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upstart writes:

Wild solar weather is causing satellites to plummet from orbit. It's only going to get worse.:

In late 2021, operators of the European Space Agency's (ESA) Swarm constellation noticed something worrying: The satellites, which measure the magnetic field around Earth, started sinking toward the atmosphere at an unusually fast rate — up to 10 times faster than before. The change coincided with the onset of the new solar cycle, and experts think it might be the beginning of some difficult years for spacecraft orbiting our planet.

"In the last five, six years, the satellites were sinking about two and a half kilometers [1.5 miles] a year," Anja Stromme, ESA's Swarm mission manager, told Space.com. "But since December last year, they have been virtually diving. The sink rate between December and April has been 20 kilometers [12 miles] per year."

Satellites orbiting close to Earth always face the drag of the residual atmosphere, which gradually slows the spacecraft and eventually makes them fall back to the planet. (They usually don't survive this so-called re-entry and burn up in the atmosphere.) This atmospheric drag forces the International Space Station's controllers to perform regular "reboost" maneuvers to maintain the station's orbit of 250 miles (400 km) above Earth.

This drag also helps clean up the near-Earth environment from space junk. Scientists know that the intensity of this drag depends on solar activity — the amount of solar wind spewed by the sun, which varies depending on the 11-year solar cycle. The last cycle, which officially ended in December 2019, was rather sleepy, with a below-average number of monthly sunspots and a prolonged minimum of barely any activity. But since last fall, the star has been waking up, spewing more and more solar wind and generating sunspots, solar flares and coronal mass ejections at a growing rate. And the Earth's upper atmosphere has felt the effects.

"There is a lot of complex physics that we still don't fully understand going on in the upper layers of the atmosphere where it interacts with the solar wind," Stromme said. "We know that this interaction causes an upwelling of the atmosphere. That means that the denser air shifts upwards to higher altitudes."

Denser air means higher drag for the satellites. Even though this density is still incredibly low 250 miles above Earth, the increase caused by the upwelling atmosphere is enough to virtually send some of the low-orbiting satellites plummeting.

"It's almost like running with the wind against you," Stromme said. "It's harder, it's drag — so it slows the satellites down, and when they slow down, they sink."

[...] "Generally speaking, increasing solar activity — and its effect on the upper atmosphere — is good news from a space debris perspective, as it reduces orbital lifetimes of the debris and provides a useful 'cleaning service,'" Lewis said.

According to Jonathan McDowell, a space debris expert at the Harvard-Smithsonian Center for Astrophysics, the positive effect can already be observed, as fragments produced by the November 2021 Russian anti-satellite missile test are now coming down much faster than before.

However, there is a downside to this cleansing process.

"The increased rate of decay of debris objects can be perceived almost like rain," Lewis said. "When solar activity is high, the 'rain' rate is higher, and missions at lower altitudes will potentially experience a greater flux of debris." A greater flux of debris means the need for even more frequent fuel-burning avoidance maneuvers and a temporarily increased risk of collisions, which could potentially generate more dangerous fragments.

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