An alien spaceship the size of an Empire State Building experiences a technical issues with its cloaking device, their duty technician needs 24 hours to repair and meanwhile the alien wants to avoid being detected by the human.

Is there any place within the solar system for the alien to hide for the next 24 hours?

With the cloaking device down they cannot mask the radiowave and gamma radiation produced by their ship effectively. If they should leave the solar system their engine has to be fully engaged to full power and thus produces more gamma radiation.

To repair the cloaking device they have to expose the ships interior as well as the engine core which will emit gamma radiation to the surrounding.
The crew will avoid using radiowave for communication.

Assume the ship is already in the blind spot when their cloaking device fails.

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    $\begingroup$ There are a lot of spots. Behind the sun (as seen from earth) seems most efficient, because i doubt that the ship could radiate anything strong enough to outshine the sun's radiation, but any object in space should be suitable to hide behind. $\endgroup$ – Burki Aug 24 '15 at 11:14
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    $\begingroup$ There is no "behind" since there are eyes on the far side too. $\endgroup$ – JDługosz Aug 24 '15 at 11:16
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    $\begingroup$ Thinking about it... maybe in front of the sun would be a better idea. such a tiny speck should be unnoticeable. $\endgroup$ – Burki Aug 24 '15 at 11:21
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    $\begingroup$ Along @Anaphory 's question, in space, acceleration (or "delta-V") is the thing you have to use your engines for. You can orbit without using power. To give you a sense of that, the Earth, without any powered engine to speak of, hurtles around the sun at 67,000mph $\endgroup$ – Cort Ammon - Reinstate Monica Aug 24 '15 at 16:15
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    $\begingroup$ @JDługosz Watch out, that's a mobile wikipedia link. $\endgroup$ – PyRulez Aug 25 '15 at 2:14

11 Answers 11


Get the ship as close to the sun as the ship can tolerate then maintain position between the sun and the earth. The sun is a strong gamma ray emitter. As long as the ship stays between the earth and the sun, any gamma rays emitted will just look like noise against the solar gamma radiation background. Any IR, visible, UV, or radio emissions will also get washed out by the sun's glare. Even if someone on Earth knew where to look, they would have significant challenges minimizing the glare of the sun. A 24 hour repair window shouldn't be difficult to achieve.

X-Class solar flares have power outputs on the order of billions of hydrogen bombs so unless your ship is really really leaky, it should escape detection.

Gamma ray imaging of the sun

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    $\begingroup$ "Hiding doesn't always involve staying in the shadows. If your enemies' eyes are adjusted to the darkness, then the best hiding place is behind the brightest light you can find." -- Michael Westen, Burn Notice $\endgroup$ – Mason Wheeler Aug 24 '15 at 14:55
  • $\begingroup$ I'd think the gamma rays would be a give away. A concentrated burst, like OP is describing, might stand out, as, we keep a keen eye on solar activity. Solar flares are a big enough for us to necessitate diligent observation and forecast. $\endgroup$ – punkerplunk Aug 24 '15 at 20:09
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    $\begingroup$ Nope. An Empire State Building sized object at 100Mm (.67AU), assuming a 50Mm stand off from the sun appears to be 2.5241e-7 or 0.000908676 arcseconds. Hubble has a resolution of 0.1 arc seconds. Even if the ships reactors went critical and exploded, the energy output from that event would be at least four orders of magnitude smaller than the smallest solar flares. A little bit of ship-based gamma ray leakage doesn't come close to how energetic the sun is. $\endgroup$ – Green Aug 24 '15 at 20:29
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    $\begingroup$ I find it amusing that the accepted answer to this question is sit in front of the most second most observed object in the solar system. First being Earth. $\endgroup$ – Wil Selwood Aug 25 '15 at 13:06
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    $\begingroup$ @Hassassin, true, the other side of the sun, Jupiter or Saturn would work just fine. Hiding in front of a giant gamma ray source seemed more interesting than hiding behind it. An answer that reads "hide behind the sun" is really boring. $\endgroup$ – Green Aug 25 '15 at 21:03

The Third Lagrangian Point

enter image description here

The Lagrangian points are those where the gravity influence of Earth and Sun and the centrifugal force balance out in a way that an orbit is possible where the object is not moving relatively to earth. The L3 point is on the opposite side of the Earth's orbit, so it will always be behind the sun when viewed from Earth.

The sun is so large that any direct line of sight from Earth or earth-orbiting satellites is blocked. Any kind of emission the ship could create will be reliably blocked out by the sun. And even those kinds of emissions which might pass through the sun (the only I could imagine would be neutrinos) will likely be mistaken for unusual sun activity. The only way to detect it might be when the emission is reflected by any other celestial objects like asteroids or planets, but the ship would have to create quite a lot of noise for that to be detectable. Unless they create some immense energy output spike, like by blowing up their main reactor, there will be no detectable echo from other celestial bodies.

The only earth-based crafts which might get a direct line of sight are interplanetary probes, but these are designed for short-range observation of the planets, moons or asteroids they are supposed to research. They usually have no reason to even look into the direction of the Earth-Sun-L3 point.

The Lagrangian points are unstable, so a ship will have to perform slight course corrections to stay exactly on the L3 point for extended periods of time, but when they just need to hide for 24 hours that will not be necessary.

  • $\begingroup$ I was thinking the second when behind the moon, but the 3rd would be a better choice. Longer, more complete coverage. Upvoting this one because exposing the engine core sounds like position efficiency is as important as position stealth. Also, Gundam uses the Lagrange points. $\endgroup$ – DeveloperWeeks Aug 25 '15 at 15:03
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    $\begingroup$ The only spacecraft that might ever take a look at L3 directly is STEREO B... but it doesn't save images during its stay behind the sun (its primary mission is real time observation), and it has such a narrow field of view that being just a few meters off of STEREO B's orbital plane means that an object at the Earth/Sun L3 will never be seen. $\endgroup$ – Ghedipunk Aug 25 '15 at 18:38
  • $\begingroup$ +1 Definitely this. Why hide in front of the sun when you can hide behind it? $\endgroup$ – piers7 Aug 26 '15 at 1:59

At the risk of offending the SE Gods - but because you asked for Hard Science:

There's no cloak in space

Many also maintain that there's no *stealth* in space

but I disagree with the absoluteness that this is often associated with the statement.

For instance, even with thousands (millions?) of telescopes and other astronomical observing equipment, astronomers are regularly surprised (only a day or two warning) by sizable rocks winging past the Earth.

The point is that if we know where & when to look, there's almost no where in a billion miles that you could hide. If we don't know where & when to look a bit of sneaking is possible.

Bear in mind that any high-powered interstellar or even inter-planetary drives will give away your location in a heart beat but a common Hohmann orbit transfer with the boost phase hidden from our direct observation would be quite difficult to detect.

There is no reason for your engines to emit different radiation at different *velocities*

Even engines like VASIMR (which can "change gears") do NOT change their radiation profile based upon spacecraft velocity. It only changes its emissions based upon rocket performance (thrust and $I_{sp}$). You can engage those changes at any spacecraft velocity that you want.

Finally, the answer

I didn't see any errors in the messages above, they all have valid points:

  1. Asteroids that surprise us usually come at us after traveling through the inner Solar System - they are harder to detect when coming at us from this direction.
  2. Most of human detection equipment is based on or near Earth. Chances are if you can obscure you ship from the direct line of sight to Earth, you'll be hidden.
  3. We do have probes all over the Solar System looking at all sorts of different things. Usually a given probe is looking at its specific object of interest (e.g. New Horizons is looking at Pluto). There's always a chance that a probe (e.g. Cassini) might be looking towards Earth or some other object at any given time - your crew will just have to take a gamble (a chance for some dramatic tension?).
  4. We also have some probes doing "Sky Surveys" - looking at an entire hemisphere of the sky at a time and looking for things out of the ordinary. You definitely need to hide your ship from these. AFAIK, these are all based around the Earth.
  5. For additional dramatic tension, we also have ground based neutrino detectors. If your ship uses extremely powerful nuclear (fission or fusion) power, those will see a blip in neutrino detections. There is simply no way to hide such emissions unless you happen to be on the opposite side of the Sun. The Sun won't block the neutrino signal (a neutrino has greater than a 50% chance of passing through 6,000,000,000,000 miles of lead), however, that signal might get lost in the neutrino "noise" coming from the Sun.
  • $\begingroup$ The neutrine argument is interesting. Note that the sun produces neutrinos, but a fission reactor produces antineutrinos. So in principle, a strong signal from a reactor could be distinguished from the solar signal. Of course if the ship uses fusion, it should produce neutrinos, just like the sun. $\endgroup$ – celtschk Aug 24 '15 at 20:56
  • $\begingroup$ The reason that rocks in space have stealth is because they produce no heat, and do not increase entropy. A ship necessarily would (indeed, life forms produce entropy all on their own.) $\endgroup$ – PyRulez Aug 25 '15 at 2:36
  • $\begingroup$ Rocks DO radiate heat and reflect light (it's the fact that they can't reflect sunlight when they come from the inner solar system that makes them hard to find). You can figure their luminosity and compare to ship luminosity and it shows that reasonable size ships NOT operating in high power mode could "hide" (aka not broadcast their presence) pretty easily. $\endgroup$ – Jim2B Aug 25 '15 at 3:47
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    $\begingroup$ But the more power they use the harder it is to remain unseen. It probably follows the inverse square law. Also if you radiate at an unusual frequency, (e.g. not thermal), you're more likely to be observed ("hey, look at that glowing blue rock!") $\endgroup$ – Jim2B Aug 25 '15 at 3:48
  • $\begingroup$ Also whether a fission reactor produces neutrinos or antineutrinos (probably) depends upon the type of "fission" (neutron, alpha, beta, positron, or true fission). IIRC, neutron -> no neutrinos, alpha -> no neutrinos, beta -> 1 anti neutrino, positron -> 1 neutrino, and fission I would think is none but could be something depending upon the reaction. $\endgroup$ – Jim2B Aug 25 '15 at 3:53

In Earth's oceans.

There is no need to hide at the end of the solar system when we can barely find anything on our own planet.

Assuming that the aliens haven't already been detected before reaching their hiding place (perhaps their cloak is just about to fail — this is a flaw in your scenario, and we must assume this otherwise everything's moot anyway), just have them dunk into the sea and drop to the bottom.

Even if they leaked a bit of radiation there, we wouldn't see it.

Then, when repaired, launch again.


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    $\begingroup$ That's how the bad guy ambushed people in H. Beam Piper's "Space Viking" when ship detectors were far better than todays. $\endgroup$ – Oldcat Aug 24 '15 at 22:08
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    $\begingroup$ A good plan assuming the aliens' ship is capable of withstanding 16,000psi external pressure (or the aliens' bodies themselves). If they're biological aliens from a planet with an atmosphere similar to Earth, space travel requires only dealing with a pressure difference of around -15psi. Not too difficult in comparison. $\endgroup$ – bcrist Aug 26 '15 at 2:22
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    $\begingroup$ But assuming 1kbar is no problem, even if humans noticed something strange, we'd never be able to do anything to investigate it in 24 hours to know it was aliens, and it'd probably be chalked up to some kind of elaborate hoax or accident on a nuclear sub. $\endgroup$ – bcrist Aug 26 '15 at 2:30
  • $\begingroup$ @bcrist: True, but without spaceship specs everything on this page is an assumption. $\endgroup$ – Lightness Races with Monica Aug 26 '15 at 9:38
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    $\begingroup$ I love this answer. But (as mentioned above) if spacecraft are designed for negative (outward force) pressure it might have difficulties resisting positive (inward force) pressure of submergence. It also needs to get to the Earth and land without being detected. This might be difficult with all the military radar around the globe. $\endgroup$ – Jim2B Aug 28 '15 at 0:15

The Oort Cloud

The far edge of the Oort Cloud is believed to be 100,000 AU from the Sun. In fact, Duncan et al. (1987) created simulations that showed the inner edge of the Cloud to be at ~3,000 AU.

Given that 1 AU $\approx$ 8 light-minutes, $$3,000\text{ AU}\times\frac{8\text{ light-minutes}}{\text{AU}}=24,000\text{ light-minutes}\gg1\text{ light-day}$$ Therefore, any signals - even from the inner edge of the Cloud - will not reach Earth for far more than one day, at which point the ship will be long gone.

Furthermore, any signals - even gamma-rays - should be reduced in strength, compared to signals from areas near any of the planets, the Sun, or the asteroid belt.


Believe it or not, the best place is just anywhere there isn't anything to point telescopes at. Most telescopes looking for or at things within the solar system are pointed in the plane of the ecliptic. Something above or below the plane of the ecliptic is quite likely to go unnoticed.

If they want further security, they could try placing themselves between Earth and a "hole" in space, an angle where there is nothing much interesting to look at within the visible universe.

They should probably take care to not thrust directly at Earth, that would be stupid, it might get picked up by a gamma ray burst detector or something, but as long as they don't aim their engine at Earth, they should be fine.

  • $\begingroup$ We have found quite a lot of stuff out to about 40 degrees from the ecliptic and a reasonable amount out to 60 degrees. So it would need to be in a very off axis orbit. I have read some where that these kinds of orbits would be ideal for mapping the solar system though so it might work quite well. wselwood.github.io/astro-grid/#Semi-Major-Axis/… $\endgroup$ – Wil Selwood Aug 24 '15 at 13:01
  • $\begingroup$ New survey systems like LSST check everywhere, at the rate of once a week or so. In the near future, the premise "lookimg at something interesting, only" is false. $\endgroup$ – JDługosz Aug 24 '15 at 14:57
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    $\begingroup$ @JDługosz yeah I'm aware there are systems designed to look everywhere. Nevertheless it is inevitable that the more area of space a telescope covers, the poorer resolution it has. The magnitude limit of LSST would be Saturn's moon Fenrir - an object which is smaller, darker or further away would go undetected. Even if detected, the ship would just be a tiny dull pixel which appears in one scan. It probably wont even be analyzed within 24 hours and after that it's too late to point a dedicated telescope or other instruments at it. The spectrum LSST works in wouldn't reveal anything interesting. $\endgroup$ – Blake Walsh Aug 24 '15 at 15:24
  • $\begingroup$ True, if it's small and passivly dark, it will "make like a rock". What does the broken active shielding do when working? $\endgroup$ – JDługosz Aug 24 '15 at 15:30
  • $\begingroup$ @JDługosz according to the OP it masks radiowaves and gamma radiation. $\endgroup$ – Blake Walsh Aug 24 '15 at 15:31

Behind Jupiter

Jupiter's big (well, unless compared to the sun). It has a lot of magnetics and radiation going on that will help mask the ship, especially if you get very close.

More importantly, because it's so far out we don't really have a lot of eyes that can see behind it compared to going behind the sun.

It's possible that one of the gas giants further out would be superior, but in general they're less active (so less masking radiation). And I would definitely avoid Saturn, if the mass of your ship or your engine perturbs the rings that would make you easily detectable.


Just put them selves in a sun centric orbit and pretend to be an asteroid for the day.

Even if they are spotted in the 24 hours it takes them to repair the ship, humans would not have much time for follow up observations to find out what the heck that faintly warm thing floating through the solar system is. Most of the sky survey telescopes operate in visible and high IR frequencies, it would take some time before one of the x-ray telescopes got pointed at the object, if ever, why point an x-ray scope at some thing unlikely to be emitting that wave length.

To add some thing to the human list of known asteroids requires multiple observations. If the object vanishes before more observations can be made people would write it off a fluke.

Of course this all assumes that the ship is not parked in earth orbit when the cloak fails.

  1. No. We even have probles on the far side of the sun.
  2. Maybe they won't be noticed. Are any all-sky surveys with high cadence operating on those frequencies, and do the aliens know the schedule?
  3. So what? The craft is where it is when the cloaking device fails. Moving now will not get very far in 24hrs and will be conspicuous by moving and increases the probability of being in the view of some instrument.
  • $\begingroup$ is there anything that would be able to notice the ship with a really,really huge fusion reactor outshining anything in the vicinity? $\endgroup$ – Burki Aug 24 '15 at 11:20
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    $\begingroup$ assuming you mean an exhaust plume that shines in all wavelengths like a hot gas, any instrument looking in that direction would see it! So how would the cloak work if the ship works that way? "hey guys, I see what seems to be rocket exhaust, but the base appears to be empty where the rocket would be expected. is that suspicious?" $\endgroup$ – JDługosz Aug 24 '15 at 12:01
  • $\begingroup$ Supposed the engine was directly between the sun and the earth: is there really any system that would be able to notice the radiation against the background of the sun? $\endgroup$ – Burki Aug 24 '15 at 12:55
  • $\begingroup$ There are probes looking at the surface of the sun continuously. That is one place they would certainly be looked at! A relatively cold cloud would show against it, too, so it doesn't need to be hotter than the sun's surface. Also, helioseismology looks at the spectrum in great detail, as do other instruments. So don't underestimate what it would take to be seen, and don't limit yourself to particular ways (e.g. by being hotter). Different is different. $\endgroup$ – JDługosz Aug 24 '15 at 13:29
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    $\begingroup$ I don't know the resolution of the instruments available. The ship will appear larger if closer. Even if not resolved, the spectra will contaminate the reading. The fusion plume will be millions of miles long, much larger than the tiny ship. $\endgroup$ – JDługosz Aug 24 '15 at 14:55

The flaw with virtually all of the schemes is that the ship is emitting gamma radiation. Unless the poster wants to indulge in some handwavium, the most common source of energetic gamma radiation for a ship sized object would be the use of antimatter. Positrons and electrons are handy antiparticles for generating energy since they mutually annihilate by releasing gamma radiation at 512 KeV. This is a pretty distinct peak, and would stand out on any instrument designed to observe gamma radiation from space.

It would be so distinct, in fact, that it would almost instantly be seen as a confirmation that some artificial source of gamma radiation was emitting the energy. Antimatter reactions using anti atoms like anti hydrogen reacting with hydrogen would make an even more interesting visible source of gamma radiation, mixed with the multitude of particles caused by the annihilation of the nucleons. So the ship would have to be somewhere that presents a pretty solid mass between the ship's reactor and the Earth (we are assuming they know about us, hence the need for secrecy), and taking care that any orbital period won't bring the ship into view of the Earth.

Given all these conditions, I would suggest the captain lands the ship on one of the ice moons of Uranus or Neptune and is physically shielded from observation, for example landing in a deep canyon or crater, or even using the ship's drive to melt into the crust if needed. After repairs are done, then wait until the Ice Giant planet is between you and the Earth before activating the drive and taking off again.

  • $\begingroup$ Nuclear reactions (both fission and fusion) also emit $\gamma$ radiation. However, the $\gamma$ radiation coming from matter-antimatter reactions is very distinctive. $\endgroup$ – Jim2B Aug 25 '15 at 16:06

The fact that you have not seen us, is enough to claim: nobody is attempting to detect us and others are not looking at all - we can stay at plain sight (claim with standard 99.999999% certainty). This is due to human selective attention, it is easy to give them something else to observe. If you want to know what that something should be in any given time, just ask this from Swarm intelligence.

Off earth

Spaceships engine room and outer shell is covered with several insulating layers of übervantablack, each absorbing standard 99.999999% electromagnetic radiation. Spaceship lacks the need for power cables, as übervantablack is superconductor and lacks standard resistance. This also acts as electromagnetic panel that is immensely more efficient then solar panels.

enter image description here

On earth

Humans think they mainly detect air vibration whose frequency between 20 to 20,000 Hz and visible light spectrum within: enter image description here but they can not differentiate subtle differences.

However other primitive creatures on the planet have superior scenes, but as they can not communicate telepathically and share similar selective attention, they can be ignored.


Humans are so ignorant that even if I tell you this, you doubt it. We actually keep several spaceship in most crowded of spaces where and they would still not notice it - even if they see it on a recorded medium like picture or video.

We can communicate telepathically and so can our computers - we can monitor what you think and change it at will, even if you detect us. For us it is as easy as saying : "you do not see me" to your subconscious and humans are too primitive to overwrite it and take it as their belief. Note however we do not do this manually, it is done by computers. It is sub system of Swarm intelligence and fully automatic in nature, but can be overridden by guardians.

enter image description here

They do not look nothing like silence (from Dr Who), but for some reason it is easiest reference humans usually understand.

  • $\begingroup$ Sorry for my poor anglish, it is not my main language, if you get what I mean. We can get English awareness from Swarm intelligence, i have not done this. $\endgroup$ – Margus Aug 25 '15 at 14:09

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