Charon

Charon is Pluto's largest moon and, relative to the world it orbits, the largest satellite in the solar system. With a radius of about 606 km it spans roughly half of Pluto's diameter, and the two bodies are so evenly matched in size that they behave less like a planet and its moon than like a bound pair. Their common centre of mass lies in the open space between them, so each swings around a point outside its own surface — the reason Pluto and Charon are often described as a binary dwarf system rather than a simple moon and host.

For nearly four decades after its discovery Charon was little more than a fuzzy companion dot beside Pluto. That changed in July 2015, when NASA's New Horizons spacecraft swept through the system and returned the first sharp portraits of a world nobody expected to be so geologically dramatic.

Discovered in a bulge on a photographic plate

Charon was found on 22 June 1978 by James Christy at the U.S. Naval Observatory's Flagstaff Station in Arizona, working with fellow astronomer Robert Harrington. Christy was not hunting for a moon at all — he was measuring photographic plates to refine Pluto's orbit when he noticed that Pluto's image was oddly elongated, a faint bulge that appeared and vanished on a regular rhythm. That periodic smear turned out to be a satellite too close to Pluto to separate cleanly with the telescopes of the day.

Christy proposed the name Charon after the ferryman who carries souls to the underworld ruled by Pluto — a fitting mythological match — while also echoing his wife Charlene's nickname, 'Char.' The International Astronomical Union formally adopted the name in 1985. The discovery immediately sharpened astronomers' picture of the system: tracking Charon's orbit finally pinned down Pluto's true mass and size, revealing it to be far smaller than earlier estimates.

A locked, mutual embrace

Charon orbits just under 19,600 km from Pluto — closer than many satellites hug their giant planets — and completes one circuit every 6.39 Earth days on an almost perfectly circular, barely inclined path. That period is not a coincidence: it exactly matches Pluto's own rotation. The system is doubly, or mutually, tidally locked, meaning each body forever shows the same face to the other.

The consequences are strange to imagine. From the Pluto-facing hemisphere of Charon, Pluto hangs motionless in the sky, never rising and never setting, slowly turning through its surface markings and phases in place. From the far hemisphere of either world, the companion is never visible at all. It is the most complete example of a synchronised planet-moon dance anywhere in the solar system.

A cracked-open crust

New Horizons revealed a surface far busier than the battered, uniform iceball many scientists had predicted. Charon's most striking feature is an enormous belt of chasms and troughs running across its face — including the canyon informally named Serenity Chasma — that stretches more than 1,600 km, roughly four times the length of the Grand Canyon and, in places, about twice as deep, plunging several kilometres to as much as 7 km. Mission geologists described it as though the entire crust of the moon had been split open.

The break may record an ancient internal ocean. If Charon once held a layer of liquid water beneath its icy shell, its freezing would have forced the moon to expand, fracturing the outer crust from within. South of the great canyons lies Vulcan Planitia, a broad, comparatively smooth and lightly cratered plain that appears to have been resurfaced — most likely flooded long ago by cryovolcanic 'lavas' of water and ammonia that welled up through the cracks and froze solid.

The red stain called Mordor

Charon's north pole is capped by a diffuse, reddish-brown patch that the New Horizons team nicknamed Mordor Macula, after the dark land in Tolkien's Middle-earth. Its origin is one of the more elegant results from the flyby. Gases escaping from Pluto's tenuous atmosphere — chiefly methane — drift across to Charon and become trapped in the deep cold of its winter pole, where temperatures fall low enough to freeze them out.

Through Charon's long seasons that frozen methane is bombarded by ultraviolet sunlight and radiation, which break it down and cook the residue into heavier, reddish organic compounds called tholins. Over millions of years these accumulate into the dusky stain we see. In effect, one world is quietly painting the pole of its companion — a chemical link between Pluto and Charon written across the surface.

An icy, airless world

Unlike Pluto, Charon has no appreciable atmosphere and shows little of Pluto's colourful variety. Its surface is dominated by water ice, with patches of ammonia-bearing compounds that hint at the cryovolcanic past, and it is grey and cratered across most of its terrain apart from the red polar cap. With half the diameter of Pluto but far less than half its mass, Charon is a lower-density, ice-rich body — consistent with formation from debris blasted off a young Pluto in a giant impact, the same broad scenario proposed for Earth's Moon.

How to glimpse it

Charon is a genuine challenge for backyard observers. At Pluto's distance the pair shines near 14th magnitude, well beyond naked-eye and small-telescope reach, and Charon sits so close to Pluto that separating the two visually demands a large amateur instrument, excellent seeing, and patient technique. Most enthusiasts instead follow Pluto and Charon as an unresolved point drifting slowly against the stars of the summer Milky Way. For a true view of Charon's canyons and its shadowy red pole, the images returned by New Horizons in 2015 remain, for now, the only window we have.