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If aliens were to bring a space ship to the solar system, and put it in orbit around a Jupiter moon, and then begin colonising the surface of the planet, what would we be able to see from using current day technology?

Presuming the best case scenario, i.e. we are at a close orbit with Jupiter, etc.

How big would their ship need to be to be visible to us? Are there any 'major' telescopes regularly pointed at Jupiter, or would it be more likely picked up by someone with a (big) telescope in their backyard? How long would it take us to develop / deploy a probe to investigate (and how long to get there)?

ETA...

The main question here is about visibility. With today's technology, what would we see?

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    $\begingroup$ Too many questions in a single post. Please pick one $\endgroup$ – L.Dutch - Reinstate Monica Mar 2 at 15:53
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    $\begingroup$ Arthur C. Clarke had a pretty good take on those things already in 2001: A Space Odyssey and the sequel. Europa was even the planet with life on it. $\endgroup$ – GrumpyYoungMan Mar 2 at 15:57
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    $\begingroup$ @L.Dutch-ReinstateMonica Those all seem to be the same questions asked slightly differently. $\endgroup$ – John O Mar 2 at 16:00
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    $\begingroup$ @L.Dutch-ReinstateMonica The hope for the answer would be something like 'with modern telescopes, we'd only be able to see x, in order to see better we would need to send something there, though that would take y/z to achieve' - it is all based on the title ''would we see aliens on Europa' $\endgroup$ – Michael B Mar 2 at 16:11
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    $\begingroup$ Arthur C. Clarke wrote almost that exact story: Jupiter Five (1953), so be sure to read it and insert a few homages in your creation. $\endgroup$ – user535733 Mar 2 at 16:41
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Probes!

Yeh, Fish suggested this in the comments. But here is a sweet image!

europa

This close-up view of the icy surface of Europa, a moon of Jupiter, was obtained on Dec. 20, 1996, by the Solid State Imaging system on board the Galileo spacecraft during its fourth orbit around Jupiter. The view is about 7 miles by 10 miles (11 kilometers by 16 kilometers) and has a resolution of 28 yards (26 meters). The Sun illuminates the scene from the east (right).

https://europa.nasa.gov/resources/110/close-up-of-europas-surface/

Check out the crater that cleared off the cracks next to it. The link explains what happened there - stuff melted and flooded! 26 meters is good enough resolution to make out an average sized middle school and you could easily see a golf course.

In any case - there has been a pretty steady presence of one probe or another out Jupiter way. A dedicated Europa mission is going to happen sometime this decade. Juno is out there now and it has taken some incidental pictures of Europa. A Michael Crichton type fiction would have the Earth folks make use of resources like Juno already on the scene to get a better look at what was going on with Europa.

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    $\begingroup$ Mind you, the surface of Europa (as shown in your image) is a VERY complex texture and any artificial adding/changing thereof might not be very obvious at all. One could superimpose the entire US Interstate highway system on Europa, and it would get lost to casual observation unless you do detailed before/after imagery comparisons. $\endgroup$ – PcMan Mar 2 at 21:36
  • $\begingroup$ @PcMan what if Europa's surface is the US interstate highway station? (united states of aliens) $\endgroup$ – user253751 Mar 3 at 16:41
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    $\begingroup$ @user253751 I'd think if anything, it'd be the highway system of Europe, all things considered. $\endgroup$ – Darrel Hoffman Mar 3 at 18:40
  • $\begingroup$ @DarrelHoffman - because of how the layout is organized? $\endgroup$ – Willk Mar 3 at 18:53
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    $\begingroup$ @Willk No, just because Europa/Europe. (I don't know how closely it resembles any Earth highway system, just making a joke.) $\endgroup$ – Darrel Hoffman Mar 3 at 19:07
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Without offering any proof for my assertions...

  1. Electromagnetic emissions that don't belong. Should your aliens use EM transmissions (optical or radiometric), then it's possible to detect them here on Earth using current tech. After all, we're still detecting both Voyagers, and they're a honking lot further away than Europa.

  2. Heat. This one is a little less believable because it would depend on the resolution of thermal detectors... but unless your aliens are using some cool tech (every possible pun intended), the exhaust from their ship(s) could be detectable.

  3. Shadows. A number of previous answers have suggested that our cameras simply aren't good enough to detect a ship of reasonable size. However, that doesn't mean that the ship orbiting Europa (and, therefore, Jupiter) at an opportune angle to the Sun wouldn't cast a long shadow that could be detected and shouldn't be there. You'd have to be watching at just the right moment for something like this, but it's possible. (Consider this in real time. A shadow suddenly appears on the surface of one of the two spheres, momentarily grows longer, then vanishes... talk about cool ju ju for your story.)

  4. Radiation. Rather than assuming a combustion-style engine as I did in #2, let's assume a nuclear-style engine. Those things burp radiation all over the place.

  5. And with a massive caveat, depending on what the surface of the ship is like, and if the angles are once again opportune, the flash of light off the ship could be detected. (I admit that this is a specialized version of #1, but #1 was meant to convey communications tech, not happenstantial "oh, crap, we didn't think about painting the darn ship!" problems.)

  6. Finally, and this one is way out into left field, consider the possibility of an exotic drive like the Albecurrie drive. We're warping space, donchaknow... and I wonder if warping space means you're warping gravity right along with it. It might be that if the navigator brought the ship out of warp just a little too late, the result would be a small but detectable shift in the orbit of Europa. Gravity, it's a harsh mistress....

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  • $\begingroup$ I like no. 1 because SETI would mostly pick up an incoming ship if it uses the 21 cm hydrogen line. $\endgroup$ – The Square-Cube Law Mar 3 at 6:25
  • $\begingroup$ This is a great answer! Thank you for answering a lot of questions I didn't ask, but definitely appreciate having answered! $\endgroup$ – Michael B Mar 3 at 12:44
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    $\begingroup$ My knowledge about this topic is rather popular-sciencey, but wouldn't 6 cause a ripple in gravitational fields that could be detected with LIGO? We may not know immediately where did it come from, but we would be alarmed nevertheless. $\endgroup$ – Yksisarvinen Mar 3 at 15:16
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    $\begingroup$ @Yksisarvinen considering that LIGO can just barely detect black holes spinning around each other a hundred times a second, I wouldn't count on it. Mind you, the distance is also a lot shorter... $\endgroup$ – user253751 Mar 3 at 16:42
  • $\begingroup$ The Voyager probes are deliberately sending focused signals directly at us. If they were just sending non-directional signals any old place (as you might expect from Europa-ian aliens), it'd likely be too weak to reach us. (What would you call aliens from Europa anyhow? Europeans seems the obvious choice, but that word is already taken...) $\endgroup$ – Darrel Hoffman Mar 3 at 18:46
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This is what Hubble can see from Europa, as published by NASA

enter image description here

This composite image shows suspected plumes of water vapor erupting at the 7 o’clock position off the limb of Jupiter’s moon Europa. The plumes, photographed by NASA’s Hubble’s Space Telescope Imaging Spectrograph, were seen in silhouette as the moon passed in front of Jupiter. Hubble’s ultraviolet sensitivity allowed for the features -- rising over 100 miles (160 kilometers) above Europa’s icy surface -- to be discerned. The water is believed to come from a subsurface ocean on Europa. The Hubble data were taken on January 26, 2014. The image of Europa, superimposed on the Hubble data, is assembled from data from the Galileo and Voyager missions.

The plume at the bottom of the planet is about 8 pixels in height. Taking for good that its height is 160 km, it means that a single pixel resolves about 20 km.

This means that anything smaller than 20 km won't be resolvable: we would just see it as a single pixel.

Forget about reaching those resolutions with a backyard telescope.

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    $\begingroup$ If we have a probe passing by (such as Galileo, Voyager, or Juno), we can get an image like the superimposed Europa or better, so we might see smaller structures. Without such a probe, yeah, this is our current best. $\endgroup$ – The Square-Cube Law Mar 2 at 16:13
  • $\begingroup$ So it would need to be a really, really big ship then! - thank you :) $\endgroup$ – Michael B Mar 2 at 16:20
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    $\begingroup$ The Juno probe is currently orbiting Jupiter. While its primary task is to observe Jupiter itself, it will make several close passes to the moons, to alter its orbit. Notably both Ganymede and Europa. So yes, we do currently have eyes on site there, although it is not nearly optimized for ground surface surveys it might well detect something. A dedicated Europa probe would be the best, of course, but the Europa Clipper is only scheduled for launch in 2024, arrival in april 2030. $\endgroup$ – PcMan Mar 2 at 21:55
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Regarding, "... would it be more likely picked up by someone with a (big) telescope in their back yard?", the Hubble Space Telescope is among the highest resolution telescopes in existence. According to https://hubblesite.org/contents/media/images/2009/12/2508-Image.html, "Hubble can see details as small as 190 miles (300 km) across on Saturn." from Earth using its Wide Field Planetary Camera 2 ([EDIT] a different instrument than the one used in the other answer), so your hypothetical spacecraft would need to be at least that large and distinguishable from the background to be detected even by Hubble. A backyard telescope has no hope of detecting such a spacecraft, short of it being extraordinarily luminous.

If you want to do estimates, this site discusses the details of Hubble's resolution and limitations: https://illuminateduniverse.org/2019/04/11/angular-resolution-and-what-hubble-cant-see/

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  • $\begingroup$ That's a useful read! - thank you $\endgroup$ – Michael B Mar 2 at 16:20
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If your aliens are technological civilization, they will probably use radio-frequency communications. Or their machinery will emit radio frequency electromagnetic waves we could detect. I think we will manage to intercept their communications, first - accidentaly ("what is this strange interferences?"), than intentionally ("we have tuned radio-telescopes and send probe to Europa to investigate strange radio interferences coming from it"). Than probe will find something unusual on orbit and surface of Europe...

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    $\begingroup$ Europa is a close moon of Jupiter. And that planet is the greatest source of radio frequency noise in the whole solar system, after the Sun itself. Small signals could get lost in the noise easily. Also: as we develop higher tech, our radio transmissions become less energetic, and more resemble pure noise. It is reasonable to extrapolate this trend for superadvanced aliens, too. $\endgroup$ – PcMan Mar 2 at 21:41
  • $\begingroup$ The assumption that advanced technology implies detectable radio emissions is not valid. A civilization might pass through a short period of using high power, narrow band radios, as humans have done, but it is more likely that an ultra-wideband, low power technology would be used. Humans, just weren't smart enough go from the early on/off spark-gap transmissions, to more effective spread-spectrum radios. We had a tendency to think that more power output was better than more sensitive receivers. We just didn't have the math. Had Shannon been born 30 years earlier, the outcome might be different $\endgroup$ – jwdonahue Mar 3 at 18:48
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The answer to your question is it depends if the aliens want to be seen.

If they want to be seen they could send out radio messages, generate unusual heat or magnetic effects and paint the ship bright orange among other possibilities.

If they don't want to be seen then there would be little chance of finding them as things stand. They could use some fairly simple camouflage to blend into the background making them virtually invisible. They could also maintain radio silence and modify the heat signature to look like something else like a diffuse plume from a crack in the crust.

If they simply don't care then its hard to say as they might or might not give themselves away by various means. All other things being equal I think we would be very lucky indeed to spot them unless the colonization process was already very advanced and covered a substantial area.

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Rocket engines are bright.

Simply put, any rocket engine capable of taking off and landing on the surface of Europa will be quite bright, whether it's a chemical rocket or some form of nuclear rocket. It's got a surface gravity of about .134 Gs, so you'd need a rocket with a thrust-to-mass ratio of at least that much in order to land there.

However, given that this is interplanetary or interstellar flight, you're going to need to burn your rocket to decelerate, and that drive plume will be extremely visible. The luminosity of the rocket would depend on how large the rocket is, how fast it's going, and its acceleration, but it's entirely possible that a torch ship pulling a 1g acceleration would become visible to the naked eye, and amateur astronomers might notice it as a new "star" - a new "star" that is moving, no less!

Something like an Alcubierre drive might not create such luminous plumes, but if you're flying FTL, you won't be able to see where you're going, so it's entirely possible that it won't show up immediately in Jupiter orbit, and it'll need to perform rocket burns for interplanetary flight to get there. It's also entirely possible that we might see such a rocket approaching by observing the distortions in the light of other stars that it would produce, though that might require a bit of luck in pointing a telescope at an area where it happened to by flying past.

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    $\begingroup$ "a torch ship pulling a 1g acceleration would become visible to the naked eye".. it would need to be about as bright as a 200kiloton nuke, per second, to be visible from Earth using human eyes. As a 7th magnitude at-the-utter-edge-of-visibility light. Which will be drowned by the light of Jupiter. $\endgroup$ – PcMan Mar 3 at 8:13
  • $\begingroup$ How bright? What would the resulting magnitude be (that would set the limit for how small the telescope could be. And also if it could actually be observed with the moon's own bright light - you will not know if a 13th magnitude star is there if very close to a 6th magnitude by observing the combined light - even observing differences over time would require a 0.3% accuracy)? $\endgroup$ – Peter Mortensen Mar 3 at 11:31
  • $\begingroup$ @PcMan "it would need to be about as bright as a 200kiloton nuke, per second, to be visible from Earth using human eyes" Sounds about right, if it's a big enough ship and it's powered by a nuclear salt water rocket or some sort of fusion torch. The designs for Project Orion involved a 6000 ton rocket that used nuclear pulse devices about a tenth of that yield for space flight, IIRC. Even a much smaller rocket would be glaringly obvious to telescopes, however. Stealth in space isn't realistically possible once you fire off your engines. $\endgroup$ – nick012000 Mar 3 at 11:55
  • $\begingroup$ Interstellar drives must be highly efficient. To achieve high efficiency, the engine exhaust plume should be very cold and high velocity, therefore defuse and far less detectable than most of our current chemical rockets. Hot ejecta, wastes energy on lateral motion of atoms/molecules. The engine would pretty much have to be pointed straight at you, to see anything at all. Likely undetectable. Even our own current landers retro rockets have invisible plumes, in the absence of an atmosphere to react with. $\endgroup$ – jwdonahue Mar 3 at 19:05
  • $\begingroup$ @jwdonahue projectrho.com/public_html/rocket/spacewardetect.php#nostealth $\endgroup$ – nick012000 Mar 3 at 19:33

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