11
$\begingroup$

Imagine a space battle happening high above Earth (~25k miles, or ~40200 km). It's right above the observers who are standing on the planet's surface.

Both armies have the same amount of spacecraft, one mothership (the size of two ISD from Star Wars), two warships (same size and weapons as a Star Destroyer), ten frigates, two hundred starfighters.

All of them shooting lasers, firing plasma torpedoes, mass drivers, letting the other side feel the pain really good. The shots, despite what science tries to tell us, are quite visible / cinematic. Just like in the movies or even brighter. Like, deliberately wasting energy to make it showy. Though they are not actually making a show for those on the surface, or projecting some image toward the planet. We postulate it's a quirk of their technology and nothing more.

Such a fight would be quite visible from the surface, even though it might be only flashes of pulsing light here and there. Under optimal conditions (i.e. at night, new moon, clear skies) people could even sit down with the ol' popcorn bucket and enjoy the free entertainment of a light show (hoping the debris won't crash on them, those space-age alloys are tough and don't burn at all in reentry). This is postulate two, to give a measure of the light the battle creates.

I'm banning overcast skies with massive notilucent or cumulunimbus miles high. Obviously that amount of water vapor / ice crystals can block even sunlight. Also, fighting against the sun (i.e. earth-fight-sun are in a "straight" geodesic gravitational line) is a no-no.

I ask then. Under what conditions would visibility of this space battle be faint, to a point a casual observer could have a 50% chance of missing it even if they were staring at the sky?

This question is regarding only optics and weather, not the space-fighting technology. Don't nitpick on my shiny laz0rs.

$\endgroup$
11
  • 25
    $\begingroup$ I can't help but think that you're contradicting yourself by wanting "science-based" but then insisting that lasers are visible. The problem stands that you're asking for science based answers, but not identifying what science you want us to base your answer on. $\endgroup$ Sep 6 at 1:31
  • 11
    $\begingroup$ The debris won't crash on them, the debris won't crash on anyone, it will just keep orbiting like the fleets before - they are orbiting, aren't they? $\endgroup$ Sep 6 at 11:17
  • 3
    $\begingroup$ starship combat should happen at such range one ship should not be visible from another ship. $\endgroup$
    – John
    Sep 6 at 19:52
  • 4
    $\begingroup$ Wouldn't a cloudy sky suffice? $\endgroup$
    – FluidCode
    Sep 7 at 14:42
  • 7
    $\begingroup$ Sorry, what's wrong with clouds? That's a very real reason why many of us are unable to witness many astronomical phenomena firsthand, even if we're looking in the right spot with huge telescopes. Saying why this is off-limits will get you much narrower answers. $\endgroup$ Sep 7 at 21:13

10 Answers 10

50
$\begingroup$

This battle is invisible, even at night

I've got bad news, and I apologize sincerely... but you did tag your question "Science-Based."

Consider the distances involved: The earth's diameter is only 8,000 miles. That's only 32% of the distance from the surface to your battle. Climb a local mountain (I have) and look down on society. From even that statistically trivial distance the ability to pick out even a large building is difficult without binoculars. You're talking about the largest object being maybe 2 miles long at a distance of 25,000 miles, which means the arc length (size of the visible object from the perspective of the viewer) is an impossible to see 4.6 millidegrees. For comparison, the blazing sun is 500 millidegrees (100X larger). And, well... the entire half-degree is blazing. For some photo evidence, consider the Pyramids of Giza, which are visible from about 1,000 miles into space, but to get a feel for your distance we need to reduce the image 25X.

About 1,000 miles up (courtesy NASA):

enter image description here

And that same image reduced 25X:

enter image description here

Unfortunately, the spread of the battle doesn't help. A lot of things you can't see spread over any distance are still invisible. It would take 10,000 ships of the largest size all bunched together to be the size of the sun, and they'd have to reflect the sunlight to be seen.

Energy bursts (e.g. lasers) aren't actually visible in space. Hollywood likes to show them, but there's simply not enough material in space to heat up to show the passage of energy.

Edit: A point needs to be made here based on several comments. In a "Science-Based" scenario, nobody would use lasers or any other form of energy weapon. (a) Too hard to focus during the heat of battle. (b) The energy requirements are unbelievable. Not just unrealistic, but absolutely unbelievable. (c) There are much simpler ways to damage your opponent. And (d) you need to either generate all that energy on the spot or store it in batteries/capacitors — which means you're either economically unrealistic (oversized engine to power the weapons) or you're a big, juicy target that's easy to destroy (probably true in both cases). Remember those hoverboards that burst into flames? Yeah, now imagine a battery large enough to do significant damage to a mile-long ship miles and miles distant and your opponent is smart enough to use kinetic weapons.... The reason hyper-realistic SciFi books and shows like "The Expanse" don't show energy weapons is that they simply fail the ruthless mathematics of practicality and usefulness. Yes, given a big enough laser a hit on another ship would likely be visible to the naked eye on the ground (at least at night). But in a "Science-Based" universe, nobody in their right mind would use one.

Whether or not something like a mass-driven projectile is visible depends on whether or not it's burning during its passage—but you tagged the question "Science-Based" and wasting energy on something burning during passage rather than burning on impact is a wasteful projectile (unlikely to be used). So none of the weapon discharges are visible to any viewer. Missile rocket engines? Unfortunately, they can't be seen either, even if they're racing away from the planet (and most of the time they'd be racing tangential to the planet).

Maybe... maybe... observers on the ground, during the night, might... maybe... see a ship explode. If it actually does explode. That's more Hollywood. There's no point to vaporizing a ship when all you really need to do is stop it from shooting at you and then wait for it to be drug down the local gravity well (people would notice that). But even if it does explode: 4.6 millidegrees and a quick blip of light.

Answer to your question:

Under what conditions would a space battle involving a relative large amount of vessels would be barely visible on the surface?

All conditions not involving binoculars or a telescope and knowing exactly where the battle is taking place. And if the observer happened to see what in the battle would be a massive explosion but on the ground would be a itty-bitty blip of light, there's a high chance they'd write it off as a flashing light from a passing high-altitude passenger vehicle and ignore it.

If the sun was at the position to best cause reflections off the largest ships, they'd appear like a handful of teenie-weenie scintillating lights in the sky... kinda like the dimmest stars.

Yeah, sorry, from a "Science-Based" perspective, there's nothing here to see.

$\endgroup$
9
  • 2
    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Sep 6 at 9:51
  • $\begingroup$ You really had to drive that home, eh? Not nice. $\endgroup$
    – Mindwin
    Sep 6 at 21:59
  • 2
    $\begingroup$ I have a remark, I may not have understand your answer fully. But could you explain why we are able to see satellites (spaceX constellation, ISS) but we would not be able to see this battle? $\endgroup$
    – RomainL.
    Sep 8 at 14:59
  • 6
    $\begingroup$ @RomainL. Most satellites are nowhere near the altitude of this battle It's easy to look up and say, "That little dot is the ISS, why can't I see the battle?" Well... the ISS is only 254 miles away... not 25,000. The enormous distance is (among other things) the problem. Next is whether or not light would reflect of a ship favorably. They're in unpredictable motion and shaped like Star Wars destroyers. If not at the right angle, they won't reflect at all. $\endgroup$
    – JBH
    Sep 8 at 15:10
  • 1
    $\begingroup$ You are wrong when you write "impossible to see 4.6 millidegrees". Have you ever seen Sirius A, the brightest star in the sky? Its angular diameter is about 6 mas (1 miliarcsecond =1/3600 milidegree). Science based: doi.org/10.1071/AS10010 @RomainL. You can see objects smaller than angular resolution of your eye. What you cannot see is their details (i.e. shape). All you can see is a shining point. $\endgroup$
    – abukaj
    Sep 9 at 9:01
14
$\begingroup$

Well, sure I saw it. But what battle?

Let's assume the most visible, flashy battle possible directly overhead in the night sky. Who knows what tech these folks have that makes such a light show? So why would no one notice it?

  • Fireworks: Your battle occurs on Global Founder's day. Or Independence from Galactic Oppressors day, or whatever. Everyone is looking to the night sky expecting to see a time-honored tradition looking like a battle - fireworks. Wow, you know the city planners really pulled out the stops this year.

enter image description here

  • Meteor showers: One of the fleets used the natural movement of asteroids to mask their advance on the other fleet. Those same asteroids are now burning up in the atmosphere. Battle? Boy, you're gullible. Didn't anyone teach you about asteroids?

enter image description here

  • Earthquake lights: So we don't know what causes random discharges of light that may or may not be geological in origin. Your battlers are using weapons that manipulate gravity, resulting in massive discharges of light around the paths of the beams. But these same gravitational weapons are triggering local earthlights on the planet's surface as the weapons stress geological features. The weapons are triggering a more spectacular display near the surface. What's a few flickers in the heavens compared to the display on the surface?

enter image description here

  • Religion/superstition: It is the worst, most terrible luck to see objects in the heavens. Most people refuse to look up for fear of being cursed accidentally. Those few poor souls forced to look up when lights are visible in the heavens avert their eyes at the first flicker of light. Even if you see something, your subconscious convinces yourself it wasn't there. Battle? Lights? You must be drunk - or high. Now shut up. I'll kill you if you spread such slanderous rumors!

enter image description here

  • Aurora Borealis: The planet itself regularly puts on a heavenly lights show. Most people don't even bother to look up when the heavens flicker. Even the ones that do look up would need to be really paying attention to realize this doesn't quite look like the usual display. Or maybe the ships attack each other by manipulating streams of high-energy particles emitted by the sun (why not?). When these beams hit the planet, they trigger a massive aurora.

enter image description here

  • There's already a war on: The locals are way behind the fleets technologically, but that doesn't mean they can't kill each other in spectacular fashion. The locals have been blasting each other with hypersonic fighters and bombers, orbital missile bases, and massive EMP attacks high in the atmosphere for days, years, months, or pick your time frame. Oh, fun - the enemy is bombing us again. Ho, hum.

enter image description here

  • This isn't the first: People have become inured to battles in space. The star system is in a giant war zone. No one even pays attention, instead looking through it like it wasn't there. Debris regularly reenters and crashed ships abound. The heavens are flashing? Well, yeah. The gods are fighting. You know, the usual.

enter image description here

  • A colorful and full moon: Your battle takes place between the viewers and the bright, richly patterned full moon of scintillating minerals. During the full moon, reflections off the surface make the heavens bright and flashy. Flickering color, phosphorescent glows, and possibly even gas emissions make the battle a trivial thing compared to the moon.

enter image description here

I must have Missed it...

Then, of course, you can have the stuff that simply blocks your view.

  • Smoke: Perhaps there are wide-spread wildfires going on. Perhaps flaming debris from a previous battle has come down, starting forest fires. But it doesn't take much smoke to block the resolution. A few flashes of light when you can't see what is flashing don't draw attention. Last year in Montana, the sky was hazy from the smoke and there was no rain. Thunderstorms were starting fires but so little water hit the ground that all you had was visible lightning. Sounds a bit like a battle...

enter image description here

  • Ash: There's an active volcano or three spilling tons of ash into the atmosphere. The sky is hazy, and even if people see lights, there's certainly no way to tell what they are. Big, small, local or global. Just decide how much to dump.

enter image description here

  • Fauna: Migrating insect swarms and bird flocks have blotted out the sun on numerous occasions. Locust swarms are a real thing. Passenger pigeons used to regularly blot out the sun until people decided to wipe them out. I imagine a giant swarm of iridescent bugs or even phosphorescent ones mating in the shining moonlight.

enter image description here

  • Flora: Your world might have thick forests and dense jungles so people can't see up. Perhaps clouds of pollen or spores are being released because it's the season for it. Maybe floating gas-filled plants drift in the sky and cause any view of the heavens to be spotty.

enter image description here

$\endgroup$
13
$\begingroup$

Full and bright daylight will surely make all the show go barely noticed or totally unnoticed.

If even the waning moon is not visible during the day, I believe the sun light of a clear day is sufficient to mask most of what you describe barely noticeable.

$\endgroup$
2
  • 1
    $\begingroup$ And NASA claims that there's only about a 33% chance that sky is clear enough to see anything, day or night. $\endgroup$
    – user535733
    Sep 6 at 21:19
  • $\begingroup$ Wait, what? The moon is prominently visible during the day, as long as there are clear skies. Only when it's very close to the new moon, or close to a full moon and therefore opposite the Sun, is that not the case. $\endgroup$ Sep 7 at 21:08
6
$\begingroup$

Nukes

Let me stay science-based: lasers won't be visible, ships won't explode. The most you can expect from "conventional" weaponry is some sort of minor explosion from a warhead on a missile, but not enough to be visible from the ground.

Which is where nukes come into play. It's quite possible that warring spacefaring nations will use nuclear warheads on their space missiles. Which will give out a brief but very visible flash. I didn't do the maths, but should be visible against the night sky, and possibly day time as well. Here for more details: http://www.projectrho.com/public_html/rocket/spacegunconvent.php#id--Nukes_In_Space

What blademan9999 said is very true as well, your capital ships will be visible at night due to the sunlight reflecting of them, but you won't be able to tell an active ship from an orbiting mass grave.

Final note - do yourself a favour and get rid of the "space fighters", those are horrendously uneconomic. All you need is a big-ass machine gun mounted to a rocket engine (+fuel, propellant, etc.) and a few electronic components. The pilot will be sitting in a comfy chair on the nearest capital ship with a remote controller and a VR headset.

$\endgroup$
1
  • $\begingroup$ A manned space fighter is also tactically inferior to a remotely controlled drone. Its maneuverability is impaired by limits of physiology of their pilots. In case of human-like pilots see en.wikipedia.org/wiki/G-force#Human_tolerance $\endgroup$
    – abukaj
    Sep 9 at 9:42
4
$\begingroup$

You should DEFINITELY by able to see it at night. The ISS has an altitude of 408km, 1% of the value here. It has a peak apparent magnitude of -5.9. So If the ISS was place in the middle of this battle, it's apparent magnitude should be 1/10000 of it's peak. Increasing the value of the apprent magnitude by 5 corresponds to a 100x reduction in brightness. This conveniently means that the Brightness of the ISS here would be equal to -5.9+10=4.1, or equivalent to a reasonably faint star. Bright enough to be visible at night in rural areas. And the cap ships here are much larger then the ISS.

$\endgroup$
4
$\begingroup$

I don't know the math to resolve it but here's the problem:

Geosynchronous orbit (~25k miles) is where you stated the battle will be but that's too far away. A 1 mile long ship at 25,000 miles has an angular size of 8 arcseconds, making it even smaller than the ISS.

As someone else stated, at 25 miles the ships will be well visible but now they're making a pretty fast transit across the sky as they orbit so no one gets to see much of the battle.

I would think a large battle in geosynchronous orbit at night with the battle not in the earth's shadow would have to be obvious enough to anyone in a rural area who happens to look up -- if nothing else, all the new "stars" fiddling around up there would look out of place. (I would think weapons fire would not be visible at that distance since the largest ships themselves are mere pinpricks.)

If we're talking modern day, though, I should think TV stations would pick it up pretty quickly and then it's a matter of telescopic zoom lenses and everyone tunes into news channels (and YouTube).

$\endgroup$
1
  • $\begingroup$ At geosynchronous height the planet's shadow would be relatively small, and so long as the battle wasn't in that shadow or close to the overhead sun, it should be visible from a range of latitudes and longitudes, many of which will be dark enough to see the reflected light. $\endgroup$ Sep 7 at 12:59
3
$\begingroup$

This is science based.

So the ships aren't from our solar system. Which means they are weapons of war of a K2 or K3 civilization.

A K3 civilizations serious weapons are the kind of weapons that destroy planets accidentally. Like, turn them into clouds of plasma.

A K2 civilizations serious weapons are the kind of weapons that sterilize entire planets.

We are K0.7. Our serious weapons are on the order of the Tsar Bomba, 50 megatonnes. Serious K1 weapons will be on the order of 1000x as energetic, and serious K2 will be 10000000000000x as energetic.

The sun deposits on the order of 10^16 W of energy. Tsar Bomba is 10^17 J of energy; if you set off a Tsar Bomba every 10 seconds for a year, you'd match the energy deposit of the sun on the Earth.

The K2 equivalent "serious" weapon is 10^30 J of energy. At 40,000 km 2% of the energy of the explosion hits Earth; call it 10^28 J of energy. Over an entire year, the sun deposits about 10^16 W * 86400 seconds/day * 365 days/year = under 10^24 J of energy - the blast is 10000x as energetic as an entire year of sunlight, concentrated over at most a few seconds.

Earth has a bit over a billion cubic km of ocean water, and it takes under 10^5 J to vaporize a gram of liquid water (half to hit boiling, half to make it to boil). The oceans weigh 10^21 kg, so 10^29 J is enough to boil all of the oceans on Earth.

That is slightly more than the back blast of a serious K2 civilization weapon.

Now, we don't fight with nuclear weapons when we fight wars.

But an actual interstellar civilization fighting an actual serious war in our actual back yard, without hand-wavey SF technology that makes interstellar travel easy, the question isn't "can we see it" but "do we survive".

$\endgroup$
1
$\begingroup$

According to Wikipedia a star destroyer is 1600 meters (5,200 ft) long. Low Earth Orbit is defined as 2000 km or less. The lowest earth observation satellite, a record set in 2019 is Japan's Tsubame satellite at 167.4 km, and super low orbits are between 200 and 300 km altitude. Generally any thing low orbit has drag from the atmosphere and doesn't stay in orbit that long.

The human eye with a pupil of about 4 mm (smaller in bright sunlight ~ 2mm, ~9mm fully dilated) resolves with an angle of about 8.74 x $10^{-3}$ degrees. So about 40 km away you can resolve with perfect vision, ignoring the haze of the atmosphere etc, objects about 6 meters apart.

So the good news is that at 25 miles altitude your mile long star destroyers about 2.3 degrees of your field of view. That seems pretty big, the moon and sun are about 0.5 degrees so there will be something to see. Although blurred by the atmosphere.

But the scattering by the atmosphere might also make your fancy lasers more visible since there is a lot of scatter ( also less light hitting the target).

For them not to be noticed, I think you can just increase the range some for them to be smaller, but reflected light from the sun can be visible for very very long distances. The very powerful laser weapons in the beam of course are very powerful, but the scattering and atmospheric attenuation is severe, and you can play with that any number of ways.

Light scatters basically by Rayleigh Scattering and Mie Scattering. Bigger particle scatter (Mie) the light mostly forward and backward, while Rayleigh scattering from very small particles and molecules scatter more in all directions. So how visible the beam might be is up to you depending on the stuff you have in the atmosphere. Remember that laser beams also expand over distance, so the spot size can be very large after a few miles.

The downside is that the space ships are fighting in the Stratosphere, or maybe mesosphere. For example the highest weather balloon made it to about 52Km. So there is a lot of drag, and even winds to contend with. But maybe since theses ships might be built to land on planets anyway that is somehow o.k.

I am not sure what the tactical reason would be for them to want to fight at low altitude.

Edited to include unit of degrees for the angle.

$\endgroup$
6
  • 6
    $\begingroup$ 25 thousand miles... not 25 miles... $\endgroup$
    – JBH
    Sep 5 at 19:13
  • 1
    $\begingroup$ Ahhhh, my mistake...That makes a lot more sense.... but at 25,000 miles probably all there would be to see is would be flares from reflected sunlight, or maybe the flash of a laser if sweeps past the eye. It would be impressive have some sort of battle in the atmosphere if the materials and structures could handle it. $\endgroup$
    – UVphoton
    Sep 5 at 20:59
  • $\begingroup$ "The human eye with a pupil of about 4 mm [...] resolves with an angle of about 8.74E-3." Maybe it does, but we cannot possibly know because you don't tell what those 0.009 stuff are. (Note how after "about" one should avoid giving three significant digits.) 9E-3 what? Gallons? Olympic swimming pools? Fluid ounces? Surely not radians -- the expected value would be about 0.3 milliradians, about 30 times smaller; maybe degrees, but still the expected value is about 1 arc-minute, about 0.017 degrees. (I suspect that you computed the Airy criterion for an ideal lens; the human eye is not.) $\endgroup$
    – AlexP
    Sep 5 at 21:51
  • $\begingroup$ In degrees a about 1.5 x 10-4 radians. Used the Rayleigh criterion sin theta =1.22 lambda/ D with D the aperture size with wavelength of 500 nm. Not a bad approximation for back of the envelope. Standard textbook approach for resolving power of the eye. How far you can see two headlights in the distance etc. Not trying to resolve high frequency components so lens quality doesn't depend as much. $\endgroup$
    – UVphoton
    Sep 5 at 23:59
  • $\begingroup$ The point is that the actual resolving power of the eye is about two times worse than what an ideal lens could do. Because the eye is not an ideal lens, not even close. $\endgroup$
    – AlexP
    Sep 6 at 0:26
0
$\begingroup$

When a laser hits a ship, it will light up the surface of the ship facing towards the laser. If the attacker is in a higher orbit, the surface that is lit up will face mostly away from the planet. The lasers might also be primarily infrared (Or alternatively, UV) so the light is not visible.

The defenders might be using a different technology, such as railguns, which are kinetic rather than energy or light. These can be very lethal, but there's no light emitted.

The ships might be visible as faint points of light, which sometimes split into smaller points or just evaporate into a faint and fading cloud.

There is a tradeoff in selecting a dark surface for the ships: It might make them less detectable (stealth) but would be more susceptible to absorbing the heat from a laser weapon. A highly reflective surface would reflect the laser's energy, but could be much more visible. A mirror surface might be less visible from some angles, but cause a bright flash at other angles. (See "Iridium flare" for an example of a real satellite with a mirror like surface. They are nearly invisible most of the time, but will randomly "flare" to be the brightest thing in the sky for a few seconds as they reflect the sun towards you.)

$\endgroup$
0
$\begingroup$

Even single kg of evaporated lithium will scatter about 150 MW of sunlight (0.1 km^2) - and the cloud will take several minutes to disappear.

And many other elements will produce similar effect - Na is about 40 times less effective, but Luna-1 managed to produce visible artificial comet with sodium. 1 kg of Na produced equivalent of 6 magnitude star (barely visible) for 5 minutes 110 thousand kilometers away.

So even without nuclear weapons any large scale space battle, with tons of matter being evaporated and hundreds of tons of gasses and liquids being thrown out will produce comet like lights that would be visible even during day.

photo of artificial comet: enter image description here

For example DART impact (the equivalent of 4.8 tons of TNT) created comet like tail after Dimorphos. Even 2 days after impact that tail was 10,000 km long and twice as bright as asteroid itself.

enter image description here

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .