The Aeronomy of Ice in the Mesosphere (AIM or Explorer 90) is a NASA satellite originally launched to conduct a 26-month study of noctilucent clouds (NLCs). Its mission was extended, and as of 2022 is still operational. It is the ninetieth Explorer program mission and is part of the NASA-funded Small Explorer program (SMEX). Mission The scientific purpose of the Aeronomy of Ice in the Mesosphere (AIM) mission is focused on the study of polar mesospheric clouds (PMCs) that form about above the surface of Earth in summer and mostly in the polar regions of Earth. The overall goal is to resolve why PMCs form and why they vary. AIM expected lifetime was at least two years. AIM measures PMCs and the thermal, chemical and dynamical environment in which they form. This will allow the connection to be made between these clouds and the meteorology of the polar mesospheric summer echoes. This connection is important because a significant variability in the yearly number of noctilucent ("glow in the dark") clouds (NLCs), one manifestation of PMCs, has been suggested as an indicator of global change. The body of data collected by AIM will provide the basis for a rigorous study of PMCs that can be reliably used to study past PMC changes, present trends and their relationship to global change. In the end, AIM will provide an expanded basis for the study of long-term variability in the climate of Earth. The AIM scientific objectives will be achieved by measuring near simultaneous PMC abundances, PMC spatial distributions, cloud particle size distributions, gravity wave activity, cosmic dust influx to the atmosphere needed to study the role of these particles as nucleation sites and precise, vertical profile measurements of temperature, , OH, , , , NO, and aerosols. AIM carries three instruments: an infrared solar occultation differential absorption radiometer, built by the Space Dynamics Laboratory, Utah State University (Solar Occultation for Ice Experiment - SOFIE); a panoramic ultraviolet imager (Cloud Imaging and particle Size Experiment - CIPS); and, an in situ dust detector (Cosmic Dust Experiment - CDE), both designed and built by the Laboratory for Atmospheric and Space Physics, University of Colorado. Ball Aerospace & Technologies Corporation constructed the spacecraft bus and GATS, Inc., Newport News, Virginia, led the data management effort. First seen in 1885, two years after the powerful eruption of the Indonesian volcano Krakatoa, scientists originally thought PMC's formed from