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The age of any asteroid fragment in Earth's surface can be found by using the radioisotope dating methods. The idea behind this method is relatively simple (although the lab protocols may be rather involved and complex). Many elements have unstable atoms due to the variable number of neutrons in them (all the atoms of an element have the same number of protons). This instability causes one or more protons to convert to neutrons (among other reactions), changing the identity of the atom to that of a different element. The original atom is called the parent atom, while the new atom is known as the daughter atom. We also know, from various related experiments, how long these conversions take for a particular element and define terms like half-life (time required for decaying to 50% of the original number of atoms). Determining the ratio between daughter and parent atoms, coupled with the knowledge of element's half-life, can be used to determine the age of asteroid fragment. Since the half life of elements is different from each other, we try different elements to get a good fit between element's half life and sample age. For example, C-14 takes 5730 years to convert half of the initial amount of carbon to N-14; while U-238 takes about 4.5 billion years for conversion of 50% of the initial amount of uranium to Pb-206.
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