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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 13 Jul 2015 15:31:03 -0700 (PDT)
Message-ID: <201507132231.t6DMV3qO006318_at_zagami.jpl.nasa.gov>

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.
Received on Mon 13 Jul 2015 06:31:03 PM PDT