The thermosphere gradually transitions to the exosphere. From lowest to highest, the atmospheric layers are the troposphere, stratosphere, thermosphere and exosphere. These particles, mainly electrons and protons, precipitate into the upper atmosphere (thermosphere/exosphere). It incorporates most of the thermosphere and all of the exosphere. The exosphere contains most of the satellites orbiting Earth. NASA's "LADEE" probe, designed to study the lunar exosphere, achieved orbit on 6 October 2013. The most common molecules within Earth's exosphere are those of the lightest atmospheric gases. The exosphere is the altitude at which the atmosphere becomes collisionless. This indication was found by detecting the extended exosphere of atomic hydrogen. The fourth species detected in Mercury's exosphere was sodium (Na). The temperature of Mercury's exosphere depends on species as well as geographical location. The exosphere is located too far above Earth for any meteorological phenomena to be possible. Any atmosphere, however, would be the minimal kind known as an exosphere. The lower boundary of the exosphere is called the "exobase". The mission will study the exosphere composition and dynamics, including generation and escape.