  We report the discovery of an extended halo (40' in diameter) around the
planetary nebula NGC7293 (the Helix Nebula) observed in 12micron band from the
Wide-field Infrared Survey Explorer all-sky survey. The mid-infrared halo has
an axisymmetric structure with a sharp boundary to the northeast and a more
diffuse boundary to the southwest, suggesting an interaction between the
stellar wind and the interstellar medium (ISM). The symmetry axis of the halo
is well aligned with that of a northeast arc, suggesting that the two
structures are physically associated. We have attempted to fit the observed
geometry with a model of a moving steady-state stellar wind interacting with
the ISM. Possible combinations of the ISM density and the stellar velocity are
derived from these fittings. The discrepancies between the model and the
observations suggest that the stellar mass loss has a more complicated history,
including possible time and angle dependences.
