Carbon dating 14, in the late 1940's Willard Libby discovered a way to determine the age of dead plants and animals remains.
All living things are made of carbon. A very small amount of this carbon is carbon-14 a radioactive isotope. One a plant or animal dies, the carbon-14 in its body begins to disintegrate. Since we know how fast carbon-14 decays, we can determine the approximate age of organic matter.
Comets
Space probe Giotto flew by the famous comet of all, Halley's comet, in 1986.
Swift Tuttle comet takes 128 years to orbit the Sun and won't pass Earth for another 100 years
Comets are big dirty snowballs of icy clumps outside Pluto's orbit