Voyage to Pluto

In July 2015, a spacecraft named New Horizons sped past Pluto, the farthest world NASA has ever visited. The spacecraft found tall mountains on Pluto and deep canyons on its largest moon. “I think the solar system saved the best for last,” said Pluto scientist Dr. Alan Stern.

Pluto is billions of miles from the Sun and Earth. It is so far away that our telescopes can’t see it well. So no one knew what the spacecraft would see.

Pluto vs. Eris

Pluto belongs to a belt of objects beyond the orbit of the distant planet Neptune. If Pluto were as big as a basketball, the typical object in this belt would be smaller than a marble.

But in 2005, astronomers found a similar world, Eris, which is much farther than Pluto. “If it’s not larger than Pluto, then I’ll eat my telescope,” the discoverer claimed. In 2010, Eris passed in front of a star and blocked its light. How long this event lasted revealed how big Eris is: 1,445 miles across.

The New Horizons spacecraft measured Pluto’s size: 1,477 miles across. So Pluto is bigger than Eris. In fact, Pluto is the largest object in the solar system beyond Neptune. 

A Cold World with a Heart

Pluto is so far from the Sun that sunlight there is weak. So Pluto is cold. The coldest temperature ever observed on Earth, near the South Pole, is -129 degrees F. But the spacecraft measured Pluto at -397 degrees F. Pluto is the coldest world a spacecraft has ever visited.

The New Horizons spacecraft surprised and delighted everyone by finding a huge white “heart” on Pluto. This heart-shaped region is a thousand miles across, bigger than Texas. Scientists named this heart Tombaugh Regio, after Clyde Tombaugh, who discovered Pluto in 1930.

Pluto’s “heart” may be white because of frozen nitrogen and carbon monoxide. Nitrogen gas makes up most of the air on both Earth and Pluto. But Pluto is so cold that most of its nitrogen freezes and turns into ice. Carbon monoxide is a poison gas on Earth, but Pluto is so cold that this gas also turns to ice. Maybe geysers or volcanoes in Tombaugh Regio erupt nitrogen and carbon monoxide gas that freezes, coloring this area white.

Pluto’s heart has immense mountains two miles high. That’s as tall as the Rocky Mountains. But Pluto’s mountains aren’t made of rock. Instead, they’re made of water ice! Pluto is so cold that its water ice is as hard as rock. Glaciers made of nitrogen ice flow across parts of Pluto. 

Mysterious Moons

The spacecraft also looked at Pluto’s five moons. The largest is Charon (pronounced “Karen” or “Sharon”). Spanning 753 miles, Charon is about half Pluto’s diameter.

While Pluto is orange, Charon is gray, and it has a polar cap. The polar caps on Earth and Mars are bright white. Charon’s polar cap is dark.

Charon has canyons that dwarf the Grand Canyon in Arizona. The Grand Canyon is about a mile deep; it formed when a river cut through rock. Charon’s canyons are hundreds of miles long and up to four miles deep. They probably formed when Charon’s crust tore apart.

Charon has craters that scientists have named for characters on the TV series Star Trek: Kirk, Spock, Sulu, and Uhura.

Like Earth, Pluto and Charon are round because they have enough mass that their gravity squeezes them into spheres. But Pluto’s other moons are so small that their gravity isn’t strong enough to force them into round shapes.

Styx is egg-shaped. Another moon, named Nix, is shaped like a jelly bean. The next moon out, Kerberos, looks like two small moons stuck together. And Pluto’s outermost moon, Hydra, is shaped like Michigan’s lower peninsula.

The New Horizons spacecraft looked for more moons around Pluto but did not find any. That disappointed scientists.

But the mission was a big success. Someday another spacecraft may visit, one that will circle Pluto again and again. Then we can study Pluto and its moons for years, discovering even more about these distant and mysterious worlds.