X-Rays Are The Next Frontier In Space Communications

Hundreds of years from now, the story of humanity’s inevitable spread across the solar system will be a collection of engineering problems solved, some probably in heroic fashion. We’ve already tackled a lot of these problems in our first furtive steps into the wider galaxy. Our engineering solutions have taken humans to the Moon and back, but that’s as far as we’ve been able to send our fragile and precious selves.

While we figure out how to solve the problems keeping us trapped in the Earth-Moon system, we’ve sent fleets of robotic emissaries to do our exploration by proxy, to make the observations we need to frame the next set of engineering problems to be solved. But as we reach further out into the solar system and beyond, our exploration capabilities are increasingly suffering from communications bottlenecks that restrict how much data we can ship back to Earth.

We need to find a way to send vast amounts of data back as quickly as possible using as few resources as possible on both ends of the communications link. Doing so may mean turning away from traditional radio communications and going way, way up the dial and developing practical means for communicating with X-rays.

The Tyranny of Physics

The essential problems with deep space communications come from two sources – the inverse-square law and information theory. The inverse-square law states that the amount of energy at the receiving end of a radio communications link is inversely proportional to the square of the distance to the transmitter. Basically, radio waves spread out from the source and at very great distances tend to diminish into the background noise. That’s why deep-space communications networks tend to have large antennas on both ends of the link, to gather and focus as much of the weak signal as possible, as well as to be able to transmit a powerful and narrowly focused beam.

Information theory tells us that more data can be packed into higher frequency signals than lower frequencies. Early satellites didn’t need much bandwidth to do their jobs, so VHF and UHF radios were generally sufficient. But as spacecraft became more sophisticated and the amount of data they needed to send back increased, their communications links began shifting gradually up the electromagnetic spectrum into the microwave region. The Voyager probes, currently in interstellar space, have an uplink using 2.1 GHz for the relatively low-bandwidth tasks of vehicle control, with a downlink at 8.1 GHz, reflecting the increased bandwidth needed to send scientific data back to Earth.

For as stunning an engineering achievement as Voyager has been, and notwithstanding the fact that it’s still working more than 40 years after launch, its radio gear only barely supports its interstellar mission. To be fair, Voyager was never meant to last this long, and every bit of data that makes it back to Earth is just icing on the cake. But for future missions specifically designed for interstellar space, sending back enough data to make such missions feasible will require more bandwidth.

Small, Bright, and Fast

The Modulated X-Ray Source experiment. The miniature source is center bottom. Source: NASA/W. Hrybyk

In late April, NASA is sending a pallet of gear up to the ISS, and one of the experiments stashed in the cargo is meant to explore the potential for X-ray communications, or XCOM, for deep space. The Modulated X-Ray Source (MXS) is a compact X-ray transmitter that will be mounted outside the space station. The receiver for this experiment is already installed; the Neutron Star Interior Composition Explorer (NICER) has been gathering X-ray spectra from neutron stars since 2017, while also gathering data about the potential for using X-ray pulsars as navigational beacons in a sort of “Galactic Positioning System”.

MXS is an interesting instrument. When one thinks of making X-rays, the natural tendency is to assume a traditional hot-cathode vacuum tube, where electrons are boiled off a filament and accelerated by an electric field in the range of 100 kilovolts to slam into a tungsten anode, would be used. But vacuum tubes like those found in a hospital X-ray suite aren’t the best space travelers, and even when ruggedized they’re too bulky and heavy to send upstairs.

So NASA researchers developed a more spaceflight-friendly X-ray generator. Rather than heating a filament to generate electrons, the X-ray source in MXS uses creates photoelectrons by bombarding a magnesium photocathode with UV light from LEDs. The few photoelectrons produced then enter an electron amplifier, an off-the-shelf component found in mass spectrometers that uses specially shaped chambers coated with a thin layer of semiconducting material. Each incident electron liberates a few secondary photoelectrons, which bounce off the other wall of the multiplier to create more electrons, greatly amplifying the signal. The huge stream of electrons is then accelerated by a 10 kV field to collide with the target anode and produce X-rays.

Comparison of hot-cathode X-ray tube to MXS. Source: NASA

While the MXS source sounds similar to a hot-cathode tube, there are important differences. First, the source can be made cheaply from off-the-shelf components and a 3D-printed metal enclosure. The whole assembly weighs only about 160 grams, fits in the palm of a hand, and has no unusual power or temperature control requirements. The big difference, though, is with how fast the X-rays can be turned on and off. A glowing filament can only heat up and cool down so quickly, meaning that effective modulation of X-ray from hot-cathode sources is difficult. In the MXS, X-rays are produced only when the UV LEDs are on, and those can be switching very quickly, in the sub-nanosecond range. The ability to modulate an X-ray beam lead to data rates in the gigabits per second range, greatly enhancing our ability to move data around in space.

What’s more, X-rays can be more tightly collimated than radio waves or even light, which is also being experimented with for space communications. The tighter X-ray beam spreads out less, making transmission more power efficient and reception easier by virtue of the strong signal from relatively bright transmitters.

Although the distance between the MXS and NICER in these XCOM experiments is only about 50 meters, they stand to position us for much better bandwidth for deep space communications. The MXS source itself has a lot of potential applications beyond XCOM too, from cheap, lightweight, low-power medical imaging on Earth and in space, navigational beacons for spacecraft, and even advanced chemical analysis by X-ray spectroscopy


90 thoughts on “X-Rays Are The Next Frontier In Space Communications

  1. I don’t understand the advantage here. Yes, higher frequencies can carry higher data rates, but the data rates for interplanetary probes are limited by the narrow bandwidth needed to give a good signal to noise ratio, not by the theoretical maximum modulation rate of the carrier.

    1. Smaller wavelengths of light spread out less with distance. We can put more signal energy on a smaller receiver by using x-rays than using say visible light or microwaves.

      1. The spreading with distance is a function of antenna gain (higher gain means a more tightly focused beam). High gain antennas are smaller at higher frequencies, but why skip directly from RF to X-Rays? We already know how to generate a very tightly focused beam of light.

        1. We already know how to generate a very tightly focused beam of light
          We already know how to generate a very tightly focused beam of X rays too. Even more importantly, we know how to make x-ray collection optics with narrow beams. The NICER instrument mentioned here has a collection beam with about the same width as a laser pointer, for example.

          Still not enough to win back the huge loss you pay by putting so much energy into a single photon or bit, compared to radio. Even light photons are crappy in terms of energy per bit.

          It’s instructive to do the arithmetic to figure out how much power you need to put in a x-ray beam to get a gigabit per second across the solar system. Friis equation gets you so far, but then you need to take into account photon statistics and background counts (and Fano noise if you want to get really picky).

    2. Wouldn’t switching the UV LEDs at sub-nanosecond scale means multi-gigabit data rates. I assume the X-ray is monochromatic, so there wouldn’t be a practical way to generate (or detect) phase modulation at such a high frequency. I’d argue that the X-ray beam isn’t the carrier, but you’d have to generate a pulse train in the modulation at some frequency and that becomes the real carrier. Meaning bandwidth is going to be equivalent to few GHz carrier, but might be fairly wideband. (I’m barely capable of understanding RF and microwave, this optical and X-ray stuff is beyond me)

      1. Not exactly true. Fast switching with steep edges gives capabilities of AM by PWM and FM by PFM. Also with simpler, smaller and more powerful switchable x-ray sources you can work toward phased array transmitters. All of that plus already invented x-ray detector arrays (side effect of LHC project) opens vast spectrum of options beyond communication field.

        1. Phased array x-ray source? Seems pretty far off, as typical x-ray wavelengths are a few angstroms at most. X-ray laser seems more likely (and those, while they work, tend to consume a lot of material).

    3. I read somewhere that this also has the potential to allow for communicating with ground control during atmospheric re-entry, when plasma builds up around the spacecraft and blocks traditional RF use.

    4. No, the data rates for interplanetary probes are limited by the antenna size and the available power. If you use radio, then the corresponding maximum gain (due to diffraction) and the transmit power defines how much bandwidth you can transmit over to get a good signal to noise on the other end. Bigger antenna means bigger gain (and tighter beam).

      In antenna speak, the gain of antenna is limited by its physical size and the wavelength of the carrier, due to diffraction. Smaller wavelength means higher gain antenna. Once you get up to the optical (so now you’re talking about lasers) you don’t think about it in terms of “antenna gain”, it’s how tightly collimated the laser is. But it’s the same principle.

  2. I disagree with the explanation for a glowing filament being too slow, after all, a traditional X ray tube could easily be manufactured with a grid to function like a vacuum tube (with a target in place of the traditional anode) and vacuum tubes can operate at pretty high frequencies.

    1. Thing is if you want any useful energy x-rays, you need several tens of kV of potential between the cathode and anode, a grid is probably not a good way to modulate that…

      Maybe an arrangement like a klystron or traveling wave tube would be better, as those are well understood and actually operate at high frequencies?

      1. Excellent point, the grid would probably melt but either the traveling wave tube or the klystron would sustain it quite handily. I’m not too familiar with them but, since they use resonance, wouldn’t that put a carrier signal in the generated X rays (though this might not necessarily be a bad thing, what with Doppler shift from a traveling space craft)? I thought of a conventional valve because of the old very ill advised experiment that can be done with a tube and a chunk of tungsten (since a few electrons will make it through the glass).

        1. No, gridded high power tubes exist. They’re even common: Your average television transmitter uses a tube transmitter running at several kV and many kilowatts, and has a grid. And it doesn’t melt. At least if the cooling water keeps flowing. Modulation bandwidths in the MHz are usual.

          1. Also the old CRTs used to operate at about 25kV – close to x-ray voltages. And they used grids to modulate the beam. Sometimes they were modulated at the cathode, but still a 25kEV electron beam was modulated.

    2. Good point. In fact, you wouldn’t even need the tube structure to have a grid. It’s already a great example of a vacuum diode, just driven really really hard. If there’s any AC component to the input to the tube, they tend to put out a modulated output all on their own. I’ve worked with plenty of tubes that produced a nice 60 hertz output pulse, from the (stepped up) household current they ran on. One good way to confirm the actual duration of the exposure was to spin a top on a film cartridge. The top would be made of lead, and have a hole drilled in it: counting the bright spots in the top’s shadow gave the actual number of pulses, which could be multiplied by the line frequency to give actual exposure time. The filament ran off DC, and the pulsing of the output had nothing at all to do with the filament heating and cooling. Modulating an X-ray tube at higher frequencies shouldn’t be very different than modulating a tesla coil’s output, and plenty of modulated tesla coils have made appearances here.

  3. Circling around a growing red-giant star, on a dried out old planet already too hot to live on but still just cool enough for short-term exploration a pair of 4-legged, 4-armed aliens explore the compendium. An ancient library built to last millennia after it’s builders have gone extinct, preserving their culture, achievements and heritage on the off-chance that somebody from somewhere else one day finds it.

    Xorg, have you found any evidence that these Hoo-Mans ever traveled the stars?

    No Xerb, not really, they only visited their moon a few times and stopped.


    Yah, this planet used to have a moon. It seems to have escaped orbit a couple of zarns ago.

    Oh. Well that figures they never went anywhere, we might have heard of them by now had they made it anywhere good.

    Hey.. wait a minute, what’s this? I think I did find something afterall.

    You mean they did travel

    No. This is really weird. They never bothered to leave their world and yet they built 1000s of robots and 10s of thousands of telescopes. They have discovered more worlds and mapped more of space than even we have. This will set our program ahead by centuries.

    Huh. That’s um… great. Shit. We are out of jobs.


    Nevermind, there was nothing here afterall. Lets get off this dry old rock before it burns away completely.

    Yeah, Let’s get out of here!

    1. “Oh. Well that figures they never went anywhere, we might have heard of them by now had they made it anywhere good.”

      “Oh well, that figures they never went anywhere. They might have still had some descendants and we might have met them had they made it anywhere good.”


  4. Personally, I think it would be cool to use quantum entanglement to make a modem. I’m sure there are tons of technical challenges that would need to be solved to make such a thing but would be worth it. That modem should over any possible distance with no latency issues, right?

    1. That’s not actually possible. Quantum theory proves that information cannot travel faster than the speed of light. In your scenario, this manifests as the No Communication Theorem, which essentially says that two different observers of a quantum state cannot communicate with each other. Nothing in quantum mechanics works the way people want to think it does.

      1. My understanding is precludes naive sci-fi style communication using the spin direction of 2 entangled particles as a signalling bit because of “noise”. Should we argue quantum computers are impossible because we can’t read qubits directly without destroying their qauntum properties?

      2. I understand using wormholes for communication is physically allowed though. That’s about all they are useful for as it takes an extreme amount of energy to create a large one.

      3. Speed of Light is all relative…

        There are two objects, A and B, travelling towards each other at 1/2 speed of light.
        Object A sends out a light pulse toward Object B, obviously at the speed of light.
        At what speed does Object B (the receiver) observe this light flash?

        Now put Object C in the middle of A and B and C is stationary. What speed is the flash?

        So, can the speed of light be broken???

        1. Observers A,B, and C each observe a constant speed of light from their own frame reference because each is experiencing time at different rates based on their relative speeds. Welcome to Special Relativity!

    2. “That modem should over any possible distance with no latency issues, right?”

      No, only the distances it is possible for us to physically travel, and latency being the time we can physically travel there in. At least photons and such move at C, so would be far better than the quantum equivalent of hand-delivered letters.

    3. Quantum entanglement doesn’t actually result in FTL communication. One good thought experiment: I take two ping pong balls, paint one red and the other one green. Without looking at the balls, I put them in envelopes along with a brief explanation of what I’ve done, and mail them to two (long lived) friends on the opposite sides of the galaxy. One friend opens his envelope, and finds a red ball. He instantly knows that the other ball is green. Did information travel faster than light?

      1. That’s not how entanglement works.

        What you did is a “hidden variable” version of quantum mechanics, where the color of the ball is chose in advance. What happens with real QM is that the color of the ball is random up to the point of opening the envelope.

        The weirdness is that when one guy opens his envelope first, it determines the color of the ball inside the other guy’s envelope as if some weird action at a distance was going on.

        The real explanation of the phenomena is that we make wrong assumptions about the situation. We treat it as if we are a classical observer observing a quantum world so we can observe both persons at once, whereas in reality nobody can observe them at once and therefore nobody can know at once which color the ball was for each. Only after a sufficient amount of time has passed, so that information has been traveling at the speed of light between them, can there be information about the differences – and it is at that point that the entanglement “chooses” who got which ball. Before that, it is essentially irrelevant who got which ball because their local realities have not yet come together, and after the point everyone’s memories of the event of opening the envelope collapse from a quantum superposition into a definite state of “I got green, you got red”.

        Technically speaking, even after that, there exists a superposition where either could have had either, but you can’t experience a superposition while you’re in it because the alternate states are mutually exclusive. It’s sufficient to say that neither state is actually real until something else in the universe demands one over the other, and when you keep piling up more and more stuff into the same local superposition, things take on the appearance of the classical universe through cyclical causal dependencies that demand things to be this way instead of that way.

        1. That is to say, what the rest of the universe really is is just what information we have of it right here and now. Take two steps to the left, and you’re in a different parallel universe that is almost, but not quite the same, because it will contain different information about the rest of the universe than someone standing where you stood before.

          So when you send the ping pong ball to the moon to an astronaut, there’s about what 2-3 light seconds of difference between you, and within this “coherence time” in principle anything could happen because you’re not entirely in sync with each other. They could open the envelope and see a red ball, but by the time they come back to report the color of the ball to you, the rest of the local universe has decided that it was green after all.

        2. The really eerie thing is, what is it about a human looking at the ping-pong ball that causes the Universe to decide once and for all on red or green — or in technobabble, what exactly causes the collapse of the state vector?

    4. Seem neutrinos are the fore front of communications and making them and receiving them at higher baud rates as well as shrinking the systems to being portable are what some labs are working on… way early in development of course.

      I was thinking the X-Ray systems will be a pimpin Spectroscopy toolkit that those annex’s or who knows WTF is tappin into and f’in with.

  5. ” The inverse-square law states that the amount of energy at the receiving end of a radio communications link is inversely proportional to the square of the distance to the transmitter”.

    I believe that electromagnetics is useful for communication because the energy of the radiation is the square of amplitude. So amplitude drops as 1/r. A drop proportional to 1/r^2 would be useless.

    And I’m pretty sure that coherent visible light, as in LASER, is better collimated than x-rays. In fact I’m fairly certain X-rays are in a XRASER/GRASER forbidden energy zone, except perhaps for Edward Teller’s one-shot device that is a bundle of tungsten rods placed beside a nuclear bomb.

    1. You should lookup the “Free space path loss” equation, the inverse square law applies to all EM radiation. It is one of the reasons that signal levels in communication systems are measured in dB, a logarithmic scale.

    2. You have brought shame upon your clan and your clone batch. X-ray lasers are a thing, and the inverse square law is a thing. Sirriously?

      The real problem is here: “humanity’s inevitable spread across the solar system”. Nothing “inevitable” about it, homeslice.

      1. Of course inverse square is a thing. It is simple geometry. However, EM detection is based on amplitude (the effect of the electric field amplitude on your antenna), which varies as 1/r. Imagine a radio telescope if it depended on the square!

  6. If I understand the article correctly, x-rays have a smaller point spread function for a given distance than other light sources. This improves the overall signal at the receiver and consequently the signal to noise ratio. I think the reason the high modulation frequency is mentioned is to explain why this light source is an improvement over regular x-ray tubes, which are already bright efficient x-ray sources.

  7. Why modulate it like ‘No Static At All’ FM, or even AM. Use it to send Morse code, you know, dits and dahs. Heck, it worked for the Titanic and saved the Earth’s bacon in ‘Independence Day’ didn’t it?

      1. Mart, CW ( Continuous Wave ) is on-off keying at a constant amplitude that requires a BFO ( beat frequency oscillator ) for audio frequency amplification. CW is not modulated. Now, there is such a thing as MCW or Modulated Continuous Wave that has a much broader bandwidth and was mainly used when sending distress messages such as an ‘SOS’. Thanks for your response. I am a HAM, an amateur radio operator (AA0PP) and former Coast Guard Radioman.

        1. With a real continuous wave (endless sine tone) you could not transmit any information. You have to modulate it and you do it. Of course your amplitude is not constant, you change it between 0% and 100% and back to 0 for each dash or dot. This is modulation, although with a really low frequency, likely <=50Hz. So you need at least 100Hz reception bandwidth.
          I don't know the "MCW", I assume that this uses some kind of sub carrier.

          I also do not see the way of mixing, demodulation or filtering as really special. With a BFO you mix your narrowband modulated "CW" signal down to an audio frequency. But this is nothing else than another (second or third) IF. An IF that happens to be in the audible range. That's one way to receive this signal.
          Theoretically (while with poor performance) you could use a simple detector receiver and connect a meter (like an S-meter) to it's DC-coupled output and read the morse code there. I think, old morse telegraphs which wrote the message down with a pen on a strip of paper worked like this, without a BFO.
          If you want you could also use an SDR, digitize your shortwave signal and filter and demodulate it in the digital domain.
          If you want to talk about qualifications: I studied HF and communication engineering at the university.
          Of course operating a radio is different from developing it.

        2. Unfortunately my first reply vanished.

          With a real CW signal, a constant sine tone, you could not transmit any information. You have to modulate it and you do this. You change the amplitude from 0 to 100% and back to 0% for each dot or dash. Although your modulation bandwidth is quite low, probably less than 50Hz..
          The BFO with it’s mixer is nothing special, it’s just another (usually second or third) IF (intermediate frequency), which happens to be in the audible range.
          If you want to talk about qualifications: I studied RF and communications engineering at the university.

    1. Hey, I’m an optimist. Snort all you like, but as long as we don’t go extinct or pass through a severe population bottleneck and attendant loss of our technological society (cf decline of the Roman Empire), we’ll eventually push out into space. If the laws of physics don’t forbid something, it’s just an engineering problem, and all engineering problems eventually get solved.

      1. All engineering problems do NOT get solved. Ever notice that, even now, we still have trouble with quality electrical or mechanical connections. Yea, we can weld or solder or make super expensive units that are reliable but nothing practical that won’t loosen up way too soon. Besides, when you are looking backward at history, of course it looks like a long sequence of successes, but we don’t see all the failures because they are naturally filtered out. What gets invented in the future is no certainty. Some problems get solved right away, others eventually, others never. Good ideas come on their own terms. We can work hard to improve the odds but we are still dependent on the mystical process of creativity, a fickle vixen at best, but well worth pursuing.

  8. Somebody didn’t do their homework.

    A single X ray photon is very costly in energy, but can transmit (at most) one bit. Statistically, to be sure you “see” a bit, you’ll need a few photons, even in the absence of background noise and with a high quantum efficiency detector. So X-ray communication will cost around 100 keV per bit, and likely much more.

    A good sat-link grade radio receiver can collect data at a kilobit per second for a WHOLE DAY, with the received energy equal to a SINGLE bit communicated by x-ray photons.

    You can win back some of that inequality with collimation of the x-rays (good luck finding an efficient way to do that, especially on the receiver), but that carries with it the requirement to point your antennas with correspondingly good tracking accuracy.

    1. Just to keep the thread alive…

      X rays suck badly for interplanetary-scale communication.

      Using the numbers from NASA’s NICER publications of the effective collection area (about 0.13 m^2) and beamwidth (1.8 mrad) for the (actually quite good) detectors and x-ray optics, and assuming negligible background counts and 100% efficient detector…

      Picking an arbitrary light-hour communication distance (about the distance to Jupiter), 1.8 mrad beamwidth is about 5x the earth-moon distance.

      To reliably see a single bit means the detector array should statistically get about 10 photons, or 100 keV per bit.

      To cover that 1.8 mrad patch of sky with 100 keV per 0.13 m^2 detector area will require a transmitted energy of 2e24 eV, or 300 kilojoules. Per BIT.

      So to transmit a gigabit per second between Jupiter space and earth will need 300 terawatts of x-ray power (which would give a nice 1g acceleration to a 10-tonne ship through photon pressure alone, but that’s beside the point).

      Even by somehow increasing antenna gain or area by a factor of 1000 on both ends will still require 300 megawatts of x-ray power.

      Yeah, someone somehow neglected to pass that tidbit along to the funding agency, I think.

        1. ”A reaction drive is a weapon effective in proportion to it’s efficiency.”

          A trope I’m disappointed James S.A. Corey didn’t employ more in The Expanse. They even are on record as saying their Epstein drive is “very efficient”.

        2. They also had giant “communication lasers” (I can’t remember that there was a hint on the frequency/wavelength) which they had to (and could) repurpose as weapons.

  9. “Information theory tells us that more data can be packed into higher frequency signals than lower frequencies…The Voyager probes, currently in interstellar space, have an uplink using 2.1 GHz for the relatively low-bandwidth tasks of vehicle control, with a downlink at 8.1 GHz, reflecting the increased bandwidth needed to send scientific data back to Earth.” – anyone care to explain?

    1. Dan’s playing fast and loose with terms, and conflating bandwidth with carrier frequency and antenna gain and SNR.

      Generally speaking, for a given coding scheme and signal to noise ratio (SNR), you require a wider signal bandwidth to carry more bits per second. A simple coding scheme might require 3 kHz bandwidth to carry 2400 bits per second. More complex coding schemes can pack 50 kilobits per second in the same bandwidth, if the SNR is good enough.

      The signal bandwidth has very little to do with carrier frequency, except that it’s tough to fit a 5 MHz bandwidth signal on a 1 MHz carrier and still call it a 1 MHz carrier.

      However, higher frequencies (shorter wavelength) also yield higher directivity and greater antenna gain for a given antenna size (assuming the antenna is built with good enough precision), yielding higher received signal power = more SNR = more bits per second (if the coding scheme supports it).

      1. Thanks Paul, that’s about what I thought. It’s not the first time I’ve seen carrier and bandwidth confused, so I wonder how widespread that misconception is.

        Interesting note with the better antenna gains at higher frequencies, I wonder if that’s the real reason for the higher downlink frequency?

  10. OK, there are lots of comments so not sure if this is already covered…

    Generating x-rays is horribly inefficient. At 10 kV, it’ll be about 0.1% max. What is much worse is that the device depicted above generates them omidirectionally. Meaning half are lost in the target and so you’re only left with a hemisphere.

    They’re not focused which is useless alone as most would zip off into deep space…

    If it’s for the feed to an x-ray laser (presumably free electron type), which can have an efficiency of ~1% at very soft x-ray energies, then it could be focused. It’s still not efficient though.

    And it’s perfectly possible to build an optical laser as focused as an x-ray laser. They all have Gaussian beams.

    This rant is not projected from an armchair, I’ve designed commercial x-rays systems.

    1. The other issue is that x-rays are absorbed by the atmosphere, so presumably they will need to launch a receiving satellite, and then relay the signal back to Earth, via radio. Also it may not make a very good two way communications setup, since I am not sure how easy it is to make a x-ray receiver small and light. Obviously it can be used as a high speed comms device and then code uploads etc could be done via lower gain radio, but again it all adds weight.

    2. I agree with you there. Generating enough flux will be difficult when it comes to power generation for the source and cooling of the anode (which isn’t even mentioned).

      A free electron x-ray laser like XFEL or LCLS is half a kilometer long. There are ways to do it in much shorter distances with plasma wakefield acceleration, but that’s still very much in the experimental stage. And I suspect that it also requires quite a bit of power and cooling.

      My background is x-ray optics for space based telescopes. Creating a collimated beam from a non-collimated source is extremely difficult. I suppose it could be done with two bend mirrors properly coated for 8 keV reflectance, but I don’t know what that would do to the coherence of the beam. Wouldn’t amplitude variations be washed out in that case?

      1. But I want a half kilometre long spaceship with a giant laser! I hereby christen it Mr Nice. Anyone who isn’t nice will be communicated at with high speed data.

  11. are you insane? xrays are dangerous and have to be carefully controlled as medical devices.

    1. you have to be certified as a medical doctor even to operate the xray machine for security (i think).

    2. xray tunnels probably have to be recertified to be safe to operate .

    3. when an xray machine is disposed of the xray tube has to be broken maybe even completely removed as anyone with glass blowing skills can blow a new envelope.

    in some worse case scenarios the firmware lock is set to prevent the machine from even operating

    1. Yes and apparently the Chairman of the FCC is insane also and public resources are become corporate only and banks/credit unions don’t have any commitments to their communities health and safety unless you’re trying to manufacture not requirements for survival jobs and economies like Attorney, Health Care, Prisons, Courts and Law Enforcement that are malicious or at least pseudo-malicious or para-malicious… yeah crazy:

      Reminds me of the fifth paragraph in this section of our space based office now with who knows what wants to molest us up the but…., I didn’t know, and more 24-7-365 on pseudo-?:

    2. It’s all about the dosage, and how it’s controlled.
      1. Nope, not at all. Operating a machine is way below a doctor’s pay grade.
      2. Probably, but so does your phone, microwave, and anything else that emits electromagnetic radiation.
      3. I’d expect someone to be capable of re-building the entire machine at that point. Or you know, ebay.

      1. ad 1.) And I am quite sure, the personnel which operates airport security x-ray machines or welding inspection equipment (independent of x-ray or with a Co-60 gamma ray source) does not need to be a medical doctor. :-)

        ad 3.) I don’t know if some old tungsten carbide tool tips would be usable as makeshift anodes. But even if not, tungsten is not that rare and other metals are usable. So somebody with glassworking skills could probably manufacture an x-ray tube, if he wants. The history of x-ray tubes even proved that a crude tube could even work with a less than perfect vacuum.

  12. They exite the XRays with an UV LED – why not use the LED’s wavelength advantage (~100nm wavelength vs ~10000000nm for microwaves…) instead of trying for a measly extra 10^3improvement from UV to 10kV Xrays (~100nm to ~ .1nm) which only provides for theoretical bonuses (not actually realized in this iteration, and technologically unsolved at such small scale)?

    Xray lasers are a thing, but so are 10ct UV lasers in any RW-disk drive – and UV allows for actual optics instead of the block-and-shallowest-of-angles-only-mirror contraptions that serve in Xray ‘optics’.

    Is it about the solar background being strong in UV and 10^3 softer in 10kV? But with good colllimation that only applies to communications directly from the direction of the sun, not even towards, so not that much of a use case, is it?

  13. The Friis equation (inverse square law) says that doubling the range requires 4x (6 dB) more transmit power or 6 dB more antenna gain to achieve the same received power. So it’s nice that antenna gain is easier to get at higher frequency. However…
    The Friis equation also describes a second inverse square law: i.e., that doubling the frequency requires 4x the power or 6 dB more antenna gain to achieve the same received power. So using higher frequency does come with its own trade offs. That said, getting 6 dB of (easier) additional antenna gain at high frequency may be more practical than coming up with 6 dB of additional power for many applications.

  14. I’m not defending the X-ray communication proposal or contesting your statements. The only claim I question is of a maximum of 1 bit per photon:

    Suppose we can both measure time intervals accurately, then I could wait N nanoseconds since the last photon I sent, and then flash another photon at you, with the information being the number N, clearly this is more than one bit per photon. Haste makes waste of course, so this is rather slow, but this scheme could be multiplexed over a large number of wavelengths/energies…

    In the face of background photons, or dark noise in the detector, if the communication scheme has error correction or checksums, it should be possible to reject the false photons , or reconstruct a missing one… I have never seen any math for this kind of communication scheme, it would appear to be novel terrain… Of course, the sender wouldn’t be literally sending one photon, but a number of photons such that the optics would ensure a few photons on average to arrive at the detector… I am merely pointing out that in principle a photon can convey many bits of information.

    In the context of searching for intelligent extraterrestrial signals, I would assume an emitter assumes a listener would sooner or later realize that long distance low energy per bit transmission schemes differ from short distance high baudrate schemes. Making a scheme 100 times more bit per energy efficient means a 10 times larger reception radius (assuming a point source broadcast) hence a 1000 times larger volume of candidate listeners, so the power is 3/2. There is little haste because the emitter doesn’t know what time the receiver wakes up and builds civilizations. You don’t need synchronized clocks, just low enough drift such that when measuring pulses from an irregular pulse source, the measured times show a clear quantization of time steps larger than the time resolution of your detector. If one is lucky one wakes up not at the single photon limit sphere of the emitter but somewhere closer by…

    According to wikipedia one of the NICER experiment goals would be investigating the possibility of X-ray navigation system.

    Someone else commented above that one of the reasons for the emitter would be to remain in communication during re-entry, which would certainly seem useful…

    1. For a back-of-the envelope Fermi approximation, a photon per bit is a reasonable place to start, but yes, you’re right, more efficient coding schemes exist. Pulse-position modulation (PPM) schemes have been in use a very long time, and a rich literature trove exists. For example http://citeseerx.ist.psu.edu/viewdoc/download?doi= shows a number of even better coding schemes built on PPM, and has a lot of good references too.

      The point is: a ballpark estimate (like one-photon-per-bit) gives you the magnitude of the problem, and it’s a very large one in this case. Shaving off a factor of ten or a hundred or even a million doesn’t save your bacon here.

      Simply put, it’s a non-starter idea for the purpose of interplanetary communication, and disingenuous (or incompetent) of the proposers of the notion.

      Now, for the re-entry communication problem, go ahead and calculate the absorption rate of those 10 keV photons in atmosphere, and figure out the transmitter power required to communicate with earth’s surface (hint: nope) or space (not likely). While you’re at it, figure out the pointing accuracy required of that x-ray transmitter antenna and estimate whether you’d be able to track a high speed rapidly-decelerating object hidden behind a plasma cloud to the required accuracy, from an orbital platform, and likewise, track the corresponding antenna on that re-entering vehicle.

      1. PPM reminds my of an old TV I had: It used a PPM modulation with no extra carrier, narrow needle pulses 1,2 or 3 T apart. It had unbelievable battery life. The zinc carbon batteries that came with it were still working >10 later.
        The only drawback was it’s sensitivity to the emission of a CFL bulb with a (new at that time) electronic ballast. While the lamp was warming up it was completely blocking the reception.

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