Author Topic: Exploring Venus by optical fibre  (Read 882 times)

Online john smith 19

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Exploring Venus by optical fibre
« on: 04/16/2017 10:32 AM »
Venus is a planet we know relatively little about. However it's atmospheric density and temperature are more in line with many of the outer planets and their moons, making it a good test environment for new techniques to study them.

That said its surface is very hostile to mechanical and electronic equipment, with life expectancy measured in hours and days, not years.

My suggestion is therefor to use use more passive sensors to investigate the planet.

The idea would be to drop a package into the atmosphere consisting of a balloon carrying an instrument package that carries a spool of optical fibre with a small weight on the end. The fibre could be winched up and down and used to collect data from its end while the balloons flight would help to supply high altitude wind data, along with any lightning formation.

While the sensor is completely passive the instrument package could use it in a variety of ways. It could be used to see what sunlight gets through to a particular level of the atmosphere, measure emission from species in the atmosphere or emit light to to probe the atmosphere. The end could also be coated to react to various components in the atmosphere in ways that could also be probed with an active light source in the package.

A more unusual approach might be use the fibre as a guide for soundwaves, either induced in it by its flight through the atmosphere or by actively generating them in the package, again to prove the environment.

A more ambitious (but possibly more limiting) technique would be to impress "Fibre Bragg Gratings" onto the fibre at fixed distances along the fibre, allowing parameters to be sampled at several heights simultaneously.


Key issues with this concept are light bandwidth, fibre diameter and breaking strain of the fibre.

So far orbital tethers up to 30Km+ have been deployed in space and the UK has laid an FO cable to Japan through the arctic sea with amplifier spacings of 30-60Km, although it seems the biggest preform (6m long) is good for a single fibre of 15Km . On Venus the atmospheric pressure is Earth Sea Level at about 65Km, however going deeper could carry a larger package, but need a higher pressure balloon. 

Ideally the fibre can hold its own weight and tests indicate Silica fibres hold most of their tensile strength below 500c. If not then it will need to be incorporated in some sort of composite "rope."  An interesting option here would be to wrap the fibre (that can stand very high temperatures) around the structural member to protect it from the reactive atmosphere (I was thinking of Carbon Fibre as the strength member), rather than the other way around of strong sheath around data carrying core.

Note that originally I was going to suggest Sapphire for the fibre. It's got close to double the the melting temperature of Silica and it's even more chemically inert (the sensor windows on the Voyager Venus mission were made of it) but I can find no indication anyone has managed to produce a single piece over 2m long.

While a cable of sensors and 2m fibre sections is possible my instinct is the huge number of splices (each one leaking just a little more light) would demand either a very powerful light source or absorb any passively detected signal before it got back to the instrument package to be detected.
« Last Edit: 04/16/2017 10:34 AM by john smith 19 »
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline mikelepage

Re: Exploring Venus by optical fibre
« Reply #1 on: 04/17/2017 04:55 AM »
Heh.  I was going predict that the cable would be torn apart by vertical wind shear.  But this paper on the subject makes it look relatively benign.

EDIT: Didn't realise I was signed into my Uni library.  Have relinked to abstract.
« Last Edit: 04/17/2017 09:26 AM by mikelepage »

Offline Rei

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Re: Exploring Venus by optical fibre
« Reply #2 on: 04/17/2017 08:57 AM »
It's an interesting idea, but the TRL for doing such a thing is far lower than that of just landing on Venus  ;)  Aka, 1) short lifespan probes, or 2) bellows or phase-change balloon probes that can rise back up, or 3)  high-temperature-operation probes; all are more mature than dangling 30km fibre optic cables on Venus, and give you a lot broader range of things you can do rather than just use it as a nephelometer / photometer. Concerning the fiber withstanding those temperatures (not just material properties, but optical  properties as well, including run length between repeaters), do you have a reference? Just from a quick search:

http://www.electronics.dit.ie/staff/tfreir/ft220/ME%201.7%20Bending%20Loss%20&%20Fibre%20Stress%20and%20Reliability%20web%20version.pdf
Quote
Effect of Temperature

At 90 degrees centigrade the fatigue susceptibility parameter is significantly higher than that at 25 degrees

Fibre strength decreases by 25% at 90 degrees compared to 25 degrees

If that's happening at 90C, then what's happening at 450-500C?  Googling for "heat resistant" silica optical fibers I find things like:

http://global-sei.com/technology/tr/bn74/pdf/74-10.pdf

Apparently "high temperature" is considered to be 200C - a far stretch from Venus surface conditions. You're presenting a rather large surface area to mass ratio, too, so you're amplifying any corrosion issues.

As Mike Lepage pointed out, there is shear, although not as much as at the cloud level (I'm not sure what paper he's referring to as it loads to a dead link to me and there's not enough info in the URL to figure it out), so comparisons to space tethers and tethers laid out on the bottom of the ocean aren't really apt.  At least there's (probably) little turbulence in the lower atmosphere. I admittedly don't follow the lower atmosphere as much as the the cloud decks (mainly the middle cloud), but if I recall correctly the layering goes:

0-~10km: possibly somewhat convective
~10-~20km: fairly stable
~20-~30km: convective
~30-~48.5km: quite stable

... and then you're well in the cloud layers, which are convective to varying degrees. The middle cloud layer for example is surprisingly reminiscent of Earth's troposphere, with 20-30km convection cells (average size on Earth = 24km). The atmosphere again becomes quite stable above ~60km.

Still, good to think outside the box  :)
« Last Edit: 04/17/2017 03:03 PM by Rei »

Online john smith 19

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Re: Exploring Venus by optical fibre
« Reply #3 on: 04/17/2017 05:04 PM »
It's an interesting idea, but the TRL for doing such a thing is far lower than that of just landing on Venus  ;)  Aka, 1) short lifespan probes, or 2) bellows or phase-change balloon probes that can rise back up, or 3)  high-temperature-operation probes; all are more mature than dangling 30km fibre optic cables on Venus, and give you a lot broader range of things you can do rather than just use it as a nephelometer / photometer. Concerning the fiber withstanding those temperatures (not just material properties, but optical  properties as well, including run length between repeaters), do you have a reference?
Just from a quick search:

https://pure.strath.ac.uk/portal/files/18200068/Mechanical_strength_of_silica_fiber_splices_after_exposure_to_extreme_temperatures.pdf

was the reference I found which suggested things would be reasonable below 500c.

This also looks interesting
http://suboptic.org/wp-content/uploads/fromkevin/program/TU1A.4%20Optical%20Fibre%20Fatigue%20And%20Submarine%20Networks%20Reliability%20-%20Why%20So%20Good.pdf

As does this
http://www.escm.eu.org/docs/eccm13/posters/2619.pdf

I was looking for data on the breaking strain of a Silica fibre to see what length a fibre could carry before snapping under its own weight.

The data on repeater distances came from a 2012 on a planned FO cable between UK and Japan reported in Popular Mechanics. Roughly 30-32 Km between repeaters, although with minimal cable stress as it was lying on the sea bed.
Quote from: Rei
http://www.electronics.dit.ie/staff/tfreir/ft220/ME%201.7%20Bending%20Loss%20&%20Fibre%20Stress%20and%20Reliability%20web%20version.pdf
Quote
Effect of Temperature

At 90 degrees centigrade the fatigue susceptibility parameter is significantly higher than that at 25 degrees

Fibre strength decreases by 25% at 90 degrees compared to 25 degrees

If that's happening at 90C, then what's happening at 450-500C?  Googling for "heat resistant" silica optical fibers I find things like:

http://global-sei.com/technology/tr/bn74/pdf/74-10.pdf

Apparently "high temperature" is considered to be 200C - a far stretch from Venus surface conditions. You're presenting a rather large surface area to mass ratio, too, so you're amplifying any corrosion issues.

As Mike Lepage pointed out, there is shear, although not as much as at the cloud level (I'm not sure what paper he's referring to as it loads to a dead link to me and there's not enough info in the URL to figure it out), so comparisons to space tethers and tethers laid out on the bottom of the ocean aren't really apt.  At least there's (probably) little turbulence in the lower atmosphere. I admittedly don't follow the lower atmosphere as much as the the cloud decks (mainly the middle cloud), but if I recall correctly the layering goes:

0-~10km: possibly somewhat convective
~10-~20km: fairly stable
~20-~30km: convective
~30-~48.5km: quite stable

... and then you're well in the cloud layers, which are convective to varying degrees. The middle cloud layer for example is surprisingly reminiscent of Earth's troposphere, with 20-30km convection cells (average size on Earth = 24km). The atmosphere again becomes quite stable above ~60km.

Still, good to think outside the box  :)
Keep in mind that atmospheric drag is the largest force on a satellite around the Earth below 1000Km and the space tether that the YES2 project tested for ESA was much more like a bare optical fibre than an electric cable.

As I noted the more ambitious approach would be to imprint multiple Bragg Gratings into the fibre to deliver multiple parameter readings. These people are known for this technology

https://www.smartfibres.com/

but I'm sure there are others.

Outside airless moons, Mercury and Mars most bodies in the solar system have pretty active atmospheres with high pressures, high temperatures or both. This looked like an approach that could supply continuous data quite deep into the atmospheres of various bodies, rather than being confined to short dives deep into the atmosphere and longish recovery periods.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline Rei

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Re: Exploring Venus by optical fibre
« Reply #4 on: 04/18/2017 12:05 AM »
https://pure.strath.ac.uk/portal/files/18200068/Mechanical_strength_of_silica_fiber_splices_after_exposure_to_extreme_temperatures.pdf

was the reference I found which suggested things would be reasonable below 500c.

500C for around a second.  They show a ~20 second heating to 500C (in Earth air), followed by passive cooling - down to under 200C by 100 seconds. And no optical testing while hot, only after cooling. Not exactly the data you need  ;)

Quote
This also looks interesting
http://suboptic.org/wp-content/uploads/fromkevin/program/TU1A.4%20Optical%20Fibre%20Fatigue%20And%20Submarine%20Networks%20Reliability%20-%20Why%20So%20Good.pdf

As does this
http://www.escm.eu.org/docs/eccm13/posters/2619.pdf

I was looking for data on the breaking strain of a Silica fibre to see what length a fibre could carry before snapping under its own weight.

That's not really the problem; you can always reinforce them with CF. The problem is longevity, whether they can operate at all at those temperatures, how they're affected by Venus's atmospheric conditions (turbulence, lightning, broad convenction patterns), etc.

We're far closer to long-term heat-stable probes than that. As well as oscillating probes. Multidropsondes are easy and have been done. And when you're only probing "things that light can probe" (like, as mentioned, acting as a nephelometer / photometer), you'd be doing a lot better science just observing from above rather than messing with a tether.

Quote
The data on repeater distances came from a 2012 on a planned FO cable between UK and Japan reported in Popular Mechanics.

That says nothing about how the fibre would behave at Venus temperatures or under great strain.

Quote
Keep in mind that atmospheric drag is the largest force on a satellite around the Earth below 1000Km and the space tether that the YES2 project tested for ESA was much more like a bare optical fibre than an electric cable.

Atmospheric drag on a satellite isn't even remotely like the wind forces experienced on a 30km cable dangling in the atmosphere  ;)

Quote
This looked like an approach that could supply continuous data quite deep into the atmospheres of various bodies, rather than being confined to short dives deep into the atmosphere and longish recovery periods.

Might want to reframe it as a way to probe gas and ice giants.  They're much colder at any given pressure level.  You can reach some pretty crazy pressures before heat becomes unbearable.  Still high risk, but I'd think it a more defensible concept  :)