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My setting is more Space Opera than science based, but I am attempting to keep a consistent set of rules for space travel. I'll try to narrow down my question to a "lowest denominator" for the benefit of Worldbuilding–SE.

In an otherwise unpopulated solar system, can I detect the arrival of another spaceship? I'll have a variety of engine types in my story, but I'll omit their specifics for this question and ask that you imagine "common" sci-fi tropes.

My constraints are:

  • Detection within the solar system only (in Star Trek they inconsistently "scan" ships that are light years away in other star systems. That seems ridiculous even for space opera)
  • All engines are "noisy". There is no special stealth technology. This one is braking from FTL and probably the noisiest it will get. Decelerating from interstellar speed to in-system speed takes days from the point of view of the traveling ship, longer from the point of view of an outside observer. (Traveling star-to-star takes weeks, if not months).
  • Ships range in size from cargo freighters to battleships. Compare roughly to modern ocean ships plus space engines. (The ship I need to detect is a military destroyer, not enormous like a colony ship, but large engines and heavy armor)
  • The "listening" ship is in the inner solar system orbiting the star. I have made no decisions about the composition of the solar system other than it is uninhabited.
  • The scenario is a one-on-one showdown between two ships. The "listener" has arrived early and is actively attempting to detect the other ship.

As you see, I am aiming for making it easy but I leave the method of detection open. I realize that asking if something is "possible" in Space Opera isn't a valid question, but what method(s) can I use to detect the ship?

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    $\begingroup$ You probably meant "braking from FTL". A drive which breaks the ship involved isn't likely to be too popular with the crews. $\endgroup$ – WhatRoughBeast Jun 11 '17 at 21:27
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    $\begingroup$ "in Star Trek they inconsistently 'scan' ships that are light years away in other star systems. That seems ridiculous even for space opera" - Not necessarily. If your space opera allows FTL travel (at speeds of several-hundred C and above), then you can "scan" a system a few LY away by dispatching a FTL drone to the area and waiting for it to report back. $\endgroup$ – aroth Jun 12 '17 at 9:31
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    $\begingroup$ Cargo freighters are the largest ships ever while battleships are the largest combat ships. Have you actually meant that your ship sizes go from "as huge as they get" to "still quite huge"? Destroyers are the smallest military vessels that can sail the ocean and not get sunk by the waves alone (although they're still so small that rough seas can incapacitate the crew). By definition, they carry no armor, they're little more than engines with weapons. Armor is the quality of a battleship. If a ship would be stealthy, it would be a destroyer. $\endgroup$ – Agent_L Jun 12 '17 at 12:05
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    $\begingroup$ Just wait in Facebook for one of the crew-members to do a check-in or to post a selfie on the new solar system. This will probably happen before any of your sensors can pick anything up... $\endgroup$ – T. Sar - Reinstate Monica Jun 12 '17 at 14:06
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    $\begingroup$ @aroth In star trek, they scan things from light years away using subspace sensors; as subspace is the source of all their FTL. $\endgroup$ – Shufflepants Jun 12 '17 at 14:51
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Oh yes! A braking rocket at relativistic speeds putting out enough thrust to do the job will be a rather bright object in the night sky. If it’s the only one, not a sky full of activity, it will stand out like a comet.

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    $\begingroup$ 'enough thrust to do the job' Yet at relativistic speeds, if you're kicking out so much power to stand out, then you could also picture the state where the observer is notified, but doesn't have long to react! $\endgroup$ – UKMonkey Jun 12 '17 at 15:22
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This one is braking from FTL and probably the noisiest it will get.

It mostly depends on the drive's characteristics (for most "realistic" drives the answer is a definite YES).

  • You might get the equivalent of a Cherenkov flash - a huge blue-white flash when the incoming starship hits an "interface" with an apparent speed equal to the speed of light in vacuum.

  • And/or you could see the Doppler-shifted radiation of the braking starship in the form of a long-duration gamma or X-ray burst.

  • the excess energy might be partially radiated as a distinct gravitational wave signature (large detectors possibly required) due to the collapse of an Alcubierre-like "twisted space" bubble. As noted in the comments, a true Alcubierre drive will not cause a sudden grav wave, but it will still give out the equivalent of a gamma ray burst, or worse.

  • a gravitational ripple is to be expected no matter what, since a FTL object can't interact with matter (not even gravitationally) without violating general relativity (essentially, going FTL must remove you from the observable universe). It follows that when the ship re-enters the sublight universe, it will appear instantly as if from nowhere, generating a very small grav wave packet with a wavelength equal to the ship's depth as seen from an observer. The observer might need to deploy a complex array of detectors to be able to observe the emergency - probably a network of sensors connected by laser beams, similar to LIGO.

  • if the vessel brakes using conventional means (mass expulsion, or even better from the ambusher's point of view, photonic thrust), it will shine like a nova in several frequencies, possibly even visible light. To detect that, you need a suitable space telescope with very large aperture, or many such telescopes. Being already in space, and having cold and vacuum readily available, will allow the listener to deploy very sensitive devices at next to no cost.

Attempt to stealth

To approach a solar system as stealthily as possible, after tweaking the propulsion to the nines, the newcomer would probably try and approach along a geodesic between the brightest or noisiest astronomic phenomenon and the likeliest position for the ambusher (or the local star). This is the equivalent of "coming in from the Sun" for starships.

To counter that, an ambusher would need to deploy farther from the Sun or use remote sensor platforms, which suffer from lightspeed lag.

Of course, if the newcomer's disturbance is too strong, too misaligned, or too unlike the noise from the far star or astronomic object, this manoeuver is pointless and the advantage goes completely back to the ambusher.

Braking could be attempted using a solar sail with the highest possible absorbance; it will give only 50% of the thrust of a maximum reflectance sail, but it will be way more stealthy. Infrared emissions will still be a problem; heat would need to be collected and dumped at right angles from the approach vector (less than 90° will contribute to braking but risk detection; more than 90° will supply an acceleration and increase detection risk from Gegenschein scatter).

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    $\begingroup$ The Alcubierre Drive would not remove a ship from the universe (it's only warping space, although technically it should form a blackhole if it's warping enough), even though it's flying FTL. However such a drive would "collect" everything in front of it - including light and radiation - and basically push it out as a gamma ray burst once it arrives. Plus such warped space would probably also generated gravitational waves. $\endgroup$ – Syzygy Jun 12 '17 at 8:17
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This may be considerably harder sci-fi than you want but from a realistic point of view: there is no stealth in space. I'm drawing quite heavily from Project Rho which is my go to resource when I'm gaming out a sci-fi scenario.

Essentially anything human (or sophont) made is a point source of energy that stands out against the galactic background.

  • Any radio signal will stand out like a sore thumb (for context the voyager 1's 20 watt radio signal is easy to pick out).
  • If it's not actively transmitting then it's still producing radiation such as heat and any radiation associated with your drives.
  • Finally just optically scanning the sky is a valid method, a computer today can do it in three days.

You probably don't want to use active sensors (radar or ladar) as these systems fall afoul of the inverse square law and have a limited detection range while making you stand out. You are going to get detected but there is no point in making it easy. You would only want these sorts of sensors for close range, high precision, detection.

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    $\begingroup$ Voyager 2 is only easy to pick out if you're looking for it with a radio telescope. $\endgroup$ – Mark Jun 12 '17 at 2:09
  • $\begingroup$ @Static Do you have any hard sources for the radio signal will stand out like a sore thumb claim, preferably with actual calculations explaining why? $\endgroup$ – AndrejaKo Jun 12 '17 at 11:48
  • $\begingroup$ I agree with the essence of this answer, but also keep in mind that it takes time for anything - even light - to cross a solar system. It takes light ~8 minutes to get from the sun to the Earth, so it could be anywhere from let's say 20 minutes to a couple of hours before sensors on the waiting ship detect the other's arrival, unless it happens to jump in very close. During that time, the other will have moved. Also, let's not write off the possibility of an intervening celestial body (like a planet) that would block the radiation from the arriving ship from ever reaching the waiting vessel $\endgroup$ – Steve-O Jun 12 '17 at 14:41
  • $\begingroup$ Alas, the Project Rho page has a major unexamined assumption - that a hot body must radiate in all directions. If the bogey has an idea of the location of a passive sensor it can provide a cooler for most of the exterior, providing a cold sensor reading at the expense of a very hot emission over a limited aspect ratio, with the hot aspect kept pointed away from the sensor. Using something like liquid helium will keep some or most of the bogey down around the CMB, and effectively invisible. $\endgroup$ – WhatRoughBeast Jun 12 '17 at 19:15
  • $\begingroup$ @AndrejaKo Nothing to hand unfortunately. Essentially the difference between cosmic background radiation and anything man-made is significant enough to be easily detectable if you're looking for it. $\endgroup$ – Static Jun 12 '17 at 21:45
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You've already stated that there is no stealth in space. Good. Anyone who wants to counter that can find an answer on this question explaining that no Jimmy, there is no stealth in space.

So rather than to answer your question of how to detect another ship in an empty system, lets ask how you remain hidden in such a scenario. We've already determined that it's not possible, so lets ask that question anyway.

Answer: don't point your drive at the person you're hiding from, or if you do, make sure there's something else between you and them.

Voila. Now you know how to detect another ship: its drive is pointed at you (or at least, in your vague general direction) and there isn't anything between you and them. How visible is it? Well, there's a nice link from the above question that states that "a single attitude control thruster of the Space Shuttle can be detected at 15 million km range and using main engines it can be detected from Uranus. With current technology."

Mind, current technology scans the sky very slowly, but pointed in the right direction it'd know. Oh it would know you were there. And how fast you were going, in what direction, and how much acceleration you had (and in what direction), and could predict your course through the solar system (to within some degree of accuracy, not due to the ship or even to the light-time delay, but rather due to the N-Body Problem). Beef up the detection tech in your story so it's got the same resolution as current tech, only it scans faster, and ta da.

The TV show The Expanse has done an amazing job of being Hard Sciency about space travel and communication and detection thereof. The only unreal thing about it is the engines, capable of providing upwards of 10G worth of thrust on virtually no fuel (which, if we're being frank, is the major limiting factor to our civilization's colonization of the other planets). Oh, and there's the...

aliens and the alien tech that does apparent magic.

But it's not terribly relevant to this answer.

Every instance of stealth in the show has been done under a hard-science approach: decoys ("this isn't the ship you're looking for, sorry!"), flak clouds ("sure, you see us, but can you pick out which 'us' is the right one amidst this cloud of radar jamming junk?"), or pure gravitational assisted flight (if no one knows to look for you and you don't fire up the GIANT GLOWING TORCH BEACON OF LOOK AT ME I'M MISTER MEESEEKS then you can drift around the system at your leisure and be impossible to spot). The gravitational stealth approach in the show had a 45 second slice that broke my suspension of disbelief, as Alex came around the curve of a moon, spotted a Martian vessel and was able to reverse course without being spotted. He was still using the attitude control thrusters, but they appear in the show as gas-vent propulsion, not torches, but the part that broke it for me was that he was able to abort his trajectory enough to slip back around the horizon. Honestly they should have omitted that scene. The quip about "oh yeah, the moons hide them from us too" is clever, but that's the only purposed it served.

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I'd say yes; in the scenario the listening ship can set up sensors that are literally miles long; like the gravity wave sensors that have recently made headlines.

Think, as an analogy, about motion detectors used in alarm systems: They have a pattern of data they are sensing, the electronics are very simple and do not "interpret" this pattern to figure out what it means: The key is they can detect if it changed, and that sets off an alarm. Your waiting ship needs to do something similar; say with gravity waves, radio waves or just plain light (including infrared, or red shift (different if something starts moving fast), etc. Once you know something is different, you can narrow down precisely where it is different with more effort or analysis.

So your listening ship can deploy a few dozen relatively inexpensive sensors in the sphere of concern; covering all sides of the sun and planets so nothing can use them for cover coming in; and those that detect change can help pinpoint the general direction and distance of a disturbance, then you can train the big telescopes on that vector.

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  • $\begingroup$ Don't forget though that such detection methods will have the time lag that the speed of light introduces over such long distances.... $\endgroup$ – ivanivan Jun 12 '17 at 14:44
  • $\begingroup$ @ivanivan Well if they have FTL travel, they should have FTL communications too! $\endgroup$ – Amadeus-Reinstate-Monica Jun 12 '17 at 17:02
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I think the answer to your question is yes, because as you are already defining the technology for FTL travel, it is reasonable to assume that you will be able to define the detection technology. If your ships are capable of FTL flight, they will need significant sensor range to avoid unmapped obstacles and this leads me to believe that the detection technology should be available.

Your concern will come with military tactics. I would assume they know approximately what direction the aggressor will be approaching from and sensors, dropped detection bouys, etc. would work fine. But what if the military expected this, overshot the star system while scanning it, braked in the next one and approached from the opposite side of the star and slower than expected.

In your design, I would expect sensors to work well toward the direction of travel, a 360 degree view of a star system is a big task, and the opposite side of the central star would be a blind spot without a sensor relay.

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Two visual telescopes separated a little bit apart and a computer can calculate the speed of any moving object they can see. There are some blind spots like a star of the system or planets, but they could be overcome with additional telescopes. Even now the telescopes from Earth can see a planet 1200 light years away. (link). The telescopes just need to move across the sky very fast to cover all the sky in a pretty short time.

There is a problem of differentiating between a ship, an asteroid or a comet if they have a small enough speed. If I remember right current aviation radars hardly differentiate between a plane and a flock of birds. Birds just don't fly at 900 km/h. High resolution telescopes can just show a photo of the object. It is beyond current tech, but sensitivity of current cameras is increasing extremely fast. There are some methods used currently that differentiate the composition of space bodies, but I do not know the details.

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