Triton is the largest moon orbiting the ice giant Neptune, and it is unique among the major satellites because it travels in a retrograde orbit, moving in the opposite direction of its planet’s rotation. This peculiar motion hints at a violent past, suggesting it was captured by Neptune’s immense gravity billions of years ago. Understanding Triton’s true physical size is the first step in appreciating its place in the solar system, especially when considering its vast distance from the Sun. This size information, primarily gathered by a single spacecraft flyby decades ago, allows scientists to establish the moon’s fundamental characteristics and begin the comparison to our home world.
Triton’s Core Physical Measurements
The precise dimensions of Triton were first accurately determined during the Voyager 2 flyby in August 1989, which provided the only close-up data we have of the moon. Before this mission, estimates of its size were much larger, but the flyby revealed that its true diameter is approximately 2,700 kilometers (about 1,680 miles). Triton is a surprisingly dense object for a moon in the outer solar system, possessing an average density of about 2.06 grams per cubic centimeter. This density suggests a composition of roughly 70% rock and metal, surrounded by an icy mantle composed of water ice and frozen nitrogen. The total mass of the moon is estimated to be 2.14 x 10²² kilograms, a figure derived directly from the gravitational influence it exerted on the passing Voyager 2 spacecraft, confirming Triton is a substantial, differentiated world.
Direct Size Comparison to Earth
Earth has an equatorial diameter of about 12,742 kilometers, meaning Triton’s 2,700-kilometer diameter is only about 21.2% of Earth’s size. This difference in diameter translates into an even greater difference in volume and mass, which fundamentally dictates the physical conditions on each world. The mass disparity is much more pronounced than the diameter difference. Triton’s mass is only about 0.36% of Earth’s mass, making our planet almost 280 times more massive. To visualize this scale, if Earth were the size of a standard basketball, Triton would be comparable to a small golf ball.
The immense gap in mass results in a significantly lower surface gravity on Triton, which is only about 8% of Earth’s surface gravity. Despite its smaller size and much lower gravity, Triton is massive enough to hold onto a thin atmosphere, primarily composed of nitrogen. This ability to retain an atmosphere is a direct consequence of its substantial size compared to most other moons and minor bodies in the outer solar system. The difference in gravity also affects the geology, as Triton’s internal structure must contend with much weaker self-compression compared to Earth’s powerful gravitational forces. The sheer magnitude of Earth’s volume allows it to sustain a molten core and thick atmosphere, a capacity Triton simply does not possess.
Contextualizing Triton’s Scale Among Solar System Bodies
Triton is classified as the seventh-largest moon in the solar system. Its size allows it to achieve hydrostatic equilibrium, meaning it has sufficient mass and self-gravity to pull itself into a nearly perfect spherical shape. Triton is often compared to the dwarf planet Pluto, as they share a similar size, composition, and likely origin. With a diameter of approximately 2,376 kilometers, Pluto is about 14% smaller than Triton, making Neptune’s moon the larger of the two icy worlds. Although Triton is slightly smaller than Earth’s Moon (3,474 kilometers across), it is more massive than all of Neptune’s other known moons combined, accounting for over 99.5% of the total mass orbiting the planet.