Detecting incoming alien spaceship.

[Sky Captain]

Suppose an alien generation ship launched centuries ago is traveling to Earth at around 0.1c. At some point when coming closer to Solar system they detected our radio emissions and found out that there is rapidly advancing technological civilization on Earth. Aliens decide best course of action would be to try to hide their presence and instead of going directly to Earth they decide to stop at some Kuiper belt object to replenish resources. They have to perform year long braking burn to slow down from 0.1 c. Their ship use fusion engines.

Would it be possible to reduce the chance of detecting braking burn? I assume no one is specially looking for active fusion engines so if discovered it would have to be mostly by accident. Although even relatively low end fusion engines on a huge multi million ton generation ship would have to operate at very high power levels maybe high terawatt to low petawatt range to produce meaningful acceleration making ship very bright against background of space.

[U.P. Cinnabar]

None. A full sky scan would detect the acceleration plume before it even reached the Kuiper Belt, then further analyses would rule out anything natural.

[Sky Captain]

Is such sky scan done regularly as a part of routine astronomical observations? Near Earth asteroid searches are done mostly by looking along the ecliptic plane. If ship comes from up or down relative to ecliptic plane then it would occupy a piece of sky that are less likely to be looked at with sensitive instruments.

[Formless]

If they are clever, I think there are a couple of things they could do. One would thing they could do, once they are close enough to the sun, is use solar sails to brake. We aren't even necessarily talking about photon sails which are reflective, but also magnetic sails that catch solar wind, and are essentially invisible. However, that technology should only be usable from relatively close in, which is important.

Another thing they could do depends on the configuration of the spacecraft. a Tetrahedral spaceship can theoretically go in either direction simply by choosing to fire one engine or the three on the opposite side of the ship from that engine. This is very inefficient of course, but the vectors allow it to change its momentum in exactly the same way as a single engine. If they start the burn from far enough away (and iirc, Earth's earliest radio transmissions should be distinguishable from natural emissions at about 40 light years away), they can likely put up an optical shield that prevents light from the three engines from being seen in the direction of the Earth. Alternatively, a highly directional engine that can limit the cone of detectable radiation to, oh lets say 15 degrees, will be extremely unlikely to be detected as long as the engine is always pointed away from earth's current position. At best we might see the leftover radioactive gasses if we were lucky, but they would likely cool down so significantly astronomers would be bending over backwards trying to find an explanation that didn't involve aliens. These are basic space stealth concepts, and while misguided hard-SF fans may complain about space stealth, it will be particularly effective against a planet bound civilization like ourselves. We really only have detectors on or around the Earth itself and they do not cover even a fraction of the celestial sphere. Most of them are not designed for SETI purposes at all, but for analyzing natural objects like stars and galaxies. In fact, you could even argue that we are unlikely to see them without them doing anything at all to hide. Especially because at only .1C their ship would still need to be capable of very long duration flight to begin with, so delaying their arrival in order to prevent detection by the local planetary civilization would likely be seen as more of an inconvenience than a mission killer. Not that they would find Humans to be very threatening to begin with.

Mike's analysis of realistic sensor technologies, i.e. fucking telescopes, is a must-read for anyone who thinks spacecraft can be detected and (just as importantly) identified at all distances without reservation (looking at you, U.P. Cinnabar). The brightness of a spaceship burning a fusion engine is pitiful compared to the brightness of the stars in the sky, especially if the ship begins slowing down before entering the solar system, which they would do regardless. In fact they would likely start decelerating exactly halfway between their home system and Sol. If they turned off their engines completely the only thing that makes them distinguishable from ordinary Kuiper belt objects or interstellar asteroids like ʻOumuamua is that they would be unusually warm from other machinery onboard. This of course means that they could only use solar sails for slowing down, and they would come in unusually fast, but with only a single known interstellar asteroid on record, we can't know that this is unusual. Astronomers are very hesitant to ascribe ANYTHING to intelligent life, so even if they could tell that the object was unusually warm from here or unusually fast, they would find it interesting, but would most likely be bending over backwards trying to justify it by natural means such as internal radiation sources like uranium. Really, that's how astronomers think. The thing that would most likely signal that they are definitively alien life is if they were using an antimatter engine rather than a fusion rocket, as that leaves a very telltale type of radiation rarely seen in nature. But literally all stars fuse hydrogen, so with the engines on it would be difficult to tell it apart from a dwarf star at interstellar distances when it first starts decelerating.

[Sky Captain]

Another thing they could do depends on the configuration of the spacecraft. a Tetrahedral spaceship can theoretically go in either direction simply by choosing to fire one engine or the three on the opposite side of the ship from that engine. This is very inefficient of course, but the vectors allow it to change its momentum in exactly the same way as a single engine. If they start the burn from far enough away (and iirc, Earth's earliest radio transmissions should be distinguishable from natural emissions at about 40 light years away), they can likely put up an optical shield that prevents light from the three engines from being seen in the direction of the Earth. Alternatively, a highly directional engine that can limit the cone of detectable radiation to, oh lets say 15 degrees, will be extremely unlikely to be detected as long as the engine is always pointed away from earth's current position.

A normal ship with all engines at the back also should be able to do it by burning at an angle, say point 20 degrees to the left and then 20 degrees to the right of Earth to compensate for sideways drift. Although this requires to have some extra fuel reserves which may or may not be there. An interstellar mission designed for maximum performance would have fuel tanks dry after most efficient possible braking burn leaving no fuel for evasive maneuvers.

Especially because at only .1C their ship would still need to be capable of very long duration flight to begin with, so delaying their arrival in order to prevent detection by the local planetary civilization would likely be seen as more of an inconvenience than a mission killer.

That is one possibility. Burn early to kill most of the velocity then coast and do a final burn to stop at desired Kuiper belt object when Sun is between ship and Earth if it comes in parallel to ecliptic. Then only space based sensors far away from Earth would have clear view.

Not that they would find Humans to be very threatening to begin with.

While humans do not have space weapons aliens would have no way to know for sure. By analyzing early TV transmissions they most likely would know we have nukes which also imply we potentially could have some nuclear pulse driven warships. A smart species would not want to burn directly for Earth orbit and come to a stop there with empty fuel tanks potentially putting whole mission in possible danger. Better stop at the edge of solar system avoiding detection if possible to resupply and got more detailed intel on what this civilization is capable of. Maybe send some small probes to flyby Earth for more detailed look.