Argthtjtr

There was just a question (actually, on Astronomy) about why the data rate from New Horizons is low.

Of course, even with the most directional radio antennas, the spread is enormous.

I guess conceptually, some sort of laser signaling system would have tremendously less spread. (Or zero spread? I don't know).

Since the Earth has crap seeing, I propose

• On New Horizons, some sort of laser modem




• On our moon, some sort of laser modem receiving base

What are the numbers here? If (1) New Horizons has a power available of 1 NHPU, how many NHPU would be needed on board for my scheme? Is it overwhelmingly too much, or just "a bit more"? (For that matter, is it much less? Like, "a really good LED" or some such? After all, directional is a fantastic energy saving in the abstract.)

How complex would Moon Base Laser need to be? What I mean is, (2) would it be "surprisingly small" rather than the huge sizes needed with radio antennas. Would it basically be ........... a telescope?? With a $200 nikon camera body glued on, or is "a laser receiver" different somehow than "a telescope with a CCD"?)

So, would Moon Base Laser be a natty device (I'm thinking, say, the size of a car) which we could easily throw on to the moon with current systems, or, would it more be "a large construction". i.e. somewhat like current large Earth telescopes??

I know we already have (tricky) devices which receive lasers bounced off a mirror on the moon ... would one of those be ready-made to do the job, or ...?

With our current awesome radio dishes, they need to get (much) bigger as the spacecraft gets further ... would (3) this be the case with Moon Base Laser or would the issue not be so bad? Would reception / data rate be pretty much the same as the craft moved through our solar system ("since lasers are directional!") or is that all wrong?

Has anyone proposed, or indeed do we (4) already use, laser-style communications in space?

Are there any other narrow-paradigm communication concepts I don't know about, other than "laser"? Which we could use instead of good old dispersing radio??

(I guess proposed multi-craft fleets like LISA indeed do this, right?)

Summary,

1. how many NHPUs of power would be needed on board for this scheme? what's the order of magnitude?




2. what size paradigm would Moon Base Receiver be? suitcase-sized? town-sized?




3. would Moon Base Receiver have to increase in size dramatically w/ distances in the solar system (as do radio dishes)




4. do we do this already?

PS I'm just assuming this would be worthless from Earth, so, I'm just assuming a Moon receiver (or I guess an orbiting receiver) is a must.

Action update! : the best current answer seems to be "it would offer 100kb/s rather than 1kb/s (using the same power, 12W)"

It absolutely could happen, but it would require a more precise pointing than New Horizons has. Lasers of some kind are the best for the high data resolution. The spacecraft to most heavily use lasers in communication is the Lunar Reconnaissance Orbiter. It has also long been talked about as a goal for a Mars communication satellite, which would allow for much more data back from Mars. The problem is the pointing requirements are pretty extreme, you even have to know which site on Earth you are going to target, the laser beam will not cover the entire planet Earth from Mars. For example, MRO has a pointing accuracy requirement of 0.0032 mrad. Pointing requirements for a laser system are in fact similar to this requirement, however, they require stability for much more time. HiRISE only requires it for a few milliseconds, while a laser communication system requires it essentially indefinitely.

In the case of New Horizons, it just isn't needed. Yes, it will take a long time to get the data back, but that isn't a problem, there's lots of time to wait.

As a reference, laser power systems generally require less power then radio based systems, because they broadcast power more directly and thus waste less power.

Let's try and do some numbers. We will need to make a few assumptions.I'm going to choose ones which make the calculations easy, varying might produce variation of a factor of 10 or 100 in the answer.

1. A near IR laser with a wavelength of $1 mu m$
   


2. NH transmitting using the LORRI telescope with an aperture of about
   20cm. We know NH can point accurately enough to keep this telescope on target.


3. A detector in Earth orbit (easier than on the Moon, I think)
   identical to the JWST with a 6m mirror.


4. A goal of 10 photons per bit hitting the detector to be reasonably
   sure of picking the signal from the noise.

So now we get a beam width of $1 mu m/ 20cm = 5 mu Rad$

At six billion km this is a beam width near Earth of $30 000 km$.

So our detector picks up $left(6m/30000kmright)^2 = 4times 10^{-14}$ of the beam.

So we need transmit about $2.5 times 10^{14}$ photons per bit.

At this wavelength, one photon has about $2times 10^{-19}$ Joules of energy, so we must transmit $5times 10^{-5}$ Joules per bit of laser light.

So now we have a tradeoff of power against data rate. Using the same power (12W) as the current radio transmitter, we could manage about $240 kb/s$ assuming a perfectly efficient laser, probably 10 to 50 % of that in practice. Using 100% of the power from the RTG (190W), we might get 3 Mb/s or so and so on.

Steve Linton's answer is excellent, although possibly a bit conservative. Information has been transmitted in the lab via laser at a rate of 1 bit per photon. For proposed uses, Error Detection and Correction codes are definitely indicated.

Has anyone proposed, or indeed do we (4) already use, laser-style
communications in space?

Yes to both, although just barely. In 2013, the Lunar Laser Communication Demonstration successfully operated in a LADEE mission. Lead times for exploiting technology in deep space missions are very long, at least a decade, so it will be a while before actual missions using the technology are launched. Nonetheless, astronomers are salivating at the thought of high-resolution, high frame rate imagery.

This is closely related to the concept of Antenna gain, which for radio transmission measures how narrow a beam the antenna can focus. The narrower the beam, the more accurately you need to point it.

However, any laser or radio beam will still be divergent - it will spread out the further away it goes. And it will spread out at the same rate, with the beam diameter doubling every time the distance doubles. This leads to area of the beam increasing by the square of the distance, giving the Inverse-square law.

A narrower beam only improves the initial signal power - e.g. 10x smaller beam area will give 10x transmission power. When you get to the Kuiper belt, signal will be only 1/400 000 000th as strong as it would be in lunar orbit. A narrow-beam improvement of 10x or even 100x pales in comparison to that.

Or to put it more simply: a narrow beam is faster, but when you go that far, it will be slow anyway.