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In my book, Oumuamua was actually a large alien invasion/colony vessel.

So, suppose we have an invading ~100 km long alien colony ship flying towards Earth. Suppose also that the aliens are unable to fully cloak themselves, but have a way of altering/reducing their visual or infrared signature.

Is there any reasonable way they could trick Earth telescopes (like Spitzer) into believing they are registering a much smaller, Oumuamua-like asteroid? The aliens probably shouldn't worry about fully mimicking the exterior of the asteroid, as Earth telescopes would only see a pale dot.

The aliens are carbon-based and 3-dimensional. They are not familiar with space-time warp (the ship was travelling at subliminal speed) and do not posess other hyper-advanced tech. So, they can for example have beam weapons, but are unable to phase out from reality or perform any other hard to understand tricks which we know nothing about. Their technology needs to be at least somewhat understandable.

How would they go about that?

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    $\begingroup$ subliminal speed sounds really slow ... $\endgroup$
    – ths
    Commented Jan 14, 2023 at 23:32
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    $\begingroup$ @ths moving only on the 25th fraction of a second :) $\endgroup$ Commented Jan 15, 2023 at 8:14
  • $\begingroup$ maybe it could tape a picture of a tiny asteroid in front of the James Webb's lens. $\endgroup$
    – Jason C
    Commented Jan 16, 2023 at 15:35

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The ship is very long.

o gif

https://en.wikipedia.org/wiki/%CA%BBOumuamua

Here is a simulation of Oumuamua tumbling. It is thought to be an elongated tumbling structure because the brightness of the single pixel that it appears as varies a lot as regards albedo. One way to explain that for a rock would be a shape that presented a much varying surface area - a cigar shape or a disc.

Your km long ship is very long and thin - a spear pointed at Earth. From the vantage point of earth it too is a single pixel. It does not tumble. On the prow of the ship they have a device which changes albedo and so presents the change perceived for Oumuamua. Maybe they were trying to fake us out. Or maybe the prow of the ship is a dancing mermaid and that is just how it looks from Earth.

From the vantagepoint of Jupiter this object might look very different.

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    $\begingroup$ So, basically, the size is hidden because you can't see how deep it is. Reminds me of this scene in Malcolm in the Middle, where what looks like one police is actually about seven police cars hidden behind the single car in front. Your spaceship's depth is hidden by it's front face. Additionally, USA spy planes use angled panels that deflect, dissipate, or absorb radar waves and give the large planes much smaller radar cross-sections, such that ""stealth"" jets appear on radar about the size of a soccer ball. $\endgroup$
    – Jamin Grey
    Commented Jan 16, 2023 at 4:32
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    $\begingroup$ one thing that most answers do not touch and i think would be pretty important is that they would need to calculate their trajectory and then let gravity bring them to us, without majorly correcting it. We would be able to see an object suddenly deviating from the natural path and that would unveil us that it is not a simple asteroid. $\endgroup$
    – bracco23
    Commented Jan 16, 2023 at 10:00
  • $\begingroup$ that animation sort of reminds me of the baby ruth bar in caddie shack. $\endgroup$
    – Jason C
    Commented Jan 16, 2023 at 15:37
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    $\begingroup$ @Jason C Good eye. "Doodie" has a long film history after Caddyshack. Check IMDB. $\endgroup$
    – Willk
    Commented Jan 16, 2023 at 15:42
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If they are only worried about observations from a single direction (Earth), 'cloaking' is simple. Have the earth-facing side be covered by a large mirror that reflects incident sunlight, both visible and infrared, away from Earth's direction. From Earth there then wouldn't be anything to see. Maybe make sure you don't accidentally reflect light from a bright star towards Earth, but I think reflected starlight would be much too weak to be observable with our current telescopes.

My guess is that for practical purposes this alone would be enough, as the majority of large telescopes detect reflected sunlight, i.e. visible and shorter infrared wavelengths. There are only a handful of large telescopes that can detect thermal infrared (only 3 on that list can observe thermal IR (around 10 µm) and are still operational), so the chances of one of those spotting the ship is minimal. If you also want to hide your thermal infrared signature you need to cool the mirror to very low temperatures. That requires a big radiator on the other side of your ship to reject the heat, but if you are only worried about observations from one direction that is not a problem.

And now your big ship is basically invisible to Earth, though not from other observation directions. The way to defeat this 'cloak' would be to have observation posts far away from earth, so the solar system could be observed from multiple vantage points. But we don't have that now.

If you want to make it look like an asteroid, maybe the 'cloak' is not quite good enough to be entirely invisible, or doing so would be too expensive, so the aliens opt to emit a little bit of radiation towards earth (on top of the radiation that leaks through the 'cloak') to simulate an asteroid.

Related: Stealth in Space: How realistic is it? (Note that defeating stealth depends on having lots of observation platforms scattered through space)

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  • $\begingroup$ Or just paint it black. That's way simpler and probably more effective. $\endgroup$
    – Graham
    Commented Jan 16, 2023 at 1:14
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    $\begingroup$ @Graham Regular black still scatters a percentage of incident light. The moon is actually very dark grey, but it doesn't look like that when the sun shines on it. I'm not sure if it would be easier to avoid stray emissions towards earth using a reflecting mirror or something strongly absorbing like vantablack, that would be an engineering question for the aliens I guess. Black has the additional problem that it heats up from the absorbed radiation and then you need to get rid of a lot more waste heat to keep your infrared emissions down. $\endgroup$
    – JanKanis
    Commented Jan 16, 2023 at 13:36
  • $\begingroup$ @Graham AND even at the same temperature, a black surface will emit much more energy than a mirror. $\endgroup$ Commented Jan 16, 2023 at 17:48
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Keep in line with the sun

Earth is the only inhabited spot in the Solar system. There is a telescope on moon/earths L2 now, but its deep infrared range does not allow it to look into the sun, or next to the sun.

An incoming alien ship could approach the solar system in such a way, that it resides behind the sun, as viewed from Earth's orbit.

During the actual invasion, the ship will proceed course: there's a turn (sling) around the sun now, which is a risky 1-3 days operation, where the ship will be visible. Proceeding after that, it could approach the planet while the sun's daylight disturbs our observation.

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    $\begingroup$ Earth rotates around the Sun in one year. If the ship arrives in a straight line from the exact opposite of the Earth with respect of the Sun, then an interesting and important plot point would be: how many months exactly does the ship have before the Earth's rotation takes it to a position where it can view the ship. $\endgroup$
    – Stef
    Commented Jan 15, 2023 at 17:14
  • $\begingroup$ @Stef t can't approach straight after it has arrived in, or near our system. The ship will travel straight to our system, until observable, say around Uranus's orbit. To approach Earth, it slows down to impulse and spirals in, along with earths orbit around the sun, keeping the sun, Earth and the ship on one line. Observers on Earth won't see the invasion coming. $\endgroup$
    – Goodies
    Commented Jan 15, 2023 at 17:50
  • $\begingroup$ That's one possibility. Spiraling requires a lot more fuel (or whatever source of energy they're using) than going in a straight line. So going in a straight line is a reasonable possibility too. It's up to the writer. But I think if I was the reader, I would enjoy realistic constraints on space travel that turn into important plot elements. For this reason, I really like your "risky sling around the sun operation", but I also really like the idea of the straight line that imposes a time window. What I wouldn't like is "the ship is completely hidden by the sun without constraints" $\endgroup$
    – Stef
    Commented Jan 15, 2023 at 18:37
  • $\begingroup$ It would also be somewhat satisfying if the ship was going in a straight line, initially hidden because it's on the other side of the sun, and then instead of the ship having to move around the sun, it's waiting for the Earth to move around the Sun, although that means that at some point it becomes visible (of course it starts accelerating towards the Earth as soon as it's visible) $\endgroup$
    – Stef
    Commented Jan 15, 2023 at 18:39
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    $\begingroup$ I was under the impression that this thing had to be mistaken for Oumuamua and so was constrained to take the approach that object actually took. $\endgroup$
    – Willk
    Commented Jan 15, 2023 at 22:17
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become the asteroid.

Theoretically a cheap and easy way to achieve this is to simply coat your invasion vessel with a coating of space rock and specialized materials capable of fooling infrared sensors.

This will both fool telescopes and protect your alien invasion vessel from exterior hazards such as asteroids that may pose a threat to the ships interior.

But, to be more realistic it doesn’t seem necessary to do anything. When humans find out the invasion ship has heat signatures, they wouldn’t be able to figure out exactly what’s causing them and how to prevent the invasion until it’s too late.

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  • $\begingroup$ Thanks. However, the ship is much much bigger than Oumuamua. $\endgroup$
    – Anton
    Commented Jan 15, 2023 at 6:32
  • $\begingroup$ I don’t see the harm in a big ship, by the time humans find out the actual size of the ‘asteroid’ it’s probably already too late. Even if they did have time to prepare they would be preparing to protect earth from an asteroid, not an alien invasion, this could be a good thing for your aliens. $\endgroup$ Commented Jan 15, 2023 at 18:08
  • $\begingroup$ I really like this answer, despite the frame challenge. The fake asteroid could be set up to look like it'll do a harmless close fly-by up until when it turns on the engines. $\endgroup$ Commented Jan 16, 2023 at 19:17
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Dark matter

Most of the alien ship, and indeed the aliens themselves, is composed of dark matter. It cannot be observed directly, and it's not nearly massive enough for us to detect its gravity.

The baryonic component, which we know as Oumuamua, is regular matter they brought along as source material from which to manufacture the weapons and other tools they'll use against us once they arrive. This matter has been specially prepared so their dark matter bodies and technology can interact with it.

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    $\begingroup$ Nice idea! However, I would like to stick to regular, carbon-based lifeforms if possble. $\endgroup$
    – Anton
    Commented Jan 14, 2023 at 21:44

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