In late 2021, operators in the European Space Agency’s (ESA) constellation Swarm noticed something worrying: satellites, which measure the magnetic field around Earth, began sinking toward the atmosphere at an unusually fast rate – up to 10 times faster than before. The shift coincided with the start of the new solar cycle, and experts think it could be the start of some rough years for spacecraft orbiting our planet.
“In the last five, six years, the satellites have sunk about two and a half kilometers [1.5 miles] a year,” Anja Stromme, ESA’s Swarm mission manager, told Space.com. “But since December of last year, they’ve been diving virtually. The sinking rate between December and April was 20 kilometers [12 miles] per annum.”
Satellites orbiting close to Earth always face the drag of the residual atmosphere, which gradually slows down the spacecraft and eventually causes them to fall back to the planet. (They usually don’t survive this so-called reentry and burn up in the atmosphere.) This atmospheric drag forces the International Space Stationcontrollers run regularly “reboot” maneuvers to maintain the station’s orbit 250 miles (400 km) above Earth.
This drag also helps clean the near-Earth environment of space junk. Scientists know that the intensity of this drag depends on solar activity – the amount of solar wind vomited by Sunwhich varies according to the 11 year solar cycle. The last cycle, which officially ended in December 2019, was quite sleepy, with a below-average number of sunspots and a prolonged minimum of almost no activity. But since last fall, the Star has awakened, expelling more and more solar wind and generating sunspots, solar flares and coronal mass ejections at an increasing rate. And Earth’s upper atmosphere felt the effects.
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“There’s a lot of complex physics that we still don’t fully understand happening 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 denser air moves to higher altitudes.”
Denser air means greater drag for the satellites. Even though that density is still incredibly low at 250 miles above Earth, the increase caused by the ascending atmosphere is enough to virtually send some of the low-orbit 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.”
Knocked down by a solar storm
The Swarm constellation, launched in 2013, consists of three satellites, two of which orbit the Earth at an altitude of 270 miles (430 km), about 20 miles (30 km) above the International Space Station. The third Swarm satellite circles the planet a little higher – about 320 miles (515 km) above the ground. The two lowest-orbiting spacecraft were hit harder by the Sun than the higher satellite, Stromme said.
The situation with the lower two became so precarious that in May, operators had to start raising the satellites’ altitude using onboard propulsion to save them.
ESA’s Swarm satellites aren’t the only spacecraft battling worsening space weather. In February, SpaceX lost 40 new Starlink satellites who were hit by a solar storm right after launch.
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In such storms, satellites suddenly drop to lower altitudes. The lower the satellites’ orbit when the solar storm hits, the greater the risk that the spacecraft will fail to recover, leaving operators powerless to watch the spacecraft fall into the atmosphere.
starlink the satellites have operational orbits of 340 miles (550 km), which is above the highest risk region. However, after release, hawk 9 rockets deposit lots of satellites very low, only about 350 km above Earth. SpaceX then raises the satellites’ orbits using onboard propulsion units. The company says this approach has advantages, as any satellite that has technical problems after launch would quickly fall back to Earth and not turn into annoying space debris. However, the sun’s growing and unpredictable behavior makes these satellites vulnerable to mishaps.
New space and the unpredictable sun
All spacecraft around the 250-mile altitude should have problems, Stromme said. That includes the International Space Station, which will have to perform more frequent reset maneuvers to stay afloat, but also the hundreds of cubesats and small satellites that have populated low Earth orbit for the past decade. These satellites – a product of the new space movement led by private entrepreneurs pioneering simple, inexpensive technologies – are particularly vulnerable.
“Many of these [new satellites] they don’t have propulsion systems,” Stromme said. “They have no way to get up. This basically means they will have a shorter lifespan in orbit. They will reenter earlier than during solar minimum.”
By coincidence (or beginner’s luck), the start of the new space revolution took place during that sleepy solar cycle. These new entrants are now experiencing their first solar maximum. But not only that. The sun’s activity last year turned out to be much more intense than meteorologists predicted, with more sunspots, more coronal mass ejections, and more solar wind hitting our planet.
“Solar activity is much higher than the official prediction suggested,” Hugh Lewis, a professor of engineering and physical sciences at the University of Southampton in the UK who studies the behavior of satellites in low Earth orbit, told Space.com . “Actually, current activity is already pretty close to the predicted peak level for this solar cycle, and we’re still two or three years away from solar maximum.”
Stromme confirmed these observations. “Solar cycle 25 that we are now entering is increasing very sharply,” she said. “We don’t know if that means it’s going to be a very difficult solar cycle. It could slow down and become a very weak solar cycle. But now, it’s increasing rapidly.”
cleaning orbits
While harsh solar activity is bad news for satellite operators, who will see the lifespan of their missions shortened (even satellites with onboard propulsion will run out of fuel much faster due to the need for frequent increases in altitude), the situation will be welcome. purifying effects in space around Earth.
In addition to becoming populated with hundreds of new satellites over the past decade, this region of space is teeming with worrying amount of space debris (old satellites, spent rocket stages and collision fragments). Researchers such as Lewis have long warned that the ubiquitous garbage scattering around the planet threatens the safety of satellite services, forcing operators to carry out frequent avoidance manoeuvres. In addition, debris can trigger an out-of-control situation known as Kessler syndromean unstoppable cascade of collisions as portrayed in the 2013 Oscar-winning film “Gravity”.
“Generally speaking, increasing solar activity – and its effect on the upper atmosphere – is good news from a space debris perspective, as it shortens the orbital lifespan of debris and provides a useful ‘clean-up service’,” he said. Lewis.
According to Jonathan McDowell, a specialist in space debris at the Harvard-Smithsonian Center for Astrophysics, the positive effect can already be observed, as fragments produced by the Russian anti-satellite missile test in November 2021 are now falling much faster than before.
However, there is a downside to this cleaning process.
“The increased rate of decomposition of debris objects can be perceived almost like rain,” Lewis said. “When solar activity is high, the rate of ‘rainfall’ is higher and missions at lower altitudes will potentially experience greater debris flow.”
A greater flow of debris means the need for even more frequent maneuvers to avoid burning fuel and a temporarily increased risk of collisions, which could generate more dangerous fragments.
Stromme and his colleagues are currently raising the orbit of the two low-orbit Swarm satellites by 45 km. Satellites may require even more tweaks later this year, he added. The aim is to help the mission, which is currently in its ninth year and beyond its originally planned lifespan, through the solar cycle. The team’s success will largely depend on the behavior of the sun.
“We still have fuel to take us through another solar cycle,” Stromme said. “If it grows like it does now, I’ll use up the fuel before the solar cycle ends. If it slows down a little, I can save them during the solar cycle.”
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