New Horizons • Pluto

2015 will be a special year for interplanetary exploration. By the end of the 1970's, humans had sent spacecraft to every planet in the Sun's family, except one.  Missions to orbit Mercury, Venus, Mars, Jupiter and Saturn, and to land four rovers on Mars, have followed. More recently, many of the lesser objects in the solar system have been studied up close: asteroids, the moons of Jupiter and Saturn, and even comets.

In 1930, Pluto was discovered by Clyde Tombaugh, working at the Lowell Observatory in Flagstaff, Arizona. Much smaller than the Earth's Moon, Pluto was found nearly one and a half billion Kilometers beyond Neptune. Pluto has always been been an emigma, its orbit dips inside Neptune's for part of its year and is tipped out of the plane of the other planets, its polar axis is tilted by nearly 90 degrees, its biggest moon, Charon, is about the same size as it is.

In fact, Pluto isn't a planet like the others, it's a member of the Kuiper Belt of objects which has strayed down into the outer regime of the regular planets. The Kuiper Belt represents a cloud of material that never joined in the serious planet making beginnings of the solar system .. some clumping of rocky and icy material happened, but probably none of the violence of melting, bombardment, destruction and collisions that created the inner planets, or the immense gravitational influences that pulled Jupiter, Saturn, Uranus and Neptune into gaseous giants. The Kuiper Belt has been called the dust bunny collection of the solar system.

Following the discovery of more Kuiper Belt objects (KBO's) by Hubble and other powerful telescopes, Pluto joined the new designation of 'dwarf planets' in 2006. Arguments revolve about this reclassification but Pluto is now in a class containing more named objects than there are classical planets (and there are hundreds more observed candidates for the class awaiting confirmation of their orbits and naming).

But Pluto remains unexplored, and enigmatic, to this day. Telescopic instruments have improved since Tombaugh's day; but even the Hubble Space Telescope can magnify the faint point of light that is Pluto barely enough to image it as a fuzzy disk. The HST has also observed four small moons (Nix, Hydra, Kerberos and Styx) in addition to Charon. Pluto is rocky and has a thin atmosphere (mostly nitrogen) that freezes, falling like snow, for the Plutonian 'winter'.

[Pluto takes about 250 years to orbit the Sun. It swings in to about 30AU (closer than Neptune) for some of that time getting close enough to heat its atmosphere into a gas, then back out to about 50AU for a century and a half cold spell. This extreme orbital swing has more impact on Pluto's temperature that its seasons .. because its rotational axis is so tilted, Pluto's "arctic circle" is, essentially, its equator!  Pluto's south pole saw the sun for the first time in 120 years in 1987. Just to completely remove it from your list of vacation spots, the temperature of the surface is 35-55 degree above absolute zero.] -- 1AU (Astronomical Unit) is the distance from the Sun to the Earth ~ 150 million Km.

Planning for a mission to Pluto began in 1989, and the New Horizons spacecraft was launched in January 2006. It will fly though Pluto's space (about 10,000Km above its surface) on July 14, 2015. Recently, after a long search, another KBO was found by Hubble, far beyond Pluto, but reachable, with little expenditure of fuel, by New Horizons in 2019.