A minor geomagnetic storm is supposed to be minor. That's why even experts were surprised on Feb. 4, 2022, when dozens of Starlink satellites started falling out of the sky. A weak CME had hit Earth's magnetic field, and the resulting G1-class (minor) storm was bringing them down:

Above: A Starlink satellite breaks up over Puerto Rico on Feb. 7, 2022. Credit: The Sociedad de Astronomia del Caribe

How could this happen? A new paper published in the research journal Space Weather provides the answer.

"Although it was only 'minor,' the storm pumped almost 1200 gigawatts of energy into Earth's atmosphere," explains lead author Tong Dang of the University of Science and Technology of China. "This extra energy heated Earth's upper atmosphere and sharply increased aerodynamic drag on the satellites."

SpaceX launched the satellites from Cape Canaveral on Feb. 3, 2022. Forty-nine (49) Starlinks were crowded inside the Falcon 9 rocket; less than a quarter would survive.

Above: The Starlink launch was sandwiched between two minor geomagnetic storms (right) possibly caused by an Earth-directed CME that left the sun on Jan. 30th (left)

As was SpaceX's practice at the time, the satellites were deployed at an altitude of 210 km--their first stop en route to an operational altitude near 600 km. In the satellite business, 210 km is considered to be low, barely above the atmosphere. SpaceX starts there in case any satellite malfunctions after launch. From 210 km, a "bad sat" can be easily de-orbited.

A little too easily, as it turns out.

Using a physics-based computer model named "TIEGCM," Dang and colleagues simulated conditions during the storm. As geomagnetic energy heated Earth's atmosphere, the air density at 210 km increased globally by 20% with "hot spots" as high as 60%. This movie shows what happened:

Starlink dodged the worst spots. "The satellites did not hit any of the 60% regions," says Dang. "But that didn't save them." The weaker 20% enhancements were enough to bring down 38 out of 49 satellites.

To prevent a repeat, SpaceX has started launching to 320 km instead of 210 km. Earth's atmosphere has to reach that much higher to drag the satellites back during a geomagnetic storm. Since the change, more than 1200 additional Starlink satellites have been launched on 24 rockets without incident.

There's still danger, though. "Air density at 320 km is an order of magnitude less (compared to 210 km), but it's not completely safe," cautions Dang's co-author Jiuhou Lei, also from the University of Science and Technology of China. "During an extreme geomagnetic storm, density could increase from 200% to 800% even at these higher altitudes."

Extreme storms may be in the offing. Young Solar Cycle 25 is just getting started. The profusion of minor storms we are observing today will intensify in the years ahead especially as we approach Solar Max around 2025.

Elon Musk's note to self: Check the space weather forecast.