Could you use the Moon to reflect sunlight onto a solar sail?

[justcurious33]

TL;DR Summary

I am not knowledgeable on physics but I am very interested in learning more about propulsion and properties of light.

I dont know much and you can dumb it down as much as you like or not. I've recently gotten very curious about light propulsion and i was reading that if a laser or beam of light was directed at a solar sail it could generate mass amounts of energy and be used to travel to deep(er) space. I was wondering if the reflective properties of a planetary body such as the moon could be used to reflect the light into a solar sail and what would it take to focus that light into the sail if possible.

Edit (I read more...) - The NASA Lunar FlashLight but pointing it at spacecraft can someone break down how that would work/look.

 

[Ibix]

Planetary bodies don't reflect much light, so you can't get much thrust from them.

Just look at the moon. Its size on the sky is the same as the Sun's, but the sky is much less bright at night than during the day. When you can see the moon during the day parts of it look blue - the brightness of the moon is similar to the tiny fraction of sunlight that is scattered by the atmosphere.

What you could, perhaps, do is build solar cells on an airless body and use them to drive lasers that would let you run solar sails in the outer reaches of the solar system. That's certainly been used in SF, although I don't know if it's been specced out by proper engineers. It's also worth noting that such devices are potentially impressively powerful weapons at relatively short ranges, so there are sociological issues as well as physics ones.

 

[justcurious33]

Not the reflection of the moon's light, rather i meant the sunlight that is bent around the moon in space.
I'm thinking of it as if a mirror in space is positioned as if the earth is during a solar eclipse and that is directed toward the lightcraft. Again I know nothing, thank you for the knowledge.

 

[Ibix]

Not the reflection of the moon's light, rather i meant the sunlight that is bent around the moon in space.
I'm thinking of it as if a mirror in space is positioned as if the earth is during a solar eclipse and that is directed toward the lightcraft. Again I know nothing, thank you for the knowledge.

I don't understand what you're getting at. Can you sketch what you have in mind? You can add images to posts with the "Attach files" button below the reply box.

 

[justcurious33]

Sorry im not good at illustration

 

[renormalize]

Not the reflection of the moon's light, rather i meant the sunlight that is bent around the moon in space.

What makes you think that sunlight "is bent around the moon in space"? The moon lacks sufficient atmosphere and gravitational pull to bend light to any noticeable degree. Instead, it simply casts a shadow.

 

[justcurious33]

Gravitational lensing and general relativity as i understand it says that all objects with mass bend and warp space and light around them. And so i thought about how that works with the moon and if using something as large as a planet to reflect such a light source would amplify or even simply make it easier to direct towards a lightcraft

 

[Janus]

Gravitational lensing and general relativity as i understand it says that all objects with mass bend and warp space and light around them. And so i thought about how that works with the moon and if using something as large as a planet to reflect such a light source would amplify or even simply make it easier to direct towards a lightcraft

It takes a huge mass to bend light by any noticeable amount. For example, a ray of light passing just over the surface of the Sun is only deflected by 1.7 seconds of an arc. (1 sec = 1/3600 of a degree). This means that the "focus point" where light passing near the Sun would converge is some 14 times the distance Pluto is from the Sun. Now keep in mind that the Sun is some 27 million times as massive as the Moon.

 

[Ibix]

Beaten to it, I see, but there's basically no gravitational lensing around the Moon or any planet. They're just way too small and light to do any significant lensing.

 

[Vanadium 50]

There is some irony posting this today. If things worked as you say, there would be no eclipse tomorrow.

 

[justcurious33]

Thank you all. I understand its probably a simple enough answer but im new to this and want to learn more.

 

[Nik_2213]

Um, unless you have a dielectric (?) sail tuned to a specific wavelength, be it RF, IR or visible, so need matched maser / laser, you'd probably do better at first to just reflect raw sun-light. No need to convert to eg AC then to coherent...

Given the remote but non-trivial danger from solar mirrors or lasers on 'our' side of Moon being hacked, weaponised and aimed Earth-wards, they're much safer on the Farside.

IIRC, this was explored by eg Larry Niven in his Kzinti Wars tales, where solar-sailing cargo pods were pushed around Solar System by launching lasers. Of course, with the 'safeties' off, they cooked Kzinti...

 

[justcurious33]

would there be a way to "catch" or harness it if it was aimed earth-ward on purpose

 

[jbriggs444]

would there be a way to "catch" or harness it if it was aimed earth-ward on purpose

You would be layering impracticality on top of impracicality. But let us set that to one side and consider a particular (impractical) proposal.

We will erect a parabolic mirror with a cross-sectional area equal to that of the moon. That is about 3500 kilometers across. Or 1750 km in radius. This is impractical because a structure that large would collapse even under the weak gravity of the moon. But we are ignoring such practical concerns.

The sun delivers light energy at a rate of about 1380 watts per square meter. Our mirror will reflect light with a total of$$P = 1380 \times \pi r^2 \approx 13 \text{ quadrillion watts}$$By contrast, total world power generation is about 3 terawatts.

Great news, right?!

But there may be a problem. The sun is a big light source. We cannot focus it down to a point. The size of the sun's focused image will be related to the sun's actual size. The two will be in a ratio corresponding to the distance from moon to earth compared to the distance from the moon to the sun. The sun is about 150 million km away. The moon is a bit about 384 thousand km away. That ratio is about 390 to one. Meanwhile, the sun has a diameter of 1.4 million km. If we divide that by our 390 to one ratio, we get an image size of about 3600 km.

By no coincidence, this is the same as the diameter of the moon. As we already knew, the moon and the sun have the same angular size when viewed from Earth!

What does this mean for the image intensity? We took the light from the sun striking the moon. Now it is striking a moon-sized region on the Earth instead. The effect is double intensity sunlight over that region.

This is great if you want to grow a field of popcorn fast. Don't even need to put it in a popper! It is not so good if you are trying to propel a space craft. You double the propulsion that you can get from a [photon driven] solar sail. But that is still not much thrust.

The thrust that you get from sunlight is given by $$E=pc$$Here $E$ is the energy of the sunlight. $p$ is the momentum imparted and $c$ is the speed of light.

Suppose that we double the sun's illumination from 1380 watts per square meter to 2760 watts per square meter. We set up a solar sail that is a square 1000 meters by 1000 meters. Now in one second we will have received 2.76 billion Joules.

This is about twice the capacity of the flux capacitor on a Delorean (1.21 gigawatts).

If we divide this by the speed of light ($3 \times 10^8$ meters/second). We will have about 9.2 newton-seconds of momentum. Or a continuous thrust of 9.2 newtons. Versus 4.6 newtons without the moon-sized mirror dedicated to accelerating this craft. [Figures updated after a sanity check]

Economics suggest that it is cheaper to make the sails 1.4 times bigger rather than erecting a mirror on the moon. [You can also buy yourself a factor of two by reflecting instead of simply absorbing].

If I have not dropped too many decimals anyway.

Edit: Cross checking propulsion numbers with the Wiki article on solar sails: "F = 4.54 μN per square metre". Multiply by one million square meters and we have 4.54 N. Versus my original figure of 460. Clearly I slipped a couple digits... Yep. There it is. I had fumbled when I divided by the speed of light.

 

[Nik_2213]

Have you over-looked factor due to modular planar reflectors being aimed at same target ??

 

[sophiecentaur]

Um, unless you have a dielectric (?) sail tuned to a specific wavelength, be it RF, IR or visible, so need matched maser / laser, you'd probably do better at first to just reflect raw sun-light. No need to convert to eg AC then to coherent...

A good reflector (shiny) will achieve more momentum change per second than any other surface with frequency selective properties.

 

[jbriggs444]

Have you over-looked factor due to modular planar reflectors being aimed at same target ??

So you propose replacing the mirrored parabolic surface with an array of panels constructed from wire grids?

"Planar reflector" is a term is normally used for a wire grid implementation that works for radio frequencies. How feasible is this for the frequency band where the bulk of the sun's energy is transmitted? Green light. What wire diameter and spacing do you assume?

By how much does the reflectance of such a panel exceed that of, for instance, a bathroom mirror?

Since the ideal reflector that I calculated with is 100% efficient, how will you improve upon it with a physical implementation?

 

[Nik_2213]

{Cough...}
Your same ideal reflector material, but arranged as individual steered panels, removing off-parabolic sagging and other mega-structure issues...
Think SKA rather than Jodrell Bank or Gold-stone Dishes...

 

[jbriggs444]

{Cough...}
Your same ideal reflector material, but arranged as individual steered panels, removing off-parabolic sagging and other mega-structure issues...
Think SKA rather than Jodrell Bank or Gold-stone Dishes...

Right. So no improvement to performance. Only a reduction in impracticality.

 

[sophiecentaur]

Right. So no improvement to performance. Only a reduction in impracticality.

Using a number of modular reflectors could probably give more directional control (the wsystem would involve real sailing, using gravity ane radiation rather than aiming away from the central radiator. There would be a need for some redundancy if there's mechanical failure. Any such sail system would need to be vast to the extent that its collision cross section with space debris and particles would be much more relevant than with present space craft designs.

 

[ShadowKraz]

Um, unless you have a dielectric (?) sail tuned to a specific wavelength, be it RF, IR or visible, so need matched maser / laser, you'd probably do better at first to just reflect raw sun-light. No need to convert to eg AC then to coherent...

Given the remote but non-trivial danger from solar mirrors or lasers on 'our' side of Moon being hacked, weaponised and aimed Earth-wards, they're much safer on the Farside.

IIRC, this was explored by eg Larry Niven in his Kzinti Wars tales, where solar-sailing cargo pods were pushed around Solar System by launching lasers. Of course, with the 'safeties' off, they cooked Kzinti...

Beat me to it. Fan of Niven's work. Presented quite the surprise for the Kzinti as their telepaths reported a totally peaceful society...

 

[ShadowKraz]

Using a number of modular reflectors could probably give more directional control (the wsystem would involve real sailing, using gravity ane radiation rather than aiming away from the central radiator. There would be a need for some redundancy if there's mechanical failure. Any such sail system would need to be vast to the extent that its collision cross section with space debris and particles would be much more relevant than with present space craft designs.

I don't know how I missed this comment earlier... Arthur C. Clarke wrote a short story, Sunjammer, based around the idea of racing light-sail spacecraft much like the America's Cup races. Even included some suggestions on the designs of the various light-sails used, using your light-sail to block another ship's light-sail, etc. Well worth a read.