... Voyager s Neptune . ... Data - Voyager 2 , NASA Planetary Data System Explanation: Cruising through the outer solar system, the Voyager 2 spacecraft made its closest approach to Neptune on August 25, 1989, the only spacecraft to visit the most distant gas giant. Based on the images recorded during its close encounter and in the following days, this inspired composited scene covers the dim outer planet, largest moon Triton , and faint system of rings. ... Triton: Neptune's Largest Moon . ...