Space Traffic Control: intercepting signals in space

[Dr Wu]

I've recently got back to writing more of my SF novel (after a lengthy interval) and picking up the threads again, I find myself instantly confounded by a problem that has been waiting for me all this time: detecting spacecraft at a distance - by which I mean distances counted in dozens of AU. One given in terms of communication is the preferred use of lasers rather than radio. That would make detection by third parties fiendishly difficult, I should imagine. I guess too that trying to spot a spacecraft 's fusion exhaust at such colossal distances is also one for the tooth fairy. All this being so, would it be theoretically possible to use radar as a means of detection? I have no objections to pushing the boat out a bit in terms of future technology, but I don't want to transgress the known laws of physics if I can possibly help it. Any tips or helpful advice gratefully received.

 

[mfb]

Fusion? 1 GW thermal emission at a distance of Pluto is brighter than Nix and Hydra, the two Pluto Moons discovered 2005.
That is a Hubble image - in the visible light, but your spacecraft would look similar in infrared.

Stealth communication is a different thing, but powerful stealth propulsion? Forget it.

 

[Dr Wu]

That's heartening news, I must say! I had no idea that thermal emissions, such as the one cited above, could be detectable across distances measured in light-hours. Many thanks again, mfb.

 

[Khashishi]

What is the exposure time for these images? Just because the Hubble telescope can image them if it knows where they are doesn't mean that the Hubble telescope can find them if it wasn't specifically looking for them. The sky is pretty big.

 

[mfb]

I don't know the exposure time Hubble used. The two moons have an apparent magnitude of about 23, the LSST currently under construction would easily see them with an exposure time of seconds, and it can scan the whole sky (night side only of course) every few days. If you have fusion-powered spacecraft s, you can probably use several of them in space, to cover the whole solid angle once per day or even more often.

 

[Loren]

I'd borrow what aviation uses today, TCAS (Traffic Collision Avoidance System).

It's simple. Each SV (space vehicle) simply globally broadcasts its identification, current 3D position, velocity vector, etc., on a periodic basis.

You will need to establish a common inertial frame reference (time-space) and on-board computers will reconstruct real-time positions of other traffic based on distances and propagation times for the radio emissions.

TCAS broadcasts via radio omnidirectionally and each vehicle has its own ID code.

Spaceports would also have their own local radar systems for traffic management, but like the current commercial aviation system, it is confined to regions near airports.

 

[newjerseyrunner]

I liked Loren's idea until I started thinking about the fact that such a system would not just have to track ships, but also rocks and comets and stuff.

How about a Von Neumann probe sent out a hundred years ago? It lands on a random asteroid, uses the materials there to build a telescope, then replicate itself and shoot off to more asteroids. If you automate the process of putting building hundreds or even thousands of telescopes at various locations in the solar system, the combined data should allow for real time tracking of every object within the solar system, be it registered or unregistered ship, as well as rocks. One telescope would have to scan the sky, thousands of telescopes with overlapping fields of view will not only allow you to see objects, but also triangulate their exact position.

 

[Loren]

Sounds expensive.

Most of these objects could be just put into a database on-board the ship. It would be no different than how we track space debris in Earth orbit. We have a database of all that stuff now.

The difference between spacecraft and asteroids, planetismals, comets, etc., is the latter orbits are fixed and essentially unchanging in their orbits. SV, on the other hand, move dynamically.

The other issue with real-time optical tracking is some objects may be too dark to see. Radar is about the only option.

 

[newjerseyrunner]

Sounds expensive.

It'd be free. The only cost would be the cost of the original ship, that's the point of self replicating machines.

 

[Khashishi]

Having Von Neumann probes will have huge ramifications to every aspect of society in your story, so you might not want that.

 

[GTOM]

Having Von Neumann probes will have huge ramifications to every aspect of society in your story, so you might not want that.

I also don't want too much AI in my setting, but still, build a swarm of recon craft is cheap compared to fusion ships. So i think stealth can be only possible in vicinity of celestial bodies, but transponders arent needed and don't used regularly.

 

[Ryan_m_b]

Even before you introduce self-replicating machines a setting in which space travel is well developed is going to be one in which mass producing telescopes is cheap. If a thousand tonne interplanetary fusion spacecraft is affordable (or even more impressive: commercially viable) then you can likely afford a few hundred telescopes to distribute around the system.

 

[Loren]

Even before you introduce self-replicating machines a setting in which space travel is well developed is going to be one in which mass producing telescopes is cheap. If a thousand tonne interplanetary fusion spacecraft is affordable (or even more impressive: commercially viable) then you can likely afford a few hundred telescopes to distribute around the system.

Assuming whatever you are looking for has an albedo that is visible.

Something cole black is not going to be very easy to detect.

The bigger question is, why is the idea of tracking objects and SVs in space interfering with the story plot?

There are probably 10s of thousands of hard and serious science fiction stories written that don't suffer from not explaining the details of some form of traffic control system.

For that matter, how many books have you read about people flying to some other city where the author feels compelled to describe the US air traffic control system, its technology, and overworked workers?

Unless your plot is centered around it, don't bring it up. Readers won't care for one thing (they are engaged with your characters and their plight, right?). For another, just like commercial flight today, the whole system just works. Leave it at that. No need to describe how the plane's landing gear system works, it just goes down with a whir and we feel a bump when we festinate.

It is interesting to brainstorm about these things, but readers (and certainly editors) will not care about them (unless the plot specifically revolves around it), so don't sweat it.

 

[mfb]

Something cole black is not going to be very easy to detect.

It still emits infrared radiation.

 

[Ryan_m_b]

Worldbuilding is important to any good speculative novel, regardless if most of it actually gets to the reader. Aside from being fun and interesting in its own respect an author who works out the back story of how his world works will be able to tell better stories. If you don't you're more at risk of running with something the reader realizes doesn't make sense. In this case for example the ramifications of working out a traffic control system highlight that stealth isn't going to be possible, which influences possible story lines.

And looking for visible light is likely less efficient than looking for IR. The background of space is quite cold after all.

 

[Loren]

Worldbuilding is important to any good speculative novel, regardless if most of it actually gets to the reader. Aside from being fun and interesting in its own respect an author who works out the back story of how his world works will be able to tell better stories. If you don't you're more at risk of running with something the reader realizes doesn't make sense. In this case for example the ramifications of working out a traffic control system highlight that stealth isn't going to be possible, which influences possible story lines.

And looking for visible light is likely less efficient than looking for IR. The background of space is quite cold after all.

I'm with you on that, Ryan.

The issue is a delicate balancing act between technical details and good story content. I have just seen too many people get stuck in the beauty of the technology and the actual story becomes secondary or even tertiary. For that reason the novel suffers badly even after great amounts of effort to create it.

 

[GTOM]

While i liked Rise of Leviathan it was irritating how easy it was to get lost in space, even when they knew where to search the rocket, not to talk about a big asteroid. Of course i don't want to go into too much tech details neither, but no stealth thing had a few effects on storyline regarding piracy or invasion.

 

[Dr Wu]

Unless your plot is centered around it, don't bring it up. Readers won't care for one thing (they are engaged with your characters and their plight, right?). For another, just like commercial flight today, the whole system just works. Leave it at that. No need to describe how the plane's landing gear system works, it just goes down with a whir and we feel a bump when we festinate.

While I entirely endorse the above comments - i.e. keeping the technical stuff firmly under the hood, unless relevant to the action - I would only add that a SF writer should also have a fair grasp of what the practical technological limits are, as it applies to the scientific knowledge base contained in any given SF novel he or she might be writing. This requirement usually demands some considerable swotting up in advance, at least for scientific semi-illiterates, such as myself. But then this research is part of the fun. It's just that one runs the risk of boring the reader rigid were it to be trotted out on the page. Keep it strictly between the lines - not in them. 'nuff said.

 

[Dr Wu]

A brief follow-on point: is there a (simple?) formula for converting power output, such as gigawatts, to absolute or apparent magnitudes? Thanks again.

 

[mfb]

If you know the spectrum of emitted radiation, it is possible to convert power to absolute magnitude. You can use a planet with a similar spectrum, if available, for comparison. Different spectra lead to different magnitude (and to make things worse, there is more than one magnitude scale).

 

[Dr Wu]

Yes, that's exactly what I did. I used Phobos as a test-run, albeit with a few guessimates thrown into ease the way forward. The results were these: nominal apparent magnitude of Phobos at 60 million km is about +12. Interestingly enough, its absolute mag turned out to be approximately +45 Mag (hardly surprising given its extremely low albedo - Enceladus it certainly isn't). As far as I can tell Phobos reflects some 33 GW of sunlight at its orbital distance of 1.5 AU.

Moving swiftly on: a 2,000 tonne spacecraft accelerating at 0.35g from rest will require 76,800 GW over a 60 minute period. This is roughly equivalent to 21 GW per second. By this stage it will have covered a distance of 22,700 km and here its thermal emissions (whether visible or in the infrared) would almost outshine Venus at its brightest (-5.0 mag). These same emissions would also yield an absolute magnitude of +45.5. This means that the ship's exhaust would cease to be a naked-eye visible object at a distance of 5,000,000 km - although I accept that such fusion-based emissions would more than likely be in the infrared. Be that as it may, by the time we reach the 5AU mark, these same emissions would have dimmed to +17 magnitude, which puts it out of range of most amateur telescopes. Of course, a professional infrared telescope would have no trouble spotting it, but at 75 -100AU? Well, unless one knew precisely where to look, I fancy any such spacecraft would stand a fair chance of remaining undetected. . . which is not what I want.

Now I'll be the first to say that I don't trust my own calculations - hence this post. Still, knowing that something's gone badly adrift is not the same as knowing the cause, more's the pity. Thus any advice would be warmly appreciated.

 

[mfb]

The spacecraft is at rest relative to what?
There are no "gigawatt per second", gigawatt is energy per time already.

There are two different definitions for absolute magnitude, with different baselines for objects in the solar system and objects outside.

I get just 11 GW for Phobos - where 1kW/m^2 is an estimate for the visible part of the solar constant at Earth. The infrared emission is much larger, because all the energy that is absorbed gets reemitted as visible light.

If it accelerates along some rail on a planet or similar, then it needs 1.6*10¹⁴ J, with an average power of 44 GW and a peak power of 88 GW. Accelerating along some rail can be done very efficiently, however, so thermal radiation can be much smaller than that.
If it accelerates in free space, it will need much more power. The total velocity change is about 13 km/s. Assuming the spacecraft wants to be able to perform two of those acceleration periods with a propellant mass of ~1000 tons (reduce this mass if you want to increase power), it needs an exhaust speed of about 50 km/s. Shooting away 500 tons of propellant at 50 km/s needs an energy of 6.25*10¹⁴ J, about four times the energy value for the rail, for an average power of 174 GW. Give the fusion reactor 50% efficiency, and your spacecraft will radiate away the same power in infrared.

Want to have 4 acceleration periods with 1000 tons of propellant mass? Double exhaust speed, double power values.

 

[Dr Wu]

The plot requires the spacecraft in question to undergo just one acceleration/deceleration phase, and this purely for the outward stage of the voyage: a (potential) fuel dump already exists at the destination. By 'rest' (a likely fiction in reality, I know) I intended this to mean pulling away from a gravitationally negligible object within Mars' trailing trojan point (the asteroid Eureka). This would add about 24km/s to the initial thrust, but for the fact that the spacecraft is pursuing a near-polar trajectory, taking it high above the plane of the solar system. Yes, I'm still getting to grips with the power terminology viz GW/GWh etc. I just wanted to make myself clear beyond all doubt - silly really, given that this is a science forum (!) A rail-style accelerator is a great idea, assuming, of course, the gees could be kept down to reasonable levels.* Solar-powered beam technology would also be applicable as a way of reducing onboard fuel mass. Unfortunately, I need this fuel mass for things other than pushing the spacecraft through the interstellar medium. That's why I initially flirted with the idea of using antimatter as a motive power source - this (in the wrong hands) for its mighty explosive potential. Antimatter, though, seems, well, somewhat hackneyed in SF. . . too 1950s? Pity really. Best stick to something that doesn't draw too much attention to itself - fusion will suffice.

mfb, I'll take onboard your calculations concerning energy requirements and see how they pan out in terms of radiation detection levels. I must say that writing 'hard' SF in this context is a great way to learn about astrophysics, indeed physics itself. It almost seems, in my case, to be the main point of the exercise :) Again many thanks.

*A decelerator rail at journey's end would be another blessing, although I would imagine a technological nightmare in view of the velocities involved. How it might be harnessed to slow down a spacecraft is for me yet another imponderable.

 

[mfb]

Rails to decelerate sound risky, you have to hit them exactly or you will ruin the spacecraft and the deceleration system. If you approach any object with an atmosphere, you can use this to decelerate.

The more you reduce the propellant mass, the higher the power has to get.

1 GW is sufficient to see it through the solar system (~100 AU). We could probably perform all-sky scans with that precision within 10 years if that would get priority, and with some science fiction it should be easy.

 

[Khatti]

The question that interests me is: Why do your characters need to know who and what is coming several AUs out? If the propulsion units your characters use is of the Star Wars variety; ship come out of hyperspace within Earth's orbit, there is no need to worry about what is going on out by Neptune--or at least not much. If the drive systems are more along the lines of the KK ships from Alan Dean Foster's Commonwealth series, the spaceships will have to continually decelerate as they approach Earth, making them easier to find.

Perhaps the way to view it is the way the British Empire would have viewed ship traffic on the Pacific and Indian Oceans. While the Brits would certainly have liked to know who all was out there, ultimately they didn't become a threat until they were off the coast of India or Australia. A ship floating around in the middle of the Pacific might have been worrisome, but not a real threat until it showed up at your door. The same is probably true of a ship floating around between Uranus and the Kyper belt.

 

[mfb]

A single ship in the Pacific was certainly not a threat, but 100 ships moving towards the coast? You probably want as much warning time as possible to prepare (e.g. get other ships/troops to come as support).

 

[snorkack]

How efficient is interplanetary space in actually converting power into detectable radiations at a specific location?

Suppose that you have an engine which is capable of producing a large thrust by issuing a high speed jet of low random velocity, ground state neutral helium atoms.

How often do they collide with the protons of solar wind? How widely would the emission be spread?
A jet which spans a large part of sky may be hard to detect above background, and even once you detect it, finding the source may take some effort (though probably less than detecting the jet).

 

[mfb]

You would try to detect the infrared emission of the spacecraft , unless something like a highly efficient direct fusion exhaust thruster is available the efficiency of the power source cannot be that good.

 

[GTOM]

You would try to detect the infrared emission of the spacecraft , unless something like a highly efficient direct fusion exhaust thruster is available the efficiency of the power source cannot be that good.

What if heat is radiated only outward the elliptic pane, with big radiator wings? And exhaust is only pure protons, with really big speed, if electrons don't change orbit, no radiation is emitted, as far as i know.
(Of course the ship also need to get rid of the electrons after a time.)

 

[mfb]

Directing the radiation means you have to radiate even more, because you need some cooled radiation shields. Not impossible, but not really practical either. Just put a few telescopes out of the ecliptic to find those spacecraft s.

And exhaust is only pure protons, with really big speed, if electrons don't change orbit, no radiation is emitted, as far as i know.

That doesn't work. Also, the discussion was not even about the exhaust, but about the heat.

 

[GTOM]

Otherwise, do i know right, that laser (or well directed radio) signals in space can't be detected unless a probe gets between the emitter and receiver?
So i can write, that a captain can calmly negotiate about backstabbing his ally?

 

[mfb]

Otherwise, do i know right, that laser (or well directed radio) signals in space can't be detected unless a probe gets between the emitter and receiver?

Right.

 

[GTOM]

Although i still wonder. Due to limited telescope accuracy, from big distance, even a long boost phase appears to be a single point.
And after thrusters stop and ship cools down, IR emission drops by many magnitudes.
Could they determine, where exactly the ships are going?

 

[mfb]

Spectroscopy from different directions could give 3D velocity data. Tracking the exhaust direction, if possible, can help as well.