NASA’s Orbiting Carbon Observatory-3 (OCO-3) instrument and the U.S. military’s Space Test Program-Houston 6 (STP-H6) payload, which carries the X-ray communications experiment, inside the trunk of SpaceX’s Dragon cargo craft before launch. Credit: NASA
A novel communications experiment developed by NASA and the Naval Research Laboratory has arrived at the International Space Station to prove data can be transmitted in space using X-ray signals, a breakthrough that could have uses in deep space exploration and military technology on Earth.
The X-ray communications experiment, known as XCOM, is one of several scientific and tech demo payloads inside a U.S. military instrument named STP-H6, which arrived at the space station Monday in the trunk of a SpaceX Dragon cargo capsule.
The space station’s robotic arm will pull the STP-H6 payload out of the Dragon’s trunk this weekend for attachment to a mounting post on the station’s truss backbone. The STP-H6 instrument package rode inside Dragon’s trunk with NASA’s Orbiting Carbon Observatory-3 experiment, or OCO-3, which will measure carbon dioxide concentrations in Earth’s atmosphere.
The OCO-3 instrument was robotically removed from the Dragon’s unpressurized trunk Thursday night using the station’s Canadian-built robotic arm, which was expected to hand off the experiment to a Japanese robot arm for placement on a mounting post outside the Japanese Kibo lab module.
The Canadarm 2 will next transfer the STP-H6 payload to pallet on the port side of the station’s truss backbone. The final robotic transfer activity will move the failed Cloud-Aerosol Transport System, an atmospheric experiment launched to the station in 2015, to the Dragon’s trunk for disposal.
The CATS instrument will burn up inside the Dragon spacecraft’s trunk during re-entry at the end of the mission June 3.
Once installed on the station, ground controllers will begin activating the STP-H6 payload’s experiments. Besides the XCOM investigation, other objectives of the STP-H6 mission include the demonstration of an infrared airglow camera to better observe the boundary between Earth’s atmosphere and space, a supercomputing experiment to evaluate new technology for image and video processing in orbit, the test of a new high-accuracy star tracker, an attitude determination and control experiment, and a spacecraft plasma diagnostic suite.
The XCOM experiment consists of a transmitter, or source, to emit X-ray signals toward a receiver on the other side of the station. The receiver is actually a NASA astrophysics instrument named NICER, short for the Neutron Star Interior Composition Explorer.
A view of the NICER instrument, with its 56 individual X-ray mirror modules, outside the International Space Station. Credit: NASA
NICER has been on the space station since 2017 measuring X-ray light coming from neutron stars, the super-dense stellar skeletons left behind after lower-mass stars exploded in violent supernovas at the ends of their lives.
The NICER experiment has the ability to log the precise time an X-ray photon falls on its silicon detectors. The time tags allowed scientists to use beams from rapidly-spinning neutron stars, called pulsars, as a tool for in-space navigation for the first time.
The regular pulses of X-ray light from a certain type of neutron star, called a millisecond pulsar, were used to derive a position estimate with NICER in late 2017.
“When we built NICER, we made use an X-ray source that we had developed at Goddard to test it, an X-ray source that we could modulate with arbitrary waveforms,” said Keith Gendreau, an astrophysicist at NASA’s Goddard Space Flight Center, and principal investigator for the NICER and XCOM experiments. “We could make simulated pulsars and make sure that NICER would actually work properly to do the X-ray timing that it was supposed to do for the science and technology applications.
“So this X-ray source we made, we called it a modulated X-ray source, or MXS,” Gendreau said. “As soon as you see that you can do any type of X-ray light curve you want, it was very clear that we could use it to transmit data. So that’s another application that we can do.”
Now NICER is a critical piece of the X-ray communications experiment, which uses an X-ray source originally developed to test the NICER instrument on the ground.
The X-ray source originally developed for the NICER instrument is also being used in a new type of CT scan machine that will give patients a tenth of the radiation dose of an existing CT scan.
“This (X-ray) source was developed to calibration of X-ray astrophysical instruments, and it’s got applications now in communications and medicine, and other areas,” Gendreau said in a recent interview with Spaceflight Now. “The core technology was developed by NASA.”
The XCOM experiment’s sponsor is the U.S. military’s Space Test Program. The Defense Department foresees X-ray communications as a more secure way to transmit data around the world, while NASA scientists believe it could be useful in deep space data transmissions.
“They had an interest in this type of experiment, and we knew how to make it,” Gendreau said of the Defense Department’s involvement. “So everybody worked together to get it to this level.”
Spaceflight Now members can read a transcript of our full interview with Keith Gendreau. Become a member today and support our coverage.
Scientists at Goddard and the Naval Research Laboratory are in charge of the XCOM experiment.
“We’re sending it up into space to actually do an in-flight demonstration of data communication in X-rays,” Gendreau said.
“There are a lot of reasons why you might want to do this,” he said. “X-rays at the very highest energies are penetrating, so they can go through materials. One concept is that it’s a way that you could possibly communicate with a hypersonic vehicle, like a re-entering spacecraft or something like that, like in Apollo 13 where they had that (radio) blackout because of the plasma.
“X-rays are above that plasma frequency and could provide a communications link,” Gendreau said.
More satellites are are beginning to use optical communication to replace conventional radio links. Optical communication, using lasers, provides faster data transmission speeds than radio communication, but it requires more precise pointing between a transmitter.
“You go from radio to laser communication, and the communication beam that’s produced is made much, much smaller,” Gendreau said. “If you can imagine going even smaller than laser communication, that would be in the direction of X-rays.
“X-rays are much smaller, about 1/1000th of the wavelength of the systems used in optical communication — laser communication — so the beam can be extremely tight,” he said.
NASA’s first-ever demonstration of X-ray communication will occur on the International Space Station. This image shows the locations of the Modulated X-ray Source and the Neutron star Interior Composition Explorer, or NICER, which are critical to the demonstration.
“There are some national security applications for that type of system,” Gendreau said. “You could imagine point-to-point communication links between geostationary spacecraft and low Earth orbit spacecraft, where the footprint of the beam is very, very small. And if anyone were to intercept it, your signal would go away. So you could really make a secure point-to-point communication link.
“On longer distances, like, say, going from the outer planets to Earth, having a very small footprint means that you’re not spending all of your power in your transmitter spreading your signal over the inner solar system,” he said. “You’re going to direct it more to the instrument you care about.”
Scientists have sought an in-space demonstration of X-ray communications for decades, but the XCOM experiment on the space station will be the first.
NICER will pause its scientific observations for several X-ray communication tests, each lasting a few hours, over the next few months, Gendreau said.
“The point of this first experiment is to kind of prove out X-ray communication, the general concept, and to elevate the (technology readiness) level of the X-ray source,” Gendreau said.
Just like laser communication provides a conduit to relay more data per second between terminals than radio waves, X-ray communication will offer another jump in bandwidth. But it comes with tighter constraints due to the narrow beam width of an X-ray communication source.
“Laser communication has a very tight beam, and the big challenges they have is pointing a laser communication transmitter accurately enough,” Gendreau said. “It gets challenging. You’re pointing at your receiver, and the spacecraft is moving, so you adjust some gimbals to change your transmitter direction, but it’s got mass, and that’s kind of causing the spacecraft to move. It gets very complicated to point a laser communication system.
“One of the long-term challenges for X-ray communication is it’s going to be even harder to point an X-ray communication system because the beam is much, much finer,” he said. “Engineers are going to do what they always do, and make something 10 times better than it needs to be. It’s just engineering to overcome these difficulties.”
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