How "hot" is the sun? This detector has "touched" it

Producer: Popular Science China

Author: Zheng Zheng has sound

Producer: China Science Expo

During the Cold War between the United States and the Soviet Union, Americans made up a lot of "stories" that smeared the Soviet Union, such as this famous "going to the sun at night":

After the americans landed on the moon, Brezhnev called soviet cosmonauts on the same day ...

"Now that the Americans have landed on the moon, the Soviet Union has now decided to send you to the sun right away." Brezhnev said.

"Don't you know, Comrade Brezhnev, we will be burned to death." The astronaut gasped and choked.

Brezhnev said angrily: "Do you think the Politburo did not consider it?" We have decided to send you to land in the sun at night! ”

Older people who are often hacked in "Soviet jokes" (Image: IMRussia)

This joke is widely circulated, and it has been repeatedly changed into an online paragraph or cross-talk line to describe a person with high status, fame and no common sense.

As we all know, the earth has day and night, but whether it is day or night, the sun is an incomparably hot fireball. As for landing or flying over? It was a pipe dream.

Earth rotation and the cycle of day and night (Image: vjshi.com)

But as science progressed, the joke of landing on the sun became a reality.

On December 14, 2021, Thomas Zubchin, deputy director of nasa's Science Mission Board, announced at the "2021 American Geophysical Union Fall Meeting" in New Orleans that the human-launched Parker Solar Probe (PSP) successfully crossed the outermost layer (corona) of the solar atmosphere in April 2021 three years after launch, becoming the first spacecraft to "touch" the sun.

Although the initial experience lasted only a few hours, it opened the chapter of human "contact" with the sun, and PSP will make 21 close-up flights to the sun over the next 7 years.

Landing on the sun, how high temperatures do you have to withstand?

We often hear, "The temperature on the surface of the sun is about 6,000 degrees", where does this data come from? Did someone test it? In fact, this temperature mainly comes from the analysis of solar radiation by scientists.

The surface of the sun is called the "photosphere", and it is from here that the visible light of the sun that we see with the naked eye is emitted. In addition to visible light, the photosphere also emits radiation of different frequencies in the whole wavelength band, such as ultraviolet rays, X-rays, etc.

Scientists analyzed the intensity of each frequency radiation and found that some frequencies radiated strongly and some were weak, which was very similar to the distribution of radiation intensity released by black bodies at about 6000 degrees (5700 Kelvin temperature to be precise), so they concluded that the surface temperature of the sun was about 6000 degrees.

However, the Parker probe is not passing through the surface of the sun, but the corona, which is much hotter than the 6,000 degrees of the surface.

The solar atmosphere is mainly divided into photospheres, chromospheres, transition zones and corona from the inside out, and the temperature changes with altitude. The curve of the following figure is the temperature change curve of the solar atmosphere from low to high, from thousands of degrees to tens of thousands of degrees on the surface, and then to the millions of Kelvin temperatures of the corona, the temperature of the solar atmosphere from the inside to the outside is getting higher and higher.

The temperature of the sun's atmosphere from the inside out, the blue Corona refers to the corona, which can reach millions of Calvin temperatures (273 degrees difference from Celsius) (Image: NASA)

Therefore, the Parker solar probe, although in the outermost region of the Sun, is instead in a place of higher temperatures.

However, the "temperature" of the "plasma" around the sun is not exactly the same as our daily experience of contact with cold and warmth. In a region of space, its temperature can reach several thousand degrees without providing significant heat to other objects.

That is, we don't feel hot in it. Why?

Before explaining this problem, we need to know that matter is composed of small to invisible particles - molecules and atoms, the so-called temperature, in fact, is a measure of the speed of movement of the particles that make up matter, and heat, a measure of the total amount of energy transferred between matters. Particles may move very quickly (high temperatures), but if the number is very small, they will not transfer a large amount of energy (low heat).

Since space is mostly empty, places like the corona are actually extremely dense, and the heat that can be directly transferred is not very high. Around the sun, what makes it easier for the probe to heat up is the sun's massive radiation. Just as we feel warm when we bask in the sun, solar radiation represented by visible light will transmit a lot of energy to the objects it hits.

So while the Parker solar probe travels through a corona space with temperatures of several million degrees, according to scientists' calculations, the surface of the insulated screen on the side of the probe facing the sun can only be heated to about 1400 degrees Celsius (2500 degrees Fahrenheit). Of course, this temperature is already very high, for example, the temperature of magma from volcanic eruptions is about 700-1200 degrees Celsius, while the melting point of iron is 1538 degrees Celsius, which is comparable to the melting edge.

Fly over the sun adventure, how to choose equipment?

At a high temperature of 1400 degrees, even if the detector is not melted, the precision equipment inside is difficult to work properly. How did the Parker solar probe successfully cross the solar corona and even conduct scientific exploration?

This is actually because scientists "wear" a heat shield for the Parker solar probe. The thermal shield consists of a composite carbon protective layer of 11.43 cm (4.5 inches) thick and coated with a white ceramic layer that allows it to reflect as much solar radiation as possible, led by sunlight.

The thermal shield is designed on the side of the detector facing the sun, on top of a large radiator constructed of a titanium frame. As the spacecraft flew to the surface of the sun, this structure kept the spacecraft's payload in a cool, dark shadow.

Workers install thermal shields (Credit: NASA)

According to scientists' estimates, although the outer layer faces a temperature of 1400 degrees Celsius, due to the role of the protective cover, the temperature of the bottom surface of the protective layer can be maintained at 315-371 degrees Celsius when the "Parker" solar probe is closest to the surface of the sun in the later part of the mission. This temperature is about the melting point of lead, which is far from the melting point of aluminum of 660 degrees Celsius.

In fact, the scientists and engineers who designed the spacecraft are more worried that the sun's back without solar radiation may be too cold, and the scientific instruments and equipment on the spacecraft may freeze during the mission, instead of melting.

To avoid going to the extreme of supercooling and freezing, the exteriors of these instruments and devices carried by the Parker detector are wrapped in thermal blankets and paired with separate solar heaters. Simply put, these instruments are covered with electric blankets just like we spend the winter! This allows them to keep the temperature of their working environment at around 28 degrees Celsius.

Whether heated or cooled, parker detectors need electricity as power and power for the instruments and other electronic equipment on board. The close sun is its "source of life". The solar panels used in the detector's power generation are mounted on wing-like flaps that protrude from under the radiator, which not only handle the heat of the sun, but are also equipped with their own cooling system, which can continuously provide power to the Parker detector.

Parker Solar Probe design appearance, the front of the white plate is a thermal shield (Image: NASA)

Back to the beginning of our article - the reason why "going to the sun at night" has become a joke is because the joke-telling public has long recognized that the sun is extremely hot, and that day and night are caused by the earth's rotation around the sun, and the sun does not become cold because it enters the night.

A little thought will show that the public perception has increased with the continuous exploration and research of scientists, and it is precisely because of the continuous study of the sun by scientists led by the scholar Eugene Parker that we know it today. With the support of the technology introduced in this article, "flying over the sun" is becoming a reality, and it can bring us more understanding of the sun, greatly expanding human understanding and reconstruction of solar physics.

Today, the "Parker" probe to get close to the sun, the precious information brought back will become the public tomorrow's "three-year-old children know common sense." It is believed that in the near future, the "Parker" solar probe will bring us more mysteries of the sun and complete more seemingly "impossible" tasks. After the Parker probe, there are many new probes ready to contribute to human exploration of space, such as China's "Xihe" probe, as well as NASA's MUSE and HelioSwarm missions, which are spacecraft observing the sun. Let's wait and see!

Bibliography:

1, Parker Solar Probe Completes a Record-Setting Swing by the Sun, https://blogs.nasa.gov/parkersolarprobe/

2, Parker Solar Probe’s Heat Shield Enters Thermal Vacuum Testing, https://blogs.nasa.gov/parkersolarprobe/2017/12/

3, The Parker Solar Probe Mission,https://blogs.nasa.gov/parkersolarprobe/2018/08/11/the-parker-solar-probe-mission/