In October 2023, NASA launched its long-awaited on-again, off-again Psyche mission. The spacecraft is on its way to study the metal-rich asteroid 16-Psyche, an M-type asteroid that could be the remnant core of a planetesimal that suffered a collision long ago. But understanding the giant, metal-rich asteroid isn’t the Psyche mission’s only goal.
It’s also testing a new laser communication technology.
The new system is called Deep Space Optical Communications (DSOC.) DSOC uses infrared lasers to communicate between spacecraft and ground stations. In this first experiment, the Psyche spacecraft communicated with the Hale Telescope at Caltech’s Palomar Observatory in San Diego County, California. Psyche was beyond the Moon when it communicated, and the distance between the spacecraft and the Hale Telescope was nearly 16 million km (10 million miles.)
The successful test took place on November 14th, and during the test, data was transmitted and received by both the spacecraft and the ground station, a phenomenon called “closing the link.” The successful test is the DSOC’s ‘first light.’
“Achieving first light is one of many critical DSOC milestones in the coming months, paving the way toward higher-data-rate communications capable of sending scientific information, high-definition imagery, and streaming video in support of humanity’s next giant leap: sending humans to Mars,” said Trudy Kortes, director of Technology Demonstrations at NASA Headquarters in Washington.
The technology may be coming to fruition just in time. As our spacecraft instruments become more powerful and as the amount of data they send back grows, current spacecraft communication systems are struggling to keep up. High-bandwidth laser communication systems should relieve the bandwidth bottleneck that hampers existing missions.
This image shows the Psyche spacecraft in a clean room. The DSOC is the silver tube extending toward the top of the image. Image Credit: NASA.
Current spacecraft communication systems are based on state-of-the-art radio systems. But the infrared laser system at the heart of DSOC works with data transmission rates from 10 to 100 times greater than radio systems.
The benefits are obvious if the system can be perfected.
Currently, spacecraft communicate with Earth using NASA’s Deep Space Network (DSN). The DSN is made up of three facilities around the world, separated by about 120 degrees. So, no matter where a spacecraft is, it can communicate with one of the facilities. The three facilities are in California, Spain, and Australia.
The three facilities that make up the DSN. Each is separated by 120 degrees. Image: NASA/JPL
The DSN is reliable, and NASA allows other spacefaring nations to use the system. But since it’s based on radio communications, it’s becoming an outdated bottleneck.
While the DSN and other space communications systems are impressive, they’re struggling to keep up with future plans. It can take up to 20 hours to transmit a 250-megabit data payload directly to Earth. And it gets worse the further a spacecraft is from Earth.
NASA’s New Horizons mission is an instructive example. When it performed its flyby of Jupiter in 2007, it transmitted data back to Earth at about 38 kilobits per second (kbps.) That’s a little slower than old telephone dial-up modems from the past. The data rate dropped precipitously when it encountered its main objective, Pluto. The data rate plummeted to approximately 2,000 bits per second (bps) at that extreme distance. That’s like the telecommunications equivalent to Morse code.
To reach those speeds, New Horizons had to use both its antennae and transmit to NASA’s largest receiving dish here on Earth. It reached Pluto in July 2015, but it took until 2016 to transmit all of the data from the historic encounter. Imagine being a member of the New Horizons team waiting for critical, career-defining data.
“More data means more discoveries.”
Artist’s impression of New Horizons’ close encounter with the Pluto–Charon system. Look how large the antenna looms. Credit: NASA/JHU APL/SwRI/Steve Gribben
DSOC’s infrared laser system will be a huge improvement. It’s similar to radio communications but uses tighter waves. This allows ground stations to receive more data, which is a critical problem with our rapidly-improving spacecraft. The DSOC on Psyche has only a 22 cm antennae, while the ground transmit antenna is 1 meter and the ground receiving antenna is 5 meters. At a distance of 0.4 AU, the uplink speed should reach 292 kbit/s, and the downlink speed should reach 100 Mbit/s.
DSOC does suffer from some drawbacks, though. For instance, downlink speeds are slower in the daytime.
Spacecraft instruments, and especially cameras, are generating more and more data. These speeds, and hopefully higher speeds in future DSOC systems, should be able to keep pace.
This graph shows the expected data rates required for future deep space missions. Image Credit: By JPL, NASA – JPL, NASA, Public Domain, https://commons.wikimedia.org/w/index.php?curid=64264594
“Optical communication is a boon for scientists and researchers who always want more from their space missions, and will enable human exploration of deep space,” said Dr. Jason Mitchell, director of the Advanced Communications and Navigation Technologies Division within NASA’s Space Communications and Navigation (SCaN) program. “More data means more discoveries.”
This isn’t NASA’s first foray into DSOC. They’ve been working on it for years, and they’ve demonstrated it in Near-Earth Orbit and out as far as the Moon. But November’s test was the first deep space test. While DSOC promises faster communication, it requires extremely precise pointing, and the precision required increases with distance. The system works by transmitting a laser beacon from Earth to the spacecraft. That helps stabilize the line-of-sight between the two and helps Psyche aim its downlink laser accurately. Further tests at greater distances are the next step.
“We were able to exchange ‘bits of light’ from and to deep space.”
There’s also the latency problem. From the Moon, it takes about 2.5 seconds for a signal to reach Earth. In this test, Psyche was well beyond the Moon, and the signal took about 50 seconds to reach Earth. But while it’s at the asteroid, a signal from the Psyche spacecraft will need up to 20 minutes to reach Earth. That latency problem doesn’t go away just because the system is based on a near-infrared laser. Infrared light moves at the same speed as radio waves.
But even though there are future challenges yet to be overcome, the test was successful, and that’s the only result NASA can hope for.
“Achieving first light is a tremendous achievement. The ground systems successfully detected the deep space laser photons from DSOC’s flight transceiver aboard Psyche,” said Abi Biswas, project technologist for DSOC at JPL. “And we were also able to send some data, meaning we were able to exchange ‘bits of light’ from and to deep space.”
This view of NASA’s Ingenuity Mars Helicopter was generated using data collected by the Mastcam-Z instrument aboard the agency’s Perseverance Mars rover on Aug. 2, 2023, one day before the rotorcraft’s 54th flight. Imagine what it would be like to watch a video of the little helicopter. Credit: NASA/JPL-Caltech/ASU/MSSS
The future of space exploration is going to be more and more data-dependent. Imagine real-time (with a signal delay, of course) video from the surface of Mars, taken by high-resolution cameras on rovers. Imagine astronauts on the surface of Mars with real-time, Mars-hardened versions of Go-Pro cameras on their helmets. Imagine subscribing to the personal YouTube channel of a Mars astronaut.
Naturally, some people won’t believe what they’re seeing. But for those of us who follow along as space technology develops year by year, it will be another crowning moment.