  It has been suggested by Dwek (2004) that iron needles could explain the
submillimetre emission from the Cas A supernova remnant (SNR) with only a very
small total mass. We investigate whether a similar model holds for the Kepler
supernova remnant, and find that its emission could indeed be explained by a
dust mass of less than 10^-2 M_sun dependent on the axial ratio (l/a) of the
needles - which we constrain to be less than 700. But the implied needle model
for Kepler is inconsistent with that suggested for Cas A since either the
needles would have to have a resistivity one or two orders of magnitude greater
than those in Cas A or the electron density in Kepler's shocked plasma must be
40 times greater than suggested by X-ray observations. An additional problem
with the needle model is that the implied thickness of the needles seems to be
implausibly small, if the emission properties are calculated under the usual
approximations.
