Was just watching a report on Osiris Rex project where the sent a rocket up to collect a sample of the asteroid and bring it back. They said that the asteroid was older than Earth itself. I think they said something like 4.5 billion years old. How can they know that?
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Various radioactive isotopes decay with rates that can vary from nanoseconds to trillions of years. Find some that are useful for the span of time that you are researching, and do the math.
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