The star of our system is much more complicated than it seems. Instead of the fixed and unchanging disk that our eyes see, the Sun is a dynamic and magnetically active star. The Sun’s atmosphere continuously ejects magnetized material that sweeps across the entire Solar System far beyond the orbit of Pluto, affecting all existing worlds along the way.

Astronomers have studied the Sun for over a century. Using ground and space telescopes that are designed to withstand the impressive radiation of the sun’s burning face, they have stared at this star at every wavelength of the electromagnetic spectrum; But no matter how hard scientists have tried, they have not been able to reveal the secrets of the sun. Perhaps this failure is because until recently no telescope had come close enough to the Sun to really study it.

However, the situation changed recently. In the fall of 2021, for the first time in human history, a spacecraft flew into the Sun’s atmosphere and passed through the super-hot particles of the Sun’s corona. This achievement provided scientists with important information, with the help of which we can finally discover the secrets of the closest star to Earth. This daring spacecraft is called Parker Solar Probe.

Artist’s impression of the Parker probe’s entry into the Sun’s atmosphere.

The original Solar Probe design in the 1990s intended to use Jupiter’s gravitational pull and enter a polar orbit that would send the spacecraft almost directly toward the Sun; But due to problems including the high cost and time of the mission, the plan was changed in the following years. In the early 2010s, the Sun exploration project was transformed into a less expensive project called Solar Probe Plus, and it was decided to use the gravitational assistance of Venus and a more direct flight path to reach the Sun. In May 2017, the spacecraft was renamed Parker Solar Probe in honor of Eugene Parker, a prominent American astrophysicist and the inventor of the term ” solar wind “. This was the first time that NASA put the name of a living person on one of its spacecraft. Finally, after decades of waiting for scientists, the Khurshid spacecraft was launched on August 12, 2018 (21 August 2018) and began its 7-year mission.

The Sun’s atmosphere is made up of different layers just like the atmosphere of our planet. The Earth’s atmosphere has three layers, the troposphere, stratosphere, and mesosphere respectively, and the layers of the Sun’s atmosphere are the photosphere, chromosphere, and corona (sun’s crown). The surface of the Sun is often thought to be hotter than other parts, But the photosphere or the surface of the sun is actually not that hot. The temperature in this layer ranges from nearly 6200 degrees Celsius in the lower part to 3700 degrees Celsius in the upper part. This temperature is almost equal to the heat of electric arc welding. It is interesting to know that the air around a lightning strike can be up to five times hotter than the photosphere.

The strange thing is that the corona, as the outermost layer of the Sun’s atmosphere, is much hotter than the photosphere. The temperature of the corona, which starts approximately 2,100 km above the surface of the sun, reaches half a million to several million degrees Celsius, or at least 80 times the surface temperature. It’s like moving away from a fire, and the further you move away from it, the hotter you get. This unusual feature is what makes the achievement of the Parker Solar Probe even more impressive.

Parker solar probe in clean room. The heat shield of the spacecraft with its white ceramic coating can be seen at the top.

Why the Sun’s corona is much hotter than the Sun’s surface is one of the unsolved mysteries of the universe, and it is the task of the Parker Solar Probe to unravel it. As a result, the probe has the task of collecting information from the magnetic fields and charged particles of the solar corona and trying to answer this puzzle.

Overall, the scientific goals of the Parker probe are to determine the mechanisms that generate the fast and slow solar winds, the heating of the sun’s corona, and the transport of energetic particles. In order to achieve these goals, the probe must approach the Sun less than 10 solar radii from the center of the Sun (the radius of the Sun is 695,500 km) and spend at least 14 hours below 10 solar radii and at least 950 hours below 20 solar radii to make in situ measurements. spend Flying around the Sun, compared to other space missions whose destination is a planet, asteroid, or comet, presents unprecedented technical challenges to the spacecraft, which we will mention below.

Parker probe travels faster than any other spacecraft; So that in its last trip around the sun, it will fly over its surface at an incredible speed of 690,000 kilometers per hour. This speed is so high that the distance between Tehran and Kermanshah can be covered in less than three seconds. However, the first big problem facing Parker was actually getting to the sun. Although the sun’s gravity acts as an anchor for the entire solar system, it is not easy to approach.

To get a satellite out of orbit around the Earth, we need to reduce its angular momentum so that it falls towards the planet. This is what we have to do when trying to remove an object from orbit around the Sun; With the difference that in this case, we are at a distance of one astronomical unit or 150 million kilometers from the sun and we are moving at a speed of 30 kilometers per second.

After placing the satellite in the earth’s orbit, we have to perform an orbital maneuver called “Hohmann transfer”. By performing this maneuver, we change the spacecraft’s orbital energy to correct its perigee (closest distance to the Sun) or apogee (farthest distance from the Sun). To meet an outer planet like Mars, we need to increase the solar apogee by adding to the orbital energy of the spacecraft; While reaching an inner planet like Venus requires reducing the solar perigee by reducing the orbital energy. To reach Mars and Venus from Earth’s orbit, we need a delta way (change in velocity) of approximately 2.9 km/s and 2.5 km/s respectively. These values ​​are obtained using the following equation:

In the equation, the Greek letter mo, similar to the English u, is called the Sun’s planetary parameter, which is the product of the Sun’s mass. R 1 is the orbital radius of the mass from which we start moving. In this case, the distance of 150 million kilometers from the Earth to the sun is considered the orbital radius, and finally, R 2 is the perigee or zenith. If we calculate the DeltaV required for the Parker Solar Probe to reach its closest distance to the Sun (6.2 million km), we will arrive at a number of 21.4 km/s, which is more than 8.5 times the DeltaV needed to reach Venus.

The obtained Delta V number is considered extremely high and exceeds the capability of all rockets built so far. But four years ago, the Parker Solar Probe launched from Cape Canaveral, Florida atop the Delta 4 Heavy, the world’s second most powerful rocket after SpaceX’s Falcon Heavy. In order to give the probe additional thrust, Delta 4 was equipped with a special solid-fuel third stage, which provided an additional three kilometers per second delta velocity for the normally two-stage rocket.

However, even with this extra power, the probe could never get close to the Sun. In order to make his record-breaking flight, which was one-seventh of the record of Helios 2, NASA’s previous solar probe, Parker was assisted by the gravity of the planet Venus in an amazing way five times, and he is going to make two more flybys of this planet in 2023 and 2024.

The path of the Parker probe. The spacecraft shortens its orbit around the Sun with each Venus flyby.

Since Venus is a relatively low-mass planet, Parker needed this number of flybys. The amount of speed a planet can change is largely determined by its gravity, which in turn is determined by the planet’s mass. As mentioned earlier, Parker’s original plan was to get a gravitational boost from Jupiter that would bring the probe three times closer to the Sun; But this path was accompanied by some problems.

Since Jupiter’s orbit is very far from the Sun, the sunlight reaching the solar panels at the peak of Parker’s orbit was reduced by 25 times, and therefore the spacecraft needed much larger panels to provide its energy. This became problematic as the spacecraft orbited Jupiter and began accelerating toward the Sun. In this situation, the panels would be destroyed by the sun’s heat and could not be folded and hidden behind the solar shield.

Other options were available. The Parker builders could have used a radioisotope thermal generator; But this would dramatically increase the cost, weight, and complexity of the spacecraft. The real strength of Parker’s new and different flight path is getting more time and data to help scientists meet the rover’s mission goal of studying the Sun.

With the original plan to fly around Jupiter, the probe had only 100 hours of time in the target region around the Sun, and could only fly past the Sun twice before reaching the end of its eight-year mission. The new shorter path means that Parker Solar Probe will take less than 150 days to complete an orbit around the Sun, allowing scientists to collect more than 900 hours of data during the probe’s 24 orbits.

The change in the program was accompanied by a change in design, abandoning the original cone-shaped heat shield and using the flat, compact, and familiar shield used in other spacecraft. This shield is made of carbon foam with a thickness of 11.4 cm; A truly amazing material that is the product of one of the most capable material innovation labs called Ultramet. Under the scanning electron microscope, this carbon foam appears to be an extremely porous material with 97% of its internal volume being empty space, thus providing amazing insulation properties for the heat shield while benefiting from the thermal stability of carbon.

But if the temperature of the Sun’s corona is at least half a million degrees Celsius, how can the probe enter it without melting? Although the Sun’s outermost layer is incredibly hot, it has a very low density. As a comparison, think of the difference between putting your hand in the oven and a pot of boiling water. (Don’t do this at home!) Hands can withstand much higher temperatures in the oven for longer than boiling water; Because they have to face many more particles in the pot of water.

Similarly, the Sun’s corona is less dense than the visible surface of the Sun; As a result, the spacecraft encounters less hot particles. In fact, while Parker is moving in an environment with a temperature of several million degrees Celsius, the heat shield facing the sun of the spacecraft only heats up to approximately 1400 degrees Celsius.

Read More: The Voyager Twins

Other parts of the spacecraft, plus some specialized sensors and solar panels, had to be designed to fit under the shield’s shadow. But there are various tools that bravely step out from under the shadow of the heat shield; Like the cup of the solar probe, which is one of several sensors on board the spacecraft. This piece is undoubtedly Parker’s most impressive piece of technology, completely outside the scope of the sun shield’s protection; As a result, designers had to be very creative in using materials.

The Parker Solar Probe cup is a Faraday cup and part of the Solar Wind Survey Instrument (SWEAP); A device that can count and measure the properties of electrons and ions radiated from the sun and actually gives the spacecraft the ability to study the solar wind and objects ejected from the sun’s crown. This device basically works by applying an electric field to the grid placed in the mouth of the cup. By changing the voltage, it is possible to select or filter the particles that are able to enter the cup, and at the same time as the charged particles hit the collector plate at the bottom of the cup, more information can be obtained about the cause of the current. In practice, the cup is a very simple device; But facing temperatures of 1,400 degrees Celsius, which is just below the melting point of pure iron, the solar probe cup required some engineering innovations.

Solar Probe Cup (SPC).

The first challenge was to choose a material for the electric grid that would create a selective electric field at the entrance of the cup. In addition to conductivity and resistance to heat, this network must also be machinable to make a spaced network on the scale of one hundred microns. For this purpose, the makers used tungsten; The same material used here on Earth in incandescent light bulbs. Thanks to tungsten, the lamps can survive the very high temperatures required to produce light. Tungsten filaments operate at a temperature of three thousand degrees Celsius; As a result, they are very durable against extreme temperatures. However, machining tungsten in a very fine mesh is difficult.

Micron-scale machining is not possible with traditional tools. By using these tools, as soon as the necessary force is applied to shave the metal, the mesh breaks. Instead, in such cases, lasers are usually used for engraving on materials; But since tungsten is very resistant to heat, the laser will not be able to melt it to form a network. Instead, the makers used acid printing.

Next, cables were needed that could supply the main power and carry the electrical signals away from the collector plate. Copper and aluminum, two common conductors on Earth, will be transformed into a pool of molten metal at the Parker Solar Probe position; As a result, they could not be used in any way. Each conductive cable in this part of the spacecraft must be made of niobium C-103, a special alloy consisting of 89 percent niobium, 10 percent hafnium, and 1 percent titanium. This strange aerospace material has also been used in the construction of all the components of the outer cover. Normally the wires are insulated with plastic outer coverings, But obviously, this option was not possible for the Parker space probe and the engineers had to use sapphire to ensure the insulation of the niobium wires.

To accomplish what is a relatively mundane task on Earth, Parker’s builders were forced to use strange materials. Other parts of the sensors beyond the solar shield are constructed in a similar manner. Magnetic field measuring instruments hidden behind the shield require antennas that extend beyond the solar shield to make measurements of the Sun’s magnetic field. These four antennas are also made of Niobium C-103.

Solar panels were the next challenge. While orbiting the sun in its distant orbit, the spacecraft can fully open its solar panels without any problems; But as the probe begins its rapid return to the Sun, heat will become an increasing problem. By collecting solar panels, this problem can be partially dealt with; But the spacecraft must maintain some power to launch its scientific equipment during this important phase of the flight.

Parker has two smaller secondary panels that face the sun and are water cooled. This water is inside the solar panels and black radiators that are like titanium trusses right under the solar shield. It is pumped. This truss is very light despite its large size. The whole truss weighs only 22.7 kg, which considering the size of the structure, is very low weight even for titanium as a metal with low density. By performing detailed stress calculations, NASA engineers have ensured that the truss can use as little material as possible. This, of course, saves the launch weight; But the materials available to transfer heat from the heat shield to the bus (main body) of the spacecraft have also been minimized.

Edilo solar furnace in France.

It is difficult to test systems on the ground in the heat they are expected to encounter. The Edilo Solar Furnace is the most similar location to the environment that the Parker Probe systems will have to endure. Built on a mountainside in southern France, the facility uses ten thousand adjustable mirrors to focus light onto a concave mirror. Edilo solar furnace has the ability to reach a temperature of 3500 degrees Celsius; More than twice the temperature that Parker’s solar shield will experience.

Parts such as the Faraday cup and the solar shield were placed at the focal point of the concave mirror of the furnace and exposed to the temperatures that they must withstand when exposed to the sun. However, a piece like Faraday’s cup had to be tested while performing its sensory functions. To that end, the engineers needed a particle accelerator to simulate the electrons and ions from the solar wind that the cup would encounter. It was not possible to combine the particle accelerator with the solar furnace; As a result, researchers from the University of Michigan proposed the smart idea of ​​using four high-power IMAX projectors to simulate the sun’s heat. They also found that the Faraday cup actually performed better when heated; Because heat cleans the system from pollutants.

Much of the information obtained from the Faraday Cup is of little interest to the casual space enthusiast. They are the raw data that provide scientists with valuable clues about the nature of the Sun. However, one of the sensors on the spacecraft, which is in charge of transmitting images to the ground, can surprise us.

During a solar eclipse, we can observe a beautiful phenomenon: bright rings of light that appear around the sun. These extraordinary patterns are the result of bright electrons that move around the Sun on magnetic field lines and have been deformed by the pressure of the solar wind. We have been able to observe streams of energetic electrons from our Earth and solar observatories located at Lagrangian point 1, But we never managed to see them up close until recently.

The next close encounter of the Parker Solar Probe will take place on June 11, 1401, and the spacecraft will circle the Sun another 15 times for a total of 24 times in the next three years. With two more flybys of Venus in 2024 and 2025, Parker will break his records and get much closer to the Sun. On the last visit, the probe will reach a distance of 16.6 million kilometers from the Sun, which is almost seven times closer than any other spacecraft.

After completing the 24th orbit, the probe may have some fuel to continue orbiting the Sun; But Parker ultimately fails to ignite the thrusters necessary to keep his heat shield facing the Sun. The probe will then begin to rotate, and parts of the spacecraft not designed to see the sun will be exposed to full radiation.

The spacecraft will first break into large pieces and then become smaller and smaller. In the end, the entire probe, which is about the size of a small car, will be left with nothing more than tiny dust scattered across the Sun’s corona. However, Parker’s legacy will live on. The probe’s observations are expected to finally help solve questions that have been puzzling scientists for decades.