Communication | Sending images with laser using sail as antenna

Images of the target planet could be transmitted by a 1Watt laser onboard the nanocraft, in a ‘burst mode’ which uses the energy storage unit to rapidly draw power for the power-intensive laser communications mode. Upon approach to the target, the sail would be used to focus the laser communication signal.

For a 4m sail, for example, the diffraction limit spot size on Earth would be on order of 1000m. A kilometer-scale receiving array would intercept 10-14 of the transmitted signal. The main challenge is to use the sail as diffraction limited optics for the laser communication system. This would be achieved by shaping the sail into a ‘Fresnel lens’ upon approach to the target. The sail structure could be different at the launch and communication phases. In order to maintain a high transmission through the Earth’s atmosphere, the communication would need to operate at a wavelength shorter than that used by the launch laser system, due to the Doppler shift of the nanocraft relative to the Earth.

Apr 12, 2016 23:58 Adam Wabeke Posted on: Breakthrough Initiatives

In watching the broadcast, I saw that the plan was to launch a series of these devices toward Alpha Centauri. Wouldn't it be possible to transmit the data from the first launched device to the devices "behind" it? Instead of having to transmit the data all the way to Earth itself, it only has to transmit data as far as the next Starchip closest to itself that is closer to Earth, creating a "mesh network" all the way back home. Not sure if that's more efficient or easier.

Apr 13, 2016 00:49 Marcus Wright Posted on: Breakthrough Initiatives

Thats exactly what i was thinking Adam and you beat me to it!! The main cost will be setting up the ground based lasers and receiving network, the Starchips themselves will be a tiny percent of the cost. Once everything else is set up thousands could be sent in a constant stream for years.

Apr 13, 2016 04:02 jim papadopoulos Posted on: Breakthrough Initiatives

I wonder about using a kerr cell retroreflector -- a corner reflector that sends a beam back toward source (Earth), with modulated reflectivity. So an earth beam would be reflected back with some encoding. The power would largely come from earth, only reflection modulation would be required of the explore-bots. Might you change plane of polarization, or simply alter reflectivity, or?? Makes me wonder if the 'sails' should be precise retro reflectors themselves -- pretty useful tracking system.

Apr 13, 2016 05:24 Karen Pease Posted on: Breakthrough Initiatives

Jim, I was thinking of something similar... but simpler.

1. You don't need a corner cube reflector if you can point. So just keep it flat (aka, spin) and keep good pointing accuracy. Making a corner cube reflector when your sail is only a couple hundred atoms thick... let's just say I have my doubts about the feasibility of that concept ;)

2. Rather than trying to modulate reflectivity, there's a simpler approach: a piezoelectric vibrator in the craft.

If mirror is effective at damping vibrations, what will happen is whenever the piezoelectric vibrator is on, you'll create tiny pointing errors in the mirror, greatly reducing the reflected light. When it's off, the vibrations will damp out and the light reflection will be restored.

Even if the mirror can't damp vibrations passively, it's probably possible to actively damp them by cancelling out the waveform.

You've got a 100GW laser on Earth, based on the project proposal... you might as well use it ;) If we go with that point source -> 1000m spot size number stated above, then a 100GW laser would reflect off of a 4m radius sail with a power of 1,6MW, or a 4m diameter sail with 400kW. That's *way* more power than transmitting with a 1W laser onboad the craft!

Apr 13, 2016 07:16 Anthony Horton Posted on: Breakthrough Initiatives

Small typo there, for a 4 metre diameter sail the diffraction limited spot size at Earth would be 10,000,000,000 m, not 1000 m as written above. The 10^-14 of the signal being collected by a kilometre sized receiver is right, though.

For the 1 W transmitter described that's no more than 10^-14 W power, about 20,000 photons/second. If there are no other sources of noise that could support maybe a few 100 bits/second downlink which sounds OK. Then again this thing will be transmitting from very near (relatively speaking) to a pair of stars, you're going to need *very* good suppression of that starlight for this to work.

Another thing to bear in mind is that biggest phased optical receivers we've built so far (astronomical telescopes) are of order 10 metres in diameter (or total area equivalent) with the largest currently planned being under 40 metres. 1 km scale is a big leap.

Edit: Ran some more numbers and the star light doesn't seem to be a total killer. Assuming a 10 pm width filter the total light from Alpha Cen A + B falling on the receiver would be about 3.5 x 10^-7 W. That gives the spacecraft signal a contrast ratio of 3.5 x 10^7 at, presumably, a few AU projected separation which isn't too far beyond the current state of the art.

Of course you'd still need to work out how to build a kilometer wide optical phased array, and find a way to keep out rampaging hordes of astronomers and planetary scientists (that thing would be orders of magnitude better than any optical telescopes likely to exist within a generation).

Apr 13, 2016 09:24 Posted on: Breakthrough Initiatives

@Adam and @Marcus - ditto. Never mind a thousand Starchips - a million starchips launched across the years would form a transmission RELAY. Someone could think up an apt phrase using RELAY as an acronym.

Apr 13, 2016 13:08 Colin Wilson Posted on: Breakthrough Initiatives

@Karen Pease - It's a good idea to modulate the reflectivity of the sail, to permit laser communications, but there may be simpler way to do this, already demonstrated in space.

The IKAROS Japanese solar sail mission, which flew past Venus, demonstrated a "Reflectivity Control Device (RCD)", these are areas of liquid crystal which can be switched from diffuse to specular reflection depending on applied voltage. This was used for attitude control (spacecraft pointing), but could conceivably also be used for modulation of retroreflected laser beam, allowing encoding of data. See for example

One would have to work out what percentage of the solar sail is covered in this material. Also, this only works when the plane of the sail is normal to the earth-spacecraft direction, which may not be convenient if trying to steer the spacecraft.

In addition to this, there's a real problem: picking up the incredibly weak signal from a retroreflected laser signal may be conceivable when the spacecraft is in deep space with a dark background - but when it approaches its target star then its signal will be completely swamped by light from the star.

In the medium term it would be challenging enough to send these nanocraft around our own solar system, especially to the outer solar system. That would be a great starting point to test some of these technologies before we can consider sending them further.

Apr 13, 2016 13:22 Karen Pease Posted on: Breakthrough Initiatives

@Colin Wilson:

"The IKAROS Japanese solar sail mission, which flew past Venus, demonstrated a "Reflectivity Control Device (RCD)", these are areas of liquid crystal which can be switched from diffuse to specular reflection depending on applied voltage."

I considered that, but when you're talking about a sail this lightweight, you don't have mass for that (ignoring all issues of how well liquid crystals would survive the initial heating phase). Creating tiny vibrations in the sail is something you can do with a point source in the center rather than needing a coating (and method to activate it) across the whole sail. The scale of the vibrations needed to shift the phase or throw off the pointing accuracy are really, really tiny.

As for being swamped, laser beams are coherent light of a single frequency; stars are nearly blackbodies. So you're only competing with the star on an extremely narrow frequency band. And you can actually tune that band by carefully choosing your spacecraft's speed - after all, this is a relativistic craft we're talking about, it's going to redshift the beam. :)

Apr 13, 2016 16:00 Karen Pease Posted on: Breakthrough Initiatives

Hmm, noting Anthony Horton's comment about the typo, that changes the picture. We're back to it being more efficient to have a laser on the craft itself, then.

So I assume we'd use a nuclear decay-pumped laser, then?

I don't know what sort of power densities you could get.. but surely direct nuclear pumping is far more efficient (from both a mass and energy perspective) than generating electricity, storing it, and then driving a laser with that. The waste heat would obviously be used for thermoelectric power production, with at least part of the sail as the radiator.

Apr 14, 2016 08:04 Posted on: Breakthrough Initiatives

Couldn't the power for the onboard laser used to beam back images be generated by using photovoltaic material for the non-reflective surface of the sail (given the spacecraft's proximity to Alpha Cen A) to save weight on the amount of nuclear fuel required?

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