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A new class of exoplanets called ultra-hot Jupiters has recently been recognized these gas-giant exoplanets are on extremely close orbits around their host stars and have dayside atmospheres which reach >2500 K. Ultra-hot Jupiters are so strongly irradiated that many of the molecules comma including the dominant constituent hydrogen comma will thermally dissociate on their daysides and may recombine on their cooler nightsides. Significant mass loss and tidal distortion have also been predicted for some ultra-hot Jupiters. However comma there are currently few ultra-hot Jupiters whose entire atmospheres are well characterized comma and additional high precision observations are required to test recent model predictions. We therefore propose to collect one HST-WFC3 G141 spectroscopic full-orbit phasecurve of WASP-12b - the prototypical ultra-hot Jupiter - in order to create longitudinally resolved temperature and chemical maps comma to understand the enormous ellipsoidal variations seen at 4.5 microns comma and to better constrain the planet.s orbital decay rate and the stellar tidal quality factor.