Atmosphere of Triton asymmetry was actually observed during star occultations by Triton in 1990s. Before Voyager 2 arrived, a nitrogen and methane atmosphere with a density as much as 30% that of Earth had been suggested. This proved to be a great overestimate, similar to the predictions of the atmospheric density of Mars, but like on Mars, a denser early atmosphere is postulated. Voyager 2 flew past Triton five hours after closest approach to Neptune in mid-late August 1989. During the flyby, Voyager 2 took measurements of the atmosphere, finding methane and nitrogen in the atmosphere. In the 1990s, observations from Earth were
Upper Atmosphere Research Satellite view of Earth’s limb. A vertical scan of the atmosphere was obtained by tracking the sun during occultation. The scan will measure the amount of solar energy absorbed by gases in the atmosphere. In order to support scanning, the instrument came in two parts, the optics unit on a two-axis gimbal and a fixed electronics unit. The optics unit contains a telescope that collects solar energy as well as the gas detectors. The electronics unit handles data, motor control and power for the instrument. HRDI observed the emission and absorption lines of molecular oxygen above the limb of the Earth,