It would be nice to find a way to detect alien starships as they crossed the sky traveling from one solar system to another. If the galaxy is populated with alien civilizations, who travel between stars for one reason or another, we could prove their existence not only from them coming here to visit us or by intercepting a message from them, but by seeing them flying around the galaxy. We would not only prove their existence, but we would also prove that no insurmountable Great Filters prevent a determined civilization from traveling through the galaxy. As far as we know now, it might be possible and it might not. No data, no conclusion.
One idea was to see the results of nosecone heating from a fast ship, meaning a fraction of light speed, intercepting interstellar gas. Unfortunately, the only time this would be visible is if the ship was ramming through a Bok globule, where gas densities were much higher than the typical interstellar gas density. And they would likely avoid these globules, which don’t occupy a very large fraction of the occupied volume of the Milky Way.
Another idea is discussed here. If they used the most efficient type of propulsion, particle beams shot out at near light speed, they might have an effect. First of all, in designing a starship, you want to minimize fuel. To do that, you want to have exhaust velocity as high as possible, and that leads to near light speeds. The only way to get matter up to those speeds is to use something like an accelerator, and for them to work, you need charged particles. For this post, we won’t discuss the choice of heavy ions as particles, but just assume protons are used.
There’s a little problem if you just shoot protons out the back end of your starship. You develop a static charge which defeats the purpose of your propulsor. So you need two accelerators, one for protons and one for electrons. You shoot out equal numbers and stay electrically neutral. Protons love electrons, and will recombine with them along the beam path behind your ship. When that happens, hydrogen recombination lines are given off. These range from the ultraviolet down to the infrared. For the purpose of discussions, let’s assume they are all at the upper end of the spectrum.
What we want to do is to assume some large IR/optical/UV dish somewhere out in space, devoted to starship detection. In order to estimate its performance, you have to assume some things about the starship, namely, how big it is and how fast it is accelerating. The larger the ship, the larger the amount of photons is, proportionally. The faster the target speed, the larger the amount of photons, proportionally. The shorter the time to get from dead stop to target speed, the larger the number of photons, inverse proportionally.
What about the dish we just built? The larger the area, the more photons can be captured, proportionally. The longer the collection time, the more photons can be captured, proportionally. A simple calculation, assuming we were staring at the back end of the ship and getting a great view of the exhaust, is that probes, of perhaps 10 metric ton weight, accelerating to 0.01 c over the course of a year, would just be detectable at less than 0.01 light years, with a 100 square meter dish and an hour of detection time. A monster emigration ship, weighing in at 100,000 metric tons, which is a bit heavier than a large aircraft carrier, would be detectable out at nearly a light year. These are not large distances, compared to the size of the galaxy, perhaps a hundred thousand light years across. But they are something to start with. Suppose we ran the detection time up to 100 hours, assuming we solved the problem of the disk not being stable enough to hold its pointing direction. Now the monster ship is detectable at something less than 10 light years.
This is the best we might do with this equipment. It assumes favorable collection of the whole spectrum and more importantly, that the ship is pointed exactly away from the collection dish. If it is not going that direction, we have the problem of proper motion. All the photons don’t go into the same pixel. In order to accumulate say, 100 photons, which we assumed was a minimum for detection against a dark galactic background, there would have to be a mountainous amount of processing to pick all possible tracks and add up photons along it. With that, the proper motion problem might be solved if we had sufficient field of view. Something moving at up to 0.01 c out at a light year cuts across many resolution cells in an hour and proportionally more in 100 hours. Detecting something moving across the field is much more difficult, but perhaps not impossible with oodles of computing power.
Where the problem comes is, where do we point the dish? If we don’t have any information on when the ship left, where its origin and destination are, we are reduced to scanning the whole sky, which, with a dish this large, would take absolutely forever. Even if we were so smart that we could detect habitable planets near ourselves, we would still have the problem of knowing where on the line between them a ship might be. This reduces the amount of scanning tremendously if there are only two candidates nearby, but still it is a major problem.
What would be a great gift is if earth was near the line between two habitable planets. Then we would be likely to see a ship from the exhaust direction, and our chance of detecting it would be much simpler. Seeing it from the nose direction would be good too, unless we were really close to the line and the ship itself obscures the exhaust. Not too likely. This is the same consideration for tapping any interstellar X-band or other communication channel between a home planet and its colony. We need to be on the beamline. If we are really, really lucky, we are, but given the density of habitable planets, this is probably not going to be a lottery winner for Earth.
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