At Vandenberg Air Force Base workers perform a fit check mating the AIM spacecraft to the Orbital Sciences Pegasus XL rocket.

At Vandenberg Air Force Base in California, workers perform a fit check mating the AIM spacecraft to the Orbital Sciences Pegasus XL rocket. Red covers are in place to protect the Cosmic Dust Experiment instrument. AIM, which stands for Aeronomy of Ice in the Mesosphere, is being prepared for integrated testing and a flight simulation. The AIM spacecraft will fly three instruments designed to study polar mesospheric clouds located at the edge of space, 50 miles above the Earth's surface in the coldest part of the planet's atmosphere. The mission's primary goal is to explain why these clouds form and what has caused them to become brighter and more numerous and appear at lower latitudes in recent years. AIM's results will provide the basis for the study of long-term variability in the mesospheric climate and its relationship to global climate change. Launch from the Pegasus XL rocket is scheduled for April 25.