Just How Big Is Pluto? Now We Know Thanks To New Horizons

Measurements of Pluto's size from ground-based telescopes have a lot of uncertainty due to the presence of an atmosphere on Pluto but thanks to NASA's New Horizons we now know that Pluto is 2,370 kilometers (1,473 miles) in diameter, confirming that Pluto is indeed the largest known Kuiper Belt Object - larger than dwarf planet Eris.

"The size of Pluto has been debated since its discovery in 1930. We are excited to finally lay this question to rest," said mission scientist Bill McKinnon, Washington University, St. Louis.

A portrait from the final approach. Pluto and Charon display striking color and brightness contrast in this composite image from July 11, showing high-resolution black-and-white LORRI images colorized with Ralph data collected from the last rotation of Pluto. Color data being returned by the spacecraft now will update these images, bringing color contrast into sharper focus. Credits: NASA/JHUAPL/SWRI

Pluto’s bright, mysterious "heart" is rotating into view, ready for its close-up on close approach, in this image taken by New Horizons on July 12 from a distance of 1.6 million miles (2.5 million kilometers). It is the target of the highest-resolution images that will be taken during the spacecraft’s closest approach to Pluto on July 14. The intriguing "bulls-eye" feature at right is rotating out of view, and will not be seen in greater detail. Credits: NASA/JHUAPL/SWRI

Charon’s newly-discovered system of chasms, larger than the Grand Canyon on Earth, rotates out of view in New Horizons’ sharpest image yet of the Texas-sized moon. It’s trailed by a large equatorial impact crater that is ringed by bright rays of ejected material. In this latest image, the dark north polar region is displaying new and intriguing patterns. This image was taken on July 12 from a distance of 1.6 million miles (2.5 million kilometers). Credits: NASA/JHUAPL/SWRI Credits: NASA/JHUAPL/SWRI

New Horizons also observed Nix and Hydra, two of Pluto's smaller moons. "We knew from the time we designed our flyby that we would only be able to study the small moons in detail for just a few days before closest approach," said New Horizons Principal Investigator Alan Stern of the Southwest Research Institute, Boulder, Colorado. "Now, deep inside Pluto’s sphere of influence, that time has come."

Nix is estimated to be about 35 kilometers (20 miles) wide with Hydra at about 45 kilometers (30 miles) wide. The sizes of the two smallest moons of Pluto - Kerberos and Styx - will be measured during the flyby and available later.

The approximate sizes of Pluto’s moons Nix and Hydra compared to Denver, Colorado. While Nix and Hydra are illustrated as circles in this diagram, mission scientists anticipate that future observations by New Horizons will show that they are irregular in shape. Credits: NASA/JHUAPL/SWRI Credits: NASA/JHUAPL/SWRI

NASA's New Horizons is now less than 1 million kilometers (620,000 miles) from Pluto. It will flyby the system on July 14 at 7:49 AM EDT and obtain the highest resolution images yet. I can't wait!

For the original Pluto press release, click here.