  We present "Tracklet-less Heliocentric Orbit Recovery" (THOR), an algorithm
for linking of observations of Solar System objects across multiple epochs that
does not require intra-night tracklets or a predefined cadence of observations
within a search window. By sparsely covering regions of interest in the phase
space with "test orbits", transforming nearby observations over a few nights
into the co-rotating frame of the test orbit at each epoch, and then performing
a generalized Hough transform on the transformed detections followed by orbit
determination (OD) filtering, candidate clusters of observations belonging to
the same objects can be recovered at moderate computational cost and little to
no constraints on cadence. We validate the effectiveness of this approach by
running on simulations as well as on real data from the Zwicky Transient
Facility (ZTF). 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 5 or more observations
and with purity between 97.7% and 100%. This includes 10 likely new
discoveries, and a recovery of an $e \sim 1$ comet C/2018 U1 (the comet would
have been a ZTF discovery had THOR been running in 2018 when the data were
taken). The THOR package and demo Jupyter notebooks are open source and
available at https://github.com/moeyensj/thor.
