Did the Parker Solar Probe really touch the Sun, or is that just a bunch of hot gas?

The Parker Solar Probe was launched in 2018 with the mission of flying closer to the Sun than any other human-made object to study the activities of our parent star. Understanding the action of stars and their impact on planets is of particular interest to astronomers both for the knowledge we can gain and for predicting solar weather for the benefit of Earth and crewed space travel.

The probe set out from Earth on an orbit which travels between the Sun and Venus, in turns. Each pass around Venus offers a gravitational assist, pushing Parker closer to the Sun’s surface. During a recent pass, which occurred in April of this year, the probe crossed an important boundary known as the Alfvén critical surface.

The results of that pass were delivered in a paper written by Justin Kasper from BWX Technologies and the University of Michigan, and colleagues. It was published in Physical Review Letters.

“At a broad level, the mission has three objectives,” Kasper told SYFY WIRE. “We want to figure out why the Sun’s corona or atmosphere is 100 times hotter than the surface, we want to figure out why some regions give off solar winds at different speeds, and we want to understand how particles are accelerated to really high energy during certain solar storms like flares or coronal mass ejections.”

A clearer understanding of those mechanisms could allow future scientists to better predict solar weather and create early warning systems for events which might negatively impact people on Earth or in space. Getting nearer to the Sun helps to illuminate the nature of those activities because solar material behaves differently at closer distances than it does nearer to Earth.

The Alfvén critical surface marks the point around the Sun within which material is gravitationally and magnetically bound. Inside of that boundary, solar material travels back and forth from the center of the star out into its atmosphere and back again. Beyond the boundary, material whips off into space where it can interact with the larger solar system but won’t ever return to the Sun itself.

While the probe hasn’t actually made contact with the Sun’s fiery surface, dipping beneath this boundary has been popularly referred to as “touching the Sun.” Whether or not that’s actually the case comes down to where we choose to draw the line.

“I understand the intuition of thinking about the Sun ending where it’s visible, which is kind of the context of the discussion about whether or not we’ve touched the Sun,” Kasper said. “I think it’s important for astronomers in particular to think about how there’s more to the universe than just the visible particles. If you’re trying to understand how the Sun slows down as it ages or loses angular momentum, that’s determined by the Alfvén surface, not the visible surface.”

Photo: NASA's Goddard Space Flight Center

If we zoom out, we can see that the Sun’s atmosphere extends well beyond this point, encompassing the solar system. From that perspective, we’ve been touching the Sun since before humans — or life of any kind — existed.

Similarly nebulous boundaries exist closer to home. The Earth’s atmosphere extends well beyond the Moon, yet we tend to think of the boundary to space as being a little nearer to the ground. From a certain point of view, Apollo astronauts both traveled to another world and never left home. When it comes to scientific definitions and the human emotional response to exploration, two things can be true at once.

In a bit of scientific poetry, we only learned that the Earth’s atmosphere extends that far as a result of observations made by an earlier solar probe known as the Solar and Heliospheric Observatory (SOHO).

Building the Parker Solar Probe was a particular challenge because of the dangerous environment it’s flying in. While testing the probe, the team built a solar furnace intended to mimic some of the conditions they would face during the course of the mission. That furnace was bright enough it could vaporize birds if they made the mistake of accidentally flying through its focus, according to Kasper. Luckily, the Parker Solar Probe is equipped to handle those conditions as long as things continue working as planned.

“When we crossed below the Alfvén point, the plasma we were flying through was at a temperature of about 1.1 million degrees Kelvin. Ridiculously hot. Fortunately, the density of the Sun’s atmosphere itself is very low. It’s kind of like putting your hand through the steam coming off a boiling pot of water” Kasper said. “The high temperatures come not from the plasma but from the photons of the light coming off the surface of the Sun. At closest approach, the Sun is about 475 times brighter than it is on Earth.”

In order to survive close approaches, the probe has to keep its heat shield pointed at the Sun, but it’s constantly fighting to do so. The pressure from the photons themselves push on the craft and, without any corrective action, it would flip. The probe is equipped with large wheels which turn in the opposite direction of the solar influence to balance it out. If the wheels start spinning too quickly, threatening to overrate the probe, it slowly fires rocket fuel to reach equilibrium again.

At the time of this writing, the Parker Solar Probe is about 95 million kilometers from the Sun and is heading for its next rendezvous in January 2022. If all goes well, it will reach its nearest approach in 2025, flying just 8.86 solar radii (3.83 million miles) above the surface. That’s roughly half the distance from the surface as the pass in April.

That’s the closest the probe can get, because gravity assists from Venus can’t push it any deeper. The short-term fate of the probe after the 2025 pass is unclear, though it might continue orbiting, gathering data until eventually it dies.

“We’ll ask NASA if we can keep the spacecraft on and see how things change over the next solar cycle. There are all sorts of environmental hazards. It could get struck by a dust grain that blows a hole through the side of the spacecraft or there could be radiation damage,” Kasper said. “Eventually we’ll run out of fuel, and we’ll get flipped and we’ll lose the spacecraft.”

Whether or not the Parker Solar Probe “touched” the Sun will likely remain a topic of some debate within the scientific community, and that’s as it should be. In the meantime, it’s important for us to allow ourselves the capacity for wonder, and sometimes that means drawing a line somewhere, defining horizons, and celebrating when we cross them.