ASTER is an advanced multispectral imager with a high spatial, spectral and radiometric resolutions for EOS-A platform. ASTER covers a wide spectral region from visible to thermal infrared by 14 spectral bands. Moreover, ASTER has a stereoscopic viewing capability by a near infrared band. Significant design challenges are realization of a pushbroom type visible and near infrared radiometer (VNIR subsystem) with a high spatial resolution of 15 m, a pushbroom type short wave infrared radiometer (SWIR subsystem) with high spectral resolution and a whiskbroom type thermal infrared radiometer (TIR subsystem) with a high spatial, spectral and radiometric resolutions. Long life mechanical coolers for the SWIR and the TIR subsystems are also components with a technical risk.