40 kilometers above the sky, scientists want to release a "balloon" to serve as an observatory

Produced by: Science Popularization China

Author: Gao Xiaofeng and Huang Wanning (Institute of Aerospace Information Innovation, Chinese Academy of Sciences)

Producer: China Science Expo

In recent years, my country's aerospace industry has achieved vigorous development. The manned space station has completed the in-orbit construction mission, the Chang'e project has completed the three major tasks of "orbiting, landing, and returning", many astronomical observation satellites have been successfully launched, and remarkable scientific results have been achieved. Tianwen-1 has also successfully landed on Mars, opening a new chapter in deep space exploration... Do you know? Behind this series of achievements, "balloons" have also played an important role.

Everyone has played with balloons, but there is one type of "balloon" that is extraordinary. It opened the door to space science research. Its related results have won the Nobel Prize in Physics twice and made great contributions to the launch of many space astronomical satellites. It is the high-altitude scientific balloon!

Figure 1 The results of the high-altitude scientific balloon experiment won the Nobel Prize in Physics twice

(Image source: self-made by the author)

Figure 2 Schematic diagram of a high-altitude scientific balloon

(Image source: self-made by the author)

Why do we have to use "balloons" to study space science?

To answer this question, we must start with the research methods of space science.

In 1912, Austrian physicist Hess discovered the existence of cosmic rays through a balloon experiment. From that moment on, detecting various rays from space became an important method of space science research.

Figure 3 Hess discovered cosmic rays

(Image source: self-made by the author)

However, cosmic rays that can be directly detected on the surface are relatively rare, because the Earth's dense atmosphere blocks most radiation sources from space, including infrared rays, X-rays, and gamma rays. If you can rise above 30km above the surface, the density of the atmosphere is only 1% of that at sea level, and you can directly observe many cosmic rays and radiation sources. Scientists believe that this is already very close to the environment of space.

Therefore, using scientific instruments that can fly to an altitude of 30-40km for observation has become the only choice for scientists.

High-altitude scientific balloons work in near-space and are close to the space environment. They also have advantages such as lower launch costs than satellites, shorter preparation cycles, more flexible launches, and convenient instrument recovery. They can often be used to effectively and cheaply verify scientific ideas, instrument principles, and technical methods for large-scale space science programs. Therefore, high-altitude scientific balloons are behind every major space research mission.

How to use high altitude scientific balloons?

Figure 4 Using high-altitude scientific balloons to detect space rays

(Image source: self-made by the author)

As shown in Figure 4, high-altitude scientific balloons use helium as the buoyancy gas and carry scientific payloads to conduct high-altitude scientific experiments through the net buoyancy generated by the sphere. A typical high-altitude scientific balloon system consists of three parts: the balloon sphere, the parachute, and the payload pod, as shown in the figure below.

Figure 5 Composition of the high-altitude scientific balloon flight system

(Image source: self-made by the author)

The difference from common latex balloons is that in order to prevent the high-altitude scientific balloon from bursting during the launch and hovering process, we need to change the structure of the balloon.

High-altitude scientific balloons are divided into zero-pressure high-altitude scientific balloons and super-pressure high-altitude scientific balloons according to the different internal and external pressure differences. The zero-pressure high-altitude scientific balloon has an open structure, and the pressure difference between the inside and outside of the balloon is close to 0. The balloon itself cannot withstand large pressure; the super-pressure high-altitude scientific balloon adopts a closed structure, and through special structural design it can withstand large pressure differences.

Take the traditional zero-pressure high-altitude scientific balloon as an example: when it is released, the buoyancy gas does not completely fill the space of the sphere, only obvious bubbles gather at the head, and the lower part of the high-altitude scientific balloon is still in a wrinkled state.

Figure 6 High-altitude scientific balloons during the release phase

(Image source: self-made by the author)

At this time, the buoyancy of the entire system is greater than the gravity, and the high-altitude scientific balloon will naturally rise. As the altitude increases, the external atmospheric pressure decreases, and the gas inside the sphere begins to expand. The higher the altitude, the lower the atmospheric density, and the smaller the buoyancy generated by the sphere displacing the air. When it reaches a balance point where the buoyancy equals the gravity, it stops rising and begins to fly horizontally. At this time, its volume expands to the maximum, taking on a teardrop shape. Scientific instruments generally work in the level flight stage.

Figure 7 The state of the high-altitude scientific balloon during the level flight phase

(Image source: self-made by the author)

The zero-pressure high-altitude scientific balloon is designed with an exhaust pipe. When it flies for a long time during the day and is exposed to the sun, the temperature of the helium inside the balloon rises and its volume expands. At this time, the excess helium is discharged into the atmosphere through the exhaust pipe, and the entire system still maintains a balance of buoyancy and gravity.

Figure 8 Zero-pressure balloon exhaust pipe during level flight

(Image source: self-made by the author)

The super-pressure high-altitude scientific balloon has no exhaust pipe. It relies on a special pumpkin-shaped spherical structure design to withstand the pressure increase caused by the temperature rise of the buoyant gas. The advantage of this design is that there is no loss of buoyant gas during the entire day and night cycle, which can greatly increase the flight time. Since the volume is almost unchanged during the day and night, its flight altitude is also very stable.

Figure 9: The appearance of a super-pressure high-altitude scientific balloon

(Image source: self-made by the author)

Figure 10 Comparison of the day and night flight altitudes of super-pressure high-altitude scientific balloons and zero-pressure high-altitude scientific balloons

(Image source: Reference 6)

Depending on the needs of the scientific payload, the working time of the high-altitude scientific balloon's level flight phase can range from a few hours to dozens of days. When the scientific payload completes the test mission, the level flight can be ended. The measurement and control center issues a command to cut the connecting cable between the parachute and the balloon. The spherical part rises rapidly because of the loss of weight, and the helium expands violently, causing the sphere to rupture. Therefore, the spherical part of the high-altitude scientific balloon is disposable and cannot be recycled.

Figure 11 The sphere breaks after cutting

(Image source: self-made by the author)

The parachute deployed quickly and slowly fell back to the ground with the payload pod below. After the scientific team recovered the payload pod, they made further iterative improvements to the instrument so that it could continue to participate in subsequent flight tests.

Figure 12 The payload pod is recoverable

(Image source: self-made by the author)

Figure 13 Mission profile of the entire high-altitude scientific balloon flight

(Image source: self-made by the author)

Our own high altitude scientific balloon technology

The high-altitude scientific balloon industry in China originated from the Institute of High Energy Physics of the Chinese Academy of Sciences (hereinafter referred to as IHEP). In June 1977, IHEP held a cosmic ray planning meeting, at which Professor Li Tibei gave a report on "Introduction to Cosmic Ray Astronomy". The meeting proposed the development strategy of "mastering high-altitude scientific balloon technology as soon as possible in the next eight years, conducting cosmic ray astrophysical observations, and then striving to launch a satellite".

1977 to 1984 was the first stage of the development of my country's high-altitude scientific balloon industry. After the unremitting efforts of the older generation of scientists, in 1983, my country's first high-altitude scientific balloon "HAPI-0" successfully took off in Xianghe County, Hebei Province, and observed gamma rays, obtaining good space observation data. This marked that my country had established its own high-altitude scientific balloon system and achieved a technological leap from scratch.

1985-1990 was the second stage of the development of my country's high-altitude scientific balloons. The main feature of this stage was "development and application at the same time". Many scientific experiments were gradually carried out, which not only met the scientific application needs, but also developed the high-altitude scientific balloon platform technology.

In 1986, my country and Japan cooperated to carry out a high-altitude scientific balloon transoceanic flight experiment. The 5,000-cubic-meter high-altitude scientific balloon took off from Kagoshima, Japan, and landed successfully in Wanli Village, Tonglu County, Zhejiang Province after about 18 hours. The Shanghai Astronomical Observatory recovered the high-altitude scientific balloon capsule intact. From 1986 to 1989, the China-Japan cooperation high-altitude scientific balloon flight experiment conducted a total of 7 successful flights.

The period from 1991 to the present is the golden period for the development of high-altitude scientific balloons in my country. The technology of zero-pressure high-altitude scientific balloons has basically matured. According to different load and flight altitude requirements, a series of models from small to large have been formed. At the same time, the technology of super-pressure high-altitude scientific balloons is also developing rapidly.

According to statistics, my country has carried out more than 200 high-altitude scientific balloon exploration and technical experiments to date, and the research fields cover astronomy, space physics, space chemistry, atmospheric physics, microgravity, space biology and remote sensing.

It can be said that China's high-altitude scientific balloons were born out of the strong demand for space science exploration. In the process of high-altitude scientific balloon development, it has also made great contributions to the establishment of many space science instruments and satellites.

The story behind "Wukong" asking the sky

The Wukong satellite (dark matter particle detection satellite), supported by the Strategic Priority Research Program of Space Science of the Chinese Academy of Sciences, is my country's first astronomical satellite. Wukong directly observed the highest energy electron cosmic rays from space for the first time, and successfully obtained the most accurate electron cosmic ray detection results in the world. Its first batch of results were published in Nature magazine at the end of 2020. Yin Gezhi, scientific director of Nature China, said: "The detection results of Wukong may change the way we look at the universe."

The launch of "Wukong" also benefited from the contribution of high-altitude scientific balloons.

In the 1990s, the high-altitude scientific balloon project led by NASA, equipped with the Advanced Thin Ion Calorimeter (ATIC), planned to conduct high-altitude cosmic ray observations in Antarctica.

In 1997, Chang Jin (now an academician of the Chinese Academy of Sciences) and his team from the Purple Mountain Observatory of the Chinese Academy of Sciences proposed to American scientists to add the observation of primordial high-energy electrons to the "ATIC" project. In order to convince American scientists, Chang Jin flew to the United States, wrote his ideas into a program in front of them, and calculated the parameters for verification.

Finally, thanks to Chang Jin's efforts, the Americans agreed to give him the test data of the Antarctic high-altitude scientific balloon for analysis. In the following seven years, Chang Jin participated in Antarctic observations three times and successfully found 210 high-energy electrons that exceeded the normal energy spectrum among more than 30 million cosmic particles. Chang Jin believed that they might come from the annihilation of dark matter in the universe.

In 2008, the results were published in Nature magazine. With the experimental verification of the ATIC high-altitude scientific balloon, the dark matter particle detection satellite project was officially included in the Chinese Academy of Sciences' space science pilot project at the end of 2011. On December 17, 2015, the "Wukong" was successfully launched.

Figure 14 Advanced Thin Particle Calorimeter (ATIC) high altitude scientific balloon experiment

(Photo credit: Strato Cat)

Figure 15 "Wukong"

(Photo source: Xinhuanet)

Insight surveys the sky, with balloons taking the lead

On June 15, 2017, my country's independently developed HXMT (Hard X-ray Modulation Telescope) satellite "Insight" was successfully launched and officially put into use on January 30, 2018. The "Insight" satellite project is a major independent innovative space science project to study cutting-edge issues of compact celestial bodies such as black holes and neutron stars. It is of great significance to enhancing my country's international status in the field of space science.

High-altitude scientific balloons have played an indispensable role in the success of Insight. As early as the 1990s, Academician Li Tibei and Researcher Wu Mei of the Institute of High Energy Physics proposed a direct demodulation imaging method, which uses nonlinear mathematical methods to directly invert the original measurement equation to achieve imaging. This method can use cheaper instruments to obtain higher-resolution images.

In 1993, a non-position-sensitive hard X-ray detector based on this "inexpensive and high-quality" imaging technology was used to conduct scanning observations of the black hole candidate Cygnus X-1 through the HAPI-4 (high altitude platform instrument) high-altitude scientific balloon experiment, and achieved much better results than its American counterparts.

Figure 16 International peers gave high praise to the work of the hard X-ray telescope at that time

(Image source: Balloon Group, Institute of High Energy Physics, Chinese Academy of Sciences)

Figure 17 HAPI-4 spherical telescope pod and observation results

(Image source: Balloon Group, Institute of High Energy Physics, Chinese Academy of Sciences)

Of course, just one experiment is not enough to convince people. The Astronomical Group of the Institute of High Energy Physics conducted a total of five flight tests of the balloon-borne hard X-ray modulation telescope in 1992, 1993, 1998, 1999 and 2001. The 2001 test flight had a volume of 400,000 cubic meters, a load of 650 kilograms, a flight altitude of 38.9 kilometers, and a flight time of 6 hours and 40 minutes. The high-altitude scientific balloon experiment of the hard X-ray modulation telescope laid a solid foundation for the establishment and successful launch of the "Huiyan" satellite.

The "Hero Behind the Scenes" of Solar Cells

Solar cell energy systems are the most basic guarantee for space flight. When designing solar cell energy systems for space, it is necessary to obtain accurate performance parameters of solar cells under standard sunlight. Especially for deep space vehicles such as lunar or Mars exploration, the weight of the energy system is very limited. Too many solar cells can easily lead to overweight, affecting and squeezing the load weight; too few solar cells cannot meet the flight mission.

The accurate calculation of solar cell performance depends on the accurate performance parameters of solar cells under standard light. The current method of calibrating cells on the ground using simulated light cannot accurately measure solar cell parameters, so it is necessary to calibrate and measure solar cells outside the Earth's atmosphere or in a close environment.

High-altitude scientific balloons can be used to calibrate solar cells because of their unique working method. 99.5% of the atmosphere is below 35km. At the upper limit of this altitude, there is no dust, no water vapor, and no major ozone belt, so the sunlight here is basically the sunlight in outer space. High-altitude scientific balloons can bring the tested cells to an atmospheric environment above 35km, and the calibrated light source state is very close to the ideal AM0 (Air Mass 0) state.

On August 8, 2018, the Space Information Innovation Research Institute of the Chinese Academy of Sciences successfully used a high-altitude scientific balloon to conduct a high-altitude calibration test on space solar cells. For the first time in China, it completed the detailed measurement of solar cell open-circuit voltage, short-circuit current, maximum power point voltage and other parameters, which promoted and facilitated the development of new solar cells for space use in my country. It also made great contributions to the energy systems of major aerospace missions such as my country's deep space exploration missions and space station missions.

Figure 18 Calibration of solar cells in the near-space of the Chinese Academy of Sciences high-altitude scientific balloon

(Image source: Reference [4])

Conclusion

The modern high-altitude scientific balloon has a history of more than 70 years. It is currently the most mature near-space flight platform and has become the cornerstone for many new technologies to move towards space science. It is also developing in multiple directions, such as longer flight time (superpressure high-altitude scientific balloon), higher pointing accuracy (planetary science observation), and controllable flight direction (sustainable cyclic altitude adjustment technology).

Balloons are an ancient technology. When we use them to meet the needs of space science development, they bring us more innovations. In the future, I believe our scientists will use them to create more miracles.

References

[1] Liao Jun, Yuan Junjie, Jiang Yi, et al. Research on the motion characteristics of high-altitude zero-pressure balloon ascent process[J]. Space Return and Remote Sensing, 2019, 40(1):9.

[2] Tencent. From high-altitude balloons to Shenzhou IV: A review of the development of space cell electrofusion devices

[3] People’s Daily Online. The origin of Wukong is a “hitchhiking” project.

[4] Xu Guoning, Tang Yu, Li Zhaojie, et al. Research on key technologies for solar cell high-altitude balloon calibration[J]. Acta Energiae Solaris Sinica, 2021(010):042.

[5] Huang Wanning, Zhang Xiaojun, Li Zhibin, Wang Sheng, Huang Min, Cai Rong. Current status and application prospects of near-space science and technology[J]. Science and Technology Review, 2019, 37(21): 46-62.

[6] Zhu Rongchen, Wang Sheng. Research and development status of super pressure balloons [C]//Abstracts of the 24th National Space Exploration Academic Exchange Conference. Chinese Society of Space Science, 2011.