A year ago, a robotic lander named Blue Ghost reached the moon.
Built by Firefly Aerospace of Cedar Park, Texas, Blue Ghost was the first American spacecraft in more than half a century to set down on the lunar surface and complete its mission.
While the mission was successful, there was a brief unscheduled danger one day before landing: Blue Ghost seemed at risk of crashing into another spacecraft orbiting the moon, potentially bringing the mission to a violent end before it ever got to the surface.
Will Coogan, the chief engineer for Blue Ghost, said the team was somewhat surprised by the events of that day, March 1, 2025.
“There’s only a few things in lunar orbit,” Mr. Coogan said. “This seemed so unlikely to be an issue, really, when it was raised to us.”
Only 11 spacecraft — from the United States, China, India and Korea — are currently in orbit around the moon. Imagine if just 11 airplanes were flying over the United States. It would seem infinitesimally unlikely that two of them would ever be flying over the same spot at the same altitude.
And yet for the past 15 years, a small team at NASA’s Jet Propulsion Laboratory in California has been keeping track of spacecraft orbiting the moon and Mars and raising alerts when it seems that two of them might cross paths.
That effort is known as MADCAP: Multimission Automated Deepspace Conjunction Assessment Process.
Think of them as space traffic controllers.
“The basic idea was to make sure that you knew where these other objects were so as to avoid any unintentional collisions,” David Berry, the lead for MADCAP, said.
Mr. Berry said the main reason that there was some probability of collisions around the moon is that the spacecrafts’ orbits are chosen for specific reasons, and not randomly. For science missions like that of Chandrayaan-2, an Indian spacecraft, it often makes sense that they fly in orbits that pass over the north and south poles, allowing them to observe the entire lunar surface.
When Blue Ghost arrived at the moon mid-February, it entered an elliptical polar orbit, gradually lowering to a circular orbit at an altitude of 100 kilometers, or about 62 miles. That was the same height as Chandrayaan-2. The trajectories of the two spacecraft were at an angle to each other, so they were generally far apart.
But over the north and south poles, the paths did potentially intersect.
Mr. Berry said the MADCAP team started highlighting “a red alert” — a potential collision on March 1 — almost a week ahead of time. Even with a red alert, the chances of a collision were very low.
”That just means more than a one-in-100,000 chance of collision,” Mr. Coogan said. “They’d give us updates every now and then. It would be red, but you’d wait a day, and it would not be red.”
This time, the red alert persisted.
MADCAP put the spacecraft teams in touch with each other to discuss whether one, or both spacecraft, should shift course slightly.
“Every time we do a maneuver, yeah, that also creates a risk,” Mr. Coogan said. “So do we think that we are creating or negating risk?”
Chandrayaan-2 had been orbiting the moon since 2019. It has a limited amount of propellant for maneuvers, and the Indian team does not want to use it unnecessarily.
With more analysis, it turned out that the spacecraft would pass each other at a safe distance, and no orbital swerving was necessary. Blue Ghost landed on March 2.
That incident was not the only time MADCAP has issued warnings.
In 2021, Chandrayaan-2 fired its thrusters to make sure it got out of the way of NASA’s Lunar Reconnaissance Orbiter.
In spring 2023, MADCAP was issuing nearly daily red-alert warnings of close passes between four orbiting lunar spacecraft — Lunar Reconnaissance Orbiter; Chandrayaan-2; a Korean spacecraft, Danuri; and Hakuto-R Mission 1, a lander from a Japanese company that later crashed into the moon’s surface.
“We held three maneuver‑coordination meetings that month to address them,” Mr. Berry said.
Last year, MADCAP handled 11 “response cases” — red alerts that were serious enough for the team of six people to step in and make sure no collisions would occur.
Currently, MADCAP largely relies on the spacecraft teams to provide it with updates of the trajectories. In the future, systems deployed at the moon and Mars could track spacecraft trajectories precisely and automatically.
With the moon attracting more interest in recent years and plans by both NASA and China to send astronauts there, the need for space traffic management will also increase.
“We don’t want to create a debris field,” Mr. Berry said.
Firefly will be sending a second Blue Ghost spacecraft to the moon, perhaps later this year. That mission includes an accompanying orbiter with a telescope, which will take pictures of the moon’s surface. But that can also be used to photograph other orbiters.
“We’ll be able to provide that as a commercial service and help with close fly-bys and tracking resident space objects around the moon,” Jason Kim, the chief executive of Firefly, said.
Mr. Coogan said they would most likely tweak the orbit slightly to avoid a repeat of the red alert situation. That might simply mean avoiding a 100-kilometer-high orbit.
“There’s no reason we can’t be at 95 kilometers, or 105 kilometers,” he said. “It’s just a nice round number that everybody picked because it’s a round number.”
