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?