Touching the Sun

 

The Sun consists largely of plasma — the fourth state of matter formed when gas is heated to the point that atoms lose their electrons.  It is a most extreme environment that nonetheless produces an object that to us looks so perfectly round, so steady-state, and so magisterial as it crosses the sky.  Past civilizations around the world made the Sun into a god, usually a benign one seen as the source of life.

It is indeed the source of life on Earth, but it is hardly only benign in its powers.

The solar winds that the Parker probe is studying, for instance, blast out of the aurora at between 600 miles per second and 200 miles per second.  They reach these hypersonic speeds (five or more times the speeds of sound) and as a result produce temperatures which break atoms in the wind into particles.   This lets the material defy the strong gravitation pull of the sun and sends particles far into the solar system.

And while the solar winds lose some of their power by the time they reach the Earth, there’s still more than enough high-energy radiation and heat to destroy all life here, were it not for the magnetic fields that surround and protect the planet.

So with the Sun as both the ultimate creator and potential destroyer of our solar system, it certainly makes sense for scientists to try to understand it better — especially since even with our magnetic fields, astronauts, space assets and power grids can be harmed by high-energy particles emanating from the sun.  And NASA does now have 20 on-going missions in its heliophysics division (or in collaboration with the European or Japanese space agencies) and a dozen more in planning.

Most are missions much closer to Earth than the Sun.  For that reason, and also its complexity and ambitions, the Parker Solar Probe is unique in terms of what it can do and what it is trying to learn.

The Parker probe launched in 2018.  Due to a lack of adequate technology or financial resources, that was many decades after it was first proposed

Of the numerous discoveries and confirmations that have come so far from Parker, many revolve around that key question of where the solar wind originates.  Study of the solar wind has until now focused largely on what of it arrives around Earth, but Raouafi said by then it has lost many of the defining characteristics from its source — the solar corona.

“With Parker, we are looking at the young solar wind being born around the Sun,” he said.  “And it’s completely different from what we see here near Earth.”

A discovery that may help explain those origins is the presence of “switchbacks” – reversals in the Sun’s magnetic field that shoot out from the corona, then rapidly flip back, like a zig-zagging mountain road. This phenomenon had been seen before in the NASA-ESA Ulysses and Helios missions, but the number of switchbacks seen by Parker is orders of magnitude greater.

As explained by Raouafi, what appears to be happening is like twisting a rope to the point where the rope can’t handle it anymore.  The magnetic fields have to reshape themselves and this excess energy released in the form of heat and speed into the plasma.

“All these switchbacks were not expected and have been detected only because we are so close to the Sun,” Raouafi said.  “They may help explain the heating and acceleration of the solar wind.”

Raouafi said there have been detections as well of what appear to be miniature explosion all over the Sun,  what are sometimes called nanoflares.  Some theorize that the tiny explosions could be a source of the otherwise unexplained super heating of the Sun’s corona, which is much hotter than the surface below despite being much farther away from the solar core.

Eugene Parker, the University of Chicago astronomer and solar wind pioneer who the mission is named after, has a particular interest in the nanoflares.  They are one billionth the size of a typical solar flare and Parker theorized in 1972 that large numbers of them could be responsible for the extra millions of degrees of heat in the corona.

The Parker probe has also explored the world of dust in the inner heliosphere – intentionally and not.

Stellar theorists have long predicted that the area close-in to the Sun will be dust free because the ubiquitous bits of matter will be incinerated by the star.  And in one of its earliest orbits, Parker Solar Probe did indeed find a region starting at 7 million miles from the Sun that is gradually more dust free until it is expected to get truly dust free at 2 to 3 million miles.

But that region is closer to the Sun than where the spacecraft has, and will, be flying.  And since the gravity of the sun pulls dust from around the solar system to it, the car-sized probe has has been traveling in quite a dusty environment, with some potentially concerning effects.

Upon impact, the dust grains and the impacted spacecraft surface get heated so much that the dust vaporizes and then fragment into electrons and ions, forming plasma, the same state of matter that makes up he Sun (and lightning too), explained David Malaspina, a plasma physicist at the University of Colorado.

“While most dust impacts cause only small effects, a few are very high energy, creating the debris and dense plasma clouds,” Malaspina said, “We identified about 250 of these very high-energy impacts during the first eight orbits of Parker Solar Probe around the Sun.”

Raouafi said that the amount of dust is greater than anticipated, though still no danger to the spacecraft. The Parker Solar Probe team at APL – which designed, built, and now operates the spacecraft – prepared for a trek through a potentially hazardous environment, designing materials and components that survive hypervelocity dust impacts and the effects of the even smaller particles created in these impacts. Engineers modeled the makeup and effects of the dust environment, tested how materials react to the dust particles, and installed fault-tolerant onboard systems that are keeping Parker Solar Probe safe in this unexplored region.

Another question the Parker team hopes to answer is how some particles can accelerate away from the Sun at such astounding speeds — more than half the speed of light, or upwards of 90,000 miles per second. These particles move so fast that they can reach Earth in under half an hour, so they can interfere with electronics on board satellites with very little warning.

The Parker Probe is named after Eugene Parker  a pioneering astrophysicist who developed a central theory about the solar wind.

In 1958, a young Parker theorized that the the Sun’s hot corona — by then known to be millions of degrees Fahrenheit — is so hot that it’s contents can overcome the star’s powerful gravity. As solar gravity weakens with increasing distance from the sun, the outer coronal atmosphere is able to escape supersonically into interstellar space. According to the theory, the material in the corona expands continuously outwards in all directions, forming a solar wind.

This was an advance on models current at the time and dovetailed with a puzzling astronomical observation:  the plasma tails of all comets blew away from the sun.  Parker reasoned those consistent comet tails pointing outward to be the result of the same phenomenon which he termed the “solar wind”.

There was strong opposition to Parker’s hypothesis on the solar wind; the paper he submitted to The Astrophysical Journal in 1958 was rejected by two reviewers, before being accepted by the editor, Nobel prize laureate and astrophysicist Subrahmanyan Chandrasekhar.

Parker has been deeply involved with stellar physics ever since and remains so;  Raouafi said the Parker probe team and Parker talk about new results all the time.

To name the mission after Parker — who is now 94 — is unusual since missions are seldom named after living scientists.  An exception was made for Parker because of his decades of work re-defining solar physics and what a solar spacecraft might look for.

Assisted by course changes resulting from two more Venus flybys, in August 2023 and November 2024, Parker Solar Probe will eventually come within 4 million miles of the solar surface in December 2024, at speeds topping 430,000 miles per hour.

Four million miles is about one-tenth the distance between the sun and Mercury, the innermost planet of the solar system at 36 million miles from the Sun.

To put the Parker probe’s proximity into a familiar context, if you put Earth and the Sun on opposite ends of an American football field, the spacecraft would get within four yards of the Sun’s end zone. The current record-holder was a spacecraft called Helios 2, which came within 27 million miles, or about the 30 yard line. Mercury orbits at about 36 million miles from the Sun.

But a 8-foot-wide, 4.5 inches deep carbon composite shield will absorb the heat and keep the spacecraft and its instruments cool. The heat shield can withstand those temperatures of up to 2500 degrees F — the melting temperature of steel — absorbing enough heat that its back reached only 700 degrees F.   With that shielding and more, the instruments in the spacecraft a meter behind will remain at room temperature even as the spacecraft enters the Sun’s upper corona.

Raouafi and the Parker team are eager to get ever closer and as a result have the opportunity to answer more unanswered questions — especially about the origins of the solar wind.

“Imagine that you’re at Niagara Falls and are standing near the bottom, where the water lands.  You can see a lot, but you don’t really know much about where the water is coming from and how it came about.”

“Now imagine that you’re at the top of the Falls and can see the water coming.  That’s where we are now in terms of the Sun and will get closer and closer until 2025.”

That’s when the prime mission officially ends, but Raouafi said he hopes and expects it to be extended to cover a whole solar cycle of 11 years.