While working on designing an X-ray navigation system for NASA's next-generation Black Hole Imager, Dr. Keith Gendreau, a physicist at the Goddard Space Flight Center, developed the world's first X-ray communication system.
NASA Tech Briefs: One of the projects you've spent considerable time on in your career at NASA is something called the Black Hole Imager. What exactly is the Black Hole Imager and what is it designed to do?
Dr. Keith Gendreau: The Black Hole Imager is one of the vision missions that's in the Beyond Einstein Program. Its objective is to actually resolve the event horizon of a super massive black hole, and it will do this by extremely high-angular-resolution x-ray imaging. When I say "extremely high," I mean better than a tenth of a microarc second. That's about a million times finer than the Hubble Space Telescope.
NTB: Tell us a little bit about the work you, specifically, did on that project. What were you personally trying to accomplish?
Dr. Gendreau: I was the study scientist on that project. That means making sure that we have a good science argument and a technology roadmap that would map to the science objectives that we want to meet. Also, as part of that, we would actually build components around the laboratory for testing things. So we've made very small-scale x-ray interferometers to try and do x-ray imaging using interferometry, which is actually the technique that would be used in the Black Hole Imager.
NTB: While you were working on that project, you came up with the concept of using x-ray sources as navigation beacons for the spacecraft that would eventually make up the Black Hole Imager. Explain that concept to us.
Dr. Gendreau: The optics for a Black Hole Imager are going to be distributed over a kilometer in the sky and spread out among 30 or 40 spacecraft. These spacecraft have to fly in formation, holding these optics at the places they need to be so that when they concentrate their light onto a detector, which is a further 20,000 kilometers behind the swarm of optics spacecraft, you would actually get the black hole image.
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.
NTB: When you discovered this, what was your initial reaction? Were you disappointed because the result wasn't what you'd expected, or did you immediately recognize the potential of what you'd discovered?
Dr. Gendreau: Well, it's not a discovery; it's a concept, and you know it's achievable. At first I was just trying to get our technology in place for the Black Hole Imager, but then it occurred to me, in the past year or two, that there are actually some advantages to doing communication in x-ray.
Basically, the advantages are all driven by the fact that the wavelength is very small, so if you can use diffraction-limited x-ray optics – and it's tough, but if you can – then you have something that is more like a laser than a laser really is. In other words, when you have laser communications, you're thinking a laser beam, right? Well, the laser beam actually diverges, and if you're going over very long distances – say from the Moon to the Earth – and your beam started off being about a foot across, in about the time it gets to the Earth it's going to diffract because of the diffraction – lambda divided by 1-foot times the distance between the Earth and the Moon – and you'll find that your spot is not a foot across when it hits the Earth, it's a kilometer across or more!
If you did that in x-ray, the beam would be very tight. So what that means is you're beaming your information exactly where you want it to go; you're not spreading your power all over the place. The upshot is that as you go to very long distances; if you can use diffraction-limited x-ray optics, you would transmit more information per unit power that you put into the system. So you can potentially beat laser-based communications if you can get all the other technical things taken care of later on. So this is a future thing.
NTB: Two of the organizations showing early interest in this new technology are the military and, of course, NASA. What are some of the potential applications they might have for it?
Dr. Gendreau: The beam is so tight that there are no side lobes, so you don't have anyone listening in. You would know, because they would interrupt your beam. That's one reason why you go to laser com.
There are some other applications. Actually, just take that application where you're taking the fact that the beam is very tight. In the future, say 50 or 100 years from now, if you want to have a high-speed data link between Mars and Earth, then this might be the energy-efficient way to transmit information over that distance. You would do it from orbiting spacecraft around Mars to orbiting spacecraft around the Earth, and do the long leg using X-com, and then you would use conventional communication techniques to go from planet surface to orbit. The idea is that with the beam being so tight, you can spend less energy transmitting a gigabit per second or more, or a hundred gigabits per second. That's a NASA application, way out in the future.
There are other applications. Let's say you have a hypersonic vehicle, like a spacecraft that's re-entering. You have this RF (radio frequency) blackout period because you have plasma that's building up because the thing is getting so hot. There are a lot of charges floating around and when you try to transmit radio radiation, it doesn't make it out because that plasma acts almost like a conducting box. That's the RF blackout period. When the Space shuttle lands, there's a blackout period when you can't talk to them. Well, if you go to high enough x-ray energy you can beat that blackout period; you can beat the plasma and potentially communicate during the blackout period, or at least pieces of it. You'd want to do this at low data rate, but usually in that situation you just want to do housekeeping-type communications. So there is also that possible application, just because of the high energy, and there are other applications, as you might imagine.
NTB: Any commercial applications?
Dr. Gendreau: Well, for example, hypersonic vehicles. If there was hypersonic transport between here and Tokyo - I'm not sure about the blackout period there - there's a potential that you could do a com link with a hypersonic vehicle that you couldn't do with radio. That's sort of speculative, but it's possible.
NTB: What about the Black Hole Imager? What's the current status of that project and will you continue to be involved with it?
Dr. Gendreau: That's a thing that's way out in the future - 30 or 40 years away. So we're going to continue to get the technology going along for it. Using X-com as motivation for developing some of the key technology, i.e. pointing an optic very precisely, making the optics big and cheap and diffraction-limited, and putting all the infrastructure in place for making that thing happen, you basically have a lot of the key components for a Black Hole Imager in the future.
One reason for pushing for x-ray communication is a way to fund the technology development, which is kind of disappearing from NASA science right now. It's a way to naturally connect with the vision for space exploration, thinking about ways to do communications over long distances.