This is the first time a human probe has been so close to the sun! Why hasn't it melted?

Produced by: Science Popularization China

Author: Chuando Space (popular science creator)

Producer: China Science Expo

Editor's note: In order to expand the boundaries of cognition, the China Science Popularization Frontier Science Project has launched a series of articles on the "Unknown Realm", which provides an overview of the exploration results that break through the limits in deep space, deep earth, deep sea and other fields. Let us embark on a journey of scientific discovery and get to know the amazing world.

According to NASA, the Parker Solar Probe recently passed through the sun's outer atmosphere at a speed of 692,000 kilometers per hour, flying at an altitude of only 3.8 million miles (6.1 million kilometers) from the sun 's surface. This is the first time that a man-made object has approached an area so close to the sun, and it moves faster than any man-made object.

According to NASA data, the second place in this record is the Helios 2 solar probe, which flew to a distance of about 43 million kilometers from the surface of the sun in April 1976. It can be seen that the Parker Solar Probe's close approach to the sun this time is closer to the solar surface than previous probes.

The Parker Solar Probe's appearance, with a heat shield on top.

(Image credit: NASA)

So what does 6.1 million kilometers mean? Let's look at a set of numbers. Compared with the parameters of the sun itself, the radius of the sun is about 700,000 kilometers, which means that the Parker Solar Probe has reached about 8.7 times the radius, which is the outermost layer of the sun's atmosphere: the corona, which can be more than millions of kilometers thick. Compared with the planets in the solar system, Mercury is the closest planet to the sun, about 58 million kilometers away from the sun, and 6.1 million kilometers is close to one-tenth of the distance between Mercury and the sun.

Artist's impression of the Parker Solar Probe entering the solar corona.

(Image credit: NASA)

Why conduct research so close to the Sun?

We study the sun. The first driving force is that the sun is the only star in the solar system. Without the sun, life forms as we know them today would not be able to exist on the surface of the earth, and human civilization would not be able to appear.

At the same time, the daily behavior of the sun directly affects human civilization. Any small change may have an impact on life on Earth. The habitable zone around the stars we are familiar with (the earth is located within the habitable zone of the solar system) is determined by multiple parameters such as the distance from the star and the type of star. Even a small change in the movement of the direct sunlight point between the Tropic of Cancer and the Tropic of Capricorn can affect the change of seasons on Earth. The influence of stars on the surrounding planets is obvious.

Image of an X-class flare released from the surface of the sun. The extremely bright flash on the right is the flare release

(Image credit: NASA)

my country's National Space Weather Monitoring and Early Warning Center predicted on May 6, 2024 that the sun erupted a strong flare of magnitude X4.5, which will cause the navigation systems, communication systems, power supply facilities, oil pipelines, aerospace activities, etc. that we rely on in our daily lives to be affected to varying degrees. When the coronal mass ejection reaches the vicinity of the earth, it may even form a geomagnetic storm.

Understanding the behavior of the sun is of great help in avoiding these extreme space weather events, especially for astronauts working and living in orbit. In the long run, when humans enter interplanetary space, such as traveling from Earth to Mars, they also need to understand the dynamics of solar activity, otherwise the lives of astronauts may be endangered.

From the perspective of the many unsolved mysteries of the sun itself, scientists have been puzzled by the bizarrely high temperature of the corona for nearly half a century. The corona area is farther away from the surface of the sun, so the temperature should be lower. But this is not the case. The temperature of the corona can be as high as millions of degrees Celsius, which is higher than the temperature of the surface of the sun (the surface of the sun is only 5,500 degrees Celsius). The two form a huge contrast. When a solar flare erupts in the corona, it will burst out more energy than normal, and eventually come into contact with the earth's atmosphere, causing great impacts on power grids, satellite communications, etc.

This is the solar corona and its surrounding extended jet, captured during the total solar eclipse of August 2017.

(Image source: WIKI)

To find the answers to these questions, the detector needs to go deep into the solar corona to detect the particles flowing through the area, study the space environment of the corona area, and find out what mechanism drives the flow of these energies. To this end, the Parker probe carries a Faraday cup, a sensor used to measure the ion and electron flux and flow angle of the solar wind. It can capture charged particles in a vacuum and measure the speed, density and temperature of these particles in combination with other equipment, thereby understanding the basic situation of the corona area.

The Parker probe also carries electric and magnetic field survey equipment, a wide-area imager, and other scientific research projects when the probe reaches the solar corona. In order to get close to the solar corona, the Parker probe uses the gravity of Venus to boost it multiple times, gradually reducing the distance to the sun during 24 orbits around the sun, and finally reaching the orbital position closest to the solar surface.

Parker Probe uses Venus' gravity to accelerate into orbit around the Sun

(Image credit: NASA)

What are the corona, solar flares, solar storms and solar wind?

The Parker probe approached a position of 6.1 million kilometers from the surface of the sun. This area belongs to the sun's corona, which is the outermost atmospheric structure of the sun. It is a hot area filled with plasma that surrounds the sun and extends to millions of kilometers above the sun's surface. During a total solar eclipse, we can observe it through a coronagraph, but the corona is not always evenly distributed on the surface of the sun. During the quiet period of solar activity, it mainly appears near the sun's equator, and during the active period, it can appear at the equator and the poles.

Solar flares are closely related to the corona. Solar flares are sudden flashes observed on the solar disk or edge, surrounding strong magnetic field areas near sunspots. When a flare forms, it releases magnetic field energy stored in the corona and forms a solar wind that can carry a large number of particles. If it is directed toward the earth, it will hit the earth's ionosphere and trigger auroras. Flares are divided into five levels: A, B, C, M, and X. Level X is the highest, and the energy can be equivalent to the energy generated by the explosion of more than billions of hydrogen bombs. A solar storm is a combination of the above events, which can refer to solar flare eruptions, coronal mass ejection events, etc.

An artist's impression of the solar wind hitting Earth.

(Image source: WIKI)

It can be seen that the activities of the sun have the characteristic of a domino effect. Coronal mass ejections are usually related to other forms of solar activities such as solar flares, and are more likely to involve energy transfer inside the sun. The connection mechanism still needs further study, which is one of the reasons why the Parker probe is close to the corona. From the perspective of the mission performed by the probe, through the data sent back by the Parker probe, scientists are trying to determine the heating mechanism of the corona, the source of the accelerated solar wind energy flow, the changing laws of the magnetic field in the solar wind source area, how the structures observed in the corona evolve into solar wind, and how high-energy particles are transferred between the corona and other solar atmospheric structures.

What defensive measures does a probe need to take to get close to the sun?

Because the temperature of the corona is extremely high, the probe needs to have effective thermal protection measures to pass through this area. Scientists have found through research that when the probe passes through the corona area around the sun, the surface temperature of the heat shield will only be heated to about 1400 degrees Celsius. For this reason, scientists designed a 11.43 cm thick carbon composite shield installed at the front of the probe, which can withstand a maximum temperature of 1650 degrees Celsius. Most of the instruments and equipment are hidden behind the shield to avoid being affected by the high temperature.

Scientists calculated that the ambient temperature behind the shield is only about 30 degrees Celsius, which can protect instruments from the high temperature of the corona. The carbon composite shield is made of carbon composite foam and carbon plates and painted with white ceramic paint to reflect as much heat as possible.

Parker probe's heat shield

(Image credit: NASA)

Some people may ask here why the corona has a temperature of millions of degrees Celsius, but the detector shield only needs to withstand about 1400 degrees Celsius. Isn't this a bit contradictory?

This involves the concepts of heat and temperature. In space, the temperature can reach thousands of degrees Celsius, but it does not provide a lot of heat to objects. Temperature is based on the measurement of the speed at which particles move, and heat is the measurement of the total energy transferred by these particles. Therefore, high temperature corresponds to the possibility that particles may move very fast, but if there are few particles, not much energy will be transferred, which will result in low heat. Since most of space is a vacuum, there are very few particles that can transfer energy to the front end of the detector's shield. When the Parker detector passes through the corona region with millions of degrees Celsius, although the temperature is extremely high, the particle density is very low. Scientists have calculated that the front end of the heat shield only needs to withstand about 1400 degrees Celsius. Even if it only needs to withstand about 1400 degrees Celsius, this is a very high heat-resistant temperature for artificial materials. In comparison, the temperature of lava from volcanic eruptions is slightly lower.

Artist's impression of the Parker Space Telescope passing through the solar corona.

(Image credit: NASA)

In order to capture enough solar wind particles, the Faraday cup that collects particles cannot hide behind a heat shield, otherwise it will not be able to collect particles. Scientists use titanium-zirconium-molybdenum alloy to make the Faraday cup, which can withstand further increases in temperature, allowing it to be directly exposed to the strong radiation environment in the corona region. From this, we can see that one of the foundations of exploring outer space is materials science. When we have materials that can withstand the extreme corona region environment, we can enter this space to carry out scientific research exploration. This route also applies to other celestial bodies in the solar system, such as Europa, which has strong radiation and extremely cold environments, and the surface of Venus, where the atmospheric pressure is nearly 100 times that of the Earth.

References:

1. Our Sun: The Facts

2. Heading to the Sun: Why the Parker Solar Probe Won’t Melt

3. Going Deeper: Parker Solar Probe

4. Parker Solar Probe makes closest flyby of the Sun in history

5. Wikipedia: Solar flare

6. Regarding solar flares, all your concerns will be clarified at once