If there is a buzzword for space in the 2020s, it’s “cislunar.” NASA talks about the lunar Gateway as the “cislunar springboard” for deep space exploration in an April white paper that is part of its broader exploration architecture. Companies discuss creating cislunar infrastructure like communications and navigation to support government and commercial missions. There is even a National Cislunar Science & Technology Strategy released by the White House last November “to guide the actions of the U.S. government in advancing scientific, exploration, and economic development activities in Cislunar space.”
But what, exactly, is cislunar space? It depends.
“There’s no commonly accepted boundary or definition for cislunar across the international or technical community,” said Jim Myers, senior vice president of the civil systems group at the Aerospace Corporation, during a briefing the organization held about cislunar space in late May.
There’s no shortage of potential definitions. The White House strategy defines Cislunar space (with a capital C) as “the three-dimensional volume of space beyond Earth’s geosynchronous orbit that is mainly under the gravitational influence of the Earth and/or the Moon.” Federal law, in the form of the NASA Authorization Act of 2010, defines “cis-lunar space” (with a hyphen) as “the region of space from the Earth out to and including the region around the surface of the Moon.”
Myers says Aerospace has its own definition: the region of space where the Earth’s gravitational influence is felt, including the moon but going beyond, to more than half a million kilometers from the Earth. Some lunar missions, like the NASA-funded CAPSTONE cubesat and ispace’s HAKUTO-R lunar lander, followed low-energy trajectories that took them more than a million kilometers from the Earth before swinging back to enter lunar orbit.
However you define it, the key point — and key concern — is that it’s a huge volume of space. George Pollock, an Aerospace astrodynamicist, illustrated that difference. If a racquetball represents the volume of space out to geosynchronous orbit, he noted, the volume of space out to the moon is a large beach ball.
That volume poses challenges for tracking objects, be they active spacecraft or debris. “It’s important to stay coordinated and vigilant about traffic and debris we’re introducing in the cislunar domain,” said Uma Bruegman of Aerospace’s strategic assessments, studies and projects division. “If there’s a collision in cislunar space, the debris could last thousands of years.”
There’s already been at least one close call. In October 2021, India’s Chandrayaan-2 lunar orbiter maneuvered to avoid coming within three kilometers of NASA’s Lunar Reconnaissance Orbiter. Both NASA and India’s space agency ISRO acknowledged the maneuver but said little about how they coordinated it.
Tracking operational spacecraft like that in cislunar space is straightforward, said Torrey Radcliffe, chief technology officer of Aerospace’s civil systems group. However, debris, already a challenge to track in Earth orbit, is much harder in cislunar space. “That’s something we don’t have the capability of doing right now,” he said. “At the moment, there’s not a lot out there, and the space is extremely large. It is something we need to have a plan for going forward.”
The national cislunar strategy did cite improving space situational awareness capabilities as one key objective. “We have some time” to develop those capabilities, he said, given the small number of spacecraft there now, but it’s unclear what needs to be developed and by whom. There are a few efforts getting started now, like the Air Force Research Laboratory’s Oracle spacecraft (previously known as the Cislunar Highway Patrol System) under development.
Prevention, though, is the best medicine for keeping cislunar space safe. “We have a chance to avoid recreating the same challenges around the moon that we created for ourselves in Earth orbit,” Bruegman said.
That will become increasingly important as more companies and countries send spacecraft to, around, or otherwise in the vicinity of the moon. That will increase traffic in cislunar space, no matter how “cislunar” is defined — or even spelled.
This article originally appeared in the June 2023 issue of SpaceNews magazine.
