The Upper Atmosphere Research Satellite (UARS) was launched in 1991 to study the ozone layer. It collected data that showed levels of ozone-depleting chemicals had stabilized by 2005, when UARS was decommissioned after exceeding its planned 3-year mission lifetime. As a satellite's orbit decays due to drag from solar wind and atmosphere, it is difficult to predict exactly where it will re-enter and debris will land, within a margin of about 4,280 miles, due to uncertainties from atmospheric effects.