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New Horizons: En Route to Pluto

New Horizons is a NASA spacecraft on its way to the dwarf planet Pluto. It scooted by Jupiter in 2007, and will pass by Pluto in 2015 before possibly heading further into the Kuiper Belt – a massive zone of icy bodies beyond Neptune.

When the spacecraft reaches Pluto, it will be only the fifth one to head so far away from Earth (the other ones being Pioneer 10 and Pioneer 11, and Voyager 1 and Voyager 2, which now fly in the outer reaches of the solar system).

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

This 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 will also be long communications delays for those staying in touch with the 1,054-pound spacecraft; at Pluto, it will take 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. Since then, we've discovered new moons — which can also be seen as dangerous obstacles for a spacecraft, if not accounted for. And in 2006 — shortly after New Horizons launched – astronomers voted to demote Pluto from its planetary status. New Horizons carries some of Tombaugh's ashes.

Design challenges for long missions

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.

"You've got to remember that it takes 9.5 years to even get to where we want to take the mission," said Glen Fountain, the New Horizons mission project manager from Johns Hopkins University Applied Physics Laboratory, in a 2006 interview with NASA.

"So we need a highly reliable system. So, we have built into the electronics nearly two of everything. We are redundant. We have two guidance control processors, computers. We have two command and data handling processors. We have two solid-state recorders. Even if there is a failure, you can switch from one to the other."

Another question, Fountain acknowledged, was how to handle power when the sun is too weak to provide solar power. New Horizons carries nuclear power (more precisely, a radioisotope thermoelectric generator) on board to solve this problem.

Mission planners planned to keep the spacecraft in deep hibernation after a quick pass by Jupiter in February 2007. Essential systems will be turned on, but the rest will stay quiet until the spacecraft gets close to Pluto.

NASA does a detailed systems check of the spacecraft once a year to make sure it's working properly and to, if necessary, make adjustments to its path to Pluto. The spacecraft also ferries a basic signal back to Earth once a week.

Zipping by Jupiter

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.

This montage of New Horizons images shows Jupiter and its volcanic moon Io, and were taken during the spacecraft's Jupiter flyby in early 2007.
This montage of New Horizons images shows Jupiter and its volcanic moon Io, and were taken during the spacecraft's Jupiter flyby in early 2007.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Goddard Space Flight Center

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.

Plans for Pluto

One of the principal aims of New Horizons is to figure out the origins of Pluto and its companion Charon – a moon that is more than half Pluto's size. At the time, Pluto and Charon were considered a double planet (although the definition of Pluto changed, as will be explained below.)

NASA believed Charon formed when Pluto hit another big object long ago, creating debris that circled around Pluto and eventually formed Charon. It's a similar theory to how Earth's moon formed, so the scientists hoped to understand the creation of our moon better by looking at Charon's origins.

Scientists are also eager to learn about the visual differences between Charon and Pluto. From Hubble observations, researchers deduced Pluto is far more reflective than Charon, and that Pluto has an atmosphere while Charon does not.

NASA further speculated that Pluto might even have volcanic activity, because the Voyager 2 spacecraft spotted possible volcanoes (to researchers' surprise) on Triton, a moon of Neptune that is of a similar size and composition.

However, no one will know for sure until New Horizons gets close enough to give Pluto its close-up. NASA is already preparing for that time. During one of its periodic check-ins on New Horizons in 2012, NASA did simulated Pluto observations to see if its plans were in tip-top shape.

Although New Horizons was still 850 million miles (1.368 billion km) away from its target, NASA practiced the essential things the spacecraft will have to do when it is near Pluto. The agency sent messages back and forth to the spacecraft, and briefly turned on Pluto's instruments and cameras to collect data.

Indications immediately after the test showed that New Horizons was performing as expected, the agency said.

Pluto's planetary status changes

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 nearby Pluto. Planners have made course corrections to keep the spacecraft away from Pluto's moons.

Further, Pluto was dethroned 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.

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.
Credit: NASA, JHU/APL

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

On Aug. 24, 2006, IAU representatives determined three features all planets must possess: they must orbit the Sun, have enough mass to form a round shape, and 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 SPACE.com. "This definition stinks, for technical reasons."

The decision is still controversial, years later. Little is known about Pluto because it is so far away from Earth, but we have been able to increase our understanding of it through peering at the planet with the Hubble Space Telescope and other observatories.

It is expected that New Horizon's arrival at Pluto will give us more data about its surface, its moons and its environment, which can better refine our knowledge of the dwarf planet and its system.

— Elizabeth Howell, SPACE.com Contributor

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