  Extremely elongated, conducting dust particles (also known as metallic
"needles" or "whiskers") are seen in carbonaceous chondrites and in samples
brought back from the Itokawa asteroid. Their formation in protostellar nebulae
and subsequent injection into the interstellar medium have been demonstrated,
both experimentally and theoretically. Metallic needles have been suggested to
explain a wide variety of astrophysical phenomena, ranging from the
mid-infrared interstellar extinction at ~3--8 micron to the thermalization of
starlight to generate the cosmic microwave background. To validate (or
invalidate) these suggestions, an accurate knowledge of the optics (e.g., the
amplitude and the wavelength dependence of the absorption cross sections) of
metallic needles is crucial. Here we calculate the absorption cross sections of
iron needles of various aspect ratios over a wide wavelength range, by
exploiting the discrete dipole approximation, the most powerful technique for
rigorously calculating the optics of irregular or nonspherical grains. Our
calculations support the earlier findings that the antenna theory and the
Rayleigh approximation, which are often taken to approximate the optical
properties of metallic needles are indeed inapplicable.
