THIS PROPOSAL PROVIDES FOR THE COLLECTION AND ANALYSIS OF INDIVIDUAL AND SIMULTANEOUS MEASUREMENTS OF THE PLASMA CHARACTERISTICS OF PROTONS ALPHA PARTICLES AND HEAVY IONS ON THE STEREO OBSERVATORIES THROUGH THE OPERATION OF THE PLASMA AND SUPRATHERMAL ION COMPOSITION (PLASTIC) SENSORS. THE PROPOSED PERIOD OF PERFORMANCE BRACKETS THE PREDICTED SUNSPOT MINIMUM (2019-2020) OF THE SOLAR SUNSPOT CYCLE. THE PRIORITIZED PLASTIC SCIENCE GOALS FOR THE THREE YEAR PERIOD OF THIS GRANT FALL INTO TWO BROAD CATEGORIES ALIGNED WITH THE CURRENT PRIORITIZED SCIENCE GOALS FOR THE STEREO MISSION: (1) CHARACTERIZE SPACE WEATHER THROUGHOUT THE HELIOSPHERE (2) STUDY THE SOLAR WIND NEAR SOLAR MINIMUM. THERE MAY BE OTHER TOPICS OF OPPORTUNITY INCLUDING PICKUP ION SCIENCE TOPICS AND ICMES OBSERVED BY MULTIPLE S/C ASSETS AT 1 AU. THE TWO PRIMARY CATEGORIES ARE ALIGNED WITH OBJECTIVES AS STATED FOR THE STEREO MISSION IN THE SENIOR REVIEW OF HELIOPHYSICS OPERATING MISSIONS (2017). THIS PROPOSAL ADDRESSES NASA HELIOPHYSICS ROADMAP (2009-2030) FOCUS AREAS F (OPEN THE FRONTIER TO SPACE ENVIRONMENT PREDICTION) H (UNDERSTAND THE NATURE OF OUR HOME IN SPACE) AND J (SAFEGUARD THE JOURNEY OF EXPLORATION).