  We present spectral calibration equations for determining mafic silicate
composition of near-Earth asteroid (25143) Itokawa from visible/near-infrared
spectra measured using the Near Infrared Spectrometer (NIRS), on board the
Japanese Hayabusa spacecraft. Itokawa was the target of the Hayabusa sample
return mission and has a surface composition similar to LL-type ordinary
chondrites. Existing laboratory spectral calibrations use a spectral wavelength
range that is wider (0.75-2.5 microns) than that of the NIRS instrument
(0.85-2.1 microns) making them unfit for interpreting the Hayabusa spectral
data currently archived in the Planetary Data System. We used laboratory
measured near-infrared reflectance spectra of ordinary (H, L and LL) chondrites
from the study of Dunn et al. (2010), which we resampled to the NIRS wavelength
range. Using spectral parameters extracted from these resampled spectra we
established a relationship between band parameters and their mafic silicate
composition (olivine and low-Ca pyroxene). We found a correlation >90% between
mafic silicate composition (fayalite and forsterite mol. %) estimated by our
spectral method and X-ray diffraction (XRD) measured values. To test the
validity of the newly derived equations we blind tested them using nine
laboratory-measured spectra of L and LL type chondrites with known composition.
We found that the absolute difference between the measured and computed values
is in the range 0.1 to 1.6 mol. %. Our study suggests that the derived
calibration is robust and can be applied to Hayabusa NIRS data despite its
limited spectral range. We applied the derived equations to a subset of
uncalibrated NIRS spectra and the derived fayalite and ferrosilite values are
consistent with Itokawa having a LL chondrite type surface composition.
