Although NASA's New Horizons spacecraft hasn't completed its primary mission yet, a flyby of the dwarf planet Pluto. The agency is planning ahead by using the Hubble Space Telescope for picking out potential target objects in the solar system's distant Kuiper belt.

The Kuiper belt is that part of the solar system that extends beyond the planets. The frozen objects it contains come from when the solar system first formed 4.6 billion years ago. Astronomers have confirmed thousands of these objects, but believe the actual number could be in the hundreds of thousands.

NASA plans to submit a proposal to extend New Horizons' mission next year so that they can study some of these objects, and now, thanks to Hubble, they've found some good candidates.

NASA astronomers spent time on Hubble searching for KBOs that were within New Horizons reach. The agency will track those objects over time, confirming that they're good candidates for flyby missions.

The team started looking for KBOs in 2011, using ground-based telescopes, but came up empty-handed, at least for those accessible to the New Horizons spacecraft's trajectory. KBOs are generally very small, and appear faint on telescopes, particularly with so many stars in their background.

"We started to get worried that we could not find anything suitable, even with Hubble, but in the end the space telescope came to the rescue," says New Horizons science team member John Spencer. "There was a huge sigh of relief when we found suitable KBOs; we are 'over the moon' about this detection."

Fortunately, astronomers identified three specific objects that are good candidates for New Horizons. Two are about 34 miles across, but the third is smaller, at about 15 miles across.

"This has been a very challenging search and it's great that in the end Hubble could accomplish a detection - one NASA mission helping another," says Alan Stern, principal investigator for New Horizons.

Once New Horizons flies by Pluto in July, 2015, it will continue traveling to the farthest regions of the solar system. If the federal government approves NASA's extended mission proposal, the spacecraft will fly by at least one of these chosen KBOs, reaching a distance of 1 billion miles beyond Pluto and a total of 4 billion miles from the sun.

If New Horizons visits a KBO, it will be the first time in history we've seen one up close. Who knows? That object may tell us some of the secrets of our solar system's birth. Whatever the case, it's also sure to raise more questions about where we come from.