When Icarus Touched the Sun

In 1958, a 30 year old scientist published his paper, titled “Dynamics of the Interplanetary Gas and Magnetic Fields”, on the famous The Astrophysical Journal, wherein he described the phenomenon of solar winds - the flow of particles (mostly protons and electrons) that shoots off the sun’s corona (the outermost part of the sun’s atmosphere, with temperatures that reach millions of degrees Fahrenheit) at about a million miles per hour due to their extreme heat that even the sun’s gravity cannot hold back. Publishing this work was not an easy task - scientists at the time were still firmly convinced of the idea that interplanetary space was a “perfect” vacuum, overlooking the existence of several isolated particles that travel throughout the solar system. The paper was dismissed as exciting but incorrect work, even though the author of said paper could not find any valid arguments against his rationale, and was rejected by the reviewers when he first submitted it in The Astrophysical Journal. However, he did not relent. A subsequent appeal was made to the journal’s editor, the famous astrophysicist Subrahmanyam Chandrasekhar who was then a professor at the University of Chicago, who overruled the initial decision arguing that the reviewers failed to point out any errors beyond their stubborn dislike of the fundamental premise.

Four years later, when the Mariner 2 (the first robotic space probe to conduct a successful planetary encounter) detected and measured particles streaming through space during its journey to Venus, the author’s predictions came true. Eugene Parker, the scientist behind the paper, was a vindicated man but that was not the only outcome of his work. Decades later, he would go on to become the first person in history after whom a NASA mission is named while the individual in question was still alive - the Parker Space Probe mission.

Footage captured from the probe

On the 28th of April 2021, the Parker Space Probe became the first spacecraft in history to touch the sun. It passed through the sun’s corona, sampling particles and collecting key information that showed that it was somewhere between 10 to 20 solar radii (4.3 to 8.6 million miles) from the surface of the sun, implying that it had crossed the Alfven Critical Zone, the point below which the sun and its gravitational and magnetic forces directly control the solar wind and beyond which it is too weak to contain the solar winds that race across the solar system.

Each and every component of the probe was carefully engineered by the John Hopkins University Applied Physics Laboratory on a budget of US$1.5 billion to deal with the brutal heat and radiation one would encounter in the Sun's atmosphere. The spacecraft's systems are protected with the aid of a 2.3m-diameter and 11.4cm-thick hexagonal solar shield made of reinforced carbon-carbon composite (which is a composite material originally developed for the reentry vehicles for ICBMs and gradually, the nose cone and wing leading edges of the Space Shuttle orbiter), which is mounted on the Sun-facing side of the spacecraft, that could withstand temperatures at over 2500 °F. A white reflective alumina surface layer minimizes absorption, whereas the primary photovoltaic array (a dual system of solar panels that power the spacecraft) is positioned to retract behind the shadow shield during the close approach to the Sun, after which the smaller secondary array takes over.

The shadow shield also protects the spacecraft systems and key scientific instruments, which are located in the central portion of the shield's shadow, by fully blocking the direct radiation of the Sun, which would be fatal to the probe within tens of seconds if exposed. The probe is also fitted with four light sensors that detect traces of direct sunlight coming from the shield limits and engage an adjustment to reposition the spacecraft within the shadow again if required.

The key scientific instruments added to the probe were done so for attending numerous scientific goals and investigations planned by the mission. Some of the aforementioned instruments (and goals of investigation) are -

1. FIELDS (Electromagnetic Fields Investigation): An instrument that consists of two fluxgate magnetometers (originally used for detecting submarines in the World War II, along with confirming the plate tectonics theory by measuring shifts in the magnetic patterns on the seafloor), a search-coil magnetometer and 5 plasma voltage sensors to measure electric and magnetic fields, electron temperature and absolute plasma density, among other metrics.

1. ISOIS (Integrated Science Investigation of the Sun): An instrument suite that contains two Energetic Particle Instruments to measure the energetic particle (including electrons, protons and heavy ions) population, with the higher energy and lower energy particles' data being considered independently.

1. WISPR (Wide-field Imager for Solar Probe): Optical telescopes to capture images of the corona and inner heliosphere (the cavity formed by the Sun in the surrounding interstellar medium, i.e., the point after which the solar plasma gives way to the interstellar plasma that permeates the galaxy).

1. SWEAP (Solar Winds Electrons Alphas and Protons): Building on the work of Eugene Parker, this investigation will count and measure key properties of particles such as electrons, protons and helium ion, with the help of its two main instruments - the Solar Probe Analyzers (SPAN, two electrostatic analyzers to limit the scanning energy and particle range targeted for detection and measurement) and the Solar Probe Cup (SPC, a Faraday Cup).

The arsenal of scientific instruments that the Parker Space Probe was equipped with wasn't the only interesting object on board the spacecraft. In March 2018, NASA invited the public to send their names to the Sun via storing it on humanity's first ever mission to potentially "touch" a star. The idea was a massive success. Over 1,137,202 names were submitted and verified over the span of seven and a half weeks before a memory card containing the names was installed on the spacecraft on the 18th of May 2018. The card also contained photographs of Eugene Parker and a copy of his seminal 1958 work on solar winds, and it was mounted on a plaque carrying his name and a quote of his - "Let's see what lies ahead".

The probe was finally launched a few months later, on the 12th of August 2018, as part of NASA's Living with a Star Program, which takes a broad look at the Sun-Earth system and attempts to understand how the star's behavior affects life on Earth. The mission's trajectory is centered on seven flybys/gravity assist maneuvers at Venus over seven years to incrementally decrease its orbital perihelion so as to near the Sun, for a total of 24 orbits. Inching closer to the Sun has its consequences - the high radiation around such environments could cause spacecraft charging effects, instrument and communications damage, so the orbit will be highly elliptical so as to restrict the time intervals spent near the Sun. On the 29th of October 2018, the spacecraft became the closest ever artificial object to the Sun, with a distance of 8.5 million kilometers as of its perihelion on the 21st of November 2021.

Getting closer to the Sun than ever before did not just capture centuries of human imagination that tried to reach the very "harbinger of light and life" - from myths of boys with wings of wax flying too close for comfort to demigods attempting to swallow the Sun - but it also enabled researchers to get a deeper look at some of our questions regarding the star.

One of the most significant events that caught the attention of the investigators was the switchbacks detected by the FIELDS instrument. These are almost complete reversals in direction of the magnetic field (such that it almost points directly back at the Sun) which flows out from the star, embedded in the solar winds. First observed faintly by the joint NASA-European Space Agency mission Ulysses, scientists were unable to identify the phenomenon and its source, but the data acquired via the Parker probe confirms the reversals and might hold clues to where it stems from in the future. This implies potential key developments on the nature of these solar winds and how they travel through the solar system down the line.

Another key observation was made regarding how quickly the solar wind rotates around the Sun as the star spins. Initial proposed models estimated the speed of the wind at a particular direction at a few kilometers per second, but the Parker probe measured the speed (with the help of the SWEAP instrument) at about 35 to 50 kilometers per second, substantially more than initially thought. This measurement has important implications for researchers to improve predictions of when a dangerous solar outburst might hit our planet and to revise ideas about how other stars in the Universe age, due to the observation that the solar wind is transporting more energy away from the Sun than previously thought and slowing down the star's rotation down at a pace that is more rapid than expected.

Other key findings include the first direct evidence of dust starting to thin out at around 11 million kilometers from the Sun (observed with the aid of the WISPR instrument), confirming the scientists' idea of a dust-free zone where the material of the dust would evaporate due to the Sun's heat and become part of the solar wind, and observations of rare particle bursts with a high ratio of heavier elements (via the ISOIS Energetic Particle instruments), which are important to study due to their implications for astronaut safety and space weather conditions near Earth.

The Parker Space Probe mission is expected to retire in 2025, by which time the probe would have had 24 close encounters with the Sun. The unique focus of the mission on solar phenomena such as solar winds will continue to help researchers apply a rigorous approach towards studying the closest star to our planet.

“The Sun is the only star we can examine this closely,” quoted Nicola Fox, director of the Heliophysics Division at NASA Headquarters. “Getting data at the source already is revolutionizing our understanding of our own star and stars across the universe. Our little spacecraft is soldiering through brutal conditions to send home startling and exciting revelations.”

- By Amrith Krishnan

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REFERENCES:

1. https://ui.adsabs.harvard.edu/abs/1958ApJ...128..664P/abstract

2. https://news.uchicago.edu/explainer/what-is-solar-wind

3. https://www.jpl.nasa.gov/nmp/st5/SCIENCE/solarwind.html

4. https://www.nasa.gov/feature/goddard/2018/parker-solar-probe-and-the-birth-of-the-solar-wind

5. https://www.qrg.northwestern.edu/projects/vss/docs/space-environment/1-what-is-in-space-besides.html

6. https://www.nasa.gov/feature/goddard/2021/nasa-enters-the-solar-atmosphere-for-the-first-time-bringing-new-discoveries

7. https://www.nytimes.com/2018/08/10/science/eugene-parker-solar-wind-nasa-probe.html

8. http://www.laboratoryequipment.com/581964-A-Spacecraft-has-Touched-the-Sun-for-the-First-Time/#:~:text=The%20sun%27s%20outer%20edge%20begins,switchbacks%2C%20emerge%20from%20this%20area.