The history of our part of the Universe is written with a fine point in the space rocks which orbit our Sun. Some of them carry small grains of material from the cloud of gas and dust from which our solar system formed. They also carry clocks. Naturally occurring radio active potassium in space rocks decays into argon gas atoms which are trapped until the rock becomes molten. By measuring the potassium to argon ratio, Dr. Tim Swindle of the University of Arizona's Lunar and Planetary Laboratory is able to determine the length of time since meteorites have solidified after an impact event.