When it comes to aliens invading Earth people usually don't find out about the invasion until it's too late. Sure, their spacecraft could use some kind of invisibility device, but that still doesn't explain things like electromagnetic signature, etc. So if an alien spacecraft were to approach Earth slower than the speed of light wouldn't Humanity notice the craft a few decades out?, if not centuries?

Also I am aware there are questions that involve alien ships hiding from Earth's view in the solar system. This question specifies that the ship hasn't entered the solar system yet, so going in a spiral with the Earth's orbit around their star doesn't seem like a viable option.


closed as too broad by Mołot, sphennings, MozerShmozer, Azuaron, Red_Wasp Nov 29 '17 at 20:29

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ I don't think this question is about world building but about the possibility of detecting real space crafts? What part of this world is fictional (and not just hypothetical like your spaceship)? $\endgroup$ – Raditz_35 Nov 29 '17 at 15:10
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    $\begingroup$ I'd suggest reading this well written answer about how there is no stealth in space. $\endgroup$ – sphennings Nov 29 '17 at 15:17
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    $\begingroup$ "A cloaking device" [...] "doesn't explain things like heat, electromagnetic signature, etc." That what does that cloaking device do? How is it useful? I thought that the very point of a cloaking device was to, you know, cloak the ship. And in space heat emission and electromagnetic emissions are the exact same thing. $\endgroup$ – AlexP Nov 29 '17 at 15:19
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    $\begingroup$ I've editted your question for clarity, but I would suggest deleting the third paragraph entirely – it's asking how the aliens could avoid detection, which is beyond the scope of asking how we would detect them. $\endgroup$ – rek Nov 29 '17 at 16:43
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    $\begingroup$ Haven't you heard? A 10km x 1km spaceship/asteroid recently passed through the solar system, and we didn't detect it until it was on its way out again. Astronomers estimated that 10 or so of these pass through the solar system every year and we don't even see them. While there is no stealth in space, space is really freaking big, and we only monitor a tiny part of it. Which is to say, you don't need stealth, you likely won't be seen no matter what. $\endgroup$ – Azuaron Nov 29 '17 at 19:22

It's extremely tough to detect many space objects and more often or not, what we do detect is from a mix of luck and tenacity. It's important to remember the size of space and that most of us hold a pretty distorted view of it...if you've ever seen a picture of the Sun and Pluto on the same piece of paper, then you've witnessed this distortion. If the sun was reduced to the size of a pea, by scale, Pluto would be a microscopic (not visible to human eye) spec of dust some 35 feet away. Scale representation of Earth wouldn't be on the same piece of paper either.

If we assume the spacecraft has taken steps to mask it's deceleration and isn't giving off any light or radiation itself, lets compare what it's taken to detect objects of smaller sizes from Earth:

In 2011, we discovered Pluto has yet another moon. https://www.nasa.gov/mission_pages/hubble/science/new-pluto-moon.html This moon 6 to 15 miles across and was discovered after a decently intense picture set was taken by the hubble telescope attempting to find out what the new horizons spacecraft would have to deal with. There are two component that were needed to find this planet...first we have to take the picture and second we need to have someone look at that picture and recognize what they are looking at.

In 2014, we discovered an asteroid between Saturn and Uranus's orbit had rings. This was entirely due to an event known as occultation, which is when an object passes by a distant star and we can see the object as a shadow in the distant star's light. https://www.space.com/25225-asteroid-rings-discovery-video-images.html It took 7 telescopes all pointed at the same object at the same time to successfully detect these rings.

As you can see, we need to focus many resources in a known area to detect something in the outer solar system. If we were going to locate it via telescopes, we would need 3 things to happen at the same time : The ship would have to traverse across the light making it to Earth from a distant star, we would need multiple telescopes pointed at that star, and we would need people actively looking at those results. Anything could happen, but this pushes past dumb luck into accidentally finding a needle in a New York sized haystack.

**Just to throw it out there, once again in the dumb luck category, but we do have a few recon spacecraft in the outer system. Not likely, but one of these crafts spotting and sending back images of an alien craft would probably be much more likely than we would be spotting it from Earth.

I'd be hard pressed to state that we'd find a 50km wide craft in the outer solar system without some extremely lucky circumstances occurring. When we get to the inner solar system, we are a bit better at detecting these bodies, but only a bit. Lets say we have a pretty good history of missing things.

http://earthsky.org/space/asteroid-2017-oo1-close-pass-undetected An object between 25 and 78 meters passed between the Earth and the moon, and we detected it 2 days after it's closest point to Earth. I don't know how large your ship is, but under 100 meters could likely land on Earth before we detected it.

More recently, an asteroid passed us that appears to be an interstellar asteroid. https://news.nationalgeographic.com/2017/10/interstellar-solar-system-asteroid-comet-space-science/ This several hundred foot object was sighted 5 days after the objects closest point with Earth and was already on it's way back out of the solar system. If your alien craft is moving at speeds required to exit the solar system, I doubt we'd detect it until after it was here.

Infact, our near earth asteroid detection has been painfully slow. http://www.cbc.ca/news/technology/nasa-near-earth-objects-asteroid-detection-program-slammed-by-audit-1.2767688

"NASA estimates that it has identified only about 10 per cent of all asteroids 140 metres and larger," Martin wrote. "Given its current pace and resources, (NASA) has stated that it will not meet the goal of identifying 90 per cent of such objects by 2020."

We have discovered upwards of 90% of all objects over 1km diameter, but this has taken several year to go over. Unless this spacecraft is multiple KM in diameter, we wouldn't stand much of a chance in discovering it what-so-ever.

If we go a bit bigger, lets take Shoemaker-Levy 9 as an example. From estimates, this 1 to 10km large object was captured by Jupiter somewhere in the 60's or 70's before it's final impact into Jupiter. We discovered it completely accidentally while looking for Near Earth objects in 1993 long after it fragmented.

We missed a 1km one that impacted Jupiter in 2009 and have to guess at it now. https://en.wikipedia.org/wiki/2009_Jupiter_impact_event This wiki page includes a note on visibility:

Assuming it was an inactive comet (or asteroid) about 1 km in diameter, this object would have been no brighter than about apparent magnitude 25.[17] (Jupiter shines about 130 billion times brighter than a 25th magnitude object.)[18] Most asteroid surveys which use a wide field of view do not see fainter than about magnitude 22 (which is 16x brighter than magnitude 25).[17] Even detecting satellites less than 10 km in diameter orbiting Jupiter is difficult and requires some of the best telescopes in the world.[19] It is only since 1999 with the discovery of Callirrhoe that astronomers have been able to discover many of Jupiter's smallest moons.[20]

Unless extremely lucky, we probably wouldn't notice a 10km wide ship until it took up orbit around the moon.

Additional note on visibility: http://www.icq.eps.harvard.edu/MagScale.html

It might actually be easier to determine what visibility rating your spacecraft would have and note from there what it would look like to us on Earth. We detected a magnitude 15.3 comet just outside of the orbit of Mars recently. https://www.universetoday.com/136564/new-comet-c2017-o1-asas-sn-takes-earth-surprise/ I'd be curious how big a space ship would have to be / what distance it would have to be to be 15.3 on this scale...this comet is actively giving off a decent amount of light that makes detecting it possible.


Might as well throw out the absolutely huge option. If this was a death star like alien ship, I'd consider it emerging into the solar system from the OORT cloud as when we would start having a chance of detecting it. Remember, we've only identified dwarf planets about half the size of Pluto out in the oort cloud in 2015 https://www.space.com/31100-most-distant-dwarf-planet-found.html and are just barely starting to see into the inner OORT cloud let alone the outer.

  • $\begingroup$ For it to land on Earth at some point it's moving at a speed that would bring it to attention to the ICBM warning systems, and the radars that track space junk. If they don't have active radar invisibility. If they don't come in directly over the south pole. If they follow a conventional orbit. (Would we spot something that came straight down from lunar distance? $\endgroup$ – Sherwood Botsford Nov 29 '17 at 19:52
  • $\begingroup$ @SherwoodBotsford - As stated in my answer, the radars that detect space junk are better at detecting space junk days after it's nearest point to us, I wouldn't much rely on it beyond luck. Always hard to speculate at what a FTL capable race is capable of doing, they may have methods of deceleration long past what we can currently consider (assuming they are accelerating and not warping space for movement). The craft would already have to be in Earth's orbit for the ICBM warning systems to kick in, no? $\endgroup$ – Twelfth Nov 29 '17 at 20:01
  • $\begingroup$ Modern radars are often set up to 'gate' the data to clean up the display. Space junk isn't junk if it's not moving at something like 2 to 12 km/s; it's not junk if it's moving more than 10 degrees a second etc. $\endgroup$ – Sherwood Botsford Nov 29 '17 at 23:57

Break it down into several scenarios:

Scenario 1. Conventional physics.

This covers anything we can think of that is primarily limited by engineering, or reasonable projections of present knowledge. For this I would include hydrogen fusion, but not the negation of gravity, or FTL travel.

Presumably they travel between stars at some fraction of the speed of light. If it's a small percentage, they are either very long lived, or have some form of hibernation. The latter may mean that they can handle massive G forces at to shed the initial amount of speed.

Suppose they travel at 10% of light speed. 30,000 kps. At 1 g it takes 36 days to decelerate. If they deliberately skewed their approach vector so Earth doesn't have a view down the throat of the exhaust nozzle, or they had a sufficiently efficient drive we might not notice. (They could also make sure that they entered the system when the Earth was on the far side of their sun from their entrance. Could we see a very thin plasma of helium nuclei recombining with electrons? A shock wave as the 25 Mev plasma interacts with the colder slower solar wind?

If they can take 10 g's it's not even 4 days. This catches everyone with their pants around their ankles.

Seeing the ship itself? Not a hope. We find asteroids because we know what to look for, and how much it should move in a day or so. We take fairly long exposures so it has time to move and show a streak, then compare that shot to one taken of that same neighborhood later. A single extra point? Pixel noise. Something moving a couple thousand times faster than the general traffic in the solar system isn't going to show, even if the path is foreshortened by it's approach.

Scenario 2: Reactionless drive.

Periodically there is a bunch of fuss and feathers about some device that apparently doesn't conserve momentum. So far they haven't panned out. But some of them have kept engineers guessing for some time. We can dream, can't we. (I really want an anti-gravity car....)

This in effect makes the side effects of the ships travel unobservable. No high speed exhaust. Because there is no mass to chuck out, you may be able to travel at much higher fractions of the speed of light. You still have to find the energy for it. Antimatter?

Scenario 3: FTL or Warpdrive, wormhole...

So much depends on the manner of FTL travel, and particularly how close to Earth can they drop below light speed. If there is a hyper limit (Niven, Weber) then ships appear in the outer system with or without a display. Travel from the hyper limit is at conventional 1-2 G accelerations. Since we have unknown physics, we have unknown side effects.

In this situation we may well see something, but fail to understand what we see until it is too late. If the hyper limit is at Pluto's distance we have about 2 weeks at 1 g before they orbit. It's it's at lunar distance we have about 3 hours.


A comment points out that we could detect an incoming body with wide angle infra-red telescopes.

A 400 K object (125C -- you have to get rid of heat somewhere, so some of the outside has to be warm enough to radiate it away.) has a black body peak radiation at about 7 microns. ( http://hyperphysics.phy-astr.gsu.edu/hbase/wien.html#c3) This is right in the middle of a bunch of telescopes. The most recent one on Wiki is WISE (https://en.wikipedia.org/wiki/Wide-field_Infrared_Survey_Explorer) which had a field of 45 arc minute, and a resolution of 6 arcsecond. )

The WISE spec claims it could detect a neptune sized object out to 700 AU Neptune is 25,000 km in diameter, or about 625 million times as large (radiating area) as a 1 km diameter space craft. 700 AU = 10^11 km

Pluto is about 40 AU from the sun/ Rounding call it 1/20 as far. So you'd be able to detect something about 1/20 the diameter of Neptune. Call it 1200 km. Nice sized moon.

On the other hand 400K is quite bright compared to other objects in the sky. This is a factor of 300 more radiation than a rock at 100K This would reduce the detection diameter by about a factor of 17-18. We're still at 60-70 km

It would take extensive engineering to detect a warm rock within a day at Plutonian distances. Not impossible, but we can't put it in place next Tuesday.

Of course at shorter distances detecting is much easier. A 1 km object should be detectable at somewhere between 1 and 2 AU. But if it moves at 1 G acceleration, that doesn't give us a whole lot of time.

WISE took an image every 11 seconds. I don't know how much of that time was collecting photons and how much was moving to the next patch of sky. I suspect that to make best use of energy and reaction mass the image was moved optically, which would be very quick. Any object that was moving fast then would have it's radiation smeared across several pixels. This would raise the detection threshold substantially.

  • $\begingroup$ Heat. None of your answers consider heat. Right now, with off-the-shelf technology, and a few billion dollars (basically a reasonably average large military program) humanity could get full spherical coverage of the solar system that would detect anyone coming in, moving around, or leaving as long as their vehicle has to obey the basic concepts of thermodynamics, is sublight, and is active. It might not provide detailed information but it would be sufficient to tell more focused detectors where they should be looking. We don't have it right now because there's no perceived need for it. $\endgroup$ – Keith Morrison Nov 29 '17 at 21:15
  • $\begingroup$ @KeithMorrison You have a point. An object at 300 K emits about 70 W of infrared radiation per square meter. But a 1 km space craft at Plutonian distances subtends a pretty small arc. My recollection from working with Dr. J. at U of Idaho many decades ago is that IR detection is horribly inefficient. Is it posible to detect what amounts to a warm rock that far out? $\endgroup$ – Sherwood Botsford Nov 29 '17 at 23:53
  • $\begingroup$ A proposed detection system would have two components: a wide field infrared telescope, and then a tracking system composed of higher resolution narrow field optical and infrared telescopes. You put at least two (so you get instant triangulation) wide field telescopes in solar orbit. They'd be able to scan the entire sphere around the sun in about a day. If they detect an anomaly, the location is sent to the tracking system (again, ideally two or more sats in solar orbit) that locks on to the anomaly and gather more data. You won't know what it is, but you would have distance and trajectory. $\endgroup$ – Keith Morrison Nov 30 '17 at 16:47

The big issue would be slowing down without detection as any braking would inevitably involve very high energy expenditure whose net force would be directed at the solar system. And this would likely be detectable. One way around this if the propulsion was sufficiently focused, would be to break at an angle to the solar system and then at an equal and opposite angle so the net force was breaking relative to the solar system but the high energy emissions would bypass the solar system on either side.

There are too many ifs to be any more precise as it would depend on the nature of the propulsion they were using among many other things.

Putting aside the issue of slowing from 99% the speed of light, I think an alien race sufficiently advanced to create a drive that could travel at 99% the speed of light should be able to avoid detection if they wanted to.

Again there are a lot of variables but measures such as painting the ship black and minimising communications to tightly focused transmissions away from earth would make any ship very hard to detect. Use of stealth like technology and the use of active cooling of the surface pointing towards the solar system to mimic the microwave background with radiators behind the ship pointing away from the solar system would make it almost impossible to detect.

The only way of detecting it would be by eclipsing of a star, but even that could be minimised by choice of trajectory to avoid eclipsing any nearby stars.


This depends on a few factors:

1) Size of the ship-

obviously the bigger the ship the more noticeable it is

2) EM Emissions-

Currently we are scouring the sky looking for all kinds of EM wavelengths, what your ship is emitting at Earth or blocking from reaching Earth greatly increases its chances of detection. Like if it blasting gamma rays at Earth or flying in the light path of a notable star like Sirus.

3) Proximity to Earth-

The closer it is the more likely it is to be discovered. This is largely because the closer it is the more sensors there are that are capable of detection as well as the more pronounced its EM signature becomes. This distance is by no means long either. I'd say our most effective detection range roughly begins at Jupiter.

4) Blind luck-

Finally, this is the biggest factor. The night sky is massive. Our celestial sphere's radius is the same as the distance from the Earth to the Sun, with half(to ~30%) of it relatively invisible at any given moment in time (occluded by the sun). Trying to find a non obvious thing in that expanse is like trying to find a piece of gold dust on a beach.

For all we know there is an ELE class meteor on the way to Earth right now we just haven't yet detected it. Our detection of such objects isn't infallible.

  • $\begingroup$ If the answer depends on that many factors, perhaps you should ask the OP for clarification in a comment. $\endgroup$ – sphennings Nov 29 '17 at 15:21
  • $\begingroup$ @sphennings not really, because the question was never really a yes or no question. I am providing the factors the OP needs in order to justify their worlds conditions. $\endgroup$ – anon Nov 29 '17 at 15:23
  • $\begingroup$ "Would humanity be able to detect an alien ship coming to earth outside the solar system?" can only really be answered "Yes they could, because....", "No they couldn't, because....", or "It depends on X, because...". If the answer depends on a lot of factors it's probably a good idea to get those factors narrowed down so that there aren't people arguing over those factors in the comments. $\endgroup$ – sphennings Nov 29 '17 at 15:29
  • $\begingroup$ Well the only real answer honestly is BLIND LUCK with other factors influencing the odds so yes this isn't a yes or no question. This would come down to the actions of individuals both the aliens and the humans which is outside the scope of WB. $\endgroup$ – anon Nov 29 '17 at 15:39
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    $\begingroup$ "Censors" are high-ranking Roman magistrates in charge of the census (among other more interesting duties), or people with black markers who censor letters and newspaper articles. Sensing devices use "sensors". $\endgroup$ – AlexP Nov 29 '17 at 19:39

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