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tokens: to the distance of 2I. The small apertures sample near-nucleus variations, whereas the 15,000 km aperture is a better measure of the total light scattered by the comet. The use of fixed linear (as opposed to angular) apertures provides a measure of the scattering cross section within a fixed volume surrounding the nucleus and obviates an otherwise necessary geocentric distance correction that is dependent on the surface brightness distribution. We obtained background subtraction from the median signal in a concentric annulus with inner and outer radii of 20,000 km and 30,000 km, respectively. The apparent magnitudes, V x , with x = 500, 1000, 2000, and 15,000 are listed in Table 2 , along with absolute magnitudes, H x , computed fromwhich corrects for the inverse square law and for the phase function, f ( α ) at phase angle α . In the absence of a measured value, we assumed f ( α ) = 0.04 mag degree −1 . The table also lists the scattering cross section, in square kilometers, computed fromwhere p V is the geometric albedo. We use p V = 0.1, as appropriate for dust in solar system comets (Zubko et al. 2017 ), while noting that the albedo of this unique interstellar object is likewise unmeasured and could be significantly different. The dust albedo is ∼2.5 times higher than the average comet nucleus albedo. Table 2 HST Fixed-aperture F350LP Photometry a Note. a For each date and aperture radius, l , the table lists the apparent magnitude, V , the absolute magnitude, H , and the scattering crossection, C e (in units of square kilometers), in the form V / H / C e . H and C e are computed using Equations ( 1 ) and ( 2 ). Temporal variations in the absolute magnitudes are plotted in Figure 4 , in which the UT 2020 March 4.3–9.3 dates of photometric outburst (Drahus et al. 2020 ) are marked by a vertical gray band. Our data show that the outburst was preceded by a progressive brightening of 2I by about 20% in all apertures. This brightening is not correlated with the phase angle, which changes modestly and nonmonotonically in this period (Table 1 ), and therefore cannot be ascribed to uncertainties in the phase function. Neither does it vary with the angle from the orbital plane. Instead, the data show a steady brightening from January 3 to February 24, indicating a period of increasing activity. We conjecture that this is due to a seasonal effect on the nucleus: according to the pole solution proposed by Kim et al. ( 2020 ), the subsolar latitude on the nucleus moved from the southern hemisphere to the northern during this period, bringing the Sun’s heat to previously unexposed ice. Figure 4. Absolute magnitudes vs. date for the 500, 1000, 2000, and 15,000 km radius apertures. Smoothed dashed lines are added to guide the eye. The shaded vertical band marks March 4–9, the dates of the reported photometric outburst. Our first postoutburst HST observation occurred about 2 weeks later (March 23), by which time the near-nucleus brightness had jumped