Prediscovery of the near-Earth asteroid 2021 DG1 in a ZTF image from 2018 August, 2.5 yr before its discovery date. For clarity, the left image is zoomed to a 500 × 500 pixel subregion of the full 3072 × 3080 difference image. Black points are locations of sample orbits consistent with the postdiscovery observations. They are used to construct a convex hull (gray region) which forms the search region for this image. Image sources detected above 4σ significance are shown as green circles (filled if they are in the search region, empty if outside it). A detected source in another image is considered to be candidate prediscovery and the resulting trial orbit is propagated to this image. The inset shows the predicted location (+) and 1σ, 2σ, and 3σ confidence regions (black contours) for this updated orbit. The trial orbit is consistent with the location of a source detected in this image (green point), contributing greatly to this particular orbit’s likelihood ratio.