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Why satellites of the future should burn

There is no shortage of opportunities for a low Earth orbit satellite to fail during its mission. Even in the best case, the vehicle must survive the bombardment of cosmic radiation and enormous temperature fluctuations. In order to survive even the worst situations, such as a hardware failure or a collision with an impure space debris, he must be equipped with robust redundancies that can keep everything running in case of system damage. Before this can happen, of course, it has to survive the wild ride to space. Add high G-loads and intense vibration to the list of things that can kill your expensive bird.

After all the meticulous technique and cost associated with putting a satellite into orbit, one might think that it would be welcome in the hero's hero to finish his mission. In fact, it is exactly the opposite. The great irony is that, after all this time and effort, a spacecraft needs to be developed that can survive the rigors of spaceflight. In the end, its operators will most likely command the aircraft to destroy itself by lowering its orbit into the earth's atmosphere. The final act of a properly designed satellite is likely to refer to the same fate that he had avoided for years or even decades.

You may be wondering how engineers design a spacecraft that is robust enough to survive years in space space environment, while at the same time just is so fragile enough that it completely burns up during reentry. Until recently, the simple answer was that it was not really something that was considered. With falling launch prices, which promise space to recover in the next few years, the race continues to develop new technologies to ensure that a satellite stays intact only as long as it needs to.

The Sky is Falling

A piece of Skylab that has crashed in Australia

The possibility of debris surviving reentry and touching the ground is nothing new, and there are documented cases in which we have been long shoot around place. Probably the most famous example was in 1979, when debris from Skylab was scattered all over the Australian Outback. The largest parts, such. For example, the tanks holding oxygen aboard America's most persnickety space station were hundreds of pounds in weight. Fortunately, these massive objects fell into one of the least populated areas of the earth, but if they had come down in a big city, the damage and danger to human life could have been considerable.

What has changed is the number of objects We can assume that we will reenter the atmosphere in the near future. When only the superpowers of the world had the opportunity to bring something into space, it was relatively easy to keep track of when things returned. However, increased competition has drastically reduced the cost of placing satellites on the market. Now, companies are looking for space investments that would have been impossible just a decade ago. Companies like SpaceX, OneWeb, and Samsung are watching mega constellations of satellites made up of thousands of individual space probes, and each will return through the atmosphere at the end of its nominal lifetime.

In a dated letter On February 26, the Federal Communication Commission specifically called on SpaceX to detail its plans to desorb the thousands of satellites in its proposed Starlink network. They wanted to know if SpaceX can make sure that the vehicle re-enters the ocean, and if not, one can estimate the likelihood that falling debris can lead to material damage or human sacrifice. The letter ends by stating that SpaceX's application for Starlink's approval could be rejected if this information was not provided to the satisfaction of the FCC.

Unacceptable Risk

It has always been common to hire a spacecraft over the ocean to make sure the residue that survives does not cause any problems on the ground. Since most of the surface of the planet is ocean anyway, this is a relatively simple thing, provided that your spacecraft works normally and can maneuver itself. Although mistakes happen; Skylab was supposed to return south of Africa, a slight error in the calculations caused it to move significantly eastward.

Two prototypes of Starlink satellites before launch

But the Starlink satellites proposed by SpaceX (and similar) are such a special case. They use a high-efficiency Hall-effect drive, ideal for occasional orbit correction and station maintenance, but lack the necessary thrust to get the plane back to a targeted reentry path.

In other words, the satellites can lower their orbit enough to ensure that they burn, but can not pinpoint where this will happen, with sufficient accuracy to warrant. This can pose an acceptable risk if only a single satellite is deployed, but long-term Starlink plans call for up to 12,000 of them. With so many skills in the game, the odds of one of them leaping over a populated area are just too high.

In their official response to the FCC, SpaceX stated that they could not guarantee that only Starlink satellites would do so. If they re-entered the atmosphere above the sea, they would have found a solution that would make it unnecessary. After "extensive research and investment," SpaceX says they have refined the design of their Starlink satellites to ensure they burn completely in the atmosphere.

Design for Demise

The Idea of ​​Designing Satellites That will burn completely during normal reentry has been circulating for decades, but has been difficult so far. Referred to in the industry as "Design for Demise" (D4D) or D4D (D4D), the biggest proponent of the European Space Agency was the central theme of their overall Clean Space initiative. As you might imagine, this is a complex issue, but there are two main approaches to the danger that things will be oversimplified: designs that break up into smaller parts when entering the atmosphere, and the use of materials that can not survive the intense reentry heat. These two ideas are not mutually exclusive and would indeed be used for best results.

Instead of holding a satellite together with screws and nuts, it could, for example, be glued to epoxides that have collapsed to high temperatures. Once the spacecraft has reached the atmosphere and begins to warm up, the epoxy gives way and the structure simply falls apart. Not only does this technique reduce the number of large objects that could potentially penetrate the atmosphere intact, it also opens the interior of the spacecraft to the flow of hot atmospheric gases, promoting more complete combustion.

A typical reaction wheel module.

There are even some components of the average satellite that are robust enough to survive. Possible candidates include titanium fuel tanks, silicon carbide mirrors (for use in optical telescopes or laser communication systems), the iron cores of the Hall effect engines, and the heavy stainless steel reaction wheels used to control the attitude of the spacecraft. These components are a much more difficult problem as the materials used have obviously been selected for a particular reason. For example, replacing a stainless steel reaction wheel with an aluminum of the same diameter may not work because of the differences in density. The entire system would need to be redesigned to accommodate the significant changes.

According to their official response to the FCC, SpaceX states that they have replaced these problem components with versions that will prevent them from re-entering. If true, this would be a major milestone for the D4D design, and probably the benchmark against which future large satellite constellations will be measured. But not everyone is convinced that SpaceX solved this complex problem as easily as they say. John Crassidis, a professor of engineering and aerospace engineering at the University of Buffalo, said in a statement to IEEE Spectrum that the allegations are that something is too bold: "Anything can come through the atmosphere when you're in the middle of it right angle meets. If you guarantee that this does not cause a problem, I have to call BS. "

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