Lift-off for space lasers

In December 2023, a payload on the International Space Station (ISS) known as ILLUMA-T began exchanging modulated infrared laser beams with a geosynchronous satellite some 35,000 kilometers above it. The experiment was designed to show how data—in this case high-definition video and photos of pets—can be relayed to the ground at high rates and without delay from spacecraft in low Earth orbit (LEO).

ILLUMA-T is the latest in a series of experiments NASA has conducted to put space-based laser communication through its paces—both for robotic and human spaceflight. Operating at optical frequencies, laser links can provide bandwidths that are orders of magnitude higher than those of conventional RF communications. The terminals used to send and receive messages generally have lower size, weight and power (SWaP) requirements than radio equipment, and because signals are transmitted in very narrow beams, they are far less susceptible to interference and eavesdropping.

Indeed, it is not just NASA and other space agencies that are embracing this technology. So too are other branches of government, especially the military, as well as industry. A number of companies, including SpaceX in the United States, are launching large fleets of satellites equipped with laser terminals into LEO to provide internet coverage across the globe—particularly in places ill-served by terrestrial links.

Challenges remain, notably in trying to mitigate for bad weather when sending signals to and from Earth. But according to Tom Wood at US IT and defense multinational CACI (which provided the modem and optical amplifiers for ILLUMA-T), satellite operators of all stripes are increasingly turning to laser communication. “Risks have [gotten] to the point where people are willing to deploy this thing,” he says.

Benefits of bandwidth

Free-space optical links involve modulated laser beams being sent between transmitting and receiving telescopes, with transceivers at either end to convert between electronic and optical signals. The links can be entirely on the ground, connect a ground station to a satellite, or connect one satellite to another. Given the constraints of space payloads, ground stations can generally accommodate larger telescopes and other features such as adaptive optics to correct for atmospheric distortions of the optical signal.

Communicating at optical rather than radio frequencies, satellites can beam data back to Earth more quickly or in greater absolute quantities. Bryan Robinson of the Massachusetts Institute of Technology’s Lincoln Laboratory, USA, cites the example of NASA’s Mars Reconnaissance Orbiter, which has sent back high-resolution images of a small percentage of the Martian surface since arriving at the red planet in 2006. “If you had an optical link,” he says, “you could map the entire surface in a few years.”

Edwin Cartlidge is a freelance science writer based in Rome, Italy.