There are two unmanned U.S. space missions currently at or preparing to arrive at dwarf planets in our solar system. Their names are Dawn and New Horizons and both have been traveling for years on long journeys getting to their target destinations. In April we discussed Dawn. This month we'll turn our attention to New Horizons.
First, let us review what a dwarf planet is. In the old days at the turn of the 21st century, our solar system had nine planets. This included the six classical planets known since antiquity lying closest to the sun, from Mercury out to Saturn (including Earth), plus three other worlds discovered in the past 234 years — Uranus, Neptune and Pluto. However, in 2006 the International Astronomical Union (IAU) shook things up by reclassifying them as major and dwarf planets.
According to various rules, available for review by simple web searches, all planets except Pluto fell into the major planet camp. Thus, there are now eight instead of nine major planets. Although Pluto is now considered a dwarf planet, it isn't alone. Several large asteroids have found themselves promoted from "minor planets" to dwarf status. These include Ceres, the largest asteroid in the main belt between the orbits of Mars and Jupiter. Three other objects located even farther out than Pluto were designated dwarf planets as well: Haumea (2004), Makemake (2005), and Eris (2005). Depending on who you ask there are up to half a dozen or so other small worlds beyond Pluto — all discovered in the current century –— which should belong in the dwarf planet category as well.
Space probes have visited all eight major planets. The most famous of these was Voyager II. Launched by the United States in 1977, Voyager II flew past Jupiter in 1979, Saturn in 1981, Uranus in 1986 and Neptune in 1989. But no spacecraft from any nation has ever visited Pluto.
That is expected to change this summer, when New Horizons — launched in 2006 — reaches its destination. New Horizons was the fastest spacecraft ever launched and covered the same distance from Earth as the moon's orbit — a journey that took Apollo astronauts three days — in less than nine hours!
During its long historic journey, New Horizons passed by several other solar system bodies. These included our own moon as well as an asteroid, plus a sling-shot maneuver past Jupiter. New Horizons is scheduled to encounter and fly past Pluto this coming July, nine-and-a-half years after its launch.
At Pluto's distance, the sun is about 1,500 times fainter than from Earth. If powered by the sun, New Horizons would need enormous solar panels. Instead, it is fueled by the decay of radioactive plutonium via a radioisotope thermoelectric generator. A similar power source was employed to power the lunar surface experiments left by the Apollo missions.
New Horizons has a wide array of scientific instruments on board. Imagers include a visible and infrared imager named Ralph, an ultraviolet imager called Alice, and a telescopic imager known as LORRI. There are also sensors for detecting dust, energetic particles and plasma from the solar wind.
Pluto has several moons. The first moon to be discovered was Charon in 1978. Nix and Hydra were discovered in 2005, less than a year before the launch of New Horizons. Two other moons were discovered after launch — Kerberos in 2011 and Styx in 2012.
Astronomers have long wanted to get a close-up look of the ninth planet, and it will finally happen on July 14 when New Horizons sails past Pluto and its ensemble of moons. Unfortunately, between launch and New Horizons' encounter with its target world, the same astronomers no longer consider Pluto to be a major planet.