Meet Pluto & its Moons: Key Facts About These Distant Worlds

New Horizons’ last look at Pluto’s Charon-facing hemisphere reveals intriguing geologic details that are of keen interest to mission scientists. This image, taken early the morning of July 11, 2015, shows newly-resolved linear features above the equatorial region that intersect, suggestive of polygonal shapes. This image was captured when the spacecraft was 2.5 million miles (4 million kilometers) from Pluto. (Credit: NASA/JHUAPL/SWRI)

MEET PLUTO
New Horizons Pluto Flyby Press Kit

General

• First dwarf planet discovered by an American, Lowell Observatory astronomer Clyde Tombaugh in 1930.
• It is the final classical planet in the solar system to be visited by a spacecraft.
• Pluto has five known moons — Charon, discovered in 1978; Nix and Hydra, discovered in 2005; Styx, discovered in 2011; and Kerberos, discovered in 2012.
• Charon is so large (half of Pluto’s size, same diameter as Texas) that the Pluto-Charon system makes up a “double planet,” the only one in our solar system. Together Pluto and Charon orbit around their common center of gravity in the space between them.
• Pluto is unusually difficult to study from Earth because it is so small and far away. It is 50,000 times fainter than Mars, with less than 1% of the red planet’s apparent diameter when viewed from Earth.
• Pluto is the largest and brightest known member of the Kuiper Belt — the solar system’s third zone — the vast region of ancient, icy, rocky bodies stretching almost 2 billion miles beyond Neptune’s orbit.
• The International Astronomical Union controversially opted in 2006 to classify Pluto and recently discovered large Kuiper Belt Objects as dwarf planets; debate continues on Pluto’s planetary classification.

Orbit

• Orbits the Sun once every 248 Earth years.
• Average distance from the Sun is 5.9 billion kilometers (3.7 billion miles), about 40 times farther out than Earth.
• Elliptical (oval-shaped) orbit; ranging from 4.4 billion kilometers (2.8 billion miles) to 7.4 billion kilometers (4.6 billion miles) from the Sun.
• Latest closest approach to the Sun was in 1989; from 1979-1999 Pluto was closer to the Sun than Neptune.
• Orbit is tilted 17 degrees from the ecliptic plane — the plane where the inner planets orbit the Sun— a higher “inclination” than terrestrial planets or gas giants (Mercury is next at 7 degrees).
• Pluto is tipped on its side — its rotational north pole is tilted 118 degrees from celestial north, or 28 degrees below the ecliptic plane.
• Pluto and Charon both rotate every 6.4 Earth days.
• Charon orbits Pluto once every 6.4 Earth days, from a distance of 19,636 kilometers (12,201 miles); Charon orbits at Pluto’s “synchronous” distance, with one side always facing Pluto.
• Pluto and Charon are locked in a gravitational resonance where not only does Charon keep the same face to Pluto (just like Earth’s moon faces Earth) but also, Pluto always sees the same face of Charon.

Physical Characteristics: Pluto

• Exact diameter is uncertain to about +/-25 kilometers, but close to 2,380 kilometers (1,500 miles); the circumference around Pluto’s equator is the same as the distance from Manhattan to Maui.
• Surface composition includes nitrogen, carbon monoxide, methane and ethane ices; many other materials may also be present, but undiscovered.
• Has a tenuous but complex atmosphere made mostly of nitrogen, with traces of methane, carbon monoxide, and some heavier hydrocarbons.
• The atmosphere undergoes extreme seasonal changes as Pluto orbits the Sun.
• Atmospheric surface pressure is currently about 50,000 times less than on Earth, about 300 times less than on Mars.
• Low surface gravity, about 6% of Earth’s.
• Estimated surface temperature is about minus-233 degrees Celsius (minus-387 degrees Fahrenheit).
• Has a density about twice that of water, indicating it is composed of a mixture of 35% ice and 65% rocky material.
• With ranges of very bright and dark areas, Pluto’s surface has more contrast than any planet in the outer solar system.

PLUTO’S MOONS

Image of Charon only from the New Horizons’ Long Range Reconnaissance Imager (LORRI), July 8, 2015. (Credit: NASA-JHUAPL-SWRI)

Physical Characteristics: Charon

• Diameter is about 1,200 kilometers (745 miles), the largest satellite relative to the planet it orbits.
• Surface known to be mostly water ice, possibly also including ammoniated hydrates.
• No detectable atmosphere (from Earth-based studies).’
• Like Pluto, its density (about twice that of water) indicates it is composed of a mixture of 50% ice and 50% rocky material.

Fast Facts: Hydra, Nix, Styx, Kerberos

Pluto’s smaller moons have been hard to study in any detail from afar. New Horizons will change that by flying past them in July 2015, but we do know a few things:

• Hydra is the outermost known moon, orbiting Pluto every 38 days at a distance of approximately 64,700 kilometers (40,200 miles).
• Nix orbits every 25 days at a distance of 48,700 kilometers (30,300 miles).
• Estimated diameters of Nix and Hydra are between 40-150 kilometers (25-95 miles).
• Styx circles Pluto every 20 days between the orbits of Charon and Nix, and is likely just approximately 7 to 21 kilometers (4 to 13 miles) in diameter.
• Kerberos orbits between Nix and Hydra with a 32-day period; estimated diameter is approximately 10 to 30 kilometers (6 to 20 miles).
• Styx and Kerberos are 20 to 30 times fainter than Nix and Hydra.

For more on what we know about the Pluto system — that is, before New Horizons revolutionizes that knowledge — visit: https://pluto.jhuapl.edu/Pluto/index.php