System Survey Equipment

[Stargate525]

So, I'm plotting out a semi-hard scifi setting, and I'm a little stumped on one aspect of the ship that this story is going to be revolving around.

It's a survey ship... sort of. It needs to be able to locate and obtain at least general planetary characteristics (orbits, atmosphere or no, water or no, obvious life) from a dead start. It arrives at a random location within... lets say the orbit of neptune, at a non-relativistic but fast vector. It's not a dedicated scientific vessel, so advanced or very knowledge-intensive methods are out.

Basically, it needs to identify the main bodies in the system (large moons would be great as well), select one that would be most like earth, and be able to navigate to it. How would you kit a ship to do that?

[Shpaget]

An infrared camera/telescope would easily and quickly detect all eight planets in our system, most of the moons too.

It would need to spend some watching time to get the orbital characteristics, but detection is trivial. Once you have orbital characteristics, you can calculate mass, and from that make an educated guess about the composition.

Some spectrometry later you cad detect if there is an atmosphere and what composition it is.

[p1t1o]

A Radar. The larger and more powerful the better. Accurate ranging is required for resolution of orbits, and I would wager my hat that some clever tricks can be done to use the radar beam to make composition analyses, such as atmospheric content, surface water etc. It can also be used as a powerful communications beam/sensitive receiver, on that note it would also be an excellent passive sensor of various EM emissions coming from planetary magnetospheres and other things along those lines.

 

[Steel]

4 hours ago, p1t1o said:

A Radar. The larger and more powerful the better...

 

I would have two main issues with radar:

1. Your power to resolve objects is very much limited by the size of your ship. If you were out at Neptune distance you might need a dish in the 100s of metres in diameter to resolve an earth-like object that is almost 30AU away.

2. Accurate radar astronomy today relies heavily on already knowing where things are in the sky. To get good information about the various bodies you would likely need some other detection system first of all.

Other than that it would be ideal for getting information about spacial position, velocity and all sorts of other things like ice concentrations

Edited October 31, 2016 by Steel

[p1t1o]

19 minutes ago, Steel said:

I would have two main issues with radar:

1. Your power to resolve objects is very much limited by the size of your ship. If you were out at Neptune distance you might need a dish in the 100s of metres in diameter to resolve an earth-like object that is almost 30AU away.

2. Accurate radar astronomy today relies heavily on already knowing where things are in the sky. To get good information about the various bodies you would likely need some other detection system first of all.

Other than that it would be ideal for getting information about spacial position, velocity and all sorts of other things like ice concentrations

1. Im sure you could get at least some use out of a dish in the tens-of-metres range, depending on the wattage and frequency band, but you can easily extend your baseline to several hundred metres (or even many kilometres) by having several remote probes with smaller antenna fly in formation at a distance, forming I think what would be an interferometer or something. Someone around here might be able to elaborate on the technicalities, but I know that a "virtual" antenna can be formed of multiple smaller units, artificially expanding the baseline.

2. This is where a nice simple optical/IR/UV telescope comes in  (Also a radar antenna can double as a radio telescope)

[Steel]

42 minutes ago, p1t1o said:

1. Im sure you could get at least some use out of a dish in the tens-of-metres range, depending on the wattage and frequency band, but you can easily extend your baseline to several hundred metres (or even many kilometres) by having several remote probes with smaller antenna fly in formation at a distance, forming I think what would be an interferometer or something. Someone around here might be able to elaborate on the technicalities, but I know that a "virtual" antenna can be formed of multiple smaller units, artificially expanding the baseline.

2. This is where a nice simple optical/IR/UV telescope comes in  (Also a radar antenna can double as a radio telescope)

I actually got curious and started to play around with some numbers. Using a 30 MHz signal with a 1 TW transmitter and assuming you can detect a minimum signal on 1 nW on the way back, you'd need roughly a 100m diameter parabolic dish to make out an Earth size object at 30AU... assuming you could track it for the 4 and a bit hours it would take for the signal to do the round trip

[Stargate525]

22 minutes ago, Steel said:

I actually got curious and started to play around with some numbers. Using a 30 MHz signal with a 1 TW transmitter and assuming you can detect a minimum signal on 1 nW on the way back, you'd need roughly a 100m diameter parabolic dish to make out an Earth size object at 30AU... assuming you could track it for the 4 and a bit hours it would take for the signal to do the round trip

That's the main problem that I'm having. You'd need to track it. This ship is dropping in at a random point in the system; it could be significantly above or below the system's orbital plane. You'd need to be able to run a 360/360 scan. (Though, I suppose, an optical track would be able to discern the relative distance from the sun, and once you pinpoint one planet, you can narrow your search.)

Launching probes is fine. I had this odd notion of firing small probes at cardinal directions off of the main ship, then tracing their trajectory to get a gravitional map of the system  as they're deflected by the various planets. I have a feeling that the deflection you'd have to measure would be so tiny that it's indecipherable from noise, though. Is that correct?

[p1t1o]

22 minutes ago, Steel said:

I actually got curious and started to play around with some numbers. Using a 30 MHz signal with a 1 TW transmitter and assuming you can detect a minimum signal on 1 nW on the way back, you'd need roughly a 100m diameter parabolic dish to make out an Earth size object at 30AU... assuming you could track it for the 4 and a bit hours it would take for the signal to do the round trip

Did a little light reading myself. It seems the frequency, and possibly other factors such as beam shaping and other involved stuff, could have a significant effect. The best example I could find was the Goldstone Solar System Radar (https://en.wikipedia.org/wiki/Goldstone_Solar_System_Radar) and whilst it does have a 70m dish, it operates at only 500kW and has been used to extensively characterise various bodies in the solar system.

So whilst I am sure that one could get a (relatively) sensible sized dish to be of use, the best bet will still be some kind of synthetic aperture using smaller, seperated antenna.

I wonder, in fact, if the best bet might not be to "seed" the system with numerous probes, each one with an antenna and several optical instruments, and a single powerful emitter on the incoming vessel (This single emitter could itself be a synthetic antenna with a baseline of many kilometres if desired).

[Steel]

14 minutes ago, p1t1o said:

Did a little light reading myself. It seems the frequency, and possibly other factors such as beam shaping and other involved stuff, could have a significant effect. The best example I could find was the Goldstone Solar System Radar (https://en.wikipedia.org/wiki/Goldstone_Solar_System_Radar) and whilst it does have a 70m dish, it operates at only 500kW and has been used to extensively characterise various bodies in the solar system.

So whilst I am sure that one could get a (relatively) sensible sized dish to be of use, the best bet will still be some kind of synthetic aperture using smaller, seperated antenna.

I wonder, in fact, if the best bet might not be to "seed" the system with numerous probes, each one with an antenna and several optical instruments, and a single powerful emitter on the incoming vessel (This single emitter could itself be a synthetic antenna with a baseline of many kilometres if desired).

This is where radar is good, out to around 10AU. Once you go further out you really get bitten hard by the fact that radar range has a fourth-root dependence, which is why there are very few radar studies beyond the orbit of Saturn. Hence radar is probably not ideal for an initial survey, rather a detailed study of bodies that are already reasonably well known (and you are within roughly 10AU of)

Edited October 31, 2016 by Steel

[p1t1o]

1 minute ago, Steel said:

This is where radar is good, out to around 10AU. Once you go further out you really get bitten hard by the fact that radar range has a fourth-root dependence on distance, which is why there are very few radar studies beyond the orbit of Saturn. Hence radar is probably not ideal for an initial survey, rather a detailed study of bodies that are already reasonably well known (and you are within roughly 10AU of)

Good point, looks as if optics might play a more significant role than I had assumed at first.

Looks like you've done the rounds on the forums a few times so you might have already heard of it, but the ubiquitous http://www.projectrho.com/public_html/rocket/ website might be of interest to you - it is specifically aimed at sci-fi writers with a mind to scientific accuracy, and I believe it has a page entirely dedicated to detection and sensors (although possibly tailored more towards ship-to-ship operations).

[Stargate525]

4 hours ago, p1t1o said:

Good point, looks as if optics might play a more significant role than I had assumed at first.

Looks like you've done the rounds on the forums a few times so you might have already heard of it, but the ubiquitous http://www.projectrho.com/public_html/rocket/ website might be of interest to you - it is specifically aimed at sci-fi writers with a mind to scientific accuracy, and I believe it has a page entirely dedicated to detection and sensors (although possibly tailored more towards ship-to-ship operations).

Ooh yes, that site. I know it well. It's about ten times harder than I intend to go (I mean, this hypothetical ship is essentially warping into the system), but I'll definitely take a look at it.

[magnemoe]

Note that even if you have FTL its probably easier to do an general survey from the home system or better systems. A cluster of +50 meter telescopes with sunshades would give decent data about good targets.
Not perfect but an good start, has some fun elements like looking at earth from 100 lightyear with an extraterrestrial planet mapper you see plenty of life, you will probably know is forrests and wherefore advanced life but no indications of civilization so you travel and expect to find an nice uninhabited planet with interesting life. 
 

[Stargate525]

7 hours ago, magnemoe said:

Note that even if you have FTL its probably easier to do an general survey from the home system or better systems. A cluster of +50 meter telescopes with sunshades would give decent data about good targets.
Not perfect but an good start, has some fun elements like looking at earth from 100 lightyear with an extraterrestrial planet mapper you see plenty of life, you will probably know is forrests and wherefore advanced life but no indications of civilization so you travel and expect to find an nice uninhabited planet with interesting life. 
 

That's another question I had, but wasn't quite related to this one.

I know that using an array gives the same resolution as a single dish as big as the two most distant pieces of the array. What I -don't- know is whether there is a practical limit to this. Could you, for instance, have scopes on Mars, Earth, and in orbit around Jupiter, for an effective telescope 11au across? What about linked telescopes in different solar systems entirely?

Edited November 1, 2016 by Stargate525

[magnemoe]

2 hours ago, Stargate525 said:

That's another question I had, but wasn't quite related to this one.

I know that using an array gives the same resolution as a single dish as big as the two most distant pieces of the array. What I -don't- know is whether there is a practical limit to this. Could you, for instance, have scopes on Mars, Earth, and in orbit around Jupiter, for an effective telescope 11au across? What about linked telescopes in different solar systems entirely?

This works for radio telescopes as their wavelength is measured in cm, problem is that you need to know the distance between the telescopes down to an faction of an wavelength. 
For optical you would need an nanometer accuracy, this get pretty tough over interplanetary distances, just getting this accuracy is a bit hard.
Having them some km between should work well however. 

[DerekL1963]

8 hours ago, magnemoe said:

This works for radio telescopes as their wavelength is measured in cm, problem is that you need to know the distance between the telescopes down to an faction of an wavelength. 

You also have to get the data from the two (or more) telescopes involved to the same place to be combined, this can be a significant challenge in it's own right.

[Stargate525]

Aha. Figured there'd be some issue like that. Thanks.

[kerbiloid]

You should be performing a spectral analysis of background radiation for some time, and to find out harmonics corresponding to the typical orbital periods (days-years).
This would give you estimated orbital periods (and so, semiaxes) of the largest planets.
When you find the giants on the predicted distances from the star, you should estimate the possible range of semiaxis values for lesser planets and try again.

P.S.
Btw, if take Pluto orbital frequency as 16 Hz.
Neptune = 16 * 247 / 165 = 24 Hz
Uranus = 16 * 247 / 84 = 47 Hz
Saturn = 16 * 247 / 29 = 136 Hz
Jupiter = 16 * 247 / 12 = 330 Hz
asteroids = 16 * 247 / 4.6 = 860 Hz
Mars = 16 * 247 / 1.9 = 2080 Hz
Earth = 16*247/1 = 3952 Hz
Venus = 16 * 247 / 0.62 = 6374 Hz
Mercury = 16 * 247 / 0.24 = 16466 Hz

Edited November 1, 2016 by kerbiloid

[magnemoe]

1 hour ago, DerekL1963 said:

You also have to get the data from the two (or more) telescopes involved to the same place to be combined, this can be a significant challenge in it's own right.

Data transfer is the easy part however I assume all the data would have to be time stamped with stupid accuracy for merging, for optical imagining I would not be surprised if relativity effects had to be calculated here. 

[DerekL1963]

Just now, magnemoe said:

Data transfer is the easy part

For certain values of "easy", as we're talking considerable amounts of data (up to gigabits per second) that have to shipped from the telescope to the processing center.  "Easy" enough terrestrially, much more difficult when we're talking installations that are multiple AU from the (presumably terrestrial) processing center, and/or which can't be connected to the processing center via hardline.

[magnemoe]

24 minutes ago, DerekL1963 said:

For certain values of "easy", as we're talking considerable amounts of data (up to gigabits per second) that have to shipped from the telescope to the processing center.  "Easy" enough terrestrially, much more difficult when we're talking installations that are multiple AU from the (presumably terrestrial) processing center, and/or which can't be connected to the processing center via hardline.

Still its something we know how to solve technically, comsats has gigabytes data rates already, ramp up antenna sizes on both end and the power and you are done, not its not trivial but mostly an need an money issue. The challenge of making large optical telescopes in space is larger, combining signals from optical telescopes far from each other is unsolved, but technical possible. 
More info about interferometry https://en.wikipedia.org/wiki/Astronomical_interferometer
You still need to gather the light so you still need an serious mirror area. 

[Stargate525]

2 hours ago, DerekL1963 said:

For certain values of "easy", as we're talking considerable amounts of data (up to gigabits per second) that have to shipped from the telescope to the processing center.  "Easy" enough terrestrially, much more difficult when we're talking installations that are multiple AU from the (presumably terrestrial) processing center, and/or which can't be connected to the processing center via hardline.

Why per second? Why can't you nab 24-48 hours of telemetry, ship the hard drives to a central location, then stitch them together after the fact?

[DerekL1963]

32 minutes ago, Stargate525 said:

Why per second? Why can't you nab 24-48 hours of telemetry, ship the hard drives to a central location, then stitch them together after the fact?

You can, and AIUI many projects do when they're dealing with "snapshots" and a small number of observing locations.  It's not a solution that scales well though, the costs mount quickly as does the difficulty and labor of handling all the drives.
 

[Shpaget]

Why would you need tens or hundreds of AUs separation? Significant gains in resolution can be had with just a few hundred meters separation, not that I think even that is needed for observation inside the solar system, especially when the mothership has warp capability. A single decent telescope with a 1 m mirror should be more than enough for detection of anything of interest in the system. Remember, Galileo discovered the moons of Jupiter with a tiny refractor using the lenses he ground and polished at home, looking through the atmosphere.

A good 1 m scope, placed in space, will produce substantially better results.

Once you have the catalog of large objects in the system, the mothership can send a small probe to each planet for a closer look.

[Stargate525]

4 hours ago, Shpaget said:

Why would you need tens or hundreds of AUs separation? Significant gains in resolution can be had with just a few hundred meters separation, not that I think even that is needed for observation inside the solar system, especially when the mothership has warp capability. A single decent telescope with a 1 m mirror should be more than enough for detection of anything of interest in the system. Remember, Galileo discovered the moons of Jupiter with a tiny refractor using the lenses he ground and polished at home, looking through the atmosphere.

A good 1 m scope, placed in space, will produce substantially better results.

Once you have the catalog of large objects in the system, the mothership can send a small probe to each planet for a closer look.

The massive array would be at the home system. Because of how the FTL works, going back isn't an option; you're in the system, and you either spend the time and resources to build back, or you die.

Galileo also knew where Jupiter was in relation to himself. This thing doesn't know where it's sitting in the system.

[radonek]

I would start with observation of central star - it's rotation should give some idea about planetary orbital plane. Dunno what would be best to measure though, maybe doppler of disk edges? I know it ain't 100% reliable but it's a start. Also, since you are probably especially interested in habitable planets, you take measurements of star energy output to establish borders of Goldilocks zone. Anything else you can divine about star can be useful. Young stars may still be in early bombardment period, ones with low metalicity may not have planetary systems at all and so on. Lot of this can be measured from afar, so your crew may have pretty good idea what to expect even before setting foot inside system.

Next step would be deploying one or more probes for paralax observation. Warping them across system or above ecliptic would be handy, but not strictly necessary. If you can't afford a dedicated probe, you will just compare sky images along the way, but this will be way slower  to catch moving objects and less precise at determining orbital parameters, so I guess probe is worth the money. Or, if you have precise star maps, you can construct image of what starfield at target system should look like, and then look for objects that do not belong. Optical/infrared observation will tell at minimum orbital parameters and light curve, which should correspond to rotation period of body. Radar could be then used to gather at least very precise distance and relative velocity info. Once you know some planets, you can start constructing orbital stabilty and resonances models. Obviously, planets are carefully explored for satellites - their orbits will provide good idea of mass. With rudimentary info about size, you can approximate planets density. All of this should give some basic idea what you are dealing with, and note that you don't need fancy equipment like kilometers sized radar arrays. Now that you know where to look and what to look for, its probably good time to make use of any and all technology you have. And obviously, you are scanning for any active sources at any wavelength all the time, in case there is something like Jupiter that shines on its own.

On Monday October 31, 2016 at 4:45 PM, Stargate525 said:

I had this odd notion of firing small probes at cardinal directions off of the main ship, then tracing their trajectory to get a gravitional map of the system  as they're deflected by the various planets. I have a feeling that the deflection you'd have to measure would be so tiny that it's indecipherable from noise, though. Is that correct?

IMO this would work nicely if there was only one unknown planet. Otherwise, you'd need a lot of probes to get some directional information and it still would be hell of a job to separate signals.  And move them pretty fast to sweep whole system. Definitely doable, but I doubt it would be faster then optical observation. Measuring orbital deviation should not be a problem though, you just equip probes with precise time beacon and measure signal delay. This is very accurate, even with current level technology. And you definitely want to observe orbits of everything AFTER you mapped system to make sure you didn't miss something.