At the end of 2021, operators of the European Space Agency’s (ESA) Swarm constellation noticed something worrying: the satellites, which measure the magnetic field around the Earth, began to descend towards the atmosphere at a speed unusually fast – up to 10 times faster than before. The change coincided with the start of the new solar cycle, and experts believe it could be the start of a tough few years for spacecraft orbiting our planet.
“Over 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 virtually diving. The sink rate between December and April was 20 kilometers [12 miles] per year.”
Satellites in near orbit Earth always face the trail of the residue atmosphere, which gradually slows down spacecraft and eventually causes them to fall back to the planet. (They usually do not survive this so-called re-entry and burn up in the atmosphere.) This atmospheric drag forces the international space stationcontrollers to perform regularly “reboost” maneuvers to maintain the station’s orbit at 400 km above the Earth.
This trail also helps clean the near-Earth environment of space junk. Scientists know that the intensity of this trail depends on solar activity – the amount of solar wind vomited by the Sunwhich varies according to 11 year solar cycle. The last cycle, which officially ended in December 2019, was rather sleepy, with a lower than average number of monthly cycles. sunspots and a prolonged low of almost no activity. But since last fall, the star woke up, spitting more and more solar wind and generating sunspots, solar flares and coronal mass ejections at an increasing rate. And Earth’s upper atmosphere has felt the effects.
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“There’s a lot of complex physics that we don’t yet fully understand in the upper layers of the atmosphere where it interacts with the solar wind,” Stromme said. “We know that this interaction causes the atmosphere to rise. This means denser air is moving upwards to higher altitudes.”
Denser air means higher drag for satellites. Even though this density is still incredibly low 250 miles above Earth, the increase caused by the rising atmosphere is enough to bring down some of the satellites in low orbit virtually.
“It’s almost like running with the wind against you,” Stromme said. “It’s harder, it drags – so it slows down the satellites, 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 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 515 km above the ground. The two spacecraft in low orbit were hit more by the action of 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 altitude of the satellites using onboard propulsion to save them.
ESA’s Swarm satellites aren’t the only spacecraft battling deteriorating 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 fall to lower altitudes. The lower the satellites orbit when the solar storm hits, the greater the risk that the spacecraft may not recover, leaving operators watching helplessly as the craft plummets through the atmosphere.
Stellar Link the satellites have operational orbits of 340 miles (550 km), which is above the most risky region. However, after launch, Falcon 9 the rockets drop the lots of satellites very low, about 350 km above the Earth. SpaceX then raises the orbits of the satellites using onboard propulsion units. The company says there are benefits to this approach, as any satellite that experiences technical issues after launch would quickly fall back to Earth and not turn into pesky space junk. However, the increasing and unpredictable behavior of the sun makes these satellites vulnerable to accidents.
New space and unpredictable sun
All spacecraft around the 250-mile altitude are bound to have problems, Stromme said. This includes the International Space Station, which will need to perform more frequent reboost maneuvers to stay afloat, but also the hundreds of cubesats and the 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, cheap technologies – are particularly vulnerable.
“Many of them [new satellites] don’t have propulsion systems,” Stromme said. “They don’t have the means to get up. This basically means they will have a shorter lifetime in orbit. They will return earlier than they would during solar minimum.”
By coincidence (or beginner’s luck), the beginning of the new space revolution occurred during this dormant solar cycle. These new operators are now facing their first solar maximum. But not only. Solar activity over the past year has been much more intense than predicted by solar meteorologists, with more sunspots, more coronal mass ejections, and more solar wind hitting our planet.
“Solar activity is much higher than official predictions suggest,” Hugh Lewis, a professor of engineering and physical sciences at the University of Southampton in the UK who studies the behavior of solar cells, told Space.com. satellites in low earth orbit. “In fact, current activity is already quite close to the predicted peak level for this solar cycle, and we are still two or three years away from solar maximum.”
Stromme confirmed these observations. “Solar cycle 25 that we are now entering is currently 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 right now it’s increasing rapidly.”
Clean the eye sockets
While harsh solar activity is bad news for satellite operators, who will see their mission lives shortened (even ship-powered satellites will run out of fuel much faster due to the need for frequent power surges). altitude), the situation will have positive effects. purifying effects on the space around the Earth.
In addition to being populated with hundreds of new satellites over the past decade, this region of space is cluttered with a worrying amount of space debris (old satellites, used rocket stages and collision fragments). Researchers like Lewis have long warned that ubiquitous trash rushing around the planet threatens the security of satellite services, forcing operators to perform frequent evasive maneuvers. Additionally, the debris could trigger an out-of-control situation known as Kessler syndromean unstoppable cascade of collisions as depicted in the 2013 Oscar-winning film “Gravity”.
“Generally speaking, increased solar activity – and its effects on the upper atmosphere – is good news from a space debris perspective, as it reduces the orbital lifetime of debris and provides a ” “helpful” 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 seen, as the fragments produced by the November 2021 Russian anti-satellite missile test descend much faster than before.
However, there is a downside to this cleaning process.
“The increased rate of debris decomposition can be felt almost like rain,” Lewis said. “When solar activity is high, the rate of ‘rain’ is higher and missions at low altitudes will potentially experience a greater flow of debris.”
A greater flow of debris means the need for even more frequent evasive maneuvers and a temporarily increased risk of collisions, which could potentially generate more dangerous fragments.
Stromme and his colleagues are currently raising the orbit of the two low-orbit Swarm satellites by 28 miles (45 km). Satellites may need even more fine-tuning later this year, she added. The goal is to help the mission, which is currently in its ninth year and past its originally planned lifespan, through the solar cycle. The success of the team will largely depend on the behavior of the sun.
“We still have fuel to get us through hopefully another solar cycle,” Stromme said. “If it grows like now, I’ll run out of fuel before the end of the solar cycle. If it slows down a bit, I could save them throughout the solar cycle.”