The school-bus-size object is the second stage of a SpaceX Falcon 9 rocket that had launched a lunar lander from Firefly Aerospace toward the moon in January 2025.
Weighing 4000kg, the rocket body is due to hit the moon around 4.35pm AEST, hitting its surface at 8690km/h.
The space junk will kick up a plume of lunar dust that will likely be illuminated by sunlight but difficult to spot with the naked eye from Earth.
The impact is unintentional, SpaceX said. The rocket piece is expected to hit Einstein Crater on the moon's western limb, which is often difficult to see from Earth.
Such stages typically fall back into Earth's atmosphere and burn up or plunge into the ocean after boosting the rocket's payload to a precise spot in orbit.
But because the January lunar lander mission required more thrust than missions closer to Earth, the rocket's second stage remained in space, floating aimlessly among thousands of other pieces of space junk that active satellites must steer clear of.
It was not until earlier this year that astronomers determined that the rocket stage, which had dumped its remaining fuel and cannot be controlled, was on an orbital trajectory ending at the moon.
"What has happened is essentially a mixture of solar activity and gravity forces have put it on a path toward the moon," Julianna Scheiman, SpaceX director of NASA science and Dragon programs, told reporters.
"This may be of some - probably minor - scientific interest, and we may learn some things from it," said Bill Gray, creator of widely used astronomy software who published a report on the stage's impact in April.
"It doesn't present any danger to anyone, though it does highlight a certain carelessness about how leftover space hardware (space junk) is disposed of."
Space junk impacts on the moon are rare. A Chinese rocket stage crashed into the moon in March 2022 after completing a lunar test mission. In 2009, NASA intentionally crashed a rocket stage into the moon to study the plume of lunar material kicked up by the impact.
Several spacecraft intending to softly land on the moon in recent years have crashed instead. Russia's nuclear-powered Luna-25 mission spun out of control and crashed in 2023. Its small power source of plutonium-238 likely remains harmlessly on the lunar surface.
NASA intentionally crashed stages from its Saturn V moon rocket into the moon in the 1970s to study the impacts' seismic effects.
NASA and SpaceX are discussing ways to prevent future lunar impacts, Scheiman said.
The US space agency plans to build a lunar base and send routine astronaut missions to the lunar surface beginning later this decade under its multibillion-dollar Artemis program. It would not want errant pieces of space junk impacting those assets.
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