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Setting:

We have alien-constructed stargates connecting different solar systems to each other, but otherwise it is a hard sci-fi universe with only known-possible technologies. We have fusion drives. But no warp drives, artificial gravity, or powerful shields.

Our protagonist is suspicious that one of the spacecraft in the system is not what it seems. He already has plenty of passively collected data from his long-distance probes. He would love to take a closer look at the spacecraft's characteristics and confirm or deny his suspicions. But wait a second, is there any way to do that?

The Question

Would it ever make sense for someone to use active sensors to look "more closely" at someone's spacecraft to determine its characteristics?

Thank you for the help in advance. You guys are super smart, and it's been fascinating lurking here.

Edit:

To clarify, we are watching a suspicious vessel - it may truly be civilian, or it may be a military vessel cleverly disguised as a civilian one. For instance, maybe it has a coilgun and additional radiators hidden in its hull. Is it likely that active sensors could give us a decisive answer to this question, where passive sensors could not?

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    $\begingroup$ You might want to remove some assumptions you are making, because you might run into a XY problem. meta.stackexchange.com/questions/66377/what-is-the-xy-problem - Don't ask about your attempted solution, ask about your problem. $\endgroup$ – Mindwin Jul 4 '17 at 6:29
  • $\begingroup$ Okay, thank you. Editing accordingly. $\endgroup$ – Ariah Jul 4 '17 at 6:31
  • $\begingroup$ What is the range between the target and the observer? At co-orbital distances around a single planet Radar will work perfectly well, but at interplanetary distances signal attenuation and light-speed-lag are going to be a major problem. $\endgroup$ – MozerShmozer Jul 10 '17 at 17:50
  • $\begingroup$ Excellent question. Our ship and the target are flying towards each other, which makes sense because of how the gates are arranged in the system. The target is a couple of light minutes away, but at its closest approach it will be less than a light second away. $\endgroup$ – Ariah Jul 11 '17 at 1:54
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Yes, to some extent.

First, there is the doppler radar that lets you know the target's relative velocity instantly, without waiting. This can be a game changer, measuring velocities using passive ways requires you to wait and allow him to fly away and get a head start.

Now for the shielding - it is cheap, but still costs money. Commercial vessels will only shield against things that are both dangerous and expected. And they will only shield as much as is needed, not more. The fact your target is more opaque than it should will tell you quite a lot - it will confirm he is hiding something. Still, in real life most of the time it is quite sufficient for us to just know the shape of our target. So if it is less reflective than it should be, be suspicious. There is literally no reason to prevent reflections for commercial crafts.

Now the main reason for using active sensors in modern military: precision. We can detect planes using passive means all right. Visible light, thermal etc. Still, when we need precise information about its location we use radar. If you will have a chance, compare a night vision goggles image with just using a flashlight. I think you can imagine the difference - image quality is much, much better with generous amounts of light. The idea that there is a need for active sensors to get details is believable all right, because that's pretty much what we are using in the real world, too. The flashlight analogy is also pretty good at illustrating side effects:

Be wary though: using an active sensor (like a radar) means the ship can see you observing it. The flashlight comparison is accurate on this issue: sure, you see better, but if you search someone, he'll know and be able to precisely locate you. – Keelhaul

Even if you use external illumination, your target will know someone is looking.

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    $\begingroup$ Be wary though : using an active sensor (like a radar) means the ship can see you observing it. The flashlight comparison is accurate on this issue : sure you see better, but if you search someone, he'll know and be able to precisely locate you. $\endgroup$ – Keelhaul Jul 4 '17 at 9:08
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    $\begingroup$ @Molot I'm just trying add some info, you already have my +1. $\endgroup$ – Cyrus Jul 4 '17 at 10:34
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    $\begingroup$ Especially in space, you can use an illuminator - dispatch a drone to shine a light/radar/whatever at the target, and record the reflections. This allows the benefits of active sensing without giving your precise location away. $\endgroup$ – Andrew Dodds Jul 4 '17 at 10:45
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    $\begingroup$ @Raditz_35 premise is something that pretends to be civilian vessel. $\endgroup$ – Mołot Jul 4 '17 at 11:42
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    $\begingroup$ @Raditz_35 For a regular civilian ship, maybe, although it could be a mean to detect a pirate having an interest on you. But a suspicious ship (wich is the actual premise) may have more reasons to know when they are under scrutiny. Also, no matter how "quiet" your active sensor is, the targeted ship receives much more particles than your ship can gather from the reflection, meaning that if you can detect the radar (or anything else) bounce, the targeted ship will easily detects it too when it hits its hull. ("it hit its". English is funny) $\endgroup$ – Keelhaul Jul 4 '17 at 11:51
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It really depends on the kind of details you want. Since you specifically asked for technologies we know its at least theoretically working, I want to focus on what we know from experience that works, followed by a wild speculation.

If you know the position of the suspicious vessel, you can use either radar or some kind of lidar (basically a laser-radar, using laser-frequency photons instead of radio photons). Both will be able to determine the current vector relative to your vessel.

Current velocity however isn't very indicative of what you want to know. But fortunately, both radar and lidar should be able to determine the spacial form of your target, specifically what kind of propulsion is used and how large it is. You might calculate an approximate maximum accelleration based on this.

What I think might be even more indicative than the exact dimensions of the drive itself would be the means of heat dissipation. Power systems produce a lot of emissions, especially with fusion-powered systems. You need to get rid of that stuff, so look for heat sinks. The vessel will need as much heat sinks as it has power available, otherwise it will fry itself when going full power.

You could use the reflection behaviour of the material you're looking at to determine it's type and guess it's use, not every surface needs to be a heat sink (see: story weakpoint)

But there is a caveat: unless you have drones in the proximity of your target, you'll probably not be able to see every heatsink, as you won't get any reflection from the sides not facing your own vessel.

Also, the range might be limited, because after bouncing from your target, the radiation will scatter much more, and radiation will, unless it is lased, diminish proportional to the second power of the distance (see: inverse-square-law). Thus, you will probably close in distance much more to have sufficient sensor resolution.

Story weakpoint

Now this is a weak point of your technology stack: Technically, it isn't too feasable to use fusion power, as the only ways to get rid of the heat in space are either dropping coolant or blackbody radiation. You could estimate needed radiator area by using Getting rid of heat SE Unless you want to overlook this scientific fact in your story (it is science-fiction after all). You could always invent some way to address this.

Other things to consider:

You probably wont find anything that is penetrating the hull of the suspicious vessel, as it will be heavily shielded against everything freely floating in space, like microwaves, gamma rays etc., since they will be facing the sun at some point (probably most of the time). Even if you could, you'll be killing the crew anyway.

However: It seems to me, that in many cases, it doesn't necessarily depend on whether you use active or passive sensors as much as your distance and the targets emissions. These, in many cases, have a much higher factor in determining detail than the type of sensor you use.

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    $\begingroup$ You're right that you need to use radiators to get rid of waste heat, but the takeaway from that isn't "fusion power isn't feasible", it's that the ship will indeed have radiators. It's perfectly feasible to deal with your waste heat that way, and there are tons of more-or-less speculative real world designs with high power power sources (what a phrase) that do so. $\endgroup$ – Elukka Jul 9 '17 at 10:05
  • $\begingroup$ @Elukka you're right, that's not the takeaway you should get. I was trying to make the point that its difficoult to create a system to blast off all that waste energy with just blackbody radiation even if your whole hull is basically a single radiator. $\endgroup$ – Doomed Mind Jul 10 '17 at 12:58
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For many purposes you can use passive sensors, since everything is being done against the 3K background of space. Ships will be radiating heat like a lighthouse against the thermal background, and the use of energy to manoeuvre, use weapons etc. will make you stand out even more.

However, very small weapons like "Soda Cans of Death" (SCoDs) will be more difficult to detect due to their small size and low thermal mass once free of a missile bus or ejected from a rail or coil gun. Devices like that will need to be tracked with a high degree of accuracy either to avoid them by jinking your spacecraft or using a Ravening Beam of Death (RBoD) laser to destroy them from as far away as possible (theoretically, up to a light second away). (For the provenance of these terms see: Rocketpunk Manifesto

Of course, the laser itself could be used on a "wide" setting or farther than a light second way. The intense thermal energy will heat up things like SCoDs, making them stand out even more against the background of space and allowing active defences to be mounted from a much greater distance.

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I say yes but for a different reason than those already given. Would it ever make sense for a cop to pull over a car that was not doing anything wrong? Sure. Why? Because the interaction that ensues can give away what the driver is really up to. An active sensor uses energy to send a signal, and derives information by how that signal interacts with the object sensed. By that definition, a radio hail from a human is a sensor too.

A good cop can sniff out when something is not right. Non-communication active sensing may provoke action that tells you more about the ship than your scans could. Suppose the incoming ship does not respond to an active scan. That is useful information. It either did not detect the scan, cannot respond, is playing dumb or does not deign to respond. Follow the scan with a courteous hail appropriate for the relative status of the two ships. The human components of that incoming ship may not be as good at concealing their true nature as the ships engineers were. If they are nervous, or do not know the mode of interaction appropriate for their cover (e.g. Navy SEALS masquerading as truck drivers) they may give themselves away. Suppose you receive back "What the !@'s $#!@ are you scanning my ^&~*@! for, you *&!?" That might be reassuring.

I hope this is a story because I like this as a narrative device: the engineer and science officers argue (as in prior answers) over how to use deflected particles to gauge mass, use radar / lidar. Then the captain opens a channel.

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In most cases, you would not need active sensors at all. All bodies radiate heat, and the amount and temperature of such radiation give a lot of information. By comparing an object's increase in radiation to its delta v, you now know it's mass and likely composition. By comparing its temperature to its brightness you know its size. There are very few things an object's own radiation won't tell you that you can determine by bouncing other radiation off of it. In addition, its very hard to mask radiation, as the only way to do so would be in a heat sink which would have to be emptied quite quickly, or to radiate in only one direction which would limit changes in velocity.

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It depends (as always) on specific circumstances but if question only is: "may active probe be more efficient to discriminate a heavily armed vessel from a civilian one?" the answer is YES!

The ultimate probe is a real (looking) nuclear attack missile. This "probe" will look like a real threat and elicit the highest counter-measures available.

In case these counter-measures prove insufficient (i.e.: we are dealing with a civilian ship) the "probe" can be stopped short of its destination, otherwise we already have a good assessment of military ship capabilities.

This can be a bit "over the board", but is a viable example of an effective "active probe" ;)

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I'm not really a hard-science guy, but I think I have a clever idea to do all of this legally in this world, it may only take the simple process of figuring out of it is plausible by our own rules of physics.

In this world, there must a ray/beam (radar) of some sort used by the authorities to automatically scan a ship anyway, consider this process/system similar to scanning someone's public records or internet history.

By using some kind of electro-magnetic wave, used to scan all sorts of electronic devices for their data and basic information. The reason this legal to used by the police (space patrol) is because anything that the ship's owner/pilot wants to stay safe and hidden from the public is automatically blocked from the scan's point of view. What exactly is going in space for the authorities to check any type of ship in the first place you might ask?

By using this basic Scan Beam, the user can see everything the ship is made out of (physical prompter) and everything that isn't blocked inside the owner's public data record (electro-magnetic prompter). Think of this as a 3-D mapping of the ship owner's place of hold, without the use of x-rays of course to keep the privacy to the owner. However, the police can still see every corner of the ship's exterior to check if it is indeed, an exact copy of the normal ship model.

And by using the "data prompter," you can see into the pilot's skill records to compare them to whoever's actually driving the civilian ship, or maybe there's very slightly wrong with the data record when someone else is driving it. Because of this system of public data, the police officer might see who's piloting the ship compared to who's actually supposed to be piloting it.

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