On July 14, the New Horizons spacecraft completed a 9.5-year-long, 4.8-billion kilometer (3-billion mile) journey to the object furthest from the Sun to be visited by a spacecraft. It is somehow fitting that the Pluto fly-by occurred 50 years, to the day, after Mariner 4 took the first images of Mars, obtained during a spacecraft encounter. New Horizons provided the first close-up views of the dwarf planet Pluto, and also the first "Kuiper Belt Object" to be visited by a robotic emissary from Earth.

When New Horizons launched in 2006, it was headed toward the ninth planet, discovered by American astronomer Clyde Tombaugh in 1930. Later that same year, the International Astronomical Union (IAU), the organization representing astronomers from around the world, reached a historic but contentious decision to define what it means to be a ”planet.“ Pluto was considered to be small enough that its gravitational pull was insufficient to ”clear out” the zone of space through which it orbited the Sun, leading the IAU to state that Pluto was now the type example of a ”dwarf planet” and representative of the numerous large, icy bodies then known to orbit the Sun beyond Neptune in a doughnut-shaped zone first hypothesized by astronomer Gerard Kuiper. There remains a vocal group of planetary scientists (including Alan Stern, the principle investigator of New Horizons), along with a large portion of the American public, who disagree with the IAU decision about the status of Pluto.

In spite of this ”nomenclature” debate, New Horizons is revealing what the denizens of the Kuiper Belt actually look like, with startling revelations presented at each New Horizons press conference. As New Horizons made its closest approach to Pluto, the dwarf planet and its large moon Charon (half the size of Pluto) were transformed from fuzzy spots visible to the largest telescopes on Earth into objects with unique geologic histories. The best New Horizons image of Pluto sent to Earth prior to the closest approach revealed a surface with many circular features thought to be large craters produced by impacts (something that had been expected), but the enormous variation in how much sunlight was reflected from the surface, along with the complex patterns of the brightness differences, were a hint that Pluto had much to tell us about its history.

Hubble Space Telescope images indicated that the brightness of Pluto was spatially variable, but there was no indication of how those variations were produced. The pre-encounter image showed Pluto has both very dark and very bright regions close together, with the brightest equatorial area found in a heart-shaped region. High resolution images released after the encounter have revealed that Pluto has mountains that rise >3.5 km (>11,000 feet) above the surrounding plains. The bright plains of the ”heart” lack obvious craters (suggesting a young age) but are broken into polygonal patterns >15 km (>10 mi.) wide, the bright plains are made up of ices of methane, nitrogen, and carbon monoxide, and flows of nitrogen ice extend from the bright plains into and around the surrounding mountains and rough terrain.

It will take 16 months to send to Earth all of the uncompressed (full-resolution) data collected during the 22-hour-long close encounter phase, due to the great distance over which the data must be transmitted. However, during this time New Horizons will be the ”gift that keeps on giving” as these precious data arrive on Earth, are assessed by the science team, and released to the public. Stay tuned! See pluto.jhuapl.edu for updates and regular data releases from New Horizons.

Check back tomorrow for a first-hand account of what it was like to be at the Johns Hopkins University Applied Physics Laboratory for the Pluto flyby.