As more satellites launch into space, the satellite industry has sounded the alarm about the danger of collisions in low Earth orbit (LEO). What is less understood is what might happen as more missions head to a more targeted destination: the moon.
According to The Planetary Society, more than 30 missions are slated to launch to the moon between 2024 and 2030, backed by the U.S., China, Japan, India, and various private corporations. That compares to over 40 missions to the moon between 1959 and 1979 and a scant three missions between 1980 and 2000.
A multidisciplinary team at Georgia Tech has found that while collision probabilities in orbits around the moon are very low compared to Earth orbit, spacecraft in lunar orbit will likely need to conduct multiple costly collision avoidance maneuvers each year. The Journal of Spacecraft and Rockets published the Georgia Tech collision-avoidance study in March.
"The number of close approaches in lunar orbit is higher than some might expect, given that there are only tens of satellites, rather than the thousands in low Earth orbit," says paper co-author Mariel Borowitz, associate professor in the Sam Nunn School of International Affairs in the Ivan Allen College of Liberal Arts.
Borowitz and other researchers attribute these risky approaches in part to spacecraft often choosing a limited number of favorable orbits and the difficulty of monitoring the exact location of spacecraft that are more than 200,000 miles away.
"There is significant uncertainty about the exact location of objects around the moon. This, combined with the high cost associated with lunar missions, means that operators often undertake maneuvers even when the probability is very low - up to one in 10 million," Borowitz explains.
The Georgia Tech research is the first published study showing short- and long-term collision risks in cislunar orbits. Using a series of Monte Carlo simulations, the researchers modeled the probability of various outcomes in a process that cannot be easily predicted because of random variables.
"Our analysis suggests that satellite operators must perform up to four maneuvers annually for each satellite for a fleet of 50 satellites in low lunar orbit (LLO)," said one of the study's authors, Brian Gunter, associate professor in the Daniel Guggenheim School of Aerospace Engineering.
He noted that with only 10 satellites in LLO, a satellite might still need a yearly maneuver. This is supported by what current cislunar operators have reported.
Favored Orbits
Most close encounters are expected to occur near the moon's equator, an intersection point between the orbit planes of commonly used "frozen" and low lunar orbits, which are preferred by many operators. Other possible regions of congestion can occur at the Lagrangian points, or regions where the gravitational forces of Earth and the moon balance out. Stable orbits in these regions have names such as Halo and Lyapunov orbits.
"Lagrangian points are an interesting place to put a satellite because it can maintain its orbit for long periods with very little maneuvering and thrusting. Frozen orbits, too. Anywhere outside these special areas, you have to spend a lot of fuel to maintain an orbit," he said.
Gunter and other researchers worry that if operators aren't coordinated about how they plan lunar missions, opportunities for collision will increase in these popular orbits.
"The close approaches were much more common than I would have intuitively anticipated," says lead study author Stef Crum.
The 2024 graduate of Georgia Tech's aerospace engineering doctoral program notes that, considering the small number of satellites in lunar orbit, the need for multiple maneuvers was "really surprising."
Crum, who is also co-founder of Reditus Space, a startup he founded in 2024 to provide reusable orbital re-entry services, adds that the cislunar environment is so challenging because "it's incredibly vast."
His research also examines ways to improve object monitoring in cislunar space. Maintaining continuous custody of these objects is difficult because a target's position must be monitored over the entire duration of its trajectory.
"That wasn't feasible for translunar orbits, given the vast volume of cislunar orbit, which stretches multiple millions of kilometers in three dimensions," he says.
By estimating a satellite's orbit using observed data and constraining the presumed location and direction of the satellite, rather than continuous tracking (a process known as continuous custody), Crum greatly simplified the process.
"You no longer need thousands of satellites or a set of enormous satellites to cover all potential trajectories," he explains. "Instead, one or a few satellites are required, and operators can lose custody for a time as long as the connection is reacquired later."
Since the team started their study, there has been a lot of interest in the moon and cislunar activity - both NASA and China's National Space Administration are planning to send humans to the moon. In the last two years, India, Japan, the U.S., China, Russia, and four private companies have attempted missions to the moon.
Why the Moon
Spacefaring nations' intense interest in exploring the lunar surface comes as no surprise given that the moon offers a variety of resources, including solar power, water, oxygen, and metals like iron, titanium, and uranium. It also contains Helium-3, a potential fuel for nuclear fusion, and rare earth metals vital for modern technology. With the recent discovery of water ice, it could be a plentiful source for rocket fuel that can be created from liquifying oxygen and hydrogen needed to launch deep space missions to destinations like Mars. In February, Georgia Tech announced that researchers have developed new algorithms to help Intuitive Machines' lunar lander find water ice on the moon.
Commercial space companies like Axiom Space and Redwire Space, as well as space agencies, are actively building lunar infrastructure, from satellite constellations to orbital platforms to support communication, navigation, scientific research, and eventually space tourism.
A key project involves the Lunar Gateway, a joint venture of NASA and international space agencies like ESA, JAXA, and CSA, as well as commercial partners. Humanity's first space station around the moon will serve as a central hub for human exploration of the moon and is considered a stepping stone for future deep space missions.
Getting Ahead of a Gold Rush to the Moon
All this activity underscores the urgency to get out in front of potential crowding issues - something that hasn't occurred in LEO, where near-miss collisions, or conjunctions, are frequent. LEO, which is 100 to 1,200 miles above the Earth's surface, is host to more than 14,000 satellites and 120 million pieces of debris from launches, collisions, and wear and tear, reports Reuters.
"Using the near-Earth environment as an example, the space object population has gone from approximately 6,000 active satellites in the early 2020s to an anticipated 60,000 satellites in the coming decade if the projected number of large satellite constellations currently in the works gets deployed. That poses many challenges in terms of how we can manage that sustainably," observed Gunter. "If something similar happens in the lunar environment, say if Artemis (NASA's program to establish the first long-term presence on the moon) is successful and a lunar base is established, and there is discovery of volatiles or water deposits, it could initiate a kind of gold rush effect that might accelerate the number of actors in cislunar space."
For this reason, Borowitz argues for the need to begin working on coordination, either in the planning of the orbits for future missions or by sharing information about the location of objects operating in lunar orbit. She pointed out that spacecraft outfitted for moon missions are expensive, making a collision highly costly. Also, debris from such a scenario would spread in an unpredictable way, which could be problematic for other objects.
Gunter agreed, noting, "If we're not careful, we could be putting a lot of things in this same path. We must ensure we build out the cislunar orbital environment in a smart way, where we're not intentionally putting spacecraft in the same orbital spaces. If we do that, everyone should be able to get what they want and not be in each other's way."
Borowitz says some coordination efforts are underway with the UN Committee on the Peaceful Uses of Outer Space and the creation of an action team on lunar activities; however, international diplomacy is a time-consuming process, and it can be a challenge to keep pace with advancements in technology.
She contends that the Georgia Tech study could provide baseline data that "could be helpful for international coordination efforts, helping to ensure that countries better understand potential future risks."
Gunter and Borowitz say that follow-on research for the team could involve looking into the Lunar Gateway orbit and other special orbits to see how crowded that space will likely get, and then do an end-to-end simulation of these orbits to determine the most effective way to build them out to avoid collision risks. Ultimately, they intend to develop guidelines to help ensure that future space actors headed to the moon can operate safely.
