Pluto: What the First Reconnaissance Taught Us

Pluto and its heart-shaped Tombaugh Regio for Valentine's Day. NASA/JHU-APL/SWRI/New Horizons mission

As the New Horizons mission flew by the small planet Pluto on July 14, 2015, collecting images and data of the planet and its moons, an amazing chapter in planetary exploration began to unfold. The actual flyby occurred early in the morning on July 14, and the signal from New Horizons telling its team that all went well arrived at Earth at 8:53 p.m. that night. The images told the story that people had been waiting on for nearly 25 years.

The spacecraft’s cameras revealed a surface on this icy world that no one expected. It has craters in some places, icy plains in others. There are chasms, dark and light areas, and regions that will take some detailed scientific analysis to explain. Scientists are still getting a grip on understanding the scientific treasure trove they've uncovered at Pluto. It took 16 months for all the data to make it back to Earth; the last bits and bytes arrived in late October 2016.

Pluto Up-Close

The mission scientists found a world with amazingly varied terrains. Pluto is covered by ice which itself is darkened in many areas by materials called "tholins". They are created when ultraviolet light from the distant Sun darkens the ices. The surface of Pluto appears to be covered with newer, fresher ice in the bright areas, along with craters and long-running cracks. Pluto also has mountain peaks and ranges, some as high as those found in the Rocky Mountains in the United States.

It now appears that Pluto has some kind of heating mechanism under its surface, which paves parts of the surface and shoves mountains up through others. One description likens Pluto's interior to a giant "cosmic lava lamp". 

The surface of Charon, the largest moon of Pluto seems to have a reddish dark polar cap, possibly coated with tholins that have somehow escaped Pluto and were deposited there.


The mission scientists knew going into flyby that Pluto has an atmosphere, and the spacecraft actually “looked back” at Pluto after it passed by, using the light of the Sun shining through the atmosphere to probe it. That data tells more precise information about the component gases in the atmosphere, as well as its density (that is, how thick the atmosphere is) and how much of each gas is there. They are looking mostly at nitrogen, which is also escaping the planet to space. Somehow, that atmosphere is replaced over time, possibly by gases escaping from beneath Pluto's icy surface.

The mission took an in-depth look at the moons of Pluto, including Charon with its distinctly gray color and dark pole. The data from the spacecraft will help them understand what the icy components are on its surface, and why it appears to be a frozen world with little of the internal activity that Pluto exhibits. The other moons are smaller, oddly shaped, and move in complex orbits with Pluto and Charon. 

What's Next?

The data from New Horizons has all arrived after 16 months of trickling back across the great distance between Pluto and Earth. The reason it took so long for the flyby information to arrive here is that there was a lot of data that must be sent.

The transmission is only 1,000 bits per second across more than 3 billion miles of space.

The data have been described as a “trove” of information about the Kuiper Belt, the area of the solar system where Pluto orbits. There are many questions that remain to be answered about Pluto, which include “Where did it form?”  “If it didn’t form where it currently orbits, how did it get there?”  and “Where did Charon (its largest moon) come from, and how did it get four other moons?”

Humans spent more than 85 years knowing Pluto only as a distant point of light. New Horizons revealed it as a fascinating, active world and whetted everybody’s appetite for more! Heck, it's probably not a dwarf planet anymore!

The Next World is in View

There's more to come, especially when New Horizons visits another Kuiper Belt object in early 2019.

The object 2014 MU 69 is along the spacecraft's path out of the solar system. It will sweep by on January 1, 2019. Stay tuned!