In science fiction a common plot device is for a planet to be surrounded by electromagnetic interference that prevents an orbiting ship from scanning it with sensors or communicating with people on the surface.

My question is: How realistic is such a plot device, assuming either current-day or near-future sensory technology? If a planet had, for example, a large-scale electrical storm or a lot of geothermal activity, would it obscure the surface of the planet from the sensors of an orbiting ship or a space telescope, aside from broad features such as continents and oceans? If not, are there any natural or artificial phenomena that would cause such interference?

  • $\begingroup$ What sort of sensors are we talking about? Are they active (shoot a signal out and measure the response/echo) or are they passive (just observe) or a combination of both? $\endgroup$ – user6511 Feb 25 '15 at 1:03
  • $\begingroup$ Hi user6511, both types. $\endgroup$ – FSharpN00b Feb 25 '15 at 1:21
  • $\begingroup$ @FSharpN00b - what technology do these sensors use? If they use visible light, then an adequate shield would be a smoke screen. If they are neutrino or quark resonance scanners, then I think only the writers that came up with such mechanisms could tell you if they can be shielded. $\endgroup$ – Johnny Feb 25 '15 at 1:37
  • $\begingroup$ Hi Johnny, I'm thinking any technology used by satellites or space telescopes today, or that might be used in the near future (i.e. where we know how it would work but it isn't practical yet). Essentially, I'm looking for a situation where it's necessary to send probes to the planet's surface (or at least through the upper atmosphere) because it's impossible to obtain detailed information from orbit or farther away. $\endgroup$ – FSharpN00b Feb 25 '15 at 2:05
  • $\begingroup$ @FSharpN00b Care to accept one of the answers? $\endgroup$ – 2012rcampion Apr 23 '15 at 20:29

Note that the 'sensors' used in science fiction TV shows are confused too easily. For example, often times a character will say that the sensors are picking up nothing at all... even when the thing is clearly visible to optical sensors (i.e. the ship's cameras and the characters' eyes). Star Trek is a particularly notable offender here.

In real life, if you can see down to the planet, and your sensors include a telescope, you can tell what's going on down there.

Basically, in order to block sensors, you need to block the mechanism they use to observe. In 99% of cases your sensors will be detectors (or emitters) of electromagnetic radiation. Remember we can use a wide variety of wavelengths to probe the surface. For example, the moon Titan is totally obscured by its atmosphere in visible wavelengths, but infrared and ultraviolet wavelengths see through somewhat, and radar penetrates handily giving us a good view of the surface.

Your planet would need to have a thick atmosphere to block all UV, visible, and IR wavelengths. Something like Venus's sulfuric acid coulds would do nicely. However, Venus can also be mapped by radar. Maybe some sort of conductive substance in the atmosphere, like metal dust, could function as chaff to block microwave, low, and medium-frequency communications, including radar.

I know this doesn't sound like a nice place to live to us, but the (non-human) aliens who live there might find it quite pleasant!

  • $\begingroup$ Question: If metallic fragments (think chaff) are added to an atmosphere like what Venus has, would that be sufficient to make radar useless in scanning? $\endgroup$ – Brad Feb 26 '15 at 20:30
  • $\begingroup$ Typical chaff for ECM purposes consists of metallic coated fibers a couple of inches long and about a mil thick (for your naturally-occurring[?] equivalent, you might explain it as some sort of asbestos-like mineral fibers), and it's very effective, although doppler radar can be used to detect fast-moving objects inside the chaff cloud. If (like Venus) the winds are slow near the surface, the chaff will do a good job of hiding the surface from radar. $\endgroup$ – 2012rcampion Feb 26 '15 at 21:02
  • $\begingroup$ Thank you very much for your answer 2012rcampion and my apologies for my slow reply. It sounds like anyone setting foot on such a planet would need a rebreather at the minimum, between the airborne metallic/asbestos fibers and thick atmosphere (I assume the more EM-blocking gasses there are, the higher the odds one of them is toxic). Thank you for those links as well, I will study them. $\endgroup$ – FSharpN00b Mar 4 '15 at 3:05
  • $\begingroup$ @FSharpN00b You'd probably need a pressure suit as well: the surface pressure on Venus is around a hundred atmospheres. The atmosphere is certainly toxic (over 95% CO2), but more importantly it is highly corrosive. (Read about the Soviet Venera probes to see the lengths they had to go to to even reach the surface.) However, on your planet the corrosive EM-blocking compounds may be present only in the clouds, and low gravity could provide a thick atmosphere with a low surface pressure. $\endgroup$ – 2012rcampion Mar 4 '15 at 3:21
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    $\begingroup$ Don't forget that SciFi confuse active (e.g. radar) and passive (telescope) sensing devices too. Active ones emit radiation and detect the reflected signal. This provides much more detailed information but is shorter ranged (the inverse square law kills you) and reveals your location to someone looking for you. Passive sensing relies upon external radiation (e.g. sunlight) to bounce off the object. You detect this in your sensor (e.g. telescope). Information (distance, size, etc.) tends to be less precise but you have a longer range and it can be done without revealing your location. $\endgroup$ – Jim2B May 14 '15 at 2:31

Building on the answer from 2012rcampion, (which was good and to the point, sensors depend on either actively or passively picking up electromagnetic, or other, energy emissions).

So given that we have agreed how sensors work and 2012rcampion has suggested some methods of blocking electromagnetic emissions, my thought in answer to your question is:
by artificial & misleading emissions of whatever energy the sensors are sampling for, (passive) or looking for reflections of (active).

The above effort might be easiest for passive sensors. As you merely have to have some form of source of emissions in whatever wavelengths of electromagnetic energy the sensors are sampling. NOTE: variations in gravity can be measured as well... this would be very difficult to fake without an unknown science & theories being applied.
There might also be forms of exotic energy that could somehow be used to gain information about the world.

Active sensors might be more difficult to counter. This is what our modern stealth technologies attempt to do for only radio waves and, it has taken us awhile and depends on the radio emitters & receivers being at a certain level of resolution. So protecting against active emissions requires (to the best of my knowledge) either:
-swamping the sample area with that type of transmission so it's difficult for the sensor to tell the difference between what was transmitted by the sensor, (though timing may become important in this regard...).
-creating some way to absorb what has been sent out by the sensor...

Whole Planet & other logistical concerns Last, we have the problem of "shielding" an entire planet.
Most planets are very large even compared to a very large space faring vessel. I would suggest NOT trying to shield an entire planet from every sensor imaginable. Instead you might do the following:

  1. Add something to the atmosphere on a regular basis that helps shield for certain types of emissions (either absorbing, redirecting, or emitting the desired type of emissions)
  2. Develop localized defensive emplacements which would, while a ship is in its area, emit misleading emissions (though this may not work well without some decentralization of the misleading emissions themselves... or it might just show the ship a broad band of emissions all coming from one source & obscuring a certain area... which they could destroy or investigate etc...).
  3. Add temporary & localized chaff to the area to throw off any ability to detect what is on that portion of the planet.
  4. Finally satellites would be the best source of all of the above but they are inherently vulnerable... so while they may work well for blocking most types of scanning as described above, they are also very easily identified and destroyed.

There are still some pretty broad problems with the above 'solution' in that covering an entire planet is still very cost prohibitive & difficult.
Additionally the problem of being able to block ALL sensors is really fairly unrealistic... you could provide active deflectors & emitters as I described but they would be most cost effective & most effective if you know what the sensors are sampling for.

Last, natural conditions:
The only thing not already mentioned as far as natural methods of making sensors less effective (not blocking, less effective) is high energy radiation belts... but they could also add to the hostility of the planet.

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    $\begingroup$ Thank you very much for your reply MER and my apologies for my belated reply. I agree with you that trying to shroud the entire planet isn't the best idea, and that it would make more sense to have some artificial (though perhaps not obviously artificial) shrouding on only a certain part of the planet, and probably devised to capitalize on existing natural conditions. Creating misleading emissions is a promising idea as well but sounds like it would be much harder to do than merely jamming or absorbing sensor emissions. Thank you for the links as well, I will follow up with those. $\endgroup$ – FSharpN00b Mar 4 '15 at 3:23
  • $\begingroup$ @FSharpN00b thanks, glad I could help some & good luck! $\endgroup$ – MER Mar 6 '15 at 7:38

Current-day or near-future sensory technology is pretty much cameras and radar. These are basically the same thing, except applied to different parts of the electromagnetic spectrum.

This is good, because I can be lazy and only need to talk about one bit of science.

How realistic are these plot devices?

Fairly realistic - but for really boring and mundane reasons. Without any special forms of interference, we’d have a lot of trouble looking at the surface of planets in all but the most ideal situations. I imagine having little interference would raise more interest than having lots of it.

Absorption spectra.

Gases, vapours, clouds, etc, all absorb parts of the electromagnetic spectrum and re-emit it as heat. Which parts of the spectrum are absorbed are determined by the chemical composition of the material (and in some cases, the size of the vapours too).

We choose to communicate and scan in parts of the spectrum where we're unlikely to find a lot of interference. Unless you're going somewhere specific, and are fore-warned of what atmosphere you'll be looking at, it's best to take the scout's motto to heart.

Be prepared for anything.

If I was a spaceship designer, I'd make sure I could communicate over as many parts of the spectrum as possible.

Scanning the composition of the atmosphere.

A common question asked by any spaceship crew would be can we breathe the atmosphere? A simple (but incomplete) test would be to measure the absorption spectra, and look for tell-tale indicators that you already have in a database.

Scanning for solid objects.

The best analogy is sonar (yes, submarines). You have passive, where you listen for noises/reflections/signals/etc. And you have active, were you make noise and listen for reflections of your noise.

On top of that, the frequency of the radiation you're using will affect the amount of detail you get. Loosely, the higher the frequency, the more detail. Lower frequency radio waves have longer wavelengths. Long wavelengths have this tendency to pass around smaller objects without interacting with them. This is true for all waves, including electromagnetic ones.

Picture a tall wooden post poking up out of the water. Imagine dropping a coin into the water next to it. The coin creates little ripples that bounce off the side of the post and reflect back out in the other direction. Now imagine a slow ocean wave rolling past – it doesn’t appear to be affected by the post in the slightest.

A similar thing happens with radio waves. If you want high-quality information about the surface of the planet, you’ll need a higher-frequency radar. But you might get unlucky and have atmosphere that absorbs exactly those radio waves.


The logic behind communication is very similar to scanning. You want to be able to receive information without interference and want to be able to transmit information without it too. You’d naturally choose similar parts of the spectrum to those of scanning.

This presents a bit of a conundrum with scanning. Communication might look like interference to scanners, and active scanners might produce interference to people communicating. Are your scanners capable of filtering-out communications?

  • $\begingroup$ Thank you very much for your answer user6511, especially the remarks on frequencies. In fact it would be helpful if lower frequencies could get through (so the crew could at least see there are places to land for instance), while higher frequencies are blocked, preventing the gathering of more detailed information - though I'd be curious as to how small objects need to be before this distinction comes into play, and whether it's affected by distance - i.e. do you need higher frequencies to maintain a level of detail the farther away you are? $\endgroup$ – FSharpN00b Mar 4 '15 at 4:06
  • $\begingroup$ In addition, once people are on this planet, it would be helpful (for plotting purposes) if communications are difficult for the same reason scanning is. Thank you for the links as well, I'll add those to my research list. $\endgroup$ – FSharpN00b Mar 4 '15 at 4:06
  • $\begingroup$ Detail-at-a-distance is governed by other factors. One thing that I forgot to consider originally was that emissions may come back 'blurry' instead of not at all. At any frequency, that will give you enough detail to say 'Hey, that's an open field we could land a spaceship on.' However, there may not be enough detail to see the man-eating plants that live there. $\endgroup$ – user6511 Mar 4 '15 at 20:12

Well, we didn't know what was on Titan until getting there directly, but radar is another story, and we have partial maps now. An orbiter would image the whole thing in short order and see visible light features only in very narrow wavelengths that happen to get through.

We don't have that because of poor budgets given to NASA. It can only be scanned from up close, and we don't have anything staying close or with more specialized instruments because Congress is not allowing money to be spent on it.

So, the reason is logistics and expense, not technology.

Look at JIMO, which ultimately did nothing at all due to the same reason (and real expense: 16 billion and technology to develop), would only work for a few months. The radiation environment is harsh.

Oh, and whoever is lurking in Europa's oceans are safe from prying Terran eyes because the physical shield of ice is an effective shield from remote sensing and physically getting a probe through it is something we don't have yet.

Even the Earth's core and the sun have been probed. Nothing is unprobable it seems, with cleverness and effort. The former is not suitable to remote sensing or rapid results.

So, the stuff to remain hidden is under a lot of mass. Buried, underwater, under ice, all prevent radar from working. Sonar requires visiting and touching the medium.

Maybe the interesting part is specific to senses you can't manage. You can detect buildings including a big dome from orbit using radar, but can't see what's painted on it through the clouds, and can't read any scent markers without getting close up and even then can't "read" them until new instruments are created and the underlying meanings learned.

  • $\begingroup$ Thank you very much for your answer jdlugosz and my apologies for my slow reply. Those are good points - presumably spaceship mass will be at a premium even in the future so I could impose some limitations on sensor packages for reasons of mass or expense. Likewise a planet could have an extensive network of caves, glaciers, etc that could hide things, especially if the rock (or ice) contains elements that distort or absorb sensor emissions as described in other answers. $\endgroup$ – FSharpN00b Mar 4 '15 at 3:33
  • $\begingroup$ You can have a character complain that if only we took the xyz scanmaster, but the budget was cut or political or management reasons. Or the sensors are not very good because of lobbyists or state-targeted contracting rather than reasonable evaluation by anybody who actually knows about it. $\endgroup$ – JDługosz Mar 4 '15 at 8:46
  • $\begingroup$ That is, if there is no good technical reason, use a business or political reason. Or it's just plain broken. $\endgroup$ – JDługosz Mar 4 '15 at 8:47
  • $\begingroup$ at is, if there is no good technical reason, use a business or political reason. Or it's just plain broken. "The repaired dish after the debris hit works just fine for x, y, and z, but without repolishing in a real service facility, this is the best image we can get for q and r is just not possible." Critical information that ought to be in the scan are lost in the blotchy picture. $\endgroup$ – JDługosz Mar 4 '15 at 8:54

Just to answer part of the question - shielding - as other's have stated, shielding an entire planet is not practical.

Also, you only describe the technological level of the sensors, not the defending planet. That could make a huge difference in how shielding is handled.

Also, others have mentioned that cameras and radar are the most likely candidates for sensors. I agree.

Here's a couple extra thoughts...

  1. Radar, even from close orbit, won't detect something a mile underground. Neither will a camera for that matter. Even if close range radar could detect an underground facility, you would have to know where to look. The larger the scanning "beam" the lower the sensor resolution will be - so you have to figure out what your lowest acceptable resolution is. For pinpointing targets from orbit, you're probably talking about a beam size of 1-mile diameter or less (guessing). For a planet the size of earth, using naive math, that's a surface area of 4.92403 x 107 (e.g. it will take longer than you want to scan for the target). Like I said, that is naive math (surface area of a circle with an earth-like diameter). But a planet is a sphere so the farther from perpendicular to the surface you are, the more distortion you will encounter. That means you have to constantly be readjusting the position of the ship.
    Burying facilities is how governments today hide things.

  2. At long range, see #1. At short range, precise targeting becomes an option. We like to imagine that the modern media's claims are true that a simple green laser pointer can blind a pilot in an airplane. Imagine what a relatively low-power, planetary wide, laser grid could do to an approaching ship's camera systems. The effects could be anything from a simple obscuring (blindness) to destruction (burning or overloading the sensor).

  3. Artificial methods of obscuring a target, such as chaff in the air, would just be a beacon. "Look! Something is hidden over here, don't look over here."

In conclusion it seems like if you want to hide something on a planet from being observed by a ship - choose a location where it is often cloudy and dig a big hole. Let math and physics handle the rest.


Let's say we are on earth and trying to keep IT from watching us. If they are using optical sensors (aka cameras), all you have to do is point an industrial strength laser pointer at it and it's blind. But if ET is using radar or something similar, like the other answer said, chaff would be one of the easiest ways to go. The main problem with the chaff is that it would be rather hard to achieve in a short amount of time and would make a bit of a mess.


Cover your whole planet in randomly blinking lights.

If you have a camera, you can prevent it from taking a good photo by having a really bright flashing light - it won't be able to set it's aperture to anything sensible. Conceivably you can apply this to all frequencies, (you could be broadcasting all the way from ELF to gamma rays) and effectively confuse just about all photographic-based sensors and disrupt any communication signals trying to get through.

The quality of your shielding with this method is down to the randomness and brightness of your lights, and the reactivity of the scanning sensors

This is, however a very un-subtle shielding technique, and the energy cost is simply astronomical. To cover the whole planet in lights/EM transmitters would also require a huge engineering effort.


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