Speeding through the outer solar system nearly 2.8 billion miles from Earth, NASA's New Horizons spacecraft was put into hibernation mode Friday for the last in a series of power-saving sleeps as scientists on Earth prepare for a make-or-break encounter with Pluto next summer.

Artist's concept of the New Horizons spacecraft encountering a Kuiper Belt Object. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
 
The hibernating spacecraft will send weekly status beacons back to Earth, with wakeup scheduled for Dec. 7 to begin the final phase of its approach to Pluto. New Horizons will stay awake for two years to prepare for the encounter, fly by Pluto, and downlink science data.

The craft's appointment with Pluto is set for July 14, 2015, when it will zoom about 6,200 miles from the icy world's unmapped surface for a one-shot chance to explore Pluto's geology and atmosphere.

New Horizons passed the orbit of Neptune on Aug. 25, the first spacecraft to reach such distances in a quarter-century.

"It's going to be a bonanza for science in so many ways, and we're going to take you along for all of this journey," said Alan Stern, principal investigator for the New Horizon mission from the Southwest Research Institute in Boulder, Colo.

Stern said the New Horizons science team will use some of the three-month downtime to fish out new destinations for the probe, which has enough fuel to fly by one of the thousands of icy worlds in the Kuiper belt, a zone of Pluto-like objects that lies beyond the orbit of Neptune.

Kuiper Belt Objects, or KBOs, are a recent discovery. Astronomers only confirmed the existence of this new frontier of the solar system two decades ago, challenging assumptions about the boundaries of Pluto, and eventually relegating Pluto off the solar system's official list of planets.

NASA advertised the $700 million mission's capability to visit multiple destinations when New Horizons launched in January 2006, assuming new targets for the probe could be found in time.

"To explore them, we first have to find them," Stern said. "They're very faint. They're very far away. These are objects that are much smaller than pluto and probably much more primitive in terms of their chemistry and their appearance. These are objects the size of counties, for example, not the size of planets."

But the search for a follow-up target for New Horizons has, so far, come up empty.

These images from the Hubble Space Telescope were taken this summer in a candidate program to confirm the telescope's capability to find suitable targets for the New Horizons spacecraft. The image at left shows a Kuiper Belt Object at an estimated distance of approximately 4 billion miles from Earth. Its position noticeably shifts between exposures taken approximately 10 minutes apart. The image at right shows a second KBO at roughly a similar distance. The positions of these newly discovered objects are not consistent with any KBOs discovered previously. In reality, they are too faint to have been seen with ground-based telescopes (magnitudes 26.8 and 27.3, respectively). Scientists still must confirm whether these objects, or other candidates, are reachable by New Horizons. Credit: NASA/ESA/SwRI/JHUAPL/New Horizons KBO Search Team
 
"We've been using the largest ground-based telescopes in the world for about four years, and although we've been discovering Kuiper Belt Objects, none have been in that region of space and and none within our fuel reach," Stern said.

In a last-ditch effort, NASA tasked the Hubble Space Telescope to search for another destination for New Horizons when ground-based surveys failed to find one.

"Hubble has done a spectacularly good job and yielded literally hundreds of images of that part of the sky, from which we've found some candidates," Stern said. "We don't know if any of them, though, are in our fuel reach and we won't still for some months."

Scientists need repeated observations of each object to measure their orbits and determine if they are within the spacecraft's reach. New Horizons carries a limited amount of fuel, so any decision on a targeted Kuiper Belt Object must account for the probe's capabilities.

Stern said a New Horizons flyby is possible from 2016 to 2021, with the most likely timeframe between 2018 and 2020.

"We hope to know before the year is out, and we will keep you posted as soon as we've made a determination of whether there are reachable objects," Stern said. "We certainly hope so."

Follow Stephen Clark on Twitter: @StephenClark1.