U.S. Air Force awards grant for cislunar constellation to track spacecraft and debris

As the stretch of space between the earth and moon becomes increasingly congested with satellite and spacecraft traffic, a new U.S. Air Force-sponsored project seeks to create a constellation of sensor-mounted satellites to increase visibility and help prevent collisions.

The Air Force Office of Scientific Research on Oct. 22 awarded a $1 million grant to researchers at Rensselaer Polytechnic Institute (RPI) and Texas A&M University to develop the algorithms that would enable such a constellation to operate unobstructed in the notoriously observation-resistant domain of cislunar space. Ground-based telescopes tend to lose sight of objects at various points amid solar glare and the Earth’s rotation, and single space-based monitors like the James Webb Telescope and the Oracle-Prime satellite in development through the Air Force Research Lab (AFRL) would not be enough to create the “diverse geometry” of measurements the Air Force seeks to obtain through this new project, said Sandeep Singh, RPI’s lead investigator for the constellation algorithms.

“If you rely on just one spacecraft, there might be a single point of failure for whatever reason,” Singh said. “But if you send a constellation out there, you have multiple measurements.”

Plans call for the effort, RCAT-CS (Reconfigurable Constellations for Adaptive Tracking in Cislunar Space), to begin with a three-year research and development period funded by the grant, Singh said. Among the problems both teams of researchers aim to determine which of the elliptical “halo orbits” around one of the Earth-moon Lagrange points would be ideal to position the constellation, which will likely contain three to 10 satellites.

AFRL’s Oracle-Prime, currently under construction, is scheduled to be launched in 2027 to an orbit near Lagrange Point 1 to monitor space commerce and junk about 85% of the distance to the moon. L-1 is also under consideration for some of the RCAT-CS satellites, Singh said, but he noted that the team would not restrict placement to a single point but search “the entire periodic orbit family database” for the best locations.

Given that resources are limited on orbit, another key focus of the study is identifying tradeoffs and efficiencies. To continuously track a single object — for instance, the lunar Gateway space station NASA plans to station in a highly elliptical orbit near the moon as part of the Artemis lunar program — it might take three satellites in different halo orbits, Singh said. And each of the RCAT-CS satellites will weigh between 1,000 and 3,000 kilograms, meaning they will have limited onboard fuel, so the governing algorithms must determine the most efficient satellite configuration to observe the tracked object and how a given tracking mission might shift between satellites.

In cases where one group of satellites may have to hand off a tracking mission to another group, the study team must determine “what tradeoff do we have in terms of fuel versus accuracy?” Singh said.

Manoranjan Majji, associate department head for Space Engineering at Texas A&M and the university’s lead on the project, said his team would focus on the optimal placement of the sensor satellites for cislunar space observation and surveillance. A participant in a 2023 Air Force Research Laboratory-led university consortium project to study the best ways to track objects orbiting the moon, Majji said he’s seen the benefits of institutional collaboration to accelerate learning.

Likewise, the challenge of cislunar air traffic control won’t be solved by a single constellation. Some 30 moon-bound spacecraft are set to transit through cislunar space between 2024 and 2030, according to the Planetary Society, in addition to existing satellites and orbital debris. Singh anticipates this will prompt multiple nations to develop and launch their own tracking constellations and ideally work together to form a more comprehensive picture of space traffic between the Earth and moon.

“The only thing you need is for [the constellations] to speak to one another and essentially transmit their measurements and the information that they’re getting from their sensors,” he said. “You can think of this as a unified object, but then it is built up of these multiple agents.”