The Mission To Touch The Sun

The daring mission of the Parker Solar Probe revealed discoveries about the Sun’s mysteries, the corona, and the solar wind and leaves an imprint in stellar understanding.

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By Aries Ho

About 93 million miles away from Stuyvesant is an active system that burns with nuclear energy and erupts with explosions of mass, excited particles, and magnetic fields. This active misunderstood system, also known as our Sun, remains a deeper mystery than what we perceive from the surface. Though it looks relatively solid from Earth, it is a hot blazing core surrounded by a dense ball of gas called the corona. The corona is a stream of particles that can be observed as solar flares or as the white ring surrounding the moon’s silhouette during a solar eclipse. Temperatures in the corona reach more than one million Kelvin, about 167 times hotter than the Sun itself. The enigma of the energy source that heats the corona’s temperature is a longstanding question in solar science, referred to as the coronal heating mystery, and is constantly misunderstood.

The corona is not only extraordinarily hot, but also the origin of a devastating space storm known as the solar wind. The solar wind consists of magnetized particles clumped together in a fast-moving wave released from the corona. This wind spins around the Sun before heading out to reach the end of Pluto’s orbit, affecting every magnetic field it encounters. This storm interferes with Earth’s magnetic fields, satellites, and power grids so often that the solar wind constitutes the most common reason for satellite failure. However, the common solar wind is hard to study from Earth due to the insufficient images produced from the dimming of the solar wind as it leaves its source, the Sun. “You're constantly asking yourself how much of what I'm seeing here is because of evolution over four days in transit, and how much came straight from the Sun?" said solar scientist Nicholeen Viall, working at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

The solar wind and the corona dump information like waterfalls onto astronomers, who from Earth can only stare at the end of the information and not the source of the waterfall. However, that changed in 2018 when Cape Canaveral Air Force Station collaborated with NASA to launch the Parker Solar Probe (PSP) with a mission to orbit the Sun 25 times by 2050 and understand the mysteries of the corona and the solar wind by analyzing the Sun right at its surface.

PSP’s mission is to observe, analyze, and understand the Sun’s many mysteries in a remarkable new way. Powered by solar panels on each of its sides, the probe is stationed 15 million miles away from the Sun as it orbits the star. After each orbit, PSP uses Venus’s gravity to increase its speed and decrease its orbit radius. Due to its six-inch-thick heat shield, the probe thrives in extreme temperatures and recently completed three out of 25 expected orbits. By the end of its mission, PSP will be 3.5 million miles closer to the Sun, traveling at a remarkable speed of 430,000 mph, making it the fastest human-made object.

After two years in the dark about the probe’s findings, papers in “Nature” documented the data transmitted from PSP and the underlying discoveries for the first time in December 2019. The data observed by PSP reformed the hypotheses and knowledge scientists had about the direction, speed, and source of the solar wind and provided a wealth of topological data about the Sun and its surrounding bodies.

In the first paper, one of the observations that the probe made was the first proof of dust dimming as it approaches the Sun. This interstellar dust, made from collisions that formed planets, asteroids, and comets billions of years ago, is everywhere in space. Scientists have long hypothesized that the Sun’s heat evaporates the dust particles, creating a dust-free zone around the Sun. There was no proof for such a hypothesis until PSP showed that the dust starts to thin a little over seven million miles from the Sun. Astronomers predict this dust-free zone will surround the Sun two to four million miles away, which means toward 2022, astronomers at NASA may be the first to either see this dust-free zone or disprove the theory supporting its existence. If this dust-free zone is forming around the Sun, does that mean all stars have a dust-free zone that no other astronomical object has? If so, at what heat does the dust-free zone occur? Is the dust-free zone related to the high heat of the corona? These are only the beginning of questions that the mission ignites in astronomers via PSP.

As I started writing this article several weeks ago on December 11, 2019, the solar probe made the first picture of the trail of debris that causes the Geminid meteor shower. Astronomers already knew this trail was present and came from the asteroid Phaethon, but they had never gotten a good look at it because it was too faint from Earth to observe. "We calculate a mass on the order of a billion tons for the entire trail, which is not as much as we'd expect for the Geminids, but much more than Phaethon produces near the sun," Karl Battams, a space scientist at the U.S. Naval Research Lab in Washington, D.C., said in a statement. The material in the stream was likely ejected in one dramatic event several thousand years ago, perhaps during one of Phaethon's close solar passes, he added. Those passes occur every 524 Earth days, and in the next year, PSP may observe them up close for the first time.

To many at NASA, the collected data of PSP shocked and altered once-solid hypotheses about the solar wind and its properties. "We think of the solar wind—as we see it near Earth—as very smooth, but Parker saw surprisingly slow wind, full of little bursts and jets of plasma," said Tim Horbury, a lead researcher on PSP's FIELDS instruments based at Imperial College London. NASA used PSP to collect data about the magnetic field of the solar wind. The astronomers recorded stable levels of magnetic fields from Earth, but at the Sun it was revealed that the magnetic field twists upon itself in an “S” shape, not unlike a whip or a rogue wave. This type of movement in the solar wind is known as a “switchback,” in which the magnetic field in the wind folds back on itself until it is pointed almost directly back at the Sun. The exact origin of the switchbacks isn't certain, but some scientists say that switchbacks may be traces of a process that heats the corona and an answer to the coronal heating mystery. Continually in the following years of the PSP mission, the understanding of the coronal heating mystery, even if it is unclear, will continue to be an important task of the mission and to NASA astronomers.

Not only did PSP pick up a totally new form of the solar wind, but it also disproved predictions about the speed of the wind. Astronomers studying the aging of the stars estimated that the solar wind should be spinning at a few kilometers per second, but PSP calculated the wind moving at speeds nearly five times faster than predicted velocities. “The large rotational flow of the solar wind seen during the first encounters has been a real surprise," Justin Kasper, the principal investigator for the Solar Wind Electrons Alphas and Protons, said. "While we hoped to eventually see rotational motion closer to the Sun, the high speeds we are seeing in these first encounters is nearly 10 times larger than predicted by the standard models."

The source of the wind cannot be understood from data collected on Earth; it’s just too far away. But 15 million miles away from the Sun, PSP gave astronomers a new clue about a possible source of the wind. Coronal holes are places in the Sun where sunspots occur that are signified with cooler hydrogen and helium atoms. Toward the equator of the Sun, these coronal holes release the more slow-spinning wind while holes near the pole release faster-spinning wind. In one of the four “Nature” articles, it states, “During the solar minimum, when the Sun is at its least active, the solar wind is observed at high latitudes as a predominantly fast (more than 500 kilometers per second), highly Alfvénic (type of wind) rarefied stream of plasma originating from deep within coronal holes.” These observations not only lead to a possible source of the solar wind, but also support the Sun’s rotation theory in which the Sun rotates faster toward each pole and slower as it approaches the equator. Though the mechanism that drives the solar wind is unresolved, PSP is lending astronomers data that could only be collected near the Sun.

Better understanding of the speed of the solar wind will help astronomers predict the effects of the solar wind, the aging of the sun, and the release of particle blobs in the solar wind. The blobs, also called coronal mass ejections, are released in the solar wind every 90 minutes as a clump of particles that are 50 to 500 times bigger than the size of Earth. Whether the Sun releases coronal mass ejections in 90-minute intervals continuously or in spurts and how much they vary between themselves is still a mystery. "The regions in front of coronal mass ejections build up material, like snowplows in space, and it turns out these 'snowplows' also build up material from previously released solar flares," Nathan Schwadron, a space scientist at the University of New Hampshire, said in the same statement. When these blobs smack into Earth’s magnetic fields, they interfere with satellites and power grids, like the solar wind. When a huge coronal mass ejection interfered with Earth’s magnetic field in 1989, it caused significant circuit failure. If such an event were to occur again, astronomers predict a global blackout of communications technology and systems.

There’s plenty more time for the discovery and gathering of data required to understand the predictions. By the end of its mission in 2025, the probe will have 21 more encounters with the Sun and will get more than three times closer to the star than it has so far. However, scientists may find that the Parker Solar Probe will pose far more questions than answers as humans expand to search the universe. Undoubtedly, the Parker Solar Probe is a leap in applied physics and mechanics, a huge step forward for astronomy, and a revolution in engineering and interstellar understanding.