This experiment seeks to probe the magnetic structure of multiferroic BiFeO3 and its evolution on lowering temperature, with the specific aim of identifying magneto-structural causes or responses to several anomalies recently reported by other techniques. These anomalies have been reported at temperatures around 463, 200 and 140 K. The magnetic structure of BiFeO3 is complex, but has been studies at both 4 K and ambient temperatures, at which points it appears essentailly the same: a G-type antiferromagnet with a long range incommensurate cycloidal superstructure. A detailed neutron powder diffraction study over the full temperature range of magnetic ordering, and in small T increments is a vital missing element in the understanding ofthis important material. WISH provides the ideal opportunity to carry out this timely experiment.