How would lighthouses work in space?

Question

In my world spaceships are guided by lighthouses floating in space instead of electronic navigational systems. A lighthouse in space has the shape of a huge sphere emitting intense red light. Like the ordinary lighthouse a space lighthouse mark dangerous things, such as : black holes, supernovas … They also mark space stations, docking bays, fuel stations and other things.

My questions are:

1. Is this system viable?

2. Is this scientifically possible?

3. How much surface should a sphere (lighthouse) have, in order to emit enough red light?

Answer 1

1 - Is this system viable ?

It is breathtakingly inefficient.

2 - Is this scientifically possible ?

Yes, the question is if you can get enough power to make it worth the trouble.

3 - How much surface should a sphere (lighthouse) have, in order to emit enough red light?

It's not surface, it's power. Because you're omni-direction you'll need a lot of it. Basically you need a star.

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The problem is making this thing bright enough to be noticed far enough away so fast-moving spaceships have time to make course corrections with minimal delta-V. I'm going to use some relatively small numbers for time and velocity by sci-fi standards to give this the best chance of working before trying to scale up.

A spaceship going 1% the speed of light is moving about 10¹⁰ (10 billion) m/h. If we want to give this ship one hour of warning (probably not enough time, but we're starting small) it needs to see the beacon 10 billion meters out. This might seem far, but it's only 1/5th the distance from Earth to Mars at their closest. Not even interplanetary scales.

First, using visible light means you're competing with everything else that's producing visible light: stars and everything reflecting starlight. It's like turning on a flashlight on a sunny day, can't see it.

An omni-directional lighthouse is just a radio transmitter. Visible light is a crowded part of the spectrum, so you can do a little better by changing to a less common frequency probably in the Microwave Window. No human is going to eyeball this thing anyway, it'll all be done with computers just like a radio. So pick a rare frequency. However, the higher the frequency the more energy required, so pick something low. Transmitting in an uncrowded, low frequency, part of the spectrum will significantly reduce the required energy. Pulsing it in a recognizable sequence will help picking it out of background noise.

But here's the problem: because this is an omni-direction beacon you can think of its energy racing outwards in an expanding sphere. The surface area of a sphere increases with the square of its radius, spreading the energy thinner and thinner. Double the distance from the lighthouse, quarter the energy. If it has 1000 lux at 1 m, at 2 m it will have just 250. At 4 m it will be down to 62.5 lux. At 10 m it's just 10 lux.

A sphere 10 billion meters in radius has a surface area 10²⁰ m² times larger than one with 1 meter radius. A light source you can spot at 1 meter needs to be 10²⁰ times brighter to be seen at 10 billion meters. And therein lies the problem: power. That's a lot of power. And it's for a relatively slow spaceship with relatively short warning. At a certain point you might as well just create a small star.

To put this in concrete terms, for your beacon to be as bright as Sirius it would have to be as bright as light bulb at 1.6x10⁴ m. A light bulb puts out about 10³ or 1000 lumens. To be that bright at 10 billion meters out, 6.25x10⁵ time further away, you'd need to increase with the square of the distance: 3.9x10¹¹ or 400 billion times brighter than light bulb: 6.25x10¹⁵ lumens. This is six orders of magnitude larger than the largest spotlight in the world, though still nine orders below the smallest red dwarf.

That's the lower bound to be seen 1 hour away by a ship going at 1% the speed of light. We're not even into interplanetary scales, much less interstellar ones. Double the speed or reaction time, quadruple the brightness required.

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Unless you have the power of a star, omni-directional transmission doesn't work at interstellar or even interplanetary scales. You need something directional. The problem for a directional beacon is to work out where to point it.

Answer 2

1. Sort of.

2. Yes

3. This would depend on how bright the light is, total radiation is surface area * brightness so to make it more visible you can increase either.

It would certainly help make things easier to see, but the thing is space is really big and empty. There isn't much to run into and it's generally easy enough to avoid the things that are there (or you need to go to them anyway).

Navigation can't really be done by eye in space since you're dealing with orbital dynamics and transfer orbits and all sorts of other very complicated maths. Most of the time in space with current or near-future tech you aren't even using your engines. You just use them occasionally to accelerate/correct course/decelerate and spend the rest of the time coasting.

A super-nova would be far more visible than your floating red sphere. Even a black hole is surrounded by an accretion disk and highly visible most of the time. If you had an isolated black hole or neutron star then you might possibly be able to argue for some sort of warning beacon but it still doesn't make a lot of sense since you would need to account for the gravity from the neutron star/black hole when navigating and if you didn't know about it would detect it by the changes to your course it generated long before impact became a risk.

Answer 3

1 - Is this system viable ?

I think the buck stops here, really. No, it's not viable.

What you are proposing is perhaps "possible" in some limited sense (but as already pointed out by others, you are up against some very fierce competition in terms of light sources). It isn't however viable.

This isn't for reasons of the amount of light you would need to put out. Throw sufficient amounts of handwavium at that, and you could explain it away, or just lampshade (pun only half intended) the whole problem.

The reason is spelled orbital mechanics combined with the finite speed of both light and space travel.

If we assume space travel by Newtonian or relativistic mechanics as currently understood (which would be an implied requirement, since you are asking for answers based in known science), then we are limited by the laws of orbital mechanics. Basically, spacecraft coast for all but a tiny fraction of their travel time. Practical spacecraft have very limited delta-v budget (ability to alter their velocity and vector) due to the tyranny of the rocket equation. In order to reduce the delta-v required for a particular position change after some amount of time, you need to increase the time between the maneuver and the time at which the position change needs to be completed. The earlier you can perform a maneuver, the less fuel you need to get the result you want. Compare the fact that in order to land, from an orbital velocity of on the order of 7-8 km/s, the space shuttle only needed to reduce its velocity by about 100 m/s under power before gravity and drag did the rest.

Objects in space generally refuse to stay put. If you take your fancy spacecraft into a low Earth orbit, park and lock the doors when you go for an EVA to grab a lunch, and look for the spacecraft about 45 minutes later, you will find it on the other side of the world. (Thankfully, you will also be on the other side of the world, which somewhat reduces the practical impact of this.) Geosynchronous orbits don't help, because you are still moving at orbital velocities; you just happen to have an orbital velocity that matches the angular velocity of the rotation of the planet beneath you (the point directly to your nadir). Lagrangian points don't help either, because as the relevant objects move those points move as well, which means you (in this case, the light source) is moving with them.

Let's say you can somehow engineer a light source that is bright enough to be possible to make out at the distance between Earth and Pluto when the two are at opposition (points farthest from each other), and we put it roughly where Pluto is in our universe. Pluto's orbit reaches out to about 49 AU from the Sun, and the Earth orbits at about 1 AU from the Sun. So we want something that is visible at 50 AU. (Pluto's orbit is in a different plane than the rest of the solar system, but for an example, this works anyway.) 50 AU really isn't far at all in terms of interstellar travel, which I take it you are concerned with because of the dangerous objects you mention in your question, but it works nicely once you approach a solar system.

Now, let's say your ships travel at 1% of the speed of light, or 3,000 km/s, relative to the light source. (This is far, far faster than anything we can accomplish with chemical rockets, but it is still somewhat within the realm of possibility with science and technology as we know them.) 50 AU is about 7500 Gm, so this distance will take your spacecraft about 2.5 million seconds to travel. (Back-of-the-envelope plausibility check: speed of light time delay from the Sun to Pluto, on the order of 7 hours. Speed of travel, 1/100 of the speed of light. Expected travel time, 700 hours. 700 hours is 2.52 million seconds. Check.)

Pluto's orbital speed averages about 4.67 km/s. In those 2.5 million seconds that the light needs to reach our intrepid spacecraft 50 AU away, Pluto (or our light source) moves almost 11.7 million km along its orbit. This is before the crew of the spacecraft even sees the light.

Spacecraft generally travel along elliptical transfer orbits selected to get it to some particular point in space at some particular time (usually at a time when an object of interest is going to, in its orbit, intersect that point, or a point near that one) within some given constraints (time, delta-v, payload mass, ...). Any time a spacecraft is going anywhere, it is doing so by assuming a transfer orbit (very often a Hohmann transfer orbit, which in many cases is the lowest-energy way we know of to get from point A to point B in space; another alternative sometimes considered is a bi-elliptic transfer orbit). In Hohmann transfer orbits, you basically trade time for delta-v; the more delta-v you can afford, the more direct a route you can take and the quicker you can get to your destination. Since as we saw above that we want to minimize the delta-v expenditure in order to reduce our spacecraft's mass ratio, this means that we need more time to get to where we are going than if we were travelling in a straight line.

So not only has the light source already moved over ten million km along its orbit by the time the light reaches the spacecraft at 50 AU away from the light source's original position, but you also have to consider how long the spacecraft will need to get to where the light source (or point of interest) is now. (And that's "now" in which reference frame?) And by the time you get there, where is the point of interest going to be then? It's a variation of the classic math trick question of halving something repeatedly:

$$ x + \frac{x}{2} + \frac{x}{4} + \cdots + \frac{x}{n} $$

This is exactly the type of problem humans are horrible at solving, and computers are excellent at solving.

Which is why anyone who wants something even remotely like this would be far more likely to use radio beacons and electronic computers than navigation by eyesight and estimation.

The type of light (or even EM) source you are using has no effect on this, because the problem is related not to the type of EM source but rather to the relative speeds of the objects involved, and orbital mechanics.

Answer 4

Apart from all the other things mentioned in other answers, there are some more relevant issues:

1. Lighthouses have to be kept in place relative to the thing they are marking. Not really an issue on earth, when marking rocks, harbour entrances etc; they don't move much, and they don't affect lighthouses.

Supernova, black holes and the like, all have gravity. This will move the "lighthouse" around. In practice, the lighthouse would probably need to be in orbit - pretty much everything smaller than a galaxy, tends to be in orbit - the moon orbits the earth, earth orbits the sun, the sun (and solar system as a whole) orbits the center of our galaxy (our galaxy is the Milky Way).

2. If you are coming from the wrong direction, the object being marked will obscure the lighthouse.

Lighthouses work great at sea, where nothing sticks up. They work ok on the coast - if you're coming from the land side, you probably don't need the lighthouse, and lighthouses are usually at the highest point, to shine above anything that might obscure it.

In space, one person's up is another person's down, and if you are unlucky enough to approach from the wrong side, that lighthouse you are looking for may be behind the object.

3. Very massive objects can bend light.

This applies primarily to black holes. Similar to mirages on earth, the light from a "lighthouse" (or, for that matter, radio signals) will bend around a sufficiently heavy object, making it appear in the wrong place, distorted, or different colour and/or brightness. See "Gravitational Lensing"

4. Light is the same thing as radio.

If this is for alien (from our perspective) beings, then be aware that light and radio are the same thing. We humans have taken a small piece of the radio spectrum, and said "that bit there is different from the rest, because we can see it". Alien beings may say the same thing about other frequencies. Or they may sense their environment in a completely different way. Similarly, if your radio could be tuned up from it's usual 91.5 MHZ to around 480 million MHZ (480 terrahertz), you would be tuned to red light. Snakes can see infrared (for a given value of "see" - they don't sense it through their eyes, but it does go through the "vision" areas of their brains), which seems "black" to us.

5. Red means "Danger" - to us! To other races of the galaxy, red may mean "safety". Beware of cultural assumptions, when working with alien species - your assumptions are bound to be wrong.

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Points 1 & 2 can be solved by putting a number of "lighthouses" in orbit, similar to the GPS satellite "constelation". GPS satellites, for example, are in 3 different orbits, all through the poles; 8 satellites in each orbit, at 120 degree intervals, for a total of 24 satellites, plus a few spares; I would think that 2 per orbit, at opposite sides of the object, for a total of 6, would be enough when seen from space. You might, hypothetically, be able to get away with 2, at opposite sides of the object, though even a small, invisible object could occlude your lighthouse.

Answer 5

As others have noted, a supernova would be putting out a lot more light than any human-built lighthouse is likely to be capable of. That's like having a guy with a flashlight to warn people away from an erupting volcano.

But as some sort of navigational marker in general ... it could work. My immediate reaction is that it would make more sense to be broadcasting a radio signal. That could be distinguished from background noise much more easily.

Is it technically possible to build a big red light and put it in orbit? Sure, why not? It's certainly possible to build satellites: humans have built plenty by now. And it's certainly possible to build large lights.

How big would it have to be? Depends how much light you want it to put out and how far away you want it to be seen. I don't think there's any formula there. Also, how visible it is will depend on how much energy is being emitted, which might be affected by the size but is not determined by it. A 100 watt light bulb and a 40 watt light bulb are often the same size. I think the bigger question is, How much power will it need, and where will that power come from?

Answer 6

More than likely they would be emitting something more powerful than red light, but the idea is sound. The Hugh Howey novel Beacon 23 is based on this idea. In that novel, it's broadcasting the location of a large asteroid field, as that world's FTL travel is similar to Star Wars (physical objects can impact travel due to gravity fields).

As for surface area, enough is largely dependent on how far you want it to be seen by. I could definitely see you wanting to mark something dark, like an asteroid field or black hole (emitting something other than Hawking Radiation), or smaller things like the aforementioned docking bays, although something more similar to Range Lights would be better for those.. Supernovas and similar events are already pretty bright all by their lonesome.

Answer 7

Have you looked into pulsars?

https://en.wikipedia.org/wiki/Pulsar

There's been some talk about using them as galactic navigation beacons.

Answer 8

I don't think this would really work.

• Wouldn't be useful
• Would cost too much energy
• There are far better ways to do things

However, I like the idea of lighthouses in space. It could lead to some nice visuals. That's something that might be worth preserving even if the notion of using them for navigation isn't practical or useful.

Are you proposing faster than light travel? Because space is really, really big. If there's no faster than light travel you don't really have to worry about running into a black hole because it's going to take you thousands of years to get to one. (and if you're traveling faster than light, using light to navigate is not going to work very well-- also the further you dig into FTL the more problems you discover). If you're positing FTL you'll be positing leaving normal space to do it. You go somewhere else, traverse some different distance and then pop out where you're going. Being able to sense some navigation beacon across the boundaries of these two types of space would be necessary. Neither that space nor anything that could penetrate it are known. So invent it ;- ) If it happens to have a nice rosy glow in the visual spectrum? Bonus.

In normal space nobody's going to spend the kind of energy needed to make a visible beacon in space. It wouldn't work well, it'd cost way too much energy and there are way better ways to do it.

You can avoid hitting rocks by mounting radar on your craft. And things you really need to worry about are traveling so fast that visual cues are kind of useless. Imagine a rock traveling faster than a bullet. It's the size of a golf ball and it comes at you from above. You're not going to see it in time to do anything about it. Radar and a computer will though.

Also there's not really much in the way of visual cues when navigating a space ship. Consider the fact that the gravity well functions like a steep slope you can't see. When you "park in orbit" you're really just doing a bunch of math about your speed, direction and placement on a steep slope. You can't see any of that with the naked eye.

What might be interesting is a heads up display which DOES show orbital dynamics and hazards visually. Show the gravity well as a lit up slope rolling down towards the planet. You could point out debris or navigation hazards in white light on your face mask or control panel when they're too far away to be able to see with the naked eye. You could represent size and speed with visual cues. The human mind is really well tuned to that. A tool which could present data in that way could be super useful. It also simplifies controls and readouts.