How hot is the sun? This probe has already touched it

During the Cold War between the United States and the Soviet Union, Americans fabricated a lot of "stories" to discredit the Soviet Union, such as this famous "Go to the Sun at Night":

After the Americans successfully landed on the moon, Brezhnev called the Soviet astronauts that same day...

"Since the Americans have already landed on the moon, the Soviet Union has now decided to send you to land on the sun immediately," Brezhnev said.

"Don't you know, Comrade Brezhnev, we will be burned to death?" the astronaut was shocked and choked.

Brezhnev said angrily: "Do you think the Politburo has not considered this? We have decided to send you to land at night at the sun!"

The old people who are often ridiculed in "Soviet Jokes"

(Image source: IMRussia)

This joke is very popular and has been modified many times to become online jokes or crosstalk lines, used to describe people who have high status and fame but lack common sense.

As we all know, the earth has day and night, but no matter whether it is day or night, the sun is an extremely hot fireball. As for landing or flying over? That is simply a foolish dream.

GIF: Earth's rotation and the alternation of day and night

(Image source: vjshi.com)

But with the development of science, the joke of landing on the sun has become a reality.

It is reported that on December 14, 2021, Thomas Zurbuchen, deputy administrator of the Science Mission Directorate of the National Aeronautics and Space Administration (NASA), announced at the "2021 American Geophysical Union Fall Meeting" held in New Orleans that the Parker Solar Probe (PSP) launched by humans successfully passed through the outermost layer of the solar atmosphere (corona) in April 2021, three years after its launch, becoming the first spacecraft to "touch" the sun.

Although this first experience lasted only a few hours, it opened the chapter of human "contact" the sun. PSP will conduct 21 close flights to the sun in the next 7 years.

How high a temperature would you have to endure if you landed on the sun?

We often hear that "the temperature of the sun's surface is about 6,000 degrees." Where does this data come from? Has anyone measured it? In fact, this temperature mainly comes from scientists' analysis of solar radiation.

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

Scientists analyzed the intensity of each frequency radiation and found that some frequency radiation is strong and some is weak, which is very similar to the radiation intensity distribution released by a black body at about 6000 degrees (to be precise, 5700 Kelvin temperature, about 5500 degrees), so they concluded that "the surface temperature of the sun is 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 on the surface.

The solar atmosphere is mainly divided into the photosphere, chromosphere, transition zone and corona from the inside to the outside, and the temperature changes with altitude. The curve in the figure below is the temperature change curve of the solar atmosphere from low to high altitude, from thousands of degrees on the surface to tens of thousands of degrees, and then to millions of Kelvin temperatures in the corona. The temperature of the solar atmosphere increases from the inside to the outside.

The temperature of the solar atmosphere from the inside to the outside. The blue corona refers to the solar corona, which can reach millions of Kelvin temperatures (273 degrees Celsius).

(Image credit: NASA)

Therefore, although the Parker Solar Probe is in the outermost region of the Sun, it is actually in a place with higher temperatures.

However, the "temperature" of the "plasma" around the sun is not exactly the same as our daily experience of cold and heat. In a certain area of ​​​​the universe, its temperature can reach several thousand degrees without providing significant heat to other objects.

That is to say, we do not feel hot when we are in it. Why is that?

Before we explain this problem, we need to know that matter is made up of invisible particles - molecules and atoms. The so-called temperature is actually a measure of the movement speed of the particles that make up the matter, while heat is a measure of the total amount of energy transferred between the matter. Particles may move very fast (high temperature), but if there are very few of them, they will not transfer a lot of energy (low heat).

Since most of the universe is empty, places like the corona actually have very low material density, so the amount of heat that can be directly transferred is not very high. Around the sun, it is the huge amount of solar radiation that makes the detectors heat up more easily. Just like we feel warm when we bask in the sun, solar radiation represented by visible light will transfer a lot of energy to the objects it irradiates.

Therefore, although the Parker Solar Probe passed through a corona space with a temperature of several million degrees, according to scientists' calculations, the heat shield surface 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 a volcanic eruption is about 700-1200 degrees Celsius, and the melting point of iron is 1538 degrees Celsius, which is just on the edge of melting.

How to choose equipment for your adventure over the sun?

At a high temperature of 1,400 degrees, even if the probe does not melt, the delicate equipment inside will have difficulty working properly. How did the Parker Solar Probe successfully pass through the solar corona and even conduct scientific exploration?

This is actually because scientists have put a heat shield on the Parker Solar Probe. This heat shield is made of a 11.43 cm (4.5 inch) thick carbon composite protective layer, which is also coated with a white ceramic layer to reflect as much solar radiation as possible, mainly sunlight.

The heat shield is designed to sit on the side of the spacecraft that faces the sun, atop a large radiator constructed from a titanium frame. This structure will keep the spacecraft's payload in cool, dark shadow as it travels toward the sun's surface.

Workers install heat shields

(Image credit: NASA)

According to scientists' estimates, although the outer layer faces a temperature of 1,400 degrees Celsius, due to the protective cover, the temperature of the bottom surface of the protective layer of the Parker Solar Probe can be maintained at 315-371 degrees Celsius when it is closest to the surface of the sun in the later stage of the mission. This temperature is about the melting point of lead, which is far from the melting point of aluminum at 660 degrees Celsius.

In fact, the scientists and engineers who designed the spacecraft were more worried that the dark side might be too cold without solar radiation, and the scientific instruments and equipment on the spacecraft might freeze instead of melt during the mission.

In order to avoid going to the extreme of supercooling and freezing, the exterior of these instruments and equipment carried by the "Parker" probe are wrapped with thermal blankets and paired with individual solar heaters. Simply put, these instruments are also covered with electric blankets like we spend the winter! This can keep the temperature of their working environment at around 28 degrees Celsius.

Whether heating or cooling, the Parker probe needs electricity as a power source and to power the instruments and other electronic equipment on board. The nearby sun is its "source of life". The solar panels used by the probe to generate electricity are installed on wing-shaped flaps extending from under the radiator. They can 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 probe.

The design of the Parker Solar Probe. The white plate on the front is the heat shield.

(Image credit: NASA)

Let’s go back to the beginning of this article - the reason why “going to the sun at night” becomes a joke is that the public who tell the joke already knows that the sun is extremely hot, and that day and night are caused by the rotation of the earth around the sun, and the sun will not get colder as night falls.

If you think about it, you will find that the public's understanding is improved as scientists continue to explore and study the sun. It is because of the continuous research of scientists led by scholar Eugene Parker that we have today's understanding of the sun. With the support of the technology introduced in this article, "flying over the sun" is becoming a reality and can bring us more knowledge about the sun, greatly expanding human understanding and reconstruction of solar physics.

Today, the Parker Solar Probe is going to get close to the Sun, and the valuable information it brings back will become "common sense that even a three-year-old child would know" tomorrow. I believe that in the near future, the Parker Solar Probe will bring us more mysteries of the Sun and accomplish more seemingly "impossible" tasks.

After the Parker probe, many new probes are contributing to human exploration of the sun, such as China's Xihe probe and the Advanced Space-based Solar Observatory (ASO-S) to be launched this year, as well as NASA's MUSE and HelioSwarm missions, all of which are spacecraft that observe the sun. Let's wait and see!

References:

[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/

Produced by: Science Popularization China

Author: Zheng Zheng You Sheng

Producer: Computer Network Information Center, Chinese Academy of Sciences