And scientists are hoping New Horizons will get the funding and approval to head to a new destination — one of those small, mysterious icy worlds in the Kuiper Belt, which could provide new information about the beginnings of our solar system.
"That encounter is potentially more valuable than the Pluto encounter," says Brett Gladman, a University of British Columbia astronomer whose research focuses on the Kuiper Belt.
Discovered in the 1990s, it is a region full of thousands of icy objects, including Pluto, beyond the eight major planets of our solar system.
Scientists think the objects in the Kuiper Belt are the remnants of the collapsing cloud that created our solar system. And many look exactly the way they did when the system first formed 4.5 billion years ago, so they would provide a glimpse into the distant past.
New Horizons' next potential destination "is an object that formed right there where it is today and basically has had all of nothing happen to it over the age of the solar system," said Gladman, who holds a Canada research chair in planetary astronomy.
"That is in some sense incredibly interesting because we've never seen an object up close in the outer solar system that is basically a primordial relic."
That would make it quite different from Pluto and its moons, which are thought to have been formed in a catastrophic collision. And it's extremely different from planets such as Earth, which have been transformed so dramatically by multiple collision, volcanoes and other events that it's almost impossible to decipher what they looked like when they were born.
Ancient building blocks
Also, the smaller objects in the Kuiper Belt are thought to be similar to the building blocks that made up larger bodies like Pluto or the major planets, says Jim Green, director of planetary science at NASA headquarters.
"We don't know how to really build a planet," he added.
But scientists do know that when small objects collide to form bigger ones, the rubble ends up strewn all over the place,and is brought back together by gravity.
A big mystery is what happens when objects are smaller than that required for gravity to bring everything back together.
Green says scientists hope New Horizons can help find the answer to that by taking gravity measurements as it flies past a smaller Kuiper Belt object. It may also reveal new insights into the size, shape and composition of some of the building blocks of our solar system.
Two potential destinations
Two potential new destinations have been identified for New Horizons using the Hubble Space Telescope. Each is a few dozen kilometres across, or about the size of the smaller Pluto moons Nix and Hydra. Both are located around 2.2 billion kilometres beyond Pluto — about 15 times farther than the distance between the Earth and the sun.
Scientists need to decide by August which one to target, Green said, so they can start steering New Horizons in the right direction. The spacecraft only has enough fuel to adjust its trajectory by two degrees.
"If they don't make the burn [course correction] by the end of November, they won't make either one of them."
Scientists hope New Horizons will get far closer to its next target — within a few tens of thousands of kilometres — than it ever got to Hydra or Nix. That way, it will get far better images.
But steering isn't the only hurdle New Horizons needs to overcome. Right now, it only has funding to study Pluto and its moons, and that runs out on October 1, 2016.
In order to pay for scientists to observe and study its next target, NASA needs to secure funding from the U.S. Congress.
Green said the team is still working out how much funding is needed, and will submit a proposal to the government early next year.
If all goes well, New Horizons will arrive at is next destination in 2018.
Solar system's edge
Gladman said the chance of it visiting a third object after that is "pretty slim" as the density of objects in the Kuiper Belt falls off further away from the sun.
However, there is one last thing scientists hope New Horizons will explore — one of the boundaries of the solar system.
The boundary of the heliosphere is where the sun's magnetic field and solar winds lose their power.
New Horizons is expected to reach that boundary in about 20 years, with a much more complete set of instruments than the Voyager 1 spacecraft did when it hit the edge of the heliosphere in 2013.
While many of Voyager's instruments had failed by that point, New Horizons' instruments are "very healthy" and are expected to be able to take very good measurements of the solar wind as it exits the heliosphere.
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