The sun is becoming increasingly "violent", how can spacecraft "defend themselves"?

Recently, there is news that due to the intensification of solar activity such as solar flares, Starlink has lost more than 200 satellites since July, and the retirement rate is faster than before. So why do distant cosmic phenomena such as solar flares threaten low-Earth orbit spacecraft such as satellites? What is the working principle behind it? What measures can researchers take to prevent and mitigate adverse conditions?

Solar activity peak exceeds expectations

What is solar activity? Simply put, it is the general term for all the active phenomena in the solar atmosphere. During solar activity, a series of amazing and spectacular scenes showing energy changes will appear one after another, including sunspots, bright spots, spectral spots, flares, prominences and coronal transient events.

Schematic diagram of solar flare

For example, sunspots are dark spots in the solar photosphere, which have stronger magnetic fields and lower temperatures than the surrounding areas, but still exceed 4,000 degrees Celsius. Solar flares are one of the most violent solar activities, also known as "chromospheric eruptions", which are mainly manifested in the ejection of a mass of coronal material from the interior of the sun within minutes to hours. Its ejection speed at its peak can exceed 1,000 kilometers per second, and the huge energy contained can be imagined.

Solar prominences are red rings around the sun, and through astronomical telescopes, you can see bright red flames dancing on the rings. Before a solar prominence explodes, it usually maintains a temperature of several thousand degrees Celsius. Once it interacts with the corona at millions of degrees Celsius, it will present a spectacular explosion effect.

To understand why these phenomena interfere with spacecraft, we need to delve into their principles. In essence, solar activity originates from the sometimes intense and sometimes weakened ionization process in the solar atmosphere, which generally has a regular cycle of 11 years. The sun in a period of intense activity is also called the "disturbed sun", which often radiates a large amount of ultraviolet rays, X-rays, particle streams and strong radio waves, which quickly impact the Earth's atmosphere, and then cause phenomena such as auroras, magnetic storms and ionospheric disturbances.

For example, coronal mass ejections are the result of the destruction of the large-scale magnetic field balance of the corona, which can seriously interfere with the flow of solar wind. The effects of high-energy particle flows and cosmic rays cannot be ignored. The state of the Earth's atmosphere is obviously not "spared."

Currently, solar activity has reached its highest level in nearly 20 years, not only reaching its peak earlier than previously predicted by astronomers, but also about 50% higher in intensity than predicted.

After years of research, scientists have discovered that as phenomena such as solar flares, prominences and coronal mass ejections become more frequent, the high-energy particles and extreme short-wave radiation (such as X-rays and ultraviolet rays) released by the sun have become stronger, the Earth's magnetic field and satellite communications have been significantly "disturbed", and even pose more catastrophic hazards to spacecraft.

Why spacecraft are "victims"

Although there is controversy over the impact of solar activity on the Earth's climate, threats such as high-energy particle flows and space radiation faced by spacecraft have been placed before aerospace researchers in various countries: In February last year, an outburst of solar radiation caused a strong geomagnetic storm, which made it difficult for about 40 SpaceX satellites to raise their orbits normally shortly after launch and forced them to re-enter the atmosphere; since this summer, the number of spacecraft failures or premature end of life in various countries has gradually increased, indicating that satellites operating in low-Earth orbits are subjected to more harsh working environments, and satellite operators in many countries are facing huge challenges.

Why does distant solar activity have such a significant impact on near-Earth orbit spacecraft? Researchers have gradually gained a deeper understanding of this issue based on lessons learned.

During early space activities, researchers gradually discovered that there are large concentrations of high-energy charged particles in the space around the Earth, called the "Earth radiation belt." These high-energy charged particles can cause radiation damage to spacecraft and lead to performance degradation of electronic devices.

As more and more satellites are launched into space, unexpected failures occur from time to time. Researchers gradually discovered that solar activity can cause the concentration of plasma in a certain area of ​​space to increase, which in turn drives the spacecraft to "charge" to a high voltage of thousands or even tens of thousands of volts, and then produce a violent "discharge" phenomenon. The strong current fluctuations often cause spacecraft components to be damaged instantly.

Schematic diagram of the solar high-energy particle flow impacting the Earth's atmosphere

Even if the spacecraft is lucky enough to "escape", the "discharge" phenomenon is accompanied by a strong electromagnetic pulse, which will still interfere with the normal operation of the spacecraft payload. Once unexpected situations such as ground-to-space communication interruption and satellite instability due to insufficient power supply occur, the ground team will face the trouble of the so-called "single particle event".

In the 1980s, small, highly integrated, and low-energy microelectronic devices were gradually widely used in spacecraft. These devices are more sensitive to electromagnetic anomalies. Therefore, what quantitative impact will the high-energy charged particles in the solar system and even the Milky Way cosmic rays, the Earth's radiation belt, especially heavy ions, have on the operation of spacecraft? This has become a key research topic for space agencies in various countries.

Later, researchers discovered that relativistic electron flux enhancement events are one of the main factors causing failures in spacecraft such as geosynchronous orbit satellites. Relativistic electrons refer to electrons that move at speeds close to the speed of light, and are not uncommon in the Earth's radiation belt. If the flux of relativistic electrons in space (which can be regarded as concentration) increases, it is likely to trigger the most harmful catastrophic space weather phenomenon in the magnetosphere, and the probability of damage to spacecraft will increase. Relativistic electrons are therefore called "killer electrons" by researchers, and related preventive measures have become a hot topic of concern in aerospace engineering in various countries.

Multi-pronged approach to circumvent threats

In order to ensure the safety of spacecraft under the influence of increasingly intense solar activity, researchers from various countries need to take a multi-pronged approach and adopt various preventive measures in different stages such as spacecraft design, testing, manufacturing and launch.

The best way to solve the problem is to work hard at the "root". A series of safeguards should be taken during the spacecraft design stage. First, researchers should clarify the applicable space environment conditions for spacecraft and formulate corresponding specifications. These specifications should not only ensure the life of the spacecraft and stable and reliable operation, but also leave room for cost control. In addition, researchers need to formulate technical conditions for ground simulation tests, specifications for device development, raw material selection catalogs, etc., and also formulate emergency plans to deal with catastrophic solar activities, satellite operation anomalies, etc. in a timely manner.

During the development phase of a spacecraft, researchers need to conduct space environment simulation tests in accordance with specifications and conduct radiation resistance tests on components and raw materials of the spacecraft. To be on the safe side, researchers need to repeatedly check the adaptability to the space environment and related measures, and verify the emergency plans previously formulated.

During the launch and in-orbit operation of a spacecraft, it seems that there is no turning back, but there are still ways to deal with the impact of solar activity. For example, researchers will choose a safe launch window that not only meets the mission requirements, but also fully considers the meteorological conditions of the target space, including the forecast of upper atmospheric density, the forecast of ionosphere state, and the forecast of the probability of meteoroids appearing.

Schematic diagram of solar activity threatening spacecraft

After the spacecraft enters orbit, the ground team must monitor the space environment in real time, "grasp" the rapidly changing key environmental parameters, help flight control personnel take timely measures to avoid or reduce the impact of space environmental events, and provide a basis for analyzing spacecraft abnormalities. In addition, establishing a space environment alarm mechanism should be the most effective countermeasure to solar activity at present.

Space plasma can easily cause "charging" failures in spacecraft, so researchers have prepared two preventive measures: one is computer numerical simulation, and the other is testing in a plasma simulation laboratory. The simulation and testing need to comprehensively consider multiple factors such as the space environment, sunlight conditions, and the shape, structure, and surface materials of the spacecraft. They can predict the impact of the space environment on the spacecraft, so as to formulate spacecraft design guidelines and test and monitoring plans, optimize control methods, and extend the life of the spacecraft. Simulation software and tests should be optimized for different orbital requirements such as geosynchronous satellites and polar orbit satellites.

In fact, to study the impact of space environmental factors such as solar activity on spacecraft, irradiation tests on components, raw materials and aerospace instruments are essential, mainly including total dose irradiation tests, single particle upset tests and accelerator irradiation tests.

The total dose irradiation test can determine the metrological indicators of components and raw materials, and provide a basis for the design of spacecraft radiation resistance. In the test, different irradiation sources are used according to the differences in the characteristics of electronic components and solar cells. Single particle upset tests are often carried out under vacuum conditions. Chip devices need to be powered on and measured online. If necessary, heavy ion accelerators are used to simulate the space environment. Accelerator irradiation tests are mainly aimed at aerospace instruments. By powering on, the ability to resist space particle radiation is tested.

In short, researchers should conduct detailed monitoring and testing throughout the entire spacecraft mission process, eliminate hidden dangers one by one, prepare emergency plans, and enable spacecraft to achieve scientific and application goals more safely and long-term. (Author: Wen Xin, Zhao Zihan, Xu Sen, Image source: NASA, Expert: Jiang Fan, Deputy Director of the Science and Technology Committee of China Aerospace Science and Technology Corporation)