NEW HORIZONS Pluto at a Glance General • First planet discovered by an American, Lowell Observatory astronomer Clyde Tombaugh in 1930. • It is the only planet in the solar system never visited by a spacecraft. • Pluto has three known moons – Charon, discovered in 1978, and at least two smaller satellites, discovered in 2005. • Charon is so large (half of Pluto’s size) that the Pluto-Charon system is the only known double planet in the solar system. • Pluto is unusually hard 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. • One of the largest and brightest members of the Kuiper Belt, the vast region of ancient, icy, rocky bodies more than a billion miles beyond Neptune’s orbit. • Currently the smallest planet in the solar system and farthest from the Sun. - International Astronomical Union is debating whether recently discovered large Kuiper Belt Objects should be called planets. 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. • Most elliptical (oval-shaped) orbit of the nine planets, ranging from 4.4 billion kilometers (2.8 billion miles) to 7.4 billion kilometers (4.6 billion miles) from the Sun. - Latest close approach to the Sun was in 1989; between 1979-1999 it was closer to the Sun than Neptune. • Orbit is tilted 17 degrees from the ecliptic plane – the plane where most of the planets orbit the Sun – the highest “inclination” of any planet (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 days. - Charon also orbits Pluto once every 6.4 days, at an orbit of 19,636 kilometers (12,201 miles) from Pluto (Charon orbits at Pluto’s “synchronous” distance). - 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) but, also, Pluto always sees the same face of Charon. - Pluto’s smaller moons are roughly two to three times farther from Pluto than Charon, and roughly 10-20% of Charon’s size. Physical Characteristics: Pluto • Exact diameter is uncertain to within about 50 kilometers, but close to 2,360 kilometers (1,466 miles); about two-thirds the diameter of Earth’s moon. • Surface composition includes nitrogen, carbon monoxide, methane and water 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 upper atmosphere has a significant amount of free hydrogen. - The atmosphere undergoes extreme seasonal changes as Pluto orbits the Sun. • Atmospheric surface pressure is currently about 100,000 times less than on Earth, about 600 times less than on Mars. 6 NASA’s First Mission to Pluto and the Kuiper Belt