Pluto and Charon, in enhanced color, thanks to observations from New Horizons’ Ralph/Multispectral Visual Imaging Camera (MVIC). Pluto’s frozen surface and intricate features are only part of its fascinating story; an ocean of subsurface water lurks far beneath the ice. (NASA/JHUAPL/SWRI)

Scientists Celebrate Pluto’s Discovery With A Retrospective Of Its Greatest Images

Happy birthday to Pluto, discovered on this day in 1930.

Pluto, first discovered in 1930, was no more than a distant dot in our most powerful telescopes.

Clyde Tombaugh’s original images identifying Pluto in 1930. The tiny, faint dot moves very slightly relative to the background stars, but sufficiently so that we’ve been able to successfully reconstruct its orbit. (LOWELL OBSERVATORY ARCHIVES)

Moving against the backdrop of fixed stars, its orbit was constructed after years of observations, revolving like no other planet.

The planets of the Solar System, along with the asteroids in the asteroid belt, all orbit in almost the same plane, making elliptical, nearly circular orbits. Beyond Neptune, things get progressively less reliable. Pluto was the oddball for decades, with its highly eccentric, out-of-the-plane orbital parameters. (SPACE TELESCOPE SCIENCE INSTITUTE, GRAPHICS DEPT.)

Pluto orbits out of the Solar System’s plane, highly inclined, and even approaches within Neptune’s orbit for a time.

In 1978, measurements of Pluto over time showed an unresolved bulge on one side that would appear and disappear periodically. That was its largest moon: Charon. The discovery of Charon has led to a much better understanding of just how tiny Pluto is. Its diameter is about 2274 km (1413 miles), and its mass is 0.25% of the mass of the Earth. Charon has a diameter of about 1172 kilometers (728 miles) and a mass of about 22% that of Pluto. The two worlds circle their common center of mass with a period of 6.387 days. (U.S. NAVAL OBSERVATORY)

In 1978, our telescopes had advanced enough to determine that it had a large satellite: the giant moon Charon.