Sometime this spring, the International Space Station will transmit its first x-ray communications signal. The astronauts on board will know immediately whether the experiment was a success. That’s because the ISS will also be the receiver.
This proof-of-principle test, traveling just 50 meters across one section of the ISS, could be an important sign of things to come for space communications. According to preliminary NASA calculations, x-ray communications (XCOM) could yield gigabit-per-second data rates across the solar system.
“The real bang for the buck on x-ray for next-gen communication is for really deep-space communication,” says Jason Mitchell, assistant chief for technology in the Mission Engineering department at NASA’s Goddard Space Flight Center, in Maryland. “To the outer planets and beyond—even outside our solar system.”
So long as you can point your x-ray source accurately (and that’s no trivial task), x-rays can remain tightly focused, keeping power budgets low for a deep-space communication link on some future planetary or extraplanetary mission.
For example, Mitchell says, a laser communications beam that’s the size of a dinner plate in low Earth orbit (2,000 kilometers above Earth) diverges out to the size of a football field by the time it reaches geosynchronous orbit (42,000 km away from the planet’s surface). An x-ray signal's beam size, by contrast, barely broadens at all. XCOM, in other words, would retain its focus some one thousand times as tightly as a laser beam.
“It’s real point-to-point comm, which for information security is important,” Mitchell says.
X-rays also can penetrate to where radio waves or microwaves simply can’t. When a spacecraft reenters the Earth’s atmosphere, for instance, the RF noise makes it impossible to communicate with the ground via its regular radio channels. (Think of those tense few minutes in the movie Apollo 13 when communications were blacked out and Mission Control didn’t know if the hobbled spacecraft had survived reentry.)
But even back during Apollo days, NASA engineers were dreaming up x-ray spacecraft communications systems that would enable uninterrupted communications even during reentry, says NASA Goddard astrophysicist Keith Gendreau. “They were thinking about using a real traditional hot-filament x-ray source,” Gendreau says. “They never did that, obviously, because it was a massive and inefficient way to do it.”
The kit to be tested this spring, the so-called Modulated X-ray Source (MXS), uses an old technology that’s finding new applications. “Because we’re NASA and everything’s got to be rock solid, we didn’t want to send up x-ray sources that were medical x-ray tubes where we had hot filaments that would boil off electrons that you accelerate into a target to make x-rays,” Gendreau says. “Filaments break; it’s power hungry. So we came up with the idea of using photoelectrically driven x-ray sources. This is what Einstein got his Nobel Prize for. It wasn’t for relativity; it was for the explanation of the photoelectric effect.”
MXS will be arriving on board the ISS as part of a palette of experiments, on a flight scheduled for late April. On the x-ray signal’s receiving end will be an experiment installed on ISS in 2017: the Neutron star Interior Composition Explorer (NICER). NICER is currently observing x-ray signals from pulsars, both for basic science and to explore the use of x-ray signals from pulsars as spacecraft-navigation beacons.
The Modulated X-ray Source, a key component in NASA’s first-ever demonstration of X-ray communication in space. Photo: William Hrybyk/NASA
All the more appropriate, then, that Gendreau originally proposed the current XCOM system as part of an ultraprecise locator in orbit that could be used by a swarm of spacecraft attempting to image the event horizon of black holes in the Milky Way, like Cygnus X-1.
Developing XCOM technology for deployment on future missions, says Gendreau, would not require technology that’s leaps and bounds beyond what’s already available.
“If you put resources into it and there’s some interest, it wouldn’t be that far away,” Gendreau says. “Diffraction-limited optics that work in the wavelengths [in question] already exist in extreme ultraviolet lithography. [It’s] the thing that makes your iPhone’s transistors smaller and smaller, [and that’s possible] because there’s been billions of dollars invested in making those diffraction-limited UV optics.”
Scaling up an XCOM system for actual deep-space communications, he says, would likely involve an MXS-like transmitter, a NICER-like receiver, additional optics, plus an ultra-accurate pointing system. “It’s engineering,” Gendreau says.
NASA shouldn’t be surprised to find China developing such promising space communications technologies independently, says strategic analyst Namrata Goswami, author of the forthcoming book “Great Powers in Space.” Says Goswami: “China has now become the first country in the world to establish an experimental Space-based Solar Power (SBSP) station, in Chongqing early this year. So, China investing in x-ray deep-space communication appears to be in step with other investments that China has been doing in the last few years.”