What's this have to do with stealth in space? Well, a spaceship that's trying to hide will still be emitting thermal radiation, and would show up very brightly in the right regions of the electromagnetic spectrum. And the amount of power radiated also increases rapidly with temperature- the Stefan-Boltzmann law tells us the power goes like the fourth power of the temperature, so if you double the temperature, you get 16 times as much radiation. This applies even to exotic objects like black holes- that's what Hawking radiation is all about- and the universe as a whole is permeated by the Cosmic Microwave Background, which fits perfectly to a blackbody spectrum with a temperature about 2.7 degrees above absolute zero, leading to the most iconic xkcd comic ever drawn.Īny object at a temperature higher than that background will emit radiation with a black-body spectrum showing a higher temperature. While you need to get objects very hot before they emit enough light in the visible part of the spectrum for us to see, every object, regardless of its temperature, will emit thermal radiation with a blackbody spectrum determined by its temperature. This radiation is a very simple and universal phenomenon, and the exact spectrum of the light emitted depends only on the temperature, not the composition of the object, or how it's heated. The most familiar form of this is the glow of a hot object, so you see it every time you turn on an incandescent light or cook breakfast. The key issue here is the phenomenon that launched quantum physics, namely black-body radiation. One of the spaceships from "The Expanse." Image from NBCUniversal. ![]() Not if you want to keep people alive inside your ship, anyway. While you can avoid radar pulses sent out by other ships, it turns out that there's no way to avoid having your own ship emit radiation that another ship could detect. There's one final obstacle to hiding ships in space, though, that comes from fundamental physics. And with travel in vacuum removing the need for space ships to be aerodynamic, it might be easier to construct ships whose shape deflects radar in useful ways. In a later volume, a key plot element is the theft of special radar-absorbing paint, which is then put to nefarious purposes. These sorts of techniques could be adapted to the problem of evading radar in space, as well, and in the original books, there's a nod in this direction. These use a combination of special shapes (surfaces put together in ways meant to reflect radar pulses off at an angle, rather than back to the emitter) and special materials to evade radar detection. The term "stealth ship" is a reference to modern military aircraft like the "stealth fighters" developed by the US Air Force, after all. Of course, hiding from radar is something we already know how to do. In space, you need to sweep a larger volume airplanes and weather radar on Earth need to worry about, but the basic principle is the same. Since light travels at a constant speed, knowing how long it takes the light to go out and come back tells you how far away something is, allowing you to build up a map of your immediate neighborhood. The obvious way to do this is something analogous to radar: you send out pulses of light, and look for the light that bounces back from nearby objects. ![]() There's also the issue that people who are expecting to (maybe) be attacked would probably be actively searching the immediate neighborhood, not just passively looking for reflected light or obstructed starlight. The range of useful places to hide is much smaller if you want to be able to attack the people you're hiding from. A lurking attack ship out in the Kuiper Belt just isn't a credible threat shipping in the Asteroid Belt. ![]() Of course, it's a little more complicated than that, because if you're doing space battle scenes, you've got less space to work with than the entire Solar System. It's a lot harder to hide something out in space than many authors seem to think, because of physics. Only in space, there's not all that much for people to hide behind (though it does happen- there are a whole bunch of hidden-ship tricks in the Star Wars movies, from the Millennium Falcon ducking into an asteroid field to the hidden battle fleet in Return of the Jedi), so it's pretty common to invent technologies for hiding ships in plain sight.Īs common as it is, though, the idea of stealth in space is inherently kind of problematic. This is a classic space-opera trope, which is basically just an update of the classic adventure-story trope of having people jump out of hiding to attack the heroes. A key plot point in the pilot of The Expanse involves a stealth ship, lying in wait for one set of unsuspecting protagonists to come along, at which point it unleashes destruction that will turn out to have disastrous consequences for everyone involved.
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