This is an picture of the Lunar Prospector spacecraft, which was used to determine the size of the Moon's core.
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Image courtesy of NASA

Earth's Moon--A Chip Off The Old Block?
News story originally written on March 19, 1999

Scientists have used data from the Lunar Prospector spacecraft to determine that the Moon has a small core. This finding supports the formation theory that the Moon was once part of Earth and was ejected following a large impact.

"This impact occurred after the Earth's iron core had formed, ejecting rocky, iron-poor material from the outer shell into orbit," explained Dr. Alan Binder, principal investigator for Lunar Prospector. "It was this material that collected to form the Moon."

Lunar Prospector used gravity measurements to determine the size of the Moon's core. It contains less than four percent of the Moon's mass. The Earth's core on the other hand contains 30 percent of our planet's mass.

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