Space weather causes satellites to fall quickly to Earth

Experts fear the new solar cycle will cause satellites flying in low orbit around the Earth to run out of fuel and fall through the atmosphere faster.

At the end of 2021, the operators of the European Space Agency (ESA) Swarm satellite cluster made a disturbing discovery. Satellites that specialize in measuring the magnetic field around the Earth are starting to fall into the atmosphere at an unusually fast rate, 10 times faster than before. This change coincides with the start of a new solar cycle. Experts say this could be the beginning of difficult times for spacecraft orbiting the Earth.

ESA must raise the orbit of the Swarm satellite cluster to deal with space weather. (Photo: ESA)

"Over the past five to six years, the satellite has been descending about 2.5km/year," said Anja Stromme, ESA's Swarm mission manager. "But from December last year, they are really going down very fast. The fall rate from December 2021 to April 2022 is 20km/year."

Satellites orbiting near Earth always face atmospheric drag, causing the craft to slow down and eventually fall back to Earth. Most spacecraft cannot survive re-entry and burn up in the air. It is this drag that forces the International Space Station (ISS) controllers to perform an elevation operation to maintain the station's orbit at 400 km above the ground.

Atmospheric drag also helps clear space debris in the near-Earth environment. Scientists know the strength of the drag depends on the activity of the Sun. The amount of solar wind ejected from the star changes every 11 years. The last cycle that ended in December 2019 was relatively quiet with a lower-than-average number of monthly black streaks and a period of barely any prolonged activity.

But since last fall, the Sun has begun to awaken, spewing more and more solar winds and forming black streaks, flares and coronal eruptions at an increasing rate. Earth's upper atmosphere is also affected.

"There are complex physical mechanisms that we still don't fully understand that are going on in the upper atmosphere interacting with the solar wind. We know this interaction causes the atmosphere to 'puff' up, which means air. especially moving to higher altitudes," Stromme said.

The denser air leads to an increased drag on the satellite. Although the density is still extremely low at 400 km above Earth, the change in the atmosphere is enough to pull down some satellites in low orbit.

The Swarm satellite cluster launched in 2013 includes 3 satellites, 2 of which orbit the Earth at an altitude of 430km, 30km higher than the ISS station. The third Swarm satellite operates 515km above the ground. The two satellites orbiting in lower orbits are more influenced by the Sun, according to Stromme. The situation of this satellite duo is so dangerous that in May 2022, the operator had to raise them using the onboard propulsion system.

ESA's Swarm satellite cluster isn't the only spacecraft struggling with worsening space weather. In February this year, SpaceX lost 40 new Starlink satellites due to a solar storm shortly after launch. In such storms, the satellite often drops suddenly to a lower altitude. The lower the satellite's orbit, the greater the risk that the spacecraft will not be able to return to orbit, leaving operators helpless to watch them fall through the atmosphere.

The Starlink satellite has an operational orbit of 550 km, above the most dangerous area. However, after launch, the Falcon 9 rocket deployed the satellite very low, only about 350 km above the ground. SpaceX then lifts the satellite's orbit with its built-in propulsion. The company says this method has many advantages such as satellites with technical problems after launch will quickly fall back to Earth and not turn into space junk. However, the Sun's increasing and unpredictable activity puts the satellite at risk.

According to Stromme, all spacecraft flying around 400km have problems, including the ISS which has to adjust altitude more and more frequently to maintain orbit. But hundreds of small low-orbit satellites with cheap and simple technology are particularly vulnerable. Many new satellites do not have propulsion systems, which means they will have a shorter operational life and an earlier re-entry.

The Sun's activity over the past year has been much more intense than experts had predicted, with more sunspots, coronal eruptions and solar winds heading towards Earth, according to Hugh Lewis, a professor engineering and physical science at the University of Southampton, UK, who studies the behavior of satellites in low-Earth orbit. In fact, current activity is close to the forecast for this solar cycle's peak, and we are still two to three years away from the peak.

Stromme and colleagues are raising the orbits of two satellites 45 km higher. The satellite cluster may need more adjustments later this year. Their purpose is to extend the mission through the solar cycle.