2015

Pluto Revealed!

Despite its 2006 demotion from full-fledged planet to dwarf planet, Pluto is still a fascinating and enigmatic place. During the 1980s, a series of eclipses between Pluto and its large moon Charon enabled the basic details of the planet to be established from telescopic observations. Pluto is about 1,430 miles (2,300 kilometers) in diameter, or about 20 percent the size of Earth, but only 0.2 percent of Earth’s mass because of its low density of 2 grams per cubic centimeter and thus its predominantly icy composition.

Further telescopic observations from the Kuiper Airborne Observatory revealed the presence of a thin nitrogen, methane, carbon monoxide atmosphere around Pluto, with a surface pressure at least 300,000 times less than Earth’s atmosphere. Spectroscopic measurements of Pluto’s surface show it to be composed of more than 98 percent nitrogen ice, with trace amounts of methane and carbon dioxide. Pluto seems in many ways to be very much like Neptune’s large moon Triton, composed of nitrogen and other very-low-temperature ices and surrounded by a thin, probably dynamically changing atmosphere.

Finding out what Pluto is like up close was the job of the NASA New Horizons space probe. New Horizons was launched in 2006 and used a gravity-assisted swingby of Jupiter in 2007 to steer it toward a flyby of the Pluto system on July 14, 2015. The encounter was a resounding success, with millions watching around the world via the internet as Pluto and its moons were transformed from mere points of light into exotic, interesting worlds of their own. Vast plains of nitrogen and carbon monoxide ice cover Pluto, with many smooth areas suggesting relatively recent geologic or atmospheric resurfacing. Tall mountains of water ice rise above these plains in places, and glaciers flow into the valleys below. Intriguingly, despite being out on the cold, lonely fringes of the solar system, Pluto displays significant evidence of an active and complex geologic history. Charon, which is half the diameter of Pluto, has also had significant geologic activity on its surface, dominated by water ice, and its reddish nitrogen/methane/carbon monoxide polar cap.

The mission of New Horizons has continued beyond Pluto, as the spacecraft is on a trajectory (like Pioneer 10 and 11 and Voyager 1 and 2) to eventually leave the solar system, after passing by one or more additional Kuiper Belt objects, including a flyby of the small object named 2014 MU69 on January 1, 2019.

Pluto (with its bright, heart-shaped Tombaugh Regio) and its darker moon Charon, imaged by the New Horizons spacecraft during the July 2015 flyby.