Posted on October 10, 2021
At 400 km from Earth, altitude of the International Space Station, the atmosphere is now 20% less dense than it was in 2000. And by the end of the century, this decrease in density will reach 80%. “It’s a big problem for satellites,” says Hugh Lewis, an astrophysicist at the University of Southampton, England. He recently did an analysis of the collision risks linked to SpaceX’s Starlink constellation, which last June had 1,800 satellites.
“End-of-life satellites and rocket debris will stay in orbit five times longer than before, because what brings them back to Earth is friction with the atmosphere,” says Hugh Lewis. The less dense the atmosphere, the less friction there is. It increases the risk of collisions by limiting the maximum number of satellites we can put into orbit. »
John Emmert, a physicist at the Naval Research Laboratory in Washington – one of the first to show this shrinking of Earth’s atmosphere – tempers: the influence of the solar cycle also affects the density of the upper atmosphere. “The increase in the concentration of COtwo has a big effect, but there’s a lot of variation over the 11-year solar cycle, says Emmert. The density at 400 km is 10 times greater during solar maximum than during solar minimum. In the long term, the increase in the concentration of COtwo density decreases at 400 km altitude by 2% or 3% per decade. »
The solar maximum corresponds to the highest activity on the Sun, with an increase in the number of solar storms – like the one that caused a disruption in the Hydro-Québec grid in 1989.
What is the connection between COtwo and the density of the atmosphere? “The COtwo at high altitudes it cools the Earth, as opposed to less than 100 km, says Emmert. At low altitude, COtwo prevents heat from escaping into space. But beyond 100 km, COtwo absorbs the sun’s infrared rays and reflects them back into space. That’s why this region of the atmosphere is called the thermosphere. Temperature decreases in the upper atmosphere by a few degrees Celsius per decade as the concentration of COtwo increase. »
As early as 1989, the first prediction that the Earth’s atmosphere would shrink with increasing CO concentrationtwo was made. “It was a theoretical model,” says Emmert. In 2000, the first concrete evidence was obtained with the observation of the descent speed of five satellites. And in 2004, I presented definitive proof of the accuracy of the 1989 model.”
The phenomenon of the shrinkage of the atmosphere is also visible in Hugh Lewis’ analysis of the collision risk linked to the Starlink satellites. The latter, which aims to provide high-speed internet, will be 12,000.
“Starlink satellites already account for half of the close passes between two satellites,” says Lewis. We’re talking about 1600 times a week where two satellites come close to less than 1 kilometer. I thought it would be much less. The theoretical model predicted a linear increase in the number of closed passages, but it is exponential. Eventually, Starlink satellites will account for 90% of the close passes. »
About 3% of these close passes require a satellite operator to perform maneuvers to reduce the risk of collision, according to the specialist website Space.com.
Since 2010, the International Organization for Standardization (ISO) has recommended that “dead” satellites return to Earth no later than 25 years after the end of their operational life. This standard is not always respected: in 2019, a report by the European Space Agency (ESA) estimated that less than a quarter of satellites orbiting beyond 500 km have enough fuel to de-orbit at the end of their lifetime. advantage of atmospheric friction to fall back to Earth. Even operators that want to meet this 25-year deadline do not take into account the shrinkage of the atmosphere caused by the buildup of CO.two emitted by humans, according to Lewis.
“We will end up with a backlog of satellites and also debris caused by collisions that will be impossible to avoid with dead satellites without propulsion. As early as last fall, the CEO of Rocket Lab warned that it was increasingly difficult to aim for free space to launch a rocket, with the rapid growth of satellite constellations like Starlink. The CEO of New Zealand company Rocket Lab, Peter Beck, told CNN in October 2020 that he now needed to “knit” between satellites to launch a rocket. Before launching a rocket, operators must make an assessment of the risk of collision with satellites and debris already in orbit. Satellites that are picked up by the Earth’s atmosphere slowly lower their orbit with atmospheric friction and eventually disintegrate in the upper atmosphere.
In 1978, NASA astrophysicist Donald Kessler predicted that Earth’s orbit could one day become “full” due to debris caused by orbital collisions. Consequence: there would be no more space to safely send additional satellites. This prediction was called “Kessler syndrome”.
One solution would be to send a spacecraft to clean in orbit. Either picking up the debris and tricking it into disintegrating in the atmosphere, or giving it a boost to go towards Earth.
Hugh Lewis is working with a British company, Astroscale, which wants to attract satellites with magnets to return them to the atmosphere after their useful life. A demonstration mission on Astroscale is underway. “In theory, it works, as do many other technologies to clean up the orbit,” says Lewis. But we’ll have to see how we finance all this. Recently, Iridium CEO Matt Desh tweeted that he was willing to pay $10,000 per satellite for such a service. It’s much, much less than what it will actually cost. Iridium is a telephone company that operates around 100 satellites.
Satellites in numbers
500: number of satellites in orbit in 1995
1000: number of satellites in orbit in 2000
6500: number of satellites in orbit in 2020