Will we ever visit Pluto?
Pluto remains the only planet that has not been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface (right). Yet an increasing amount of information is unfolding about this peculiar planet. The uniqueness of Pluto's orbit, rotational relationship with its satellite, Charon, spin axis, and light variations all give the planet a certain appeal. For this and other reasons, the Pluto-Kuiper Express mission was planned by NASA in the late 1980s to reach Pluto by 2010. Unfortunately, in Sept 2000, a stop work order was issued by NASA. A new mission, to reach Pluto by 2020, is being developed.

Pluto is usually farther from the Sun than any of the nine planets; however, due to the eccentricity of its orbit, it is closer than Neptune for 20 years out of its 249 year orbit. Pluto crossed within Neptune's orbit January 21, 1979, and made its closest approach September 5, 1989, remaining within the orbit of Neptune until February 11, 1999. This will not occur again until September 2226.

Pluto is the only planet to rotate synchronously with the orbit of its satellite. Thus being tidally locked, Pluto and Charon continuously face each other as they travel through space. Average separation and orbital period are used to calculate Pluto and Charon's masses. Pluto's mass is 7 times the mass of Charon, approximately 0.0021 Earth mass, or a fifth of our moon.

Pluto's composition is unknown, but its density (about 2x water) indicates that it is probably a mixture of 70% rock and 30% water ice much like Triton. The bright areas of the surface seem to be covered with ices of nitrogen with smaller amounts of (solid) methane, ethane and carbon monoxide.

Pluto's atmosphere probably consists primarily of nitrogen with some carbon monoxide and methane. It is extremely tenuous. and may exist as a gas only when Pluto is nearest the Sun (perihelion). For the majority of Pluto's long year, the atmospheric gases are frozen into ice.

Pluto: A planet or not?
There are some who think Pluto would be better classified as a large asteroid or comet rather than as a planet. Some consider it to be the largest of the Kuiper Belt objects (also known as Trans-Neptunian Objects, which we will learn about in the next lecture). There is considerable merit to the latter position, but historically Pluto has been classified as a planet and it is very likely to remain so.

Click HERE for the press release from the International Astronomical Union on this topic.