The Orphan Stream earned its name by losing its parent: a long, narrow stellar stream arcing through the Galactic halo on a strongly tilted, great-circle-like path, with distance and velocity measurements tracing a coherent orbit whose progenitor dwarf is fully dissolved or at best weakly detected (Belokurov et al. 2007; Newberg et al. 2010).
Its diagnostic power is the problem it creates. Each stream in the halo is an independent probe of the Galactic potential, and a consistent model must fit them all with one halo. The Orphan Stream refuses to cooperate: halo shapes and substructure populations tuned to reproduce the Sagittarius stream or GD-1 mispredict the Orphan Stream's curvature, precession, and kinematics, and configurations that fit Orphan break the others (Lux et al. 2012; Koposov et al. 2019). The Gaia-era analyses deepened the puzzle by revealing a velocity component perpendicular to the stream track, conventionally attributed to a recent perturbation by the Large Magellanic Cloud, which adds another tuned ingredient to an already strained fit. A halo that must change shape depending on which stream is asked is not a measurement converging; it is a model degenerating.
The standing makes the Milky Way's stream ensemble a stringent joint test: whatever the Galactic potential is, it is one thing, and the streams collectively insist it is not a tuned triaxial particle halo. LSST will add dozens of new streams to the cross-examination.