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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.
(Score: 3, Informative) by dalek on Monday June 27 2022, @05:04AM (3 children)
The upper atmosphere is so thin that it just isn't going to absorb a lot of infrared radiation that would otherwise escape to space. The upper atmosphere doesn't heat up much because it can't absorb a lot of the heat.
That's incorrect. I'll try to explain the relevant processes again.
The Earth is always very nearly in radiative balance, meaning it emits as much radiation to space as it receives from the sun. In the absence of any greenhouse effect and the current solar constant, the global average temperature would be around 255K or -18°C. In this scenario, 100% of the radiation from the surface of the Earth would escape to space. We know how much radiation the Earth should emit if it's in radiative balance and solve for the temperature using the Stefan-Boltzmann law.
Instead, the global average temperature is around 288K, or 15°C. Because this is roughly 33°C warmer, it means the Earth will emit more radiation. However, some of that radiation gets trapped in the atmosphere because of the greenhouse effect. The portion of that radiation that is able to escape from the atmosphere into space will be equal to the amount of radiation that the Earth receives from the sun.
If you increase the greenhouse effect further, more heat will be trapped and a smaller portion of the heat will escape to space, warming the Earth even more. As the Earth warms, it emits more radiation, and some of that extra radiation will also escape to space. Eventually the Earth will heat up enough to restore the radiative balance.
If radiative balance was never restored, where would the extra energy go? The first law of thermodynamics says that energy cannot be created nor destroyed. If it's not escaping from the Earth, it has to go somewhere. The scenario you describe would cause temperatures on Earth to continue increasing in perpetuity because radiative balance would never be restored. We know that doesn't happen. The warming stops once radiative balance has been restored.
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest just whinge about SN.
(Score: 1) by khallow on Monday June 27 2022, @01:37PM (2 children)
The upper atmosphere is so thin it just doesn't need to.
In other words, the Earth indeed is radiating less heat to the upper atmosphere than it was before.
You already acknowledged the imbalance. It doesn't need to be a permanent effect in order to happen.
(Score: 3, Interesting) by dalek on Monday June 27 2022, @06:39PM (1 child)
The dominant mechanism for heating the thermosphere is the absorption of x-rays and ultraviolet light by molecular oxygen. This energy can be transferred to other gas molecules (e.g., molecular nitrogen, carbon dioxide, etc...) when the molecules collide. Unlike the molecular oxygen and nitrogen, when carbon dioxide absorbs some of the energy in a collision, it radiates this energy much more rapidly. Because the atmosphere is so thin at this altitude, it's very unlikely that the energy that has been radiated will be absorbed by other greenhouse gases.
Any heat absorbed in the thermosphere due to the greenhouse effect is a drop in the bucket. It's a tiny contribution, just as the thermal expansion of the troposphere would make a very small contribution to increasing pressures in the thermosphere.
Temperatures in the thermosphere range from roughly 600K to 3,000K. The vast majority of that heat isn't from absorbing radiation that's been emitted by the surface or gas molecules in the lower atmosphere. It's from absorption of x-rays and ultraviolet light. When that energy gets transferred to carbon dioxide molecules, they radiate it faster than the molecular oxygen and molecular nitrogen. That's by far the dominant mechanism for cooling the thermosphere. If you had an equation for the heat budget of the thermosphere, the process you're describing is a small change (an imbalance in the neighborhood of 0.2%) of a term of the equation that's very small to begin with.
Now, a persistent imbalance of 0.2% in the energy budget can have large impacts on climate. An energy deficit of that magnitude is more than sufficient to cause something like the Little Ice Age. An energy surplus of that magnitude can melt ice sheets over a period of centuries. It's larger than the radiative imbalance needed to melt the Laurentide Ice Sheet and bring the Earth out of the last glacial maximum over a period of 10,000 years. But the effects of this imbalance are very small in the thermosphere compared to the dominant processes that affect that layer of the atmosphere.
And before anyone asks, here's a source for the radiative imbalance at the end of the last glacial maximum: https://www.pnas.org/doi/10.1073/pnas.1905447116 [pnas.org].
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest just whinge about SN.
(Score: 1) by khallow on Tuesday June 28 2022, @11:30AM