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I was gonna ask a very similar question to a very recently updated post from 5 years ago, sorry if considered a duplicate.

My question is : how long can a 15-150 km-long extrasolar asteroid coming from 25-100 A.U.s straight to Earth (or through 'accidental' gravity assists ballistic manoeuvers) stay unnoticed with 'optimal' and 'natural' stealth properties (e.g. low albedo and and crystal-like shape offering low radar cross section) and what is the best possible 'natural' stealth coating for an asteroid (I'm guessing some carbonaceous chondrite like Phobos) ?

Also, this asteroid needs to travel from the edge of the heliosphere to Earth in a 12 years period, meaning an average speed of 144,000 kph (about 4 times faster than most asteroids I found). Is it possible regarding known extrasolar objects ?

If that asteroid is detected around the second half of July, 1994 (as the sixth object discovered this second half of the month), and it's colision course with Earth confirmed in the meantime, is D/1994N6 a convention-accurate name ?

Is stealth even possible for an object that big (considering a 99% light absorbing material and Nighthawk style shape to trick radars) or it's own mass and gravity will be enough to significantly (= detected by modern telescopes) alter the path of other objects in the Solar System to predict its existence and even triangulate its position ?

With all these special properties in mind (4 times velocity of most asteroids crossing the Solar System, reduced RCS and albedo through geometric shape and blackness/light-absorption due to natural chemical elements composing it, and being aimed at Earth, either directly on its expected location in the future or from accidental interaction with other celestial bodies like gas giants altering its course like a slingshot), will it's nature as an artificial threat launched by an extrasolar civilization be suspected immediately after discovery ? Or its true nature can stay ambiguous ?

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    $\begingroup$ Welcome to the site and there's an interesting concept here. However, there is a strict "one question per post" policy here, please take the tour and see what the conventions are. I suggest that you narrow down this question to focus on just the stealth considerations - which means that you need to provide information on how it was launched. The question of what its convention-accurate name would be probably belongs on Astronomy SE along with the believability of its velocity. What scientists might suspect based on its characteristics is a story question that is up to you. $\endgroup$ Apr 14 at 22:11
  • $\begingroup$ Unnoticed by what? This depends massively on what's being used to look for it. $\endgroup$ Apr 14 at 22:19
  • $\begingroup$ Are you seriously looking for absolute truth? The deep space radar network has been in business since 1963. Therefore, it's possible that the incoming asteroid could be detected. I'm not even going to search to see if anything we've seen moves that fast, because we're constantly surprised by what we find in space. So that's possible, too. Radar, just another kind of photon, travels 10,000X faster than your proposed speed, so you're clear there, too. Naming convention? Are you expecting to be judged by a NASA pro? ... $\endgroup$
    – JBH
    Apr 14 at 23:29
  • $\begingroup$ And whether or not humanity figures out that it's artificial and an attack by an alien species is storybuilding, not worldbuilding. It's definitely not an absolute truth. I've not voted to close, but you're on your way to having this question closed. Please read the following two Help Center pages to better understand our strengths and weaknesses (help center and help center). BTW, a 30-sec Google search listed a dozen places to learn about asteroid naming conventions. Research is expected on Stack Exchange. $\endgroup$
    – JBH
    Apr 14 at 23:32
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    $\begingroup$ Does it matter? Even if it were blatantly artificial (radar showed it was made of processed metals and it was emitting modulated EM signals) it's not like humanity would be able to do anything about a properly-sized planet-killer. $\endgroup$ Apr 15 at 4:23

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A good starting point for this sort of information might be 'Oumuamua, aka. 1I/2017 U1. The 1I prefix shows it is the first identified interstellar object. The only other confirmed one with a name right now is 2I/Borisov, I believe.

Speeds are slippery things when considering gravity and orbits, and an interstellar object will have a hyperbolic orbital trajectory about the Sun, accelerating as it approaches perihelion. I don't believe there's anything particularly unusual about the speeds you're thinking of, however. Borisov had a hyperbolic excess velocity (the speed at which it "entered" and "left" the solar system) of 32 km/s which isn't far off the 40 km/s you want, and it would be accelerating on the inbound leg. Computing its actual speeds at whatever point on its orbit is left as an exercise for the reader.

Is stealth even possible for an object that big

The question you should be asking is "is stealth even necessary for an object that small, visible for such a short period of time, that no-one is actively hunting for, using technology similar to humans today?". Detecting asteroids is hard, but there tend to be many opportunities to see them on their orbits, and those opportunities are very much lacking for interstellar objects which are strictly one-run-and-done. 'Oumuamua was basically detected at its perihelion. Borisov generated a nice cometary tail to aid detection. Your object could be a regular space rock and we might not actually spot it until (in astronomical terms) it was right on us. Radar astronomy only works on very close objects, occultation only works by luck if you don't already know where the body is going to be or it flies in front of something really obvious like Jupiter... but I guess its originators could predict and avoid that.

it's own mass and gravity will be enough to significantly (= detected by modern telescopes) alter the path of other objects in the Solar System to predict its existence and even triangulate its position ?

It is far too tiny for that.

blackness/light-absorption due to natural chemical elements composing it

As always, there ain't so such thing as a free lunch. If you paint it black, it is going to heat up, and that makes infrared astronomy more practical. In all likelihood though, I would expect it to get pretty close to Earth (maybe within an AU) before anyone spotted it, and there's a reasonable chance that people on the other side of the world detect it via seismometers before anyone actually saw it with a telescope or radar.

Remember that 90s astronomy wasn't nearly as capable as 2020s astronomy, and 2020s stuff isn't really up to detecting regular interstellar objects let alones ones that are actively trying to avoid being detected. Your stealth seems like overkill, and in any case it could come in glowing red hot and broadcasting "we're gonna murder you all, lol" messages across the spectrum at high power, and I suspect it'd still be impractical to effectively deflect it in time.

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    $\begingroup$ This. There is no stealth in space, if someone is looking with halfway-decent instruments. But for the most part, there isn't anyone looking, or they're looking too infrequently to matter. It's more likely to be noticed if it passes near something else that people frequently observe. It's also so routine for an object to be located in earlier observations after its discovery that there's a term specifically for it, "precovery". $\endgroup$ Apr 15 at 13:02
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This question is entirely subjective and only depends on what you want to have happen in the story. Asteroid scouting is such a niche field right now, i would not be surprised if we did not notice the rock till it got a trail.

Is it possible regarding known extrasolar objects ?

Yes.

Is stealth even possible for an object that big

No. An asteroid 15-150 kilometers across is larger than mount Everest.

(considering a 99% light absorbing material and Nighthawk style shape to trick radars)

By that point it is a Missile and not anything natural. The overall shape of an object does not determine its stealth capabilities. Radar waves are around 1-2 cm long. Anything larger than that will reflect some of the emitted signal. Stealth is all about creating a surface with geometry that either absorbs or reflects the waves such that they dont come back to the emitter.

Asteroids are made of rock and ice. Their surface looks like a moonscape. Its way to rough to absorb anywhere near 99% of anything. Leaving aside visual light is not how you would spot such a thing.

If the Asteroid is 150 kilometers across, it is a dwarf planet which will have some thermal emission from the core.

will it's nature as an artificial threat launched by an extrasolar civilization be suspected immediately after discovery ?

I have not even read this before making my previews point. So, yeah. Absolutly no asteroid could ever look like what you describe. Even a black hole would be more obvious than this thing.

The fact it is artificial would be apparent the nanosecond anyone took a spectral reading of the surface to figure out its composition. There is just no other explanation left than it being artificial.

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  • $\begingroup$ '> No. An asteroid 15-150 kilometers across is larger than mount Everest.' My question was when will it be spotted as it go through the Solar System. We know some dwarf planets/big asteroids like Sedna and Orcus exist in the System beyond Pluto but can't see them, and they are bigger than this rock. '> By that point it is a Missile and not anything natural.' 99% was an overkill. 93% like Phobos with its 0.07 albedo. '> Asteroids are made of rock and ice. Their surface looks like a moonscape.' Is there no alternative, like a mostly crystal like structure ? (like a big diamond ?) $\endgroup$
    – Kaliem
    Apr 15 at 13:52
  • $\begingroup$ I answered that. Whenever you want it to be spotted. The way we currently spot things is not very uniform. If you want it to be spotted by some amateur 2 lighthours out, go ahead. If you want NASA to freak out when it suddenly pops up next to Jupiter, do it. You can see Phobos from earth with a good telescope. 0.07 is still very bright. No. How would that form ? Besides, if it was a crystal it would reflect more light. $\endgroup$
    – ErikHall
    Apr 15 at 14:25
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Keep it away from the ecliptic so there's a lot less looking towards it. Let it pass the sun and come at Earth on the way back out. Most everything that is looking at the sky will be looking out because the sunlight hitting the atmosphere messes with anything pointed anywhere near the sun. (Not a problem for space-based telescopes but for safety reasons they don't point those anywhere near the sun.)

You don't need to get fancy with your rock, just make sure it is free of materials that would form a tail. I could easily see such a rock not being detected until it was very close indeed, perhaps in the atmosphere.

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