A MAJOR GOAL IN CONTEMPORARY ASTROPHYSICS IS TO EXTEND THE STUDY OF STELLAR ATMOSPHERES TO THE MUCH COOLER ATMOSPHERES OF GAS-GIANT EXTRASOLAR PLANETS. ONE KEY PATHWAY IS THE IDENTIFICATION OF FREE-FLOATING SUBSTELLAR OBJECTS THAT NOT ONLY SHARE COMMON TEMPERATURES WITH EXOPLANETS BUT ALSO COMMON MASSES AND THUS SURFACE GRAVITIES. IN RECENT YEARS SEARCHES FOR EVER COLDER FREE-FLOATING OBJECTS HAVE STEADILY PUSHED THE CENSUS TO EVER LOWER MASSES AND FINALLY INTO THIS LONG-SOUGHT PLANETARY-MASS REGIME (5-15 JUPITER MASSES; Y DWARFS). THE EMPIRICAL BACKBONE OF THE EXPLORATION OF THESE NEW SUBSTELLAR SAMPLES IS THE ACCURATE MEASUREMENT OF DISTANCES VIA TRIGONOMETRIC PARALLAXES. DISTANCES ALLOW US TO MAKE EMPIRICAL SEQUENCES OF ABSOLUTE MAGNITUDE VERSUS COLOR AND SPECTRAL TYPE TO COMPARE TO BETTER STUDIED NORMAL BROWN DWARFS AS WELL AS PROVIDING ABSOLUTE BOLOMETRIC FLUXES AND THEREBY LUMINOSITIES AND TEMPERATURES. HOWEVER UNLIKE SIMPLE PHOTOMETRY OR SPECTROSCOPY PARALLAX MEASUREMENTS ARE TIME CONSUMING AND PRONE TO LARGE SYSTEMATIC ERRORS WHEN ATTEMPTED BY COMBINING DATA FROM MULTIPLE TELESCOPES AND BANDPASSES. SPITZER/IRAC IS POISED TO DELIVER THE DEFINITIVE SAMPLE OF PARALLAX DISTANCES FOR THE COLDEST BROWN DWARFS. IT IS THE ONLY FACILITY CAPABLE OF MEASURING HIGH-PRECISION ASTROMETRY EFFICIENTLY FOR A LARGE SAMPLE OF SUCH FAINT SUBSTELLAR OBJECTS AS VALIDATED BY OUR PAST WORK. SPITZER WAS NEVER INTENDED TO DELIVER HIGH QUALITY ASTROMETRY BUT USING OUR CUSTOM BUILT ASTROMETRIC PIPELINE WE HAVE SUCCESSFULLY PRODUCED THE FIRST PRECISE IRAC ASTROMETRY AND MEASURED DISTANCES FOR A SMALL INITIAL SAMPLE OF THE COLDEST BROWN DWARFS (DUPUY&KRAUS 2013). WE SHOWED THAT IN SPITE OF THE SEEMINGLY SUBTLE SPECTRAL CHANGES FROM T8 TO Y0 (ONLY 2 SUBTYPES) THE MEAN EFFECTIVE TEMPERATURE PLUMMETS BY ALMOST A FACTOR OF 2 FROM ~700 K FOR T8 DWARFS TO ~400 K FOR Y0 DWARFS. EVEN IN THIS SMALL SAMPLE WE FOUND SOME OBJECTS THAT SEEMED TO DEFY THE EXPECTATION THAT LATER SPECTRAL TYPE CORRESPONDS TO COOLER TEMPERATURE. THIS MAY POINT TO COMPLEXITY IN THE ATTEMPT TO SPECTRAL TYPE PLANETARY MASS OBJECTS AS OTHER PARAMETERS LIKE GRAVITY OR NONEQUILIBRIUM CHEMISTRY COULD BECOME JUST AS IMPORTANT AS TEMPERATURE. HOWEVER A SAMPLE LARGER THAN ONE OR TWO OBJECTS PER SPECTRAL TYPE BIN IS REQUIRED TO MAKE ANY DEFINITIVE STATEMENTS ABOUT PHYSICAL TRENDS. WE PROPOSE A COMPREHENSIVE STUDY OF THE COLDEST KNOWN BROWN DWARFS BY MEASURING ACCURATE DISTANCES FROM SPITZER PARALLAXES. WE WILL USE ALL DATA IN THE SPITZER ARCHIVE SUITABLE FOR PARALLAXES TO PERFORM A HOMOGENEOUS ANALYSIS OF A COMPLETE SAMPLE OF THE LATEST T AND Y DWARFS. OUR PROPOSED WORK WILL ROUGHLY TRIPLE THE SAMPLE SIZE AND FILL IN MAJOR GAPS AT THE T9.5 AND>Y0 SPECTRAL TYPES (~300-350 K) THAT PRODUCED SOME OF THE MOST PUZZLING RESULTS IN OUR PREVIOUS WORK. THE LONGER TIME BASELINE WILL ENABLE>3X MORE PRECISE PARALLAX MEASUREMENTS THAN IS POSSIBLE FROM THE GROUND FOR SIMILARLY FAINT OBJECTS. THE RESULTING EMPIRICAL SEQUENCES OF ABSOLUTE MAGNITUDE WILL PROVIDE A TOUCHSTONE FOR COMPARING BOTH MODELS AND DATA E.G. FROM DIRECTLY IMAGED EXOPLANETS. WE WILL ALSO USE THE ABSOLUTE FLUXES TO DERIVE BOLOMETRIC LUMINOSITIES FOR OUR SAMPLE. COMBINING THESE WITH WELL-ESTABLISHED MODEL RADII WILL RESULT IN ROBUST TEMPERATURE DETERMINATIONS THAT WILL ALLOW DEEPER SCRUTINY OF WHETHER TEMPERATURE OR A MORE COMPLEX MIX OF PHYSICAL PROPERTIES DRIVES SPECTRAL VARIATIONS IN THE PLANETARY MASS REGIME. FINALLY OUR ASTROMETRIC MONITORING WILL NATURALLY ALLOW US TO EXAMINE THE POPULATION S KINEMATICS AND THE LONG TIME BASELINE WILL ALLOW A NOVEL SEARCH FOR LOWER MASS COMPANIONS VIA NONLINEAR PERTURBATIONS. OUR PROPOSED WORK WILL RESULT IN A DEFINITIVE BENCHMARK SAMPLE FOR BOTH THE BROWN DWARF AND EXOPLANET COMMUNITIES PROVIDING CONTEXT FOR CURRENT AND FUTURE EFFORTS TO CHARACTERIZE BROWN DWARFS DIRECTLY IMAGED PLANETS AND THE COOL TRANSITING PLANETS PROBED BY SECONDARY ECLIPSE MEASUREMENTS.