Assessing NASA Science Mission Extensions Act - Instructs the Administrator of the National Aeronautics and Space Administration (NASA), in conducting assessments of the costs and benefits of extending the date of the termination of data collection for those science missions that have exceeded their planned mission lifetime, to consider: (1) the potential continued benefit of instruments on such missions; and (2) the cost and schedule impacts, if any, of mission extension on other NASA activities and science missions.