Occultations from SOFIA

Occultations occur when a relatively near object (like Pluto) passes in front and blocks light from a more distant object. These events can be useful in determining an object's size, its precise celestial coordinates, the vertical temperature, pressure and density profiles of its atmosphere, the vertical distribution of aerosols, and in cases where Fresnel fringes can be resolved, the distance to the occulting object. We are currently in an occultation golden age: the Gaia star catalog provides us with unprecedentedly accurate stellar positions, but these positions will be degraded by stellar proper motions in the years to come. A SOFIA occultation campaign often requires extensive pre-flight observations to determine a very accurate ephemeris for the occulting object, as well as adaptive optics observations of the star itself to make sure it is not double. For objects with sizes of a few 10s of km (like Arrokoth), a milliarsecond error in the object's RA and DEC relative to the occulted star is equivalent to missing the event by more than the width of the shadow path. Other science goals, like observing central flashes, also require that observers be placed within a few 10s of km of the center of the shadow path. SOFIA is an effective platform from which to observe occultations. It can be deployed with extreme precision to locations where ground-based observations are impossible, as it was for Arrokoth (then 2014 MU69). The permanent guide camera, FPI+, has properties that are well-suited to occultation observations: high quantum efficiency, low read noise and high frame rates. SOFIA's large aperture means that occultations of faint stars may still provide lightcurves with productive signal-tonoise ratios. We report on a few examples of SOFIA's successful occultation campaigns, including the first occultation detection of Arrokoth and occultations of Triton and Pluto with central flash lightcurve features. We briefly describe where central flash features come from and why they are so sensitive to haze opacity and temperature profiles. Finally, we discuss SOFIA's prospects for upcoming occultations.