How does the modern "Kuafu" satellite chase the sun?

Produced by: Science Popularization China

Author: Wang Zheng

Producer: China Science Expo

At 7:43 am on October 9, 2022, my country successfully launched the "Kuafu-1" using the Long March 2D carrier rocket at the Jiuquan Satellite Launch Center, and the satellite successfully entered the predetermined orbit.

The launch scene of Kuafu-1

(Photo source: Xinhua News Agency)

With a name like Kuafu, friends can easily think of a mythological story of ours - Kuafu chasing the sun.

Kuafu in mythology

In ancient times, there lived a group of giants with great strength. Their leader was called Kuafu. One year, the weather was extremely hot. The scorching sun shone directly on the earth, burning crops, scorching trees, and drying up rivers. The heat was unbearable, and people of the Kuafu tribe died one after another. Looking at his people in pain, Kuafu decided to catch the sun and get everyone's life back on track.

Kuafu chased the sun all the way to where the sun sets. After running for so long and being tired and scorched by the sun, he felt thirsty and wanted to drink water. He drank the water from the Yellow River and the Wei River, but the water from these two rivers was not enough to quench his thirst, so he set out to drink water from the big lake in the north. However, Kuafu died of thirst halfway before he reached the big lake. After his death, the walking stick in his hand was abandoned and turned into a peach forest. This peach forest is lush all year round, providing shade for passers-by, and the fresh peaches it produces quench the thirst of hardworking people.

The original text of this story is from the Classic of Mountains and Seas: Overseas Northern Classic: "Kuafu chased the sun and entered it. He was thirsty and wanted to drink. He drank from the Yellow River and the Wei River. The water was not enough, so he went north to drink from the Great Lake. Before he reached the lake, he died of thirst on the way. He threw away his staff, which turned into a forest."

Kuafu Chasing the Sun

(Image source: Textbook scan)

The myth defines "Kuafu" as a person who chased the sun, reflecting the ancient people's strong desire and tenacious will to explore nature. So, the use of this name for the spacecraft used by humans to observe and explore the sun has a lot of cultural connotations.

The real-life "Kuafu" - Advanced Space-based Solar Observatory (ASO-S)

The subject of this article is Kuafu-1 satellite, and the unit where the author works is the lead unit.

The scientific name of this satellite is Advanced Space-based Solar Observatory (ASO-S), which was independently proposed by Chinese scientists as early as 2011. With the support of the Chinese Academy of Sciences' Space Science Pilot Program, ASO-S has undergone preliminary research, background model research and comprehensive demonstration. At the end of 2017, it was approved by the Chinese Academy of Sciences for formal project establishment and finally developed and launched. The satellites we are familiar with, such as "Wukong", "Mozi", "Hui Yan", "Practice 10", "Taiji 1", and "Huairou 1", were all developed with the support of this pilot program.

The name "Kuafu" has actually been used for China's solar observation program, the "Kuafu Mission" proposed by Academician Tu Chuanyi of Peking University in 2003, but unfortunately the plan was not implemented. The Kuafu Mission was once planned to be launched and detect solar activity in the peak year of solar activity every 11 years (2012-2014), and the ASO-S satellite also has this intention, aiming at the peak year of the 25th week of solar activity that we are experiencing now, and achieving a breakthrough in my country's comprehensive solar satellite detection.

What is the breakthrough? ASO-S is a scientific satellite, and its scientific goals can be summarized as "one magnetic field and two storms" - "one magnetic field" refers to the solar magnetic field. The sun has a magnetic field like the earth, and it is even stronger than the earth's magnetic field. The solar prominences, sunspots, etc. on the sun are all related to its magnetic field; "two storms" refer to the two most violent explosion phenomena on the sun, namely solar flares and coronal mass ejections. Flares are dazzling spots on the sun, and "coronal mass ejections" refer to the large amount of material in the outer layer of the sun called the "corona" that is ejected into space when the sun explodes. ASO-S's research on the sun is multi-faceted, that is, observing and studying the origins of the solar magnetic field, solar flares and coronal mass ejections, and the possible causal relationship between the three.

The sun under 171Å (17.1nm) radiation imaging, flares and coronal mass ejections can be seen

(Image source: US Solar Dynamics Observatory SDO)

As for the name of ASO-S, in July 2022, the satellite engineering department solicited names from all over the country and collected more than 25,000 nominations, one-third of which suggested naming it "Kuafu". Finally, "Kuafu-1" was chosen as the name of the satellite. It can be said that the Kuafu project, which was not implemented before, has been reborn in this new series of solar observation satellites.

How does this satellite detect the sun?

We know that the Kuafu-1 satellite is helping humans better understand the sun through observations and research on the solar magnetic field, solar flares and coronal mass ejections. However, it is just a satellite orbiting the earth. How can it detect these things on the sun?

First of all, the Kuafu-1 satellite adopts a sun-synchronous dawn-dusk orbit with an altitude of about 720 kilometers and a period of about 99 minutes. The so-called sun-synchronous orbit is because the satellite is affected by the non-uniform gravity of the earth when flying, and the orbital plane will slowly rotate. From the perspective of the universe, on the one hand, it flies around the earth and follows the earth's revolution around the sun, and on the other hand, the orbital plane also rotates around the sun, keeping this surface facing the sun.

We have heard that the moon always faces the earth with its front side, and the back side can never be seen. The same is true for the satellite's sun-synchronous orbit plane, just like the front and back sides of a mirror, with only one side facing the sun all the time. The orbit plane adopted by the "Kuafu-1" satellite is even along the earth's terminator, which allows it to be exposed to the sun almost all year round - there is only a shadow from mid-May to August each year, a total of about 2.5 months, and the longest shadow time is 18 minutes a day. Therefore, it can almost always observe the sun.

So what does it observe? That's about its payload. The Kuafu-1 satellite carries a full-disk vector magnetograph (magnetograph), a Lyman-alpha solar telescope and a solar hard X-ray imager.

The magnetic field is an invisible and intangible substance, but in fact, after "polarization" of sunlight, two monochrome images of equal brightness are obtained in areas without magnetic fields, and two images of different brightness are obtained in areas with magnetic fields. The stronger the magnetic field, the greater the brightness difference between the two images. Therefore, the full-disk vector magnetograph of the "Kuafu-1" satellite consists of three functional blocks: imaging optical system, polarization optical system, and data acquisition and processing system. In the conventional observation mode, the observation of a single magnetic field component is completed by 128×2 frames of images, and a set of vector magnetograms takes 2 minutes. In this way, information on the solar magnetic field is obtained.

Solar vector magnetogram of an area observed by the Huairou Solar Observatory of the National Astronomical Observatory of the Chinese Academy of Sciences

White and black represent the positive and negative magnetic fields, and blue and red arrows represent the transverse magnetic fields at the positive and negative polarities, respectively.

(Image source: Advanced Space-based Solar Observatory official website)

"Lyman Alpha" (Lyα) is a band of radiation. The solar Lyman Alpha emission line is the strongest emission line in the solar ultraviolet spectrum. Early studies have shown that during solar flares, this spectrum line shows obvious radiation enhancement. Later, scientists found that it can well characterize the characteristics of the solar outer chromosphere and corona. The Lyman Alpha full-disk imager of "Kuafu-1" conducts imaging observations of the sun from the center of the solar disk to 1.2 solar radii at a time interval of 4-40 seconds in the band (121.6±4.5 nm), which can help us uncover the mystery of the sun.

When we have a physical examination, we sometimes use "X-rays" with strong penetrating power to take pictures to check the inside of the body. "Hard X-rays" are high-energy X-rays with a wavelength between 0.01nm and 0.1nm (if the wavelength is greater than 0.1nm, it is called soft X-rays). Hard X-rays are also an important radiation from the sun. Many activities on the sun can be better seen with hard X-rays. Therefore, the solar hard X-ray imager's photography of the sun is also helpful in studying activities such as solar flares and coronal mass ejections.

With these three instruments, the Kuafu-1 satellite can detect solar flares from multiple aspects, which will help study the scientific principles of solar activity, and will be more helpful in predicting space weather and providing protection for the safety of my country's space environment.

The sea of ​​stars above our heads has always been the yearning of mankind. The story of "Kuafu" chasing the sun tells us that mankind has never stopped on the long road of exploring deep space and nature.

Editor: Sun Chenyu