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Astronomers are a little unsure of the applicability of this index, but NASA’s Planetary Protection Officer is all in favor.
Alt text:
Astronomers are a little unsure of the applicability of this index, but NASA’s Planetary Protection Officer is all in favor.
This is not based on mass but on volume. For example the moon is only 1% of the earth’s mass yet considerably less dense therefore it makes a thicker layer.