[Astronomy help] Likelihood of discovery?

[Jareth Valar]

OK, I, at best have Quirk level "Interest in Astronomy", but no real skill or true knowledge, so...

In my game I am having a massive alien "world/generation" ship appear in system about 14-15 AU from Earth, kinda mid way between Jupiter and Saturn.

The "ship" is between SM +28 to SM+31 or so (if I calculated correctly between 56ish and 170ish miles long, not accounting for shape), still playing with Spaceships and campaign ideas, but I do want it "many miles long" type big. Solid TL11^ (same race that was involved in helping defend its hidden interests in my previous question. I'm thinking it does have some stealth from sensors, but given my understanding of the rules in Spaceships I don't really see any point.

My question is, would a TL9 Earth, at the early stages of solar exploration (near Earth mostly) be able to detect it? I know there's always that chance the someone just happens to point a telescope in the right direction at the right time, but what is the general likelihood it would be randomly detected?

I mean the powers-that-be on Earth are looking, but IIRC our own TL8 Earth didn't detect Oumuamua until it was about a quarter AU away or so, and it was moving (extremely fast as I recall), but they still are unsure of it's actual size (anywhere from 100 to 1000 meters or so).

With this "ship" it will be much farther, not within the orbit of a planetary body, so any gravitational affect it would have should be minimal (I think), it's not moving (it arrived via Hyperspace styled FTL drive), I wouldn't say the hull all that super reflective and minimal visible spectrum light (the race see's mainly in visible and UV spectrums (from a world that rarely gets brighter than dusk on Earth).

I'm asking because one or two of the player will likely end up helping in that department during my downtime/time jump. Just wondering if it's even something I should have them even roll for. It will get detected eventually, when it becomes story relevant, but I have no issue if it gets detected early. This is mostly a question for if the powers-that-be would have any real way of finding it before I'm ready, as the players probably wouldn't mention it even if they found it, LOL. They don't really trust those-in-charge.

Sorry for all the odd questions. I my brain needs guidelines and general benchmarks to work from. Like "if an average Soldier is X and an "average" Special Forces is Y, then if I want a character that is comparable or better (but not superhuman) I have something to compare. Just how my brain works, *shrug*.

[Varyon]

Going entirely off the rules from Spaceships, as I'm probably no more qualified to discuss astronomy than you are (possibly less-so; I find it interesting but have only a bare-minimum of knowledge about the subject):

It's at between +28 and +31 to spot thanks to its large size, at +10 for being in plain sight (unless it's cloaked), +24 for being silhouetted against deep space, and -73 for being 15 AU away. That's a net -11 to -8 for a 20-second scan from an SM +2 TL 9 ship that only has the sensors that came with the Control Room. If it's got a TL 11 stealth hull, that becomes -21 to -18 (I believe it still counts as In Plain Sight), while if it has a cloaking device that instead becomes -31 to -28 (-10 for cloaking, and the +10 for In Plain Sight now doesn't apply, for net -20). In the former case, it's also got to be functioning on minimal power; auxiliary is a +3 (-18 to -15), I think most superscience perpetual-motion engines would be +4 (like solar panels, energy cells, etc), but anything more energetic makes the stealth hull useless. The cloaking device requires +4 or more from power generation (it's a high-energy system, which cannot be run on auxiliary power). So, all said, you're probably looking at anywhere between -27 (SM +28, power cell-equivalent IR signature, cloaking device) to -1 (SM +31, no stealth hull or one that's compromised from too high of IR signature, fusion power-equivalent IR signature, fusion power plant); if it doesn't have a cloaking device but does have a stealth hull, it's probably at -18 to -15.

By contrast, ʻOumuamua was SM +10 to +16, possibly at -1 to -2 for streamlining (although with it tumbling I suspect that wouldn't apply*), also had the net +34 for plain sight and silhouetted, and was detected 0.22 AU away, which would be around -62; total net is around -18 to -12 (not accounting for any modifier for being streamlined) - or -19 to -13 keeping in mind that TL 8 sensors are at -1 relative to TL 9 ones. So, if it uses a cloaking device, it's markedly less visible than ʻOumuamua and you can readily justify it being missed until it becomes plot relevant (and may want it to be discovered by a vessel that gets a lot closer than Earth is, or due to a temporary failure of the cloaking device). If it's using a stealth hull at auxiliary power, it's probably roughly as difficult to spot at ʻOumuamua was, so it likely is going to be found, although you can probably get away with waiting until it's plot relevant. If it lacks a stealth hull or has an IR signature high enough that the stealth hull doesn't work, it probably should have been detected shortly after showing up.

*Given the extreme streamlining, one may be justified in setting the SM based on width rather than length, like for a rope. That's somewhere around a -6 to SM - although I'd make it -4, so that a really long rectangle isn't harder to see or hit than a square of equal width - and means even the stealth-hulled vessel is easier to detect than ʻOumuamua was.

[johndallman]

Originally Posted by Jareth Valar

if I calculated correctly between 56ish and 170ish miles long, not accounting for shape

My question is, would a TL9 Earth, at the early stages of solar exploration (near Earth mostly) be able to detect it?

That depends how hard they are looking. At present, we are looking fairly hard for asteroids that might collide with Earth, which means that we're also finding more distant ones. Your alien ship is the size of a reasonably big asteroid, and we'd probably notice it within a few weeks, via automated telescope systems such as LINEAR, the Catalina Sky Survey, Spacewatch and others listed here.

Even if your TL9 society feels they have listed all solar system bodies of significant size, interstellar ones don't seem to be terribly rare, so they might well feel a survey project on our current scale was worthwhile.

[thrash]

A 100 km long object with a 5:1 aspect ratio and an albedo of 0.05 (about as black as soot) would have an absolute magnitude H ~ 10.3 . At 14.5 au in conjunction (phase angle = 0), such an object would have an apparent magnitude V ~ 21.8 . Ground-based astronomical surveys today routinely scan down to V = 24 or 25 (higher number is dimmer, hence better in this case), which means that there is little chance of not spotting the object.

Factors that would make it harder to spot:

• Different phase angle (off to one side of the Sun-Earth line).
• Not on the ecliptic (the plane the planets orbit in).
• Behind or close to the Sun, as seen from Earth.
• Backlit by the densest part of the Milky Way.
• Small end pointed at the observer.

Originally Posted by Jareth Valar

...it's not moving (it arrived via Hyperspace styled FTL drive), ...

I have to correct this: everything in space moves. The question is how?

If the object arrives at rest relative to the Sun and doesn't maneuver, it will start to fall into the Solar System from the Sun's gravity. This will make it look a bit like a long-period comet at first, but orbital plots will highlight the funny business pretty quickly (days).

If the object arrives with some latent velocity or maneuvers, it could enter an orbit anywhere from circular at its current radius from the Sun to elliptical to hyperbolic. A circular or broadly elliptical orbit would look like a (somewhat odd) Centaur asteroid. A narrowly elliptical orbit would look like a comet, shading towards longer periods the more narrow. A hyperbolic orbit would look like an interstellar body (e.g., 'Oomuamua) but would also eventually carry the object out of the Solar System unless it maneuvers. In each case, there is the option of an inward vs. outward initial vector; the latter would be less alarming but might excite more interest due to the limited observation window.

If the object hovers in place (only possible by maneuvering somehow), it might look like a background star -- but one that suddenly appeared, without having shown up previously in catalogs. This would also excite interest from astronomers.

Depending on your technology, maneuvering could light up the object for observers like a flare on a dark night. At a minimum, any radical change in its orbit would point to artificial origin. (At that distance from the Sun, there essentially aren't any natural forces that would do it, short of an actual collision. This, in turn, would leave a large and very visible cloud of dust.)

[RyanW]

This object is about the same size as Saturn's moon Phoebe, and unlikely to be much darker, and Phoebe was discovered in 1899. Many much smaller moons have been discovered since then (some by Cassini, but others by Earth-based observation).

Unless it has magical stealth, an object this size will be seen if somebody is looking in the right area. If it's drifting, it'll be immediately flagged as special because it's apoapsis is infinity (it's extrasolar). If it's thrusting, every telescope in the half of the planet facing it will be pointed at it in about ten minutes.

[SilvercatMoonpaw]

You could have the ship arrive and maneuver to keep something between it and Earth.

[KarlKost]

Originally Posted by SilvercatMoonpaw

You could have the ship arrive and maneuver to keep something between it and Earth.

Exactly. At TL 11, such a ship will only be spotted by a TL 9 civilization if it either wants to be found, or just simply doesnt care. If it decides to stay away at orbit on plain sight in the most obvious way, it will be quickly spotted.

If however this thing is actively trying to hide it's presence it can do so, even if it doesnt have some miraculous magic-superscience stealth field. All it had to do would be to hide behind the Sun or behind Jupiter or Saturn or even Uranus, in relation to Earth. In such case, it would be practically impossible to find it.

Otherwise, it would be almost a given that it would be spotted, particularly due to all the weirdness that it would be doing (clearly artificial size/shape and movement)

[Anthony]

Originally Posted by Jareth Valar

My question is, would a TL9 Earth, at the early stages of solar exploration (near Earth mostly) be able to detect it?

For this, I would treat it as an asteroid.

The absolute magnitude of an asteroid is around 33±2 - (SM*5/6) (remember to add +2 SM for typical asteroid shapes; thus, a 1 km asteroid is SM+18). If we call your ship SM+30, absolute magnitude is thus ~8.

This is adjusted for distance from the observer: add 5*log10(distance in AU). This adds 5.75 to apparent magnitude.

This is also adjusted for distance from the sun: again, add 5*log10(distance in AU). Again, 5.75.

Final apparent magnitude is likely between 17 and 22, which is unlikely to be spotted by a random observer but is within the capabilities of automated sky surveys used for detection of asteroids.

[Pursuivant]

Presumably, a TL9 society will have a number of ships in orbit around Earth and other planets as well as communications and observation stations, which might reduce range between the nearest observers and the alien ship making it even easier to spot. (E.g., if there's a deep space observatory or asteroid-detection system in orbit around Mars).

Another thing that might make the alien ship obvious is heat, assuming that its drives and power plants generate heat which has to be radiated into space. Something that shines like a beacon in the infrared spectrum would really stand out against the relative cold of the outer planets even if its albedo is low in the visible spectrum.

A potentially fun idea is Who Spots It First? An alien ship first detected by an amateur astronomer is going to be introduced to the world in a very different way than one detected by a U.S. (or Chinese) Space Force deep space control vessel or a Martian-registered robotic asteroid mining ship.

A potentially fun "prequel" adventure could be various human factions all rushing to figure out what the newly discovered celestial feature is and then reacting in different ways once they realize it's an advanced alien ship. (Open fire? Run away? Try to destroy or disable rivals so you're the one making first contact? Initiate contact? If you try to initiate contact what do you say and how do you say it? Meanwhile, what do the aliens think of all the quaint locals operating the interstellar equivalent of outrigger canoes?)

[Varyon]

Originally Posted by SilvercatMoonpaw

You could have the ship arrive and maneuver to keep something between it and Earth.

Unless it has a means of "looking" through whatever it keeps in the way, it's not going to be able to tell what's happening on Earth either - and depending on just how much the TL 9 civilization has spread out into space, it may not be able to keep everything on the other side of Jupiter (or whatever) anyway. A swarm of small machines with sensors on them would work for the former (they aren't seeing through what's in the way so much as they are deploying things that are too small to detect to do the looking for them; normally that's going to result in poor resolution, but slaving enough of them together into a network could create something akin to a space-based version of the Very Large Array, which will help markedly with that). But for the latter, they'd likely need some superscience cloaking or similar - if the humans have sensors out in the asteroid belt (hitching a ride on Ceres, for example), the ship is going to be visible to someone at some point, it's just not possible to stay fully hidden without getting really close to Jupiter (and with the size of the ship, I think this might influence Jupiter's orbit and/or those of its moons in a detectable way).