[meteorite-list] How Big Is Pluto? New Horizons Settles Decades-Long Debate

[Ron Baalke]

http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20150713

How Big Is Pluto? New Horizons Settles Decades-Long Debate
July 13, 2015

[Image]     
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).

[Image] 
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.

NASA's New Horizons mission has answered one of the most basic questions 
about Pluto - its size.

Mission scientists have found Pluto to be 1,473 miles (2,370 kilometers) 
in diameter, somewhat larger than many prior estimates. Images acquired 
with the Long Range Reconnaissance Imager (LORRI) were used to make this 
determination. This result confirms what was already suspected: Pluto 
is larger than all other known solar system objects beyond the orbit of 
Neptune.

"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.

Pluto's newly estimated size means that its density is slightly lower 
than previously thought, and the fraction of ice in its interior is slightly 
higher. Also, the lowest layer of Pluto's atmosphere, called the troposphere, 
is shallower than previously believed.

Measuring Pluto's size has been a decades-long challenge due to complicating 
factors from its atmosphere. Its largest moon Charon lacks a substantial 
atmosphere, and its diameter was easier to determine using ground-based 
telescopes. New Horizons observations of Charon confirm previous estimates 
of 751 miles (1208 km) kilometers) across

LORRI has also zoomed in on two of Pluto's smaller moons, Nix and Hydra.

"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."
 
[Graphic] 
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.

Nix and Hydra were discovered using the Hubble Space Telescope in 2005. 
Even to Hubble, they appeared as points of light, and that's how they 
looked to New Horizons until the final week of its approach to Pluto. 
Now, the latest LORRI images show the two diminutive satellites not as 
pinpoints, but as moons seen well enough to measure their sizes. Nix is 
estimated to be about 20 miles (about 35 kilometers) across, while Hydra 
is roughly 30 miles (roughly 45 kilometers) across. These sizes lead mission 
scientists to conclude that their surfaces are quite bright, possibly 
due to the presence of ice.

What about Pluto's two smallest moons, Kerberos and Styx? Smaller and 
fainter than Nix and Hydra, they are harder to measure. Mission scientists 
should be able to determine their sizes with observations New Horizons 
will make during the flyby and will transmit to Earth at a later date.