LONDON — A discarded upper stage from a SpaceX Falcon 9 rocket is believed to have slammed into the Moon's surface near the Einstein crater early Wednesday, giving planetary scientists an unusual opportunity to study the effects of a high-speed impact whose exact size, speed and location are already known in advance.

The empty rocket stage struck the lunar surface at approximately 7:35 a.m. British Summer Time, or 6:35 a.m. GMT, traveling at roughly 5,400 miles per hour, or about 8,700 kilometers per hour, according to tracking data cited by researchers. The impact occurred in broad daylight across much of the world, meaning the brief flash the collision produced would have been far too faint to see, even for observers using high-end amateur telescopes.

A Predictable Piece of Space Debris

The rocket stage originated from a Falcon 9 launch that lifted off from Florida in January of last year, carrying two lunar landers into space. The mission's primary objective, successfully sending both landers beyond Earth's orbit and onto a trajectory toward the Moon, was accomplished at the time. What remained afterward was the spent upper stage itself, a discarded piece of hardware roughly the size of a five-story building and weighing at least 4,000 kilograms, left drifting in a long, looping orbit that repeatedly swung out toward the Moon and back again.

Over the following 18 months, the combined gravitational pull of the Earth, Moon and Sun, along with the faint but persistent pressure of sunlight itself,........

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