  Flat rotation curves v(r) are naturally explained by elongated (prolate) Dark
Matter (DM) distributions, and we have provided competitive fits to the SPARC
database. To further probe the geometry of the halo one needs out-of-plane
observables. Stellar streams, poetically analogous to airplane contrails, but
caused by tidal dispersion of massive substructures such as satellite dwarf
galaxies, would lie on a plane should the DM-halo gravitational field be
spherically symmetric. We aim at establishing stellar stream torsion, a local
observable that measures the deviation from planarity in differential curve
geometry. We perform small-scale simulations of tidally distorted star clusters
to check that indeed a central force center produces negligible torsion.
Turning to observational data, we identify among the known streams those that
are at largest distance from the galactic center and likely not affected by the
Magellanic clouds, as most promising for the study, and by means of polynomial
fits we extract their differential torsion. We find that the torsion of the few
known streams that should be sensitive to most of the Milky Way's DM Halo is
much larger than expected for a central spherical bulb alone. This is
consistent with non-sphericity of the halo. Future studies of stellar stream
torsion with larger samples and further out of the galactic plane should be
able to extract the ellipticity of the halo to see whether it is just a slight
distortion of a spherical shape or rather ressembles a more elongated cigar.
