12-01-2008 At the edge (Article 120)

[Simon Kenny]

At the edge (Article 120)

Pluto is a tiny world that lies 3664 million miles from the Sun. Its orbit of our star takes 248 earth years, so it has not yet made one complete orbit since it was discovered in 1930. Only one fifth the diameter of Earth, this dwarf planet spins on its axis every 6.4 earth days. It has a highly tilted axis of 123 degrees, which causes its north pole to appear almost upside down compared to Earth. Pluto has a highly elliptical orbit, hence for short periods it’s path takes it closer to the Sun than Neptune, and this orbit is again tilted at 17 degrees to that of the Earth, so it ‘strays’ further from the Solar System Plane (the Ecliptic) than the planets. NASA’s New Horizons probe was launched in January 2006 to visit Pluto, whose structure is as yet unknown, for arrival July 2015. From observations using optical and radio telescopes, and also radar, it is believed to have a rocky core covered in ice. With a surface of frozen methane, some of which evaporates to form a thin, short lived, atmosphere for a few years when the planet is closest to the Sun, as it warms up Pluto’s average surface temperature is minus 223oC.

Pluto’s moon, Charon, with more than half the diameter of Pluto itself, makes it the largest moon, in relation to its parent. Discovered in 1978, it orbits Pluto every 6.4 days, and as this is the same as Pluto’s spin rate, both bodies always present the same face to each other, in a synchronised orbit. This means from one hemisphere of Pluto, it is always invisible, - there would never be a moonrise, whereas from the opposite hemisphere, it’s always visible and remains motionless, 11400 miles above the surface. An observer on each object looking at each other would also never see the far side of either body. Charon is believed to have a surface of water ice at minus 220oC, making this ice as hard as stone. Two smaller moons were discovered in late 2005.

Observing Pluto is difficult. The image below shows the best that ground based telescopes can show of Pluto and even in the Hubble Space Telescope image alongside, both still appear as star-like objects! The easiest way to identify Pluto is by studying its motion against the background stars over a period of weeks, or from photographs, a similar technique used for its discovery. Over the next four years, Pluto’s path takes it across the star rich summer constellation of Sagittarius, making amateur observations virtually impossible. Though not known by the ancient Greeks, Pluto represents their god of the Underworld, and from its dark and distant domain at the edge of our Solar System, the name is indeed most appropriate.