WFIRST Ultra-Precise Astrometry I: Kuiper Belt Objects

Abstract: I show that the WFIRST microlensing survey will enable detection and precision orbit determination of Kuiper Belt Objects (KBOs) down to H_=28.2 over an effective area of about 17 sq.deg. Typical fractional period errors will be about 1.5% x 10^0.4(H-28.2) with similar errors in other parameters for roughly 5000 KBOs. Binary companions to detected KBOs can be detected to even fainter limits, H_vega=29, corresponding to $R\sim 30.5$ and effective diameters D~7 km. For KBOs H~23, binary companions can be found with separations down to 10 mas. This will provide an unprecedented probe of orbital resonance and KBO mass measurements. More than a thousand stellar occultations by KBOs can be combined to determine the mean size as a function of KBO magnitude down to H~25. Current ground-based microlensing surveys can make a significant start on finding and characterizing KBOs using existing and soon-to-be-acquired data.