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Comment by superposeur | Hacker News Reader

The overwhelming loss in this calculation is from the antenna’s radiated energy spreading out over a larger and larger area (despite the directional “gain” factor).

I’m wondering: would a probe launched today instead employ a laser to communicate? This would seem to offer many orders of magnitude improvement in the directionality of the signal.

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The main challenge is the earth to probe comms for distant probes, since the earth is often very close (in an angular sense) to the sun from the probes perspective, and the sun gives out a lot of black body radiation.

However, due to the shape of the black body radiation curve, the sun gives out relatively less microwave radiation than it does visible light, which might outweigh the advantages of more directionality given by using a laser.

Further, we're good at building really big radio transceivers here on Earth, we don't have nearly the same technical experience with lasers of that scale.

Ok what about using a maser instead of a laser?

The big dish antennas do use ruby masers, but not to transmit. The maser is used as the LNA on the receive side. Check out the picture on page 41 of the pdf, clearly this a flux capacitor, mislabeled to deceive us ;)

https://descanso.jpl.nasa.gov/monograph/series10/03_Reid_cha...

https://www.rfcafe.com/references/popular-electronics/amazin...

Cool use of maser for receive.

Not having thought this through before, I see now that while a transmit maser may have efficiency advantages, it may not improve directionality relative to a standard parabolic radio transmitter. All methods of producing microwaves will have basically the same diffraction-limited gain for a given “aperture” (dish) size. That darn uncertainty principle! (However, an optical laser would still give way better directionality.)

Presumably though it would be useful to have a high bandwidth link back to earth even if we had to use conventional microwave transmitters to send data back.

We want to download high resolution images/spectrographs whereas we only want to upload code/instructions.

There's several projects for laser-based communication and research. It would also make it really difficult to aim since you can miss your target now.

https://www.jpl.nasa.gov/news/nasas-deep-space-optical-comm-...

https://en.m.wikipedia.org/wiki/Laser_Interferometer_Space_A...

Interesting about the JPL program and I’m amazed this prototype was only launched last year! Apparently the answer to comms laser use is “not yet but soon”.

Actually the answer is 'right now'. The Psyche spacecraft is actively using a laser to provide data on it's way to the Psyche asteroid.

Its certainly exiting that we'll have laser based communication in the near future. its like all ingredients for interplanetary missions are slowly coming together. all we're missing now is a very high specific impulse engine that could push the several tons for human habitation to another planet.

All space agencies have optical comms in their road maps. Largely they are thinking about inter satellite communications (the atmosphere causes significant issues when going back to earth). So the main application is to have some relay satellite that can then transmit to earth via RF. The application is not mainly deep space ropes but Leo or meo satellites, the typically only have very short transit times over the ground stations, so can't get all their measurement data down. By using e.g. a geo relay they can transmit lots of data optically and the geo relay can more slowly transmit the data to earth until the leo satellite comes back in view.

I'm curious about the feasibility of combining the two problems of propulsion alway from Earth and communication with Earth into beam-powered propulsion aimed directly at Earth, pulsed for use as communication.

Probably infeasible for several reasons (only useful when accelerating DIRECTLY away from Earth, incoming light to power spaceship is probably coming from the sun and therefore likely also in the directly of Earth, so net zero acceleration at best from firing the photons back towards the sun), but it'd be pretty neat.

Not an expert but I imagine it just doesn't matter much. It's able to do incredible things with just 24 watts of power.

I imagine it'd certainly employ some type of beamforming at the least.

Assuming you don't need fast steering, is a 3.7m transmitter array doing beamforming really better than a 3.7m dish transmitting at the same power?

My intuition would have been that you are better off using a fairly standard transceiver and spending your engineering budget either increasing power or getting a bigger dish (either by launching on a wider rocket or with a folding design).

Lasers might interesting for the downlink, but receiving a laser signal on the probe sounds difficult (earth is pretty bright).

Diffraction scales inversely proportional to wavelength so you gain significantly by going to optics, i.e. you can use a much smaller aperature in optics.

There's some value in getting rid of mechanical devices (or reducing the need to rotate the entire spacecraft).

The array doing beamforming can be spread out much farther. If the DSN were being built now, I'd think its antennas would look more like the Square Kilometer Array.

Seems they're already on to something like that[1][2]:

We envision deployment in three sites at or near the longitudes of the existing DSN sites.

As an example, a potential initial phase could deploy 40 12-m elements at each complex to duplicate the X-band performance of a DSN 70-m antenna.

Second and third phase deployments may bring the number of 12-m antennas to 200, then perhaps 400 per complex.

[1]: https://ieeexplore.ieee.org/document/4374100

[2]: https://ieeexplore.ieee.org/document/4374103

Perhaps we should consider relaying the signal through a third satellite.

Improving directionality also makes aiming much harder.

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