49 Starlink satellites encountered geomagnetic storm. Is it the sun's fault?

◎ Science and Technology Daily reporter Tang Fang

Recently, the official website of SpaceX announced that due to a geomagnetic storm, as many as 40 of the 49 Starlink satellites launched by the company on February 3 had already or would fall into the atmosphere and be destroyed the next day. The company emphasized that the risk of these deorbited satellites colliding with other satellites is "zero".

Luo Bingxian, a researcher at the National Space Science Center of the Chinese Academy of Sciences, told Science and Technology Daily that as of February 12, public data showed that five Starlink satellites launched in this batch have been confirmed to have crashed, more than 10 are still in orbit, and the situation of the remaining satellites is still unclear.

"The probability of these deorbited satellites colliding with other satellites is relatively low, but it is a bit absolute to say that the risk is 'zero'." Lei Jiuhou, a professor at the School of Earth and Space Sciences of the University of Science and Technology of China, told Science and Technology Daily that if a satellite is launched in the near future, it is still necessary to avoid passing through the airspace of out-of-control satellites.

At an orbital altitude of about 210 kilometers, the Starlink satellite encountered the lowest level of geomagnetic storm

What is a geomagnetic storm? Luo Bingxian explained that when solar wind reaches the Earth, it causes disturbances in the Earth's magnetic field. If the disturbance is severe, it is a geomagnetic storm. In years with high solar activity, geomagnetic storms occur more frequently and at higher levels.

"The geomagnetic storm encountered during the Starlink satellite damage incident was of the lowest level," Luo Bingxian told Science and Technology Daily. The geomagnetic Kp index is usually used to characterize the strength of geomagnetic disturbances. The Kp index is divided into 0 to 9 levels, and Kp=5 indicates a low-level geomagnetic storm. Space weather monitoring shows that on February 4, local time, this batch of Starlink satellites encountered a geomagnetic storm of Kp=5 at about 210 kilometers.

"Geomagnetic storms of this level have occurred an average of more than 50 days per year from 1957 to now. In years with high solar activity, they occur more than 120 days a year." Luo Bingxian said that low-level geomagnetic storms have little impact on ordinary satellites in orbit and are basically controllable.

Kp=9 is a strong geomagnetic storm. The Earth's radiation environment and atmospheric environment will undergo drastic changes. Relevant agencies will make corresponding early warning, protection and avoidance measures for strong geomagnetic storms.

Talking about the principle and process of the generation of this geomagnetic storm, Lei Jiuhou said that along with the explosion on the surface of the sun, the earth will generally be attacked by three rounds: enhanced electromagnetic radiation (flares, reaching the earth in about 8 minutes), high-energy particle flows (reaching the earth in about 1 hour) and high-speed plasma clouds ejected from the solar corona.

"This Starlink satellite event was caused by an M-class flare from the sun on January 29, accompanied by a moderate-intensity coronal ejection event on January 30." Lei Jiuhou went on to say that the plasma mass carried by the solar wind arrived near the earth on February 3, interacting with the earth's magnetic field, and through processes such as compressing the top of the earth's magnetosphere and causing magnetic field reconnection, it caused severe disturbances in the magnetosphere and ring current system in the earth's space, thereby generating a geomagnetic storm.

Experts: Geomagnetic storm is only a trigger, damage may be caused by "satellite propulsion strategy"

Although Elon Musk and his companies have made many innovations and breakthroughs in space exploration, the Starlink project he proposed in 2014 has been widely criticized. The project originally planned to launch 12,000 satellites into space, but the company has been approved to build about 30,000 more satellites, which may increase the total number of satellites to 42,000. According to reports, as of the end of December 2021, the company has launched nearly 1,900 satellites.

"It's a combination of 'natural disasters' and 'man-made disasters'." According to Lei Jiuhou's analysis, the Starlink satellite incident was caused by a geomagnetic storm that heated the Earth's atmosphere, increasing the drag on the satellites and preventing them from ascending to orbit. However, Lei Jiuhou pointed out that the damage to these satellites may be mainly caused by the satellite propulsion strategy and satellite design, and the increase in atmospheric drag is only a trigger.

It is understood that Starlink satellites operate in an orbit around 500 kilometers with an inclination of about 55 degrees. Why did it fail at an orbital altitude of 210 kilometers this time?

Luo Bingxian pointed out that the propulsion strategy for this batch of Starlink satellites is to first use rockets to send the satellites into a predetermined orbit of 210 kilometers, and then use electric thrusters to send the satellites to a higher altitude. However, the satellites encountered a geomagnetic storm when they stayed at 210 kilometers.

"The satellite is in an orbit of 210 kilometers and uses electric propulsion. The atmospheric drag and propulsion force may be roughly equivalent. However, this geomagnetic storm caused the atmospheric drag to increase and the propulsion effect failed. In addition, the satellite is designed to enter a safe mode and cannot return to the orbit raising state," said Lei Jiuhou.

SpaceX said that a geomagnetic storm on February 4 had a serious impact on these satellites, causing the atmosphere in the area where the satellites were located to warm up and increase its density. The atmospheric drag on the satellites increased by up to 50% compared with previous launch levels.

Luo Bingxian analyzed the rationality of this data. "After calculation and observation, it is confirmed that this geomagnetic storm caused the atmospheric density to increase. In addition, as the satellite descends in altitude due to atmospheric drag, it will encounter an increasingly dense atmosphere. Overall, it is reasonable for the atmospheric drag on the satellite to increase by 50% in a short period of time," he said.

In Lei Jiuhou's opinion, if the satellite is sent to a higher orbit from the beginning, even if it encounters a stronger geomagnetic storm, or even a disturbance many times stronger, it will not cause the satellite to be quickly damaged. He added that satellites are generally pushed to a higher orbit and will not be parked at an altitude of 210 kilometers. This is because the atmospheric density decays exponentially with altitude. When the orbit is raised by 100 kilometers, the atmospheric drag is 1-2 orders of magnitude lower. The atmospheric drag at an orbit altitude of 400-500 kilometers is relatively small.

"These deorbited Starlink satellites will fall from about 210 kilometers and burn up after re-entering the atmosphere. No space debris will be generated, and no satellite parts will hit the earth's surface," said Luo Bingxian.

Regarding the risk of deorbiting Starlink satellites, Lei Jiuhou said that these satellites are in a low orbit of about 210 kilometers, and are subject to greater resistance and a shorter lifespan. Without control, they will burn up in a dozen days or more during the fall. In addition, there are few other satellites in this orbit, so the chance of collision is relatively low.

How big an impact do geomagnetic storms have on artificial satellites?

Lei Jiuhou said that during geomagnetic storms, high-energy particles from the sun fall and (electric current) Joule heating occurs, which makes it easy to see enhanced auroral activity in the polar regions, and will also cause the global upper atmosphere temperature to increase. The atmosphere expands, and the atmospheric density near the satellite orbit increases, which increases the satellite drag.

In addition, there are many other influences, such as changes in the composition of the atmosphere, changes in the atmospheric circulation, and the corresponding area of ​​atmospheric ionized plasma, commonly known as the ionosphere, will also undergo drastic changes, affecting satellite and ground communications.

It is worth emphasizing that during geomagnetic storms, the magnetic field structure protecting the Earth will change. Many satellites in higher orbits, such as synchronous satellites, will be exposed to different magnetic fields and plasma environments, and thus be directly attacked by high-energy particles, and even cause some satellite components to fail.

Luo Bingxian told reporters that powerful solar eruptions could cause severe geomagnetic storms and affect satellites in orbit or infrastructure on the ground, such as two famous examples in the 20th century.

In 1979, the United States' Skylab was in orbit during a year of high solar activity. Due to the cumulative effect of multiple geomagnetic storms, the expansion of the Earth's atmosphere caused a sharp increase in atmospheric drag on Skylab, and its orbit decayed faster. Skylab fell into the atmosphere and burned up faster than expected.

In 1989, a geomagnetic storm overloaded the power grid in Quebec, Canada, causing a large-scale power outage in the region that lasted for 9-12 hours. It was also reported that at almost the same time, thousands of targets in the U.S. space target tracking system needed to be repositioned, making it difficult to track and identify newly launched flying objects.

Source: Science and Technology Daily

Editor: Wang Yu

Review: Yue Liang

Final judge: He Yi