Dr. Keith Gendreau, Physicist, Goddard Space Flight Center, Greenbelt, MD

NTB: Would it be like a grid of spacecraft?

Dr. Gendreau: Grid is too good a word. Have you ever heard of VLBI or sparse aperture? Basically it’s a sparse aperture x-ray imager. It wouldn’t be a perfect grid. You want to cover independent baselines, so it would almost look semi-random within this disc. You might have more in the middle and less on the ends across this virtual aperture that’s about a kilometer in diameter.

NTB: A significant breakthrough came when you tried modulating x-rays by switching them on and off many times per second to change their intensity, because at that point you realized you might have the basis for a new type of communication system. Can you explain that to us?

Dr. Gendreau: What actually happened was that communication came along later. When we needed a way to reference the spacecraft relative to each other, you have to have a beacon system. Normally, you would put little lights on your various spacecraft and then put another telescope in another spacecraft to look at the lights and see where they are relative to each other. The problem is, if you do this in optical light, the wavelengths are so long that even with diffraction-limited optics, you could not get high enough angular resolution on the separation of these spacecraft. So the spacecraft in our swarm have to be held relative to each other. You actually have to have knowledge of where these spacecraft are to within a few microns, and you’re doing this as viewed from, say, 20,000 kilometers away. There’s no way to do that optically. You would need tens-of-micro-arc-seconds of angular resolution, and that is orders of magnitude away from the best optical telescope flying right now – the Hubble – which gets about a tenth of an arc-second

You couldn’t do it optically, so we thought about putting x-ray sources up there and doing it using diffraction-limited x-ray optics. What we’re doing here is we’re using the fact that x-ray has a very short wavelength. There’s a broad definition for what an x-ray is. It ranges from anywhere from a 100-angstrom wavelength down to much smaller than an angstrom. Optical light is more like, say, 5000 angstroms. Much smaller wavelengths mean that the diffraction limit for the same diameter optics becomes much smaller. It scales like lambda. If you use a 10-centimeter optic to look at 1-micron radiation, then the diffraction limit would be about 1 micron divided by 10 centimeters, and that’s in radians. That would come out to a few arc-seconds or so. But if, instead of 1 micron, the radiation had a wavelength of 10 angstroms, then the diffraction limit gets smaller by the wavelength and would be more like 2 milliarcseconds.

We wanted to have an x-ray beacon system as our way of registering the relative positions of these spacecraft. So it was a relative navigation system that we were proposing. Then it occurred to us that if we could actually modulate the x-rays, you would get a communication system for free out of the beacons.
