THOR: Searching for Asteroids Without Tracklets

We present “Tracklet-less Heliocentric Orbit Recovery” (THOR), an algorithm for linking observations of Solar System objects across multiple epochs that does not require intranight tracklets or a predefined cadence of observations within a search window. THOR sparsely covers regions of interest in orbital phase space with “test orbits”, then transforms nearby observations over a few nights into the corotating frame of each test orbit at each epoch. By performing a generalized Hough transform on the transformed detections followed by orbit determination filtering, candidate clusters of observations belonging to the same objects can be recovered at moderate computational cost with little to no constraint on cadence. We test the effectiveness of this approach by running on data from the Zwicky Transient Facility (ZTF). When applied to a short, 2-week, slice of ZTF observations, we demonstrate THOR can recover 97.4% of all previously known and discoverable objects in the targeted (a > 1.7 au) population with five or more observations and with purity between 97.7% and 100%. This includes 10 likely new Main Belt discoveries, and a precovery of an e ~ 1 comet C/2018 U1 (the comet would have been a ZTF discovery had THOR been running in 2018 when the data were taken). Applied to the same ZTF observations, algorithms requiring intranight linkages for Solar System searches would at best (assuming 100% completeness) have recovered 1.5-2x fewer minor planets. The THOR package and demo Jupyter notebooks are open source and available at https://github.com/moeyensj/thor. We’re also working on making THOR available as a scalable, cloud-based service.