Answer :
Final answer:
The estimation of the amount of astatine on Earth is based on indirect means such as studying the decay products of other radioactive elements.
Explanation:
The estimation of the amount of astatine present on Earth at any given time, despite not having any samples to directly investigate the element, is based on indirect means of prediction. Astatine is a highly radioactive element with a very short half-life, so it would quickly decay and disappear. However, scientists can study the decay products of other radioactive elements and calculate the expected abundance of astatine as a result of those decays. This allows them to estimate that the total amount of astatine in the Earth's crust is less than one gram at any given time.