Charon, the Water Half

The body

Charon Is Half Pluto’s Radius and an Eighth of Its Mass, Which Makes It a Partner

Large enough to be a real ground, small enough to be cheap to leave.

Charon has a radius of 606 kilometers and a density of 1.702 grams per cubic centimeter, marginally lighter than Pluto’s 1.854 and consistent with a higher ice fraction. Surface gravity is 0.29 meters per second squared — 0.030 g, essentially the same as Ceres. Escape velocity is 591 meters per second.

The surface is water ice with ammonia-bearing material detected in and around the younger craters, which is the signature of relatively recent cryovolcanic resurfacing. There is no nitrogen: Charon is too small and too warm at its peak subsolar temperature to have retained any, which is precisely what makes the pairing useful. Observed

The complement

One Body Holds the Carbon and Nitrogen. The Other Holds the Water.

The two are chemically complementary and 19,591 kilometers apart, which is closer than Earth’s surface is to geostationary orbit. Because both are mutually locked, the crossing is flown against a fixed geometry rather than a changing phase angle, so the transfer vehicle is designed once and flown forever. Modeled

The energy case

Bulk Mass Leaves Charon for a Quarter of the Energy It Takes to Leave Pluto

Launch energy scales with the square of escape velocity, so Charon’s 591 meters per second against Pluto’s 1,210 is a factor of 4.2 in specific energy for anything headed to orbit. Water for shielding, water for propellant feedstock, regolith for mass — all of it is cheaper to lift from the smaller body, and all of it is present there.

Charon is also the better anchor ground. Pluto’s surface is substantially volatile ice that creeps and sublimates at 40 kelvin, so a structure there has to reach stable water-ice bedrock and stay clear of nitrogen and methane terrain. Charon’s crust is water ice and ammonia hydrate throughout with no volatile glaciers, which makes the Charon-side foundation of the tidal bridge the more forgiving of the two even though it does the same job.

Charon is not a candidate for a rotating space elevator. It is tidally locked, and the locked-body result places the synchronous radius 44 percent outside the Hill sphere for every tidally locked body without exception. The bridge works because it is a different structure.

Mordor Macula

The North Polar Cap Is Pluto’s Escaped Atmosphere, Cold-Trapped

Charon’s north pole carries a dark reddish cap named Mordor Macula, roughly 500 kilometers across. The accepted explanation is that methane escaping Pluto’s upper atmosphere is captured by Charon, freezes out in the polar winter — which lasts about 100 Earth years — and is processed into tholins by ultraviolet light and Lyman-alpha radiation.

The industrial reading is minor and worth one line: the binary already runs a material transfer between its two bodies without anyone building anything, and the product accumulates at a known, fixed address. Whether a cold trap 500 kilometers wide is a feedstock or a curiosity depends on measurements nobody has made. Inferred

Record

Provenance and Data Sources