ASTER is an advanced multispectral imager with a high spatial, spectral and radiometric resolutions for EOS-AM1 platform which will be launched in June 1998. 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. Excellent observational performance can be expected by 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 a high spectral resolution and a whiskbroom type thermal infrared radiometer (TIR subsystem) with high spatial, spectral and radiometric resolutions. Long life mechanical cryocoolers are developed to enhance the performances of the SWIR and the TIR subsystems.