The spacecraft's planned rendezvous with the ancient object named 2014 MU69 - considered one of the early building blocks of the solar system - is January 1, 2019.
"The New Horizons mission to Pluto exceeded our expectations and even today the data from the spacecraft continue to surprise," said Nasa's Director of Planetary Science Jim Green.
"We're excited to continue onward into the dark depths of the outer solar system to a science target that wasn't even discovered when the spacecraft launched," he added.
In addition to the extension of the New Horizons mission, Nasa said the Dawn spacecraft should remain at the dwarf planet Ceres rather than changing course to the main belt asteroid Adeona.
"The long-term monitoring of Ceres, particularly as it gets closer to perihelion - the part of its orbit with the shortest distance to the sun - has the potential to provide more significant science discoveries than a flyby of Adeona," Green noted in a statement.
The Mars Reconnaissance Orbiter (MRO), Mars Atmosphere and Volatile EvolutioN (MAVEN), the Opportunity and Curiosity Mars rovers, the Mars Odyssey orbiter, the Lunar Reconnaissance Orbiter (LRO), and Nasa's support for the European Space Agency's Mars Express mission have also got extension.
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