Spaceflight Insider

Pluto found bigger than previously thought as excitement builds over flyby

Pluto and Charon display striking color and brightness contrast in this composite image from July 11.

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. Image Credits: NASA/JHU-APL/SwRI

On the eve of New Horizons’ historic Pluto flyby, the decades-long question of Pluto’s size has finally been answered: Pluto is larger than previously thought – it has a diameter of 1,478 miles (2,370 km). The finding comes from images taken by the spacecraft’s Long Range Reconnaissance Imager (LORRI). It means Pluto is the largest known object orbiting beyond Neptune.

“That settles the debate about the largest object in the Kuiper Belt,” principal investigator Alan Stern announced at a flyby celebration at the Johns Hopkins University Applied Physics Laboratory in Laurel, MD.

“It also has important scientific implications,” Stern continued. “Because it’s larger, and we’ve known its mass very precisely for a long time, that means it’s a little less dense, which will raise the fraction of ice in the interior that modelers will need to compensate the rock in the interior.”

Pluto being larger means the lower layer of its atmosphere, known as the troposphere, is shallower than predicted by some computer models.

Graphic representing a view of Pluto and Charon as they would appear if placed slightly above Earth's surface and viewed from a great distance.

Graphic illustrating a view of Pluto and Charon as they would appear if placed slightly above Earth’s surface and viewed from a great distance. Pluto has a diameter of 2370 km, 18.5% that of Earth’s, whereas Charon has a diameter of 1208 km, 9.5% that of Earth’s. Credits: NASA/JHU-APL/SwRI

For decades, determining Pluto’s size proved elusive to scientists because of its complicated atmosphere. In contrast, Charon’s diameter of 751 miles (1208 km) has been known for some time, as the moon’s lack of a significant atmosphere made it easier to measure using ground-based telescopes.

New Horizons confirmed earlier ground-based estimates of Charon’s size.

LORRI also took close-ups of Nix and Hydra, two of the system’s four small moons. The images enabled scientists to determine the sizes of both. Nix is approximately 20 miles (35 km) in diameter while Hydra is about 30 miles (45 km) in diameter.

The approximate sizes of Pluto’s moons Nix and Hydra compared to Denver, Colorado.

Graphic showing the approximate sizes of Pluto’s moons Nix and Hydra compared to Denver, Colorado. (Click to enlarge) Image Credits: NASA/JHU-APL/SwRI

Although the above graphic illustrates Nix and Hydra as circles, planetary scientists anticipate that further close-up images will show them to be irregular in shape. Their sizes indicate both moons have bright surfaces that could be composed of ice.

New Horizons will image Kerberos and Styx, the system’s smallest moons, during flyby; data taken will allow scientists to determine their sizes as well.

In the picture below, taken by New Horizons on July 12 from a distance of 1.6 million miles (2.5 million km), Pluto’s bright, enigmatic heart-shaped feature is rotating into view, ready for its close-up on closest approach. However, the mysterious “bullseye” feature on Pluto’s right side will remain so as it is rotating out of view and will not be imaged during the flyby.

Image taken by New Horizons on July 12 from a distance of 1.6 million miles (2.5 million km).

Pluto on July 12 at 2.5 million kilometers. (Click to enlarge) Photo Credits: NASA/JHU-APL/SwRI

The latest data has also confirmed that Pluto has a north polar cap; compositional spectroscopy indicates it has a higher methane ice composition than the planet’s dark equatorial regions. The polar cap also contains nitrogen ice.

Nitrogen was detected escaping from Pluto’s atmosphere five days earlier and much farther out than had been expected.

In the photo below, also taken on July 12, Charon shows a system of chasms followed by a substantial impact crater along the equator. Surrounding that crater are rings of bright ejecta. Charon’s strange, dark “anti-polar cap” shows unusual patterns not previously seen.

Image of Charon was taken on July 12 from a distance of 1.6 million miles (2.5 million km).

Charon on July 12 at 2.5 million kilometers. (Click to enlarge) Photo Credits: NASA/JHU-APL/SwRI

Stern confirmed that Alice, New Horizons’ ultraviolet instrument, is observing the surfaces of both Pluto and Charon.

As a fail safe mechanism, the probe sent back data and images gathered over the last few days to Earth for release on Tuesday. Should the probe be destroyed as it flies through the system, this assures the survival of as much information as possible.

That worst case scenario is improbable. According to Stern, the chances of the spacecraft being impacted by debris in the system is less than one in 10,000.

Beginning Monday night, the probe’s antenna will focus solely on Pluto to collect as much data as possible. This means New Horizons will not be in contact with Earth for 22 hours.

Closest approach to Pluto will be on Tuesday morning at 7:49 a.m. EDT. The anticipated “phone home” contact from New Horizons will not come until around 9:00 p.m EDT Tuesday night.

Google is celebrating the historic flyby with a special Google Doodle featuring a rotating Pluto as the second letter “o” in “Google” with a fast flying spacecraft against a background of stars.

Video courtesy of VideoFromSpace – Space.com

 

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Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

Reader Comments

Excellent recap of the Pluto encounter thus far. Let’s keep our fingers crossed for the images expected at 9 pm tonight.

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