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The means by which a satellite in low Earth orbit can fail during its mission are not lacking. Even in the best case, the craft must survive cosmic ray bombardment and huge temperature changes. To even have a chance to survive the worst, such as a hardware failure or a collision with a misplaced waste, it must be designed with robust redundancies that allow everything to remain operational despite the systemic damage. Of course, before such a situation can occur, she will have to survive the mad rush in space. So add high loads and intense vibrations to the list of things that can kill your expensive bird.
After the meticulous engineering and expense of putting a satellite into orbit, you might think that it will be welcomed by a hero at the end of his mission. But in fact, it's the opposite. The great irony is that after all the time and effort required to develop a spacecraft capable of surviving the rigors of a space flight, its operators will more than likely order the craft to destroy itself in plunging its orbit into the Earth's atmosphere. . The last act of a well-conceived satellite will probably be to engage in the same fate that he had spent years, even decades, to avoid.
You may be wondering how engineers design a spacecraft robust enough to survive for years in the space environment while staying just fragile enough for it to be consumed completely during the school year. Until recently, the simple answer was that it was not really something that had been taken into account. But with the drop in introductory prices that should make space much more crowded in the coming years, the race to develop new technologies will ensure that a satellite is intact as long as necessary.
The sky is falling
The possibility of debris surviving re-entry and hitting the ground is not new. There are documented cases going back about as long as we filmed objects in space. The most famous example is probably that of 1979, when Skylab wrecks were scattered in the Australian hinterland. The largest parts, such as the tanks that held oxygen on the most intelligent space station in the United States, weighed hundreds of kilograms. Fortunately, these voluminous objects fell on one of the least populated land on the planet, but if they had fallen in a big city, the damage and risk to human life could have been considerable.
What has changed is the number of objects we can expect in the atmosphere in the near future. When only the superpowers of the world had the ability to put something in space, it was relatively easy to follow the moment when things came down. But the intensification of competition has dramatically reduced the cost of putting satellites into orbit and companies are now considering space investments that would have been simply impossible just ten years ago. Companies such as SpaceX, OneWeb and Samsung are considering mega-constellations of satellites composed of thousands of individual spacecraft. Each of them will eventually go back down into the atmosphere at the end of its nominal life.
In a letter dated 26 February, the Federal Communications Commission specifically asked SpaceX to clarify its plans for the de-orbit of thousands of satellites in the proposed Starlink network. They wanted to know if SpaceX could ensure that the craft would fly over the ocean and, if not, estimate the likelihood that the debris drop would result in property damage or loss. human lives. The letter ends by stating that SpaceX's request for Starlink approval could be rejected if this information is not provided to the satisfaction of the FCC.
Unacceptable risk
Leading a spaceship to a tenant over the ocean has always been the usual way of ensuring that any surviving debris will not cause any problem on the ground. In any case, most of the planet's surface is ocean, so this is a relatively simple task, assuming your spacecraft is operating normally and able to maneuver. Although errors occur; Skylab was actually supposed to return to the south of Africa, a slight error in the calculations made it overflow considerably to the east.
But the Starlink satellites proposed by SpaceX (and others similar) are a case apart. They use a high efficiency Hall effect propulsion, ideal for occasional orbit adjustments and holding in position, but lacking the thrust needed to place the craft on a targeted reentry path.
In other words, satellites are able to lower their orbit enough to make sure they are burning, but they can not locate the situation with sufficient precision to be able to give guarantees. This could be an acceptable risk for a single satellite, but Starlink's long-term plans involve 12,000. With so much in-game crafting, the odds are that it's going to happen. between them separates from a populated area are just too high.
In its official response to the FCC, SpaceX explained that, if they could not guarantee that the Starlink satellites would only enter the atmosphere over the ocean, they had developed a solution which rendered it useless. After "extensive research and investment", SpaceX claims to have refined the design of its Starlink satellites to ensure they will completely burn in the atmosphere.
Design for Demise
The idea of designing satellites so that they are consumed entirely when a normal return reappear for decades, but until now, they have struggled to understand. Nicknamed "Design for Demise" or D4D in the industry, its main defender was probably the European Space Agency, which made it a central theme of their "Clean Space" initiative. As you can imagine, this is a complex subject, but at the risk of over simplifying things, there are essentially two main approaches: designs that break up into smaller pieces as they enter the atmosphere and the environment. use of materials that can not survive the intense heat of re-entry. These two ideas are not mutually exclusive and would actually be used together to achieve the best results.
For example, rather than holding a satellite with nuts and bolts, it could be glued with epoxies formulated to decompose at high temperatures. Once the spaceship hits the atmosphere and begins to heat up, the epoxy gives way and the structure simply falls apart. This technique not only reduces the number of large objects likely to cross the atmosphere intact, but it also opens the interior of the probe to the flow of hot atmospheric gases that will promote more complete combustion.
Even in some cases, some of the medium satellite components are robust enough to survive. Probable candidates are titanium propellant tanks, silicon carbide mirrors (used in optical telescopes or laser communication systems), iron cores of Hall effect thrusters, and heavy stainless steel reaction wheels. for the control of the attitude of the spacecraft. These components pose a considerably more problematic problem because the materials used in their construction have obviously been chosen for a reason. For example, replacing a stainless steel reaction wheel with an aluminum wheel of the same diameter would simply not work because of differences in density. The whole system should be redesigned to take into account the important change.
According to their official response to the FCC, SpaceX claims that they have replaced these problematic components with versions that they can guarantee will not survive. If this is true, it would be an important milestone for the design of D4D, and probably the criterion by which future large-scale satellite constellations will be measured. But not everyone is convinced that SpaceX has solved this complex problem as neatly as they say. In a statement to IEEE Spectrum, a professor of mechanical and aerospace engineering at the University of Buffalo, John Crassidis, said the claims were a bit too bold: "Everything can come from the atmosphere if you hit the right angle . If they guarantee that this will not be a problem, then I will need to call BS. "
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