Assume that an alien starship pops into existence at the L2 Lagrange point of the Earth-Moon system, behind the Moon as seen from Earth.

When and how is the starship detected and recognized as extraterrestrial?

  • For purposes of the story, Earth has real-world, present-day science and technology.
  • The starship has a cross-section of 1,000 to 10,000 square metres, depending on the aspect. It consists of steel and composites with a matte-black paint job and no lights or radio beacons, but no deliberate stealth technology.
  • How the starship arrives behind the Moon is space opera technobabble, the means are not detectable by real-world science. It cannot be seen on arrival.
  • I'm aware that space debris is tracked by various agencies, but I don't know if they have means to look at L2.
  • I'm aware that there is a satellite at L2. No idea if it has navigation sensors or if it is watched by relays off the direct line.
  • L2 is unstable. The starship can maneuver to stabilize a halo orbit or park on L2, again by means not detectable by Earth science. It has collision-avoidance systems to dodge satellites already there.
  • Assuming that somehow the presence of an "anomaly" was detected, what assets could be used to take pictures of the object "behind" the Moon?

For narrative purposes, the best answer would be a civilian scientists detecting "weird" readings, followed by an attempt to get a peek with civilian instruments, followed by civilians telling one or more air forces about "something big-but-fuzzy at L2," followed by hectic activity in various capitals. What can be done to make this plausible, if it isn't?

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  • $\begingroup$ Definitely related, but with tighter constraints: How far from the Sun could we detect an alien spacecraft similar to the Voyagers? Full disclosure: My own question. $\endgroup$ – a CVn Jul 14 '19 at 18:22
  • $\begingroup$ Okay so I know whether I'm editing or cutting a whole new answer what details in particular are you looking for? $\endgroup$ – Ash Jul 16 '19 at 18:50
  • $\begingroup$ @Ash, editing is fine. I'm looking for a plausible, interesting chain from a random/lucky initial find, over instruments a civilian researcher might get re-tasked to confirm or disprove the anomaly, to the systems an air force or government agency would use to follow up. A credible researcher would hardly go "this blip in my astrophysics data means there is an alien spacecraft behind the Moon, let's redirect your expensive telescope and take a look." It would be more like "funny, what's that blip, I'll ask some grad students to analyze it, fit it into established theory, and write a paper." $\endgroup$ – o.m. Jul 16 '19 at 19:00
  • $\begingroup$ Okay I'll have a fresh look at it from that angel tomorrow, it's time to logout for me. $\endgroup$ – Ash Jul 16 '19 at 19:12
  • $\begingroup$ Several days left on the bounty. $\endgroup$ – o.m. Jul 17 '19 at 4:08

There are space telescopes sensitive to various frequencies in various orbits. Even a telescope in low Earth orbit would be at a right angle to the direction between Earth and the Moon twice in each 90 minute orbit. And at that angle a low orbit telescope (and one on the ground below it) should almost be able to see the L2 position.

According to a rough scale diagram I drew a low orbit telescope might possibly be able to see the exact L2 point when the Moon is at perigee, it's closest distance. And an object, like your alien space ship, at the L2 point should have a halo orbit around the L2 point and sometimes be thousands of miles from the L2 point. If any of that movement is perpendicular to the line between the L2 point and the centers of the Earth and the Moon, the alien spaceship should sometimes be in line of sight as seen from low Earth orbit or even the surface of the Earth.

Any telescope able to view L2 with enough resolution at the right wavelength - like the infrared wavelengths of the heat emitted by the space ship - should detect the space ship.

This looks like a job for the James Webb Space Telescope!

It is scheduled to launch in March, 2021 into an orbit near the Sun-Earth L2 position. It will be near the L2 point 1,500,000 kilometers (930,000 mi) from Earth, directly opposite to the Sun. Actually it will be in a sort of halo orbit around the L2 point, an orbit with a radius of kilometers or 500,000 miles. So the Earth-Moon L2 point should almost always be visible from the James Webb Space Telescope.

And the James Webb Space Telescope is designed to detect infrared wavelengths, and all objects in outer space emit infrared radiation based on their temperatures.


If there are aliens in that spaceship that come from an Earth-like environment the spaceship will have an earth-like temperature and emit Earth-like infrared radiation.

And even if the aliens come from a planet so cold their bio chemistry uses liquid ammonia or liquid methane instead of water, that planet - and thus their spaceship - will still have a temperature above absolute zero and will glow in some infrared wavelengths due to that temperature.

And if the alien spaceship is only occupied by machines, those machines will have an optimum working temperature, and the spaceship will need to be at that optimum working temperature, which will be above absolute zero, and so the spaceship will emit infrared radiation at that temperature.

At the present time the Spitzer Space Telescope, launched in 2009, is orbiting the Sun and some of its infrared instruments are still working. The Earth-Moon L2 point would only rarely be hidden behind the Earth or the Moon from the Spitzer Space Telescope, so if it has enough resolution at the right wavelengths it could detect the infrared light emitted by the alien spaceship.


Someone more familiar with those infrared space telescopes might be able to calculate if they could accidentally detect the alien spaceship in the L2 position or confirm its existence if accidentally detected by other methods.

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    $\begingroup$ Another point. Even if the exterior of their ship is as cold as space itself, it still will be detectable because at such a relatively close range, it would occlude a relatively wide arc of the sky. We have detected a few Kiuper Belt objects by noticing them block the light from distant stars. Earth-Moon L2 is much closer than the Kiuper Belt, so even a small ship would have comparatively large apparent size. $\endgroup$ – Ryan_L Jul 14 '19 at 22:59
  • $\begingroup$ I get a feeling that the two answers so far could be combined into a solution. The Low-Frequency Explorer for the initial clue, the Spitzer to nail it down. $\endgroup$ – o.m. Jul 15 '19 at 16:39
  • $\begingroup$ This answer covers who could see the ship, but not whether any of them are actually looking at it. Space is big and just because you are a telescope in the same orbit as an object, does not mean you notice it. $\endgroup$ – Muuski Jul 15 '19 at 20:55
  • $\begingroup$ From just some quick estimates, I get similar to user58697's comment to you in the other answer. Basic trig shows approximately 12000km for the side of the triangle going from Earth core out to where you could possibly see L2, so that's about 6000km beyond the planet surface. A satellite out in geosynchronous orbit meets that criteria, but not low earth orbit. $\endgroup$ – Loduwijk Jul 16 '19 at 19:25

The Queqiao satellite parked at the Earth-Moon L2 point may detect the appearance of the object if there is an accompanying radiation spike in the 80 kilohertz to 80 megahertz range as the satellite hosts the Netherlands-China Low-Frequency Explorer which is running astrophysical studies focused on that frequency range. It's also possible that the ship may break said satellite on arrival.

Alternately there are several other satellites planned for that area one of which may observe or impact on the ship. In terms of visual observation from Earth, no that's not going to happen.

Also depending on the mass of the ship in question very finely calibrated satellite systems such as GRACE may detect it purely due to gravitational anomaly.

I believe that in order to make a good visual observation we would need to launch a new asset.

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  • $\begingroup$ Nothing in a high orbit that could get line-of-sight and get retasked? $\endgroup$ – o.m. Jul 14 '19 at 19:03
  • $\begingroup$ @o.m. Nothing is that high up, and in fact the farther up an object's orbit the less able it is to see past the moon to the anomaly, you might be able to retask SOHO or one of the other Earth-Sol L1 satellites they're 0.1 AU from Earth and should be able to observe the anomaly as the Earth-Moon L2 passes Aphelion. $\endgroup$ – Ash Jul 14 '19 at 19:23
  • $\begingroup$ There are space telescopes sensitive to various frequencies in various orbits. Even a telescope in low Earth orbit would be at a right angle to the direction between Earth and the Moon twice in each 90 minute orbit. And at that angle an low orbit telescope and one on the ground below it) should almost be able to see the L2 position Any telescope able to view L2 with enough resolution at the right wavelength - like the infra red wavelengths of the heat emitted by the space ship - should detect the space ship. $\endgroup$ – M. A. Golding Jul 14 '19 at 19:38
  • $\begingroup$ @M.A.Golding L2 is behind the Moon as seen from Earth, the lunar disk occludes most direct lines of sight. $\endgroup$ – Ash Jul 15 '19 at 10:37
  • $\begingroup$ @Ash The Earth is about 4 times as wide as the Moon, and a low Earth Orbit is even wider compared to the Moon. And the alien space ship should oscillate around the exact L2 point. Thus if the alien space ship is far enough beyond the Moon it will be visible from low Earth orbit or even the "side" of the Earth. See my answer and try creating a scale diagram of the Earth and the Moon. $\endgroup$ – M. A. Golding Jul 15 '19 at 16:28

Your preference is for initial observation by Mr. Average Joe. We can see how plausible this is by creating some categories of how likely Average Joe is to have access to certain vantage points.

From most plausible to least plausible (list is not exhaustive):

  • Surface of Earth - very easy, just walk outside
  • Low Earth Orbit (hudreds of km up) - not as easy but still plausible; Average Joe can get viewing time on such telescopes
  • Geosynchronous Orbit (10s of thousands of km up) - even less likely, possibly implausible; can Average Joe get viewing time on such satellites? Not that I know of, but I'm not an expert
  • Lunar Orbit - Not unless Average Joe has some special job, connections, or story plot hook
  • Something else farther away, such as Earth-Sun L-points instead of Earth-Moon L-points, or other non-Earth/Moon-bound objects - Not feasible for Average Joe

Now let's do some simple, rough trig estimations to see how far from Earth you'd need to be to see around the moon to that point.

Distances in thousands of km:

+ < Moon edge
+ ~2
+            ~61
^Moon center                ^L2

All we have to do is scale that triangle up by multiplying the sides. We know that the distance Earth-to-Moon is ~384000km, so the distance Earth-to-Lunar-L2 is ~(384000+61000)km = ~445000km. That is a factor of ~7.3 increase from 61000km, so...

(again distances in 1000s of km)

+ < X (which is where we want to be at or past to see Earth-Moon-L2)
+                    + < Moon edge ~2
+                    +
^ Earth center ~445  ^Moon center ~61    ^L2

This has now become a "Solve for X" math problem. X / 2 = 445 / 61 After solving for X by shifting the "/2" over, we have X = 14

So that is 14000km from the center of the Earth, so for orbit distance about half that number of km from the surface of the Earth. A calculation that I did shortly before writing this answer put it at a 6000km orbit, and I think that quick calculation was slightly more accurate than this one.

Roughly, 6000km is about 10 times farther than low earth orbit, but is only about one-tenth as much as geosynchronous orbit. So Average Joe cannot view L2 from his backyard, nor from a low earth orbit telescope. Average Joe would need to beg, borrow, steal, or pay for time on something farther out. Possible, but not quite as feasible.

It might still work for your story if this is Academic Astronomer Joe or Wealthy Hobby Astronomer Joe rather than just Average Joe. Either that, or maybe a plot hook that gets Average Joe some experience and some time on the appropriate telescope. Plot hooks could include being a college astronomy major or having a friend or relative that can get Joe the telescope time.

As Joe rode his bike past the observatory, he saw the last car leave their parking lot. Joe had wanted to get back into there ever since he got a tour of the place for a 9th grade STEM event. They even had a computer in there that has access to a telescope in geostationary orbit that they got to play with for a few minutes - it was as if they were sitting out in space and looking out at the cosmos!

Joe couldn't shake the feeling. Despite being on detention again this week for breaking into the high school's science lab after hours, he just could not pass this opportunity up. With the holiday tomorrow, nobody would be back at the observatory for days. It's decided! The only question now is where to hide the bike from street view for the next hour.

(break away to another set of characters, then next chapter come back to...)

Joe could hardly believe it, the password was written down on a sticky note in a drawer nearby! Didn't these guys ever get a computer security lesson when they were in school? Oh that's right, that wasn't a thing when they were Joe's age.

After punching in the password, Joe was in. Luckily, Joe was paying close attention on the tour when the astronomers were controlling the orbiting satellite. Hmm, it's trained on one of Saturn's moons - so far away, so tiny, so pixelated... so boring. Let's look at something closer. The moon should be easy to find. What is that tiny patch there? It's black, but it still stands out, just barely...

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  • $\begingroup$ "Mr. Average Joe" is not quite what I mean. More like "Prof. Average and his team on a research grant that has nothing to do with alien starships." $\endgroup$ – o.m. Jul 17 '19 at 4:07

We do have a telescope observing the Earth from beyond the Moon, the Deep Space Climate Observatory. It orbits itself at Earth-Sun L1 point. When moon aligned right it already captured the moon's far side, potentially revealing any objects above it or at Earth-Moon L2.

However, 10000 square metres means diameter of about 100 metres. This is roughly 0,00005 of moon's diameter, so should the alien object be visible as 1 pixel, the image of the moon must have at least 20k*20k pixels.

Sadly DSCOVR with its 90's technology downlinks only 1024x1024 pixels. Launching another satellite with muchly improved resolution telescope is technically fully feasible.

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I just got a crazy idea reading at people answers.

What if a meteorite or comet is about to pass really close to the Earth (and thus, the Moon), and, as so many people are looking at it, they detect the shadow of the alien starship when it goes between the Sun and the meteorite?

I know is pretty impossible that it just happen to align for the shadow to hit on the meteorite, but that way a lot of people with their telescopes would see it, and with the professional ones even calculate the size of the starship.

Maybe too crazy for your question, but I think is somewhat plausible.

Hope I inspired something!

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  • $\begingroup$ Nice idea, but I doubt that is feasible. An asteroid or comet is so irregular-shaped that some grainy blotches have many better explanations than an alien starships. I think the initial clue will have to be in a data dump from an orbital or deep-space instrument. $\endgroup$ – o.m. Jul 18 '19 at 15:27
  • $\begingroup$ @o.m. Yeah, you are right. Maybe then a moon satellite? We have 3 now in orbit, they can maybe help with it. $\endgroup$ – Miki4444 Jul 19 '19 at 8:21
  • $\begingroup$ That would require people tracking the sat with optical instruments. Wouldn't radio interruptions be more noticeable? $\endgroup$ – o.m. Jul 21 '19 at 20:32
  • $\begingroup$ @o.m. Well, it would be possible for people to be looking at those sats, and yes, radio interruptions would be more noticeable (although an advanced enough starship in stealth mode would avoid those). But I was meaning that those sats would see the starship. A simple picture with the black starship in the middle of the white moon would be noticeable fast. In fact, those pictures at first would be out of focus, so the warning around the scientist would rise, and they would have to change the focus of the satellite, or use another one, to see directly to it. $\endgroup$ – Miki4444 Jul 22 '19 at 6:34

I think the sudden gravitational influence of a very large ship would impact the stability of every geostationary satellite orbiting the Earth. Since these satellites have limited maneuvering fuel for correcting minor perturbations to their orbit, the corrections are applied very judiciously.

So, a sudden perturbation to every satellite would cause people to look for a reason. And they’d all point to something massive at L2. So, they’d go look using telescopes, spacecraft, or whatever was handy. It's reasonable to assume they’d ask NASA and other global space agencies for help figuring out what happened.

I think after that point, the discovery of the Thing at L2 would be inevitable.

Edit --------------------------------------------

Based on an estimate of 100E6 tons for the mass of the spaceship and a distance of 1E6 miles between L2 and Earth, the spaceship would exert 1E-14 Newtons on a satellite. In a week of the additional gravity field, the satellites could experience as much as a ~1E-3 m/s delta-V -- which is very small. But, after a month it would be significantly greater at just under 0.1 m/s and after a year > 1 m/sec to delta-V.

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  • $\begingroup$ Good idea, but are the disturbances great enough? A cross section up to 10000 square metres suggests something less than a million cubic metres, and that should mean less than a million tons. $\endgroup$ – o.m. Jul 18 '19 at 5:05
  • $\begingroup$ @o.m. I added some calculations for the perturbations to the delta-V of satellites in orbit. Given sufficient time it will is obvious. But, in the short term, I think it is that every satellite suddenly changed in a very small degree that would catch peoples attention $\endgroup$ – EDL Jul 18 '19 at 6:28

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