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 EARTH-LEADING AND EARTH-LAGGING STEREO OBSERVATORIES THROUGH THE OPERATION OF THE PLASMA AND SUPRATHERMAL ION COMPOSITION (PLASTIC) SENSORS. THE PROPOSED PERIOD OF PERFORMANCE IS DURING THE DECLINING PHASE OF THE CURRENT SOLAR SUNSPOT CYCLE. THE PRIORITIZED PLASTIC SCIENCE GOALS FOR THE TWO YEAR PERIOD OF THIS GRANT FALL INTO THREE BROAD CATEGORIES ALIGNED WITH THE CURRENT PRIORITIZED SCIENCE GOALS FOR THE STEREO MISSION: (1) CHARACTERIZE SPACE WEATHER THROUGHOUT THE HELIOSPHERE; (2) STUDY THE CORONA AND SOLAR WIND AT SOLAR MINIMUM; AND (3) DETERMINE WHAT WE CAN LEARN FROM COVERAGE OF THE FULL HELIOSPHERE. THESE CATEGORIES ARE THE SAME OBJECTIVES AS STATED FOR THE STEREO MISSION IN THE PROPOSAL SUBMITTED TO 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).