The Parker Solar Probe’s Ultimate Maneuver (Updated)
by Sam Atkins
The Parker Solar Probe is set to ring in the holiday with an insane dive into the Sun’s corona where it will reach the fastest speed ever by a human-made object!
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Image credit: NASA
Today is Christmas Eve and NASA’s Parker Solar Probe (PSP) will perform its historic 22nd dive down into the Sun’s upper atmosphere, known as the corona! When it does this, the probe will be reaching a record-breaking speed of about 690,000 km/h (430,000 mph). This is fast enough to travel the width of the Earth in just over a minute! This speed will shatter the probe’s own record that it set on three different occasions since last year. The PSP is currently the fastest object ever built by humans, dethroning the previous 1976 record holder, Helios 2.
The Parker Solar Probe was launched back in 2018 with the mission to “touch the Sun,” and in 2020, it became the first spacecraft to fly through the Sun’s corona, which is essentially the upper atmosphere. The probe is named after the solar and plasma physicist Dr. Eugene Parker who shaped much of our modern understanding of the Sun. Though he has since passed away, it was the first NASA spacecraft to be named after a living person and he got to be the first person to witness the launch of a spacecraft bearing his name. The probe carries on his work to this day and today will make the biggest stride in its journey yet.
The elliptical shape of the Parker Solar Probe’s orbit stretches out to the orbit of Venus. Image credit: NASA’s Eyes on the Solar System
Artist’s impression of the Parker Solar Probe swinging around Venus. Image credit: NASA / Johns Hopkins APL / Steve Gribben
These incredible speeds are thanks to the probe’s uniquely close approaches to the Sun. Our star is massive, making up 99.85% of the entire solar system’s total mass. With all that mass, the Sun has immensely powerful gravity and the closer you get to it, the more forcefully the Sun’s gravity pulls you toward it. To make such a close approach to the Sun requires a very complex application of math and physics.
The PSP is in a continuous highly-elliptical orbit around the Sun. This orbit resembles a long oval shape with the Sun very close to one end. When the probe reaches the point in its orbit that is closest to the Sun, that is called perihelion. This part of the orbit requires the highest speeds or else the probe would get yanked inward by the star’s gravity and be vaporized. Even with its state-of-the-art thermal protection, being so close to the Sun is punishing to the probe’s material and electronics so this elliptical orbit also minimizes the time spent near perihelion.
Still, getting close to the Sun is the whole point here. To achieve incrementally closer and closer perihelion, the PSP must use repeated gravity assists from Venus, which passes right through the PSP’s orbital aphelion (the point in its orbit furthest from the Sun). Gravity assists can alter the trajectory of spacecraft which itself will affect the shape of its orbit. In the case of the Parker Solar Probe, Venus is used to decelerate the probe with each pass by leaving some of its momentum with Venus (ala Newton’s third law) and allowing it to fall closer and closer to the Sun.
The PSP’s most recent, and in fact seventh and final, gravity assist with Venus was this last November 6th, which brought the spacecraft less than 400 km above the planet’s surface. For comparison, that’s about how high the International Space Station orbits above Earth’s surface. This pass by of Venus was the windup to the mission’s final pitch.
Parker Solar Probe’s 22nd flyby of the Sun sped up to one day per second. Video credit: NASA’s Eyes on the Solar System
Today, less than two months after its final flyby with Venus, the Parker Solar Probe will come within 6.1 million km (3.8 million mi) above the Sun’s surface. This distance is just 4.3 Sun widths away. This close proximity will require the probe to reach unfathomable speeds to avoid falling into the star. The probe is expected to top out at about 690,000 km/h (430,000 mph) which is 0.064% the speed of light. That’s fast enough to travel from Philadelphia to Washington D.C. in just a single second!
As it cruises around the star, the probe will pass through the corona, cutting through plumes of plasma still connected to the Sun. It will be close enough to pass inside a solar eruption, like a surfer diving under a crashing ocean wave. The probe will endure solar radiation 475 times more intense than what we receive on Earth and temperatures of up to 1,370°C (2,500°F). Thanks to the probe’s hexagonal solar shield, which is always pointing towards the Sun, the probe’s internal equipment is protected. Were the shield not there, the probe would become inoperative within tens of seconds.
Another hurdle presented by the Sun’s radiation is the radio interference. Communication is already limited by the over 150 million km distance which creates a more than 8-minute delay between signal transmission and signal reception. However, the Sun’s corona itself is a significant source of radio wave emissions. This essentially washes out signals between the probe and the Earth for several days when making close passes to the Sun. For these reasons, the Parker Solar Probe requires a significant level of automation. The team working with the probe have described it as “the most autonomous spacecraft that has ever flown.” This includes the automatic retraction of solar panels to regulate temperature, attitude control to keep the solar shield pointed towards the Sun, etc. While the probe is out of contact with Earth during perihelion, it stores the data it gathers then waits until it gets far enough away from the Sun before it transmits the data back to us, sometimes months after it is procured.
Speaking of which, what data is the Parker Solar Probe gathering and how does it obtain it?
Diagram of the Parker Solar Probe showing where the main equipment is located. The Sun-facing heat shield is pointed to the right. Image credit: NASA / Sam Atkins
NASA’s mission to “touch the Sun” is about much more than just breaking a speed record. It is a science mission intent gathering data directly from the corona as the PSP makes its fateful dive. The probe has been fitted with four primary science instruments to uncover a series of mysteries:
FIELDS (Electromagnetic Fields Investigation): Consists of five plasma voltage-sensing antennas and three magnetometers. Four antennae are made of an extremely heat-resistant alloy and stick out into the scorching sunlight to measure the flow of solar particles. Attached to a rear boom in the shadows is the fifth antenna, which takes a three-dimensional picture of the electric field, and the three magnetometers, which assess how the Sun’s magnetic field changes.
IS☉IS (Integrated Science Investigation of the Sun): Measures energetic particles flowing from the Sun such as electrons and ions. It consists of two main components. The EPI-Lo studies lower-energy particles found in the solar wind. The EPI-Hi studies higher energy particles that are associated with more sporadic solar activity such as coronal mass ejections and solar flares. One of the major things these instruments will do is help us distinguish between energetic particles from the Sun that originate with a particular energy versus energetic particles that obtain that energy somewhere in interplanetary space as they move outward. This one reason it’s so important for PSP to get so close to the Sun.
WISPR (Wide-Field Imager for Solar Probe): Consisting of two telescopes that are meant to peek over the edge of the heat shield and take 3D images of the solar wind and corona as the probe is flying into them. Specifically, they are looking for the passing light scattered by electrons. Getting close to the Sun will provide a purer view, free from the irrelevant background noise that propagates through much of interplanetary space such as dust.
SWEAP (Solar Wind Electrons Alphas and Protons): Consists of three separate instruments. The first is a forward-facing faraday cup (SPC) that peeks around the side of the probe’s heat shield. It measures the solar wine’s ion and electron fluxes (the quantity of ions and electrons passing through over a given time) and flow angles. The other two instruments sit at the back on either side of the probe, staring out into the night sky and counting all the most abundant particles and determining their temperature, speed and density. These particles — consisting mostly of electrons, protons and helium ions — make up the bulk of the solar wind that escapes the Sun’s corona and travels across the rest of outer space.
All of this data is stored in the Parker Solar Probe’s memory banks and then transmitted directly to Earth via a high-gain radio antenna fitted to the side of the probe once the probe is free of the Sun’s interference.
All of these instruments and electronics are highly sensitive and require protection from the punishing solar radiation. This is the job of the Thermal Protection System (TPS) which is the eight-foot-wide, 4.5-inch-thick carbon heat shield mounted to the front of the probe. The sun-facing side is also coated with white ceramic paint to maximize reflectivity. This is the same reason astronauts wear white spacesuits. The heat shield will endure up to 1,370°C (2,500°F) but the rest of the probe’s equipment resting within the shield’s shadow will enjoy a comfortable 30°C (85°F) which is basically room temperature. This is because out in space there is virtually no atmosphere so there’s no gas to transfer heat between directly-radiated areas and shadowed areas. This is also why the atmosphere-free Mercury is extremely hot in the day side but icy cold on the night side.
This works both ways, however. This inability to transfer heat through an atmosphere means you can get rid of the heat you have either. This is why the PSP has an built-in cooling system that circulates water through its solar arrays to absorb heat then funnels it into large radiators which cools the water and radiates the excess heat out into the vacuum of space.
So, we have all these high-tech science instruments and we have 21 flybys of the Sun across six years. What has the Parker Solar Probe actually taught us about our star?
Diagram focusing on the Sun’s atmosphere.
The photosphere is more or less the Sun’s visible surface and stretches up to 400km.
The chromosphere is a low-density region that reaches up to 2,000 km above the surface where we find the Sun’s flickering spicules.
The corona is the vast outermost region of the Sun’s atmosphere which, like the chromosphere, is only visible when the Sun is blocked out by the Moon during total solar eclipses. The corona appears as streams of ionized gas reaching millions of kilometers out into space.
Image credit: Sam Atkins
The PSP has been diving deeper and deeper into the Sun’s upper atmosphere. Known as the corona, this region around the Sun reaches temperatures between one and two million degrees Celsius. This is hundreds of times hotter than the photosphere, which is the closest thing the Sun has to a surface. A consequence of the second law of thermodynamics (Homer: which we obey in this household), is that heat always transfers from a hotter medium to a colder medium. Therefore, the Sun’s cooler surface cannot possibly be supplying extra heat to the more distant regions surrounding the Sun. Learning why the corona is so much hotter has been one of the biggest mysteries they hope to solve at the Johns Hopkins Applied Physics Laboratory (which is just down the street from us in Laurel, Maryland, by the way).
Artist’s impression of the S-shaped switchbacks found in the solar wind flowing out into space. Image credit: NASA / Goddard / CIL via AP
Some of the first major observations that the Parker Solar Probe made were regarding switchbacks — S-shaped kinks in the magnetic field of the solar wind.
Most of the magnetic field is made up of closed loops of magnetic energy with both ends anchored to the Sun. Less common are open field lines which stream out from the photosphere into interplanetary space. Over the course of the Sun’s solar cycle (which you can learn more about here), these open field lines migrate between the Sun’s magnetic poles. Along the way they will encounter the more common closed loops which triggers something called an interchange reconnection. This is when the opposing magnetic field lines “collide” which results in them breaking apart and subsequently reconnecting which releases a lot of energy into the surrounding plasma, heating up the corona and accelerating the solar wind. This reconnection can also cause reconfigured and reversal of the magnetic field which is what creates the S-shaped switchbacks observed by PSP around the Sun. This is in line with theories proposed by the probe’s namesake, Eugene Parker, back in the late 1980’s. While it still remains an open question, we are gradually honing in on an answer. Perhaps the Park Solar Probe will illuminate us further when it makes its historically close and historically fast 22nd flyby of the Sun.
UPDATE: Two days after PSP’s fateful dive into the Sun’s corona, its beacon signal was recieved, indicating it had indeed survived the perilous approach. The first data from its instruments are expected on New Year’s Day 2025!