Greg Brennecka is lead author on a paper appearing in the January 22nd, 2010 issue of Science. Coauthored with Stefan Weyer, Meenakashi Wadhwa, Phil Janney, J. Zipfel and Ariel Anbar, this paper measures the 238U/235U ratio in meteoritic material. This ratio has been assumed to be invariant, and is a critical assumption when calculating the age of the Solar System by the lead-lead dating method. These results show that the 238U/235U variations seen in these samples could cause miscalculation of the Solar Sytems oldest solids by as much as five million years. Calcium-Aluminum-rich Inclusions (CAI's) of the Allende meteorite have variable 238U/235U ratios, and correlations with curium analogs of thorium and neodymium suggests the variability is caused by decay of 247Cm to 235U in the early Solar System. This exciting result was profiled in a Science Perspective article by J.N. Connely in the same issue.