We propose to use the WFC3 G141 grism to measure the near-IR thermal spectrum (1.1-1.7 microns) of the transiting exoplanet WASP-33b. WASP-33b is the most strongly irradiated and hottest known exoplanet, and it is key to testing hypotheses on the origin of temperature inversions in the atmospheres of giant hot exoplanets. Moreover, it orbits an A-type star whose spectrum is quite different from most planet-hosting stars. Consequently, the existence or non-existence of a temperature inversion in this planet is key to understanding what are the principal wavelengths where stellar irradiance drives the inversion process. The observational signature of an inversion in WASP-33b will be the presence of the strong 1.4-micron water band in emission, rather than in absorption as for non-inverted exoplanets. That signature will be very clear and obvious in our high-precision spectroscopy of this scorching world, and there will be no degeneracies in the interpretation. Our observations will span the time of secondary eclipse for this transiting system. We will use a ratioing technique - proven by our ongoing Cycle-18 program - to cancel instrumental errors and achieve photon-limited precision for the spectroscopy. We propose utilizing the new WFC3 driftscan capability to collect vast numbers of stellar photons from this bright (V= 8.3) system with high observing efficiency. Our observations will thereby produce some of the most precise exoplanet spectroscopy ever achieved. eclipses of WASP-33 EXTRA-SOLAR PLANETS 12 false 0.0 0.0 false false false false SUPPORTED STAR A4-A9 V-IV F3-F9 J2000 J2000 WFC3/IR Imaging No No F139M WFC3/IR Spectroscopic No No G141 WFC3/IR Imaging No No F139M WFC3/IR Spectroscopic No No G141 Phase windows are not calculated when the period is less than 1.5 days. Assuming no scheduling restrictions for Phase 0.340/0.348. The scheduling data was generated by CASM 20.2. CASM was run in Phase II mode. first eclipse of WASP-33 (Tic-Visit :proposal-id 12495 :visit-id "01" :proposal-version -1 :trans-version "20120802" :date "11-Nov-2012 21:54:53.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 5 :saa-model "30" :si-bright-earth-limb-angle 20.0 :data-volume 1294544 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12495 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12495 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2916 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3287 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12495 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 2966 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3287 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12495 :visit-id "01" :orbit-number 3 :pcs-time 222 :si-vis-utilization 2966 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3287 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12495 :visit-id "01" :orbit-number 4 :pcs-time 222 :si-vis-utilization 2966 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3287 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12495 :visit-id "01" :orbit-number 5 :pcs-time 222 :si-vis-utilization 2966 :al-overhead-at-end 0 :internal-dur-at-end 468 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3287 :pcs-gap 0 ) false 5 0 Phase windows are not calculated when the period is less than 1.5 days. Assuming no scheduling restrictions for Phase 0.318/0.326. The scheduling data was generated by CASM 20.2. CASM was run in Phase II mode. second eclipse of WASP-33 1353836083000 121.24182603213558 (Tic-Visit :proposal-id 12495 :visit-id "02" :proposal-version -1 :trans-version "20120802" :date "11-Nov-2012 21:55:58.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 5 :saa-model "30" :si-bright-earth-limb-angle 20.0 :data-volume 1294544 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12495 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12495 :visit-id "02" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2916 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3287 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12495 :visit-id "02" :orbit-number 2 :pcs-time 222 :si-vis-utilization 2966 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3287 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12495 :visit-id "02" :orbit-number 3 :pcs-time 222 :si-vis-utilization 2966 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3287 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12495 :visit-id "02" :orbit-number 4 :pcs-time 222 :si-vis-utilization 2966 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3287 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12495 :visit-id "02" :orbit-number 5 :pcs-time 222 :si-vis-utilization 2966 :al-overhead-at-end 0 :internal-dur-at-end 468 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3287 :pcs-gap 0 ) false 5 0