New Horizons: Exploring Pluto and Beyond
This artist’s concept shows NASA's New Horizons spacecraft during its 2015 encounter with Pluto and its moon, Charon.
Credit: Southwest Research Institute

New Horizons is a NASA spacecraft that was the first to visit dwarf planet Pluto in July 2015. Its pictures of the dwarf planet's icy surface, as well as observations of Pluto's moon Charon, are revolutionizing our understanding of solar system objects far from the sun.

The mission is now en route to an object deeper in the Kuiper Belt, called 2014 MU69. It will reach this object on Jan. 1, 2019.

The spacecraft's extreme distance from Earth makes it only the fifth to venture so far from home (the other ones being Pioneer 10 and Pioneer 11, and Voyager 1 and Voyager 2, which are either in the outer solar system or in the case of Voyager 1, interstellar space.)

Pluto's distance — about 3 billion miles (5 billion kilometers) from Earth — presented power challenges for New Horizon's designers, since the sun's rays are too weak to generate power. There are also long communications delays for those staying in touch with the 1,054-lb. (478 kilograms) spacecraft; at Pluto, it took 4.5 hours for a one-way message to get there from Earth. 

Further, our understanding of the Pluto system keeps changing. The planet was discovered in 1930 by astronomer Clyde Tombaugh at the Lowell Observatory. (New Horizons carries some of Tombaugh's ashes.) Since then, we've discovered new moons around Pluto — which can also be seen as dangerous obstacles for a spacecraft, if not accounted for. 

There are also ongoing debates about whether Pluto is a planet or not. The International Astronomical Union voted to change its status to “dwarf planet” in 2006, following the discovery of several similarly sized objects in the Kuiper Belt. 

However, New Horizons principal investigator Alan Stern has repeatedly said he is not in favor of the decision – especially after the flyby revealed more a more complex formation history than initially anticipated. Stern and others argued that such complexity is more representative of a planet, and plan to make a formal submission to the IAU explaining that in 2017.

This is a montage of New Horizons images of Jupiter and its volcanic moon Io, taken during the spacecraft’s Jupiter flyby . The image was released in Oct. 2007.
This is a montage of New Horizons images of Jupiter and its volcanic moon Io, taken during the spacecraft’s Jupiter flyby . The image was released in Oct. 2007.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Spacecraft typically have a set design lifetime, similar to warranties on electronics or cars. Over time, solar particles,cosmic rays and other phenomena can degrade the surface of the spacecraft or mess up the electronics. This makes long missions such as New Horizons especially challenging, requiring backup systems and a source of power (nuclear power) to keep the spacecraft alive far away from the sun.

New Horizons launched Jan. 19, 2006, on an Atlas V rocket from Cape Canaveral Air Force Station in Florida. A power outage and high winds delayed two previous launch attempts, but New Horizons made it safely into space on the third try.

The spacecraft's first destination was Jupiter, in February and March 2007. New Horizons passed by less than 1.4 million miles (2.4 million km) of the solar system's largest planet, making it the first spacecraft to swing by since the Galileo probe finished its mission at Jupiter in 2003.

Among New Horizons' first pictures were some of Io, Jupiter's volcanic moon. The spacecraft captured the clearest pictures ever of the Tvashtar volcano on Io, showing volcanic fallout that was bigger than the state of Texas.

Additionally, the spacecraft flew through a stream of charged particles swirling behind Jupiter. It found large bubbles of charged particles, or plasma, and also revealed variations in the stream.

At the time, astronomers said the observations could help with understanding the environment around "hot Jupiter" planets found at other stars.

To conserve energy and lessen the chance of anything breaking, controllers kept the spacecraft in hibernation save for periodic wakeups for navigation and systems checks. It emerged from hibernation in December 2014 to get it ready for the Pluto encounter and sending data back to Earth afterwards.

New Horizons was so busy gathering data in its July 2015 encounter that, as planned, the spacecraft didn't communicate with Earth during its closest approach to Pluto and Charon. Controllers celebrated when New Horizons phoned home and they knew that data was on the way.

Early pictures from New Horizons showed a surprisingly young surface, with a mountain range on Pluto as high as 11,000 feet (3,500 meters). Believed to be about 100 million years old at most, this range likely pointed to recent geological activity on the surface, but how was a mystery.

“This may cause us to rethink what powers geological activity on many other icy worlds,” said John Spencer, New Horizons geology, geophysics and imaging team deputy leader, in a statement shortly after the encounter.

More youthful terrain — a huge plain bereft of craters, just north of the mountainous range — popped up in pictures sent back in mid-July. The zone has been informally named Sputnik Planum and is a region of intense scrutiny, given that geologists are still trying to figure out what caused this.

This high-resolution image from New Horizons shows the “shoreline” of Sputnik Planum on Pluto.
This high-resolution image from New Horizons shows the “shoreline” of Sputnik Planum on Pluto.

Evidence of ionized gas tens of thousands of miles beyond Pluto found around the same time pointed to the planet's atmosphere being lost to space after the solar wind crashes into it. Later in July, team members presented evidence of a haze above Pluto's surface — another surprise. At the time, the models they had suggested the haze is created when sunlight breaks up methane in Pluto's atmosphere.

From time to time, New Horizons still does periodic data dumps back to Earth, revealing new insights about Pluto and Charon. Among them: evidence of a past subsurface ocean on Charon, and strange water ice hills on Pluto floating in frozen nitrogen. Some scientists have also suggested that Pluto could have the ingredients for life on its surface, even at its great distance from the sun. 

Ten years can be a long time in planetary science, and that is particularly true of Pluto. Since New Horizons left our planet in 2006, we've discovered another moon near Pluto. Planners have made course corrections to keep the spacecraft away from Pluto's moons.

Further, Pluto was demoted from its position as the ninth planet in our solar system. In August 2006, members of the International Astronomical Union (IAU) — the global body that governs astronomy names and other matters — met in a general assembly to decide on the definition of a planet.

This vote was called in response to the recent discoveries of large bodies in the Kuiper Belt, an area beyond Neptune believed to contain trillions of objects.

On Aug. 24, 2006, IAU representatives determined three features all planets must possess: 

  1. They must orbit the sun (and not another body, as a moon orbits a planet). 
  2. They must have enough mass to form a round shape. 
  3. They must be large enough to clean out bits of rock and other matter in the area around their orbits. 

Pluto didn't meet all the classifications, and was reclassified as a dwarf planet.

The decision drew fire from Alan Stern, the principal investigator of the New Horizons mission. "I'm embarrassed for astronomy. Less than 5 percent of the world's astronomers voted," he said in a 2006 interview with "This definition stinks, for technical reasons." 

In 2017, Stern and other scientists announced plans to go again to the IAU to redefine Pluto (and other objects) once again as planets, based on the complexity of Pluto’s terrain and formation history.

An overhead view of the New Horizons spacecraft's path across Uranus' orbit.
An overhead view of the New Horizons spacecraft's path across Uranus' orbit.

Over the Northern Hemisphere summer of 2014, investigators used the Hubble Space Telescope to see if there were any Kuiper Belt objects within reach of New Horizons after it concluded its Pluto mission. Scientists identified three candidates, with each of them at least 1 billion miles (1.6 billion km) beyond the dwarf planet.

With NASA approval in 2016, New Horizons’ mission was extended to take a closer look at one of these worlds, a Kuiper Belt object dubbed 2014 MU69. The New Horizons team is now determining how to operate the scientific instruments during the flyby, and the best way to orient the spacecraft as it goes over. The flyby distance has not been determined as of May 2017, but it is expected to be somewhere between 3,000 and 20,000 kilometers (1,875 to 12,500 miles).  

Meanwhile, even after the mission ends, a group of scientists, artists, engineers and more are proposing placing a sort of message from Earth on the free hard drive space on the New Horizons spacecraft.

"When New Horizons gets past Pluto, [and] has done all its data and is going on the slow boat to the heliopause [the boundary between the solar system and interstellar space], then it might be possible to just reprogram about 100 megabytes of its memory and upload a new sights and sounds of Earth that are not created by a small group of scientists but, in fact, are globally crowdsourced," said Jill Tarter, who is the co-founder of the SETI (Search for Extraterrestrial Intelligence) Institute, in 2013.