The NASA news article NASA’s NavCube Could Support an X-ray Communications Demonstration in Space — A NASA First mentions the potential use of X-ray wavelengths for improved range of deep space communications. So far the only advantage I can see over optical communications is the shorter wavelength, which means potentially less divergence for a given aperture.

For example, using round numbers, a 1 eV photon has a wavelength of roughly 1 micron. Transmitting back to Earth with a 10cm aperture, the divergence angle would then be 1E-05 radians (about 2 arc seconds), but use of that requires:

  • Diffraction-limited optics
  • Diffraction-limited source size, (e.g. optical fiber semiconductor laser)
  • Nicely filled aperture
  • Mechanical alignment within the system to micron precision
  • Beam pointing accuracy and stability to single-digit arc-seconds

These are all certainly doable. Fine control of beam pointing could be done for example with an actuated MEMS tracking device at the focal plane, but locking on to and tracking an optical beacon would get hard because of the significant optical delays. Let's assumed this is solved somehow — perhaps by tracking incidental stars through the same optical path (off-axis).

For X-ray energies of for example 100 eV, 1 keV and 10 keV, the wavelengths are of order 100, 10, and 1 ångström! To take advantage of these, it seems many if not all of the items above would have to be between 100 and 10,000 times better than the optical system.

Question: Is there any research, or even speculation of what an X-ray transmitter in deep space would look like? Or is the advantage of X-rays for deep space not actually related to diffraction-limited optics?

  • $\begingroup$ One limitation I can think of is that many celestial objects emit X-rays, so this could make it hard to receive a specific X-Ray signal. In addition, X-Ray is between 16 and 19 orders of magnitude of frequency. We commonly use X-band and S-band for deep space and C/K/Ka bands for Earth orbiting vehicles: the highest frequency here is the Ka band which goes up to 26 GHz (IIRC). The higher the frequency, the more power is needed to generate the signal. This is such a concern that the extreme majority of spacecraft use a lower frequency for the downlink than for the uplink. $\endgroup$ – ChrisR Jan 24 '17 at 1:11
  • $\begingroup$ @ChrisR is that true for deep space communication? For example the Voyagers now only downlink to Earth via X-band (~8.4 GHz) and receive uplink from Earth via S-band (~2.4 GHz). This is because the high gain dish antenna has a gain of +48dBi for X-band but only +36 dBi for S-band. After they got very far from Earth they stopped the S-band downlink. And that is because of the same diffraction effect discussed in the question. For the same signal strength at earth, the higher frequency would require less power, not more. $\endgroup$ – uhoh Jan 24 '17 at 1:57
  • $\begingroup$ @ChrisR there is plenty of interference from astronomical sources at RF and optical frequencies too - check quantitatively. Since X-ray photons are so much higher in frequency, the issues related to thermal noise limit of the front end are very different than for an RF front-end. $\endgroup$ – uhoh Jan 24 '17 at 2:02
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    $\begingroup$ Building a X-ray dish antenna with very low beam width is a problem of the optics used. Using lenses is not possible and mirrors may be used only if the angle from the plane of reflection is very low. Modulating an X-ray with high bandwidth data is another problem. $\endgroup$ – Uwe Jan 24 '17 at 14:08

Something different to a X-ray tube would be needed.

  • The efficieny of X-ray tubes is very low, about 1 % or less.

  • The frequency range of the emitted X-rays is very broad.

  • Beam width is very poor.

  • Pulse modulation is slow, about a millisecond.

  • $\begingroup$ There are certainly X-ray lasers and more recently electron undulators or even better. I'd also focus on the time domain; receiving 10 pulses in one second could be noise, but if they all came within 1 millisecond, followed by 999 milliseconds of silence, then that's a clear signal for example. $\endgroup$ – uhoh Dec 16 '18 at 10:53
  • $\begingroup$ But is there a lightweight compact X-ray laser or a Synchrotron radiation source? $\endgroup$ – Uwe Dec 16 '18 at 11:42
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    $\begingroup$ My posted question is not an easy one! $\endgroup$ – uhoh Dec 16 '18 at 11:48

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