sun, it is generally agreed it comes at anywhere between 4.3 and 8.6 million miles (6.9 to 13.8 million km) from the star. That, by all accounts, presently puts the Parker past the Alfvén, and right into the Sun’s atmosphere, something that was never done before. And NASA even has proof of that, in the form of the detected magnetic and particle conditions specific to the corona past that point. Controversy aside, the fact this probe is where it is, and in working order, should benefit us all greatly. Already the machine “sampled particles and magnetic fields there,” and that should “help scientists uncover critical information about our closest star and its influence on the solar system.” It also proved the Alfvén critical surface isn’t a smooth ball, but an area with spikes and valleys that may influence solar wind and how it eventually impacts us. And, the cherry on the cake, it even moved through something called a pseudostreamer, a loop-like structure we can see from Earth during solar eclipses. More flybys in this region of space are planned for the future (the next one in January 2022), and they should unlock even more mysteries for us to dissect and marvel at. Untll that time, the first video below shows a stunning recording of the probe’s journey through the corona, as seen from on board Parker. The second video explains all of the above in easy-to-understand images.

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