When You’ve Been To Pluto, What Do You Do for an Encore?

When it blasted off for Pluto back in 2006, NASA’s New Horizons probe was poised to achieve several major milestones at once. It would be visiting the last planet still unexplored at close quarters (and yes, Pluto was still a planet when the mission began). It would also be the first mission to explore a class of planet vastly different from the Solar System’s rocky inner worlds, and also from the gas giants further out. Even after it was demoted to “dwarf planet,” Pluto represented the nearest of the ice worlds that lurk at the edges of the Sun’s influence. Understanding their true nature called for a close encounter—and New Horizons was designed to provide it.

But once the probe whips past Pluto and its moon Charon next July, it will still have functioning instruments and fuel to burn. And now, says NASA, it may have someplace to go and another scientific milestone to achieve. An intensive search with the Hubble Space Telescope has revealed three icy bodies more or less along New Horizons’ post-Pluto path and a billion miles (1.6 billion km) further out. Sometime in 2018 or 2019 the probe could be getting a close look at one of these so-called Kuiper Belt Objects (KBO)—a primordial remnant left from the very earliest days of the Solar System.

“The objects Hubble has identified are much smaller than Pluto,” says Alan Stern, a planetary scientist with the Southwest Research Institute, in Boulder, CO, and New Horizons’ Principal Investigator. “They’re the building blocks Pluto was made of.”

Hubble got this discovery in just under the wire. The rocket burn that will readjust New Horizons trajectory to intercept one of the KBO’s won’t happen until after the Pluto encounter. But in order to calculate that complicated maneuver, ground controllers need to know precisely how the KBO’s themselves are moving. “We need to make a series of observations,” says Stern, “to connect the dots.” And if they didn’t have a first set of observations by now, they wouldn’t have enough dots to connect.

In one sense, researchers have already gotten a close look at a KBO: Europe’s Rosetta probe went into orbit around a comet in August, with plans to set down a lander on its surface on November 9—and a comet is essentially a KBO that has wandered into the inner Solar System.

But that means it’s been exposed to the Sun’s heat, so unlike its cousins further out, it’s not truly primordial. Beyond that, these three new objects are between 15 and 35 miles (24 and 56 km) across. That’s about ten times bigger and a thousand times more massive than Rosetta’s comet, while still a thousand times less massive than Pluto. Whichever KBO New Horizons visits will therefore fill in a huge gap, helping scientists understand how Pluto itself formed.

It will, that is, if NASA approves the extended mission, funding the probe for longer than was originally planned. That kind of second act is not unusual—Hubble itself has had its mission extended several times, and so did the Spirit and Opportunity rovers on Mars. It’s not guaranteed, though. “We have to make a proposal,” says Stern, “but at least we now have something concrete to propose.”

Even if the mission is green-lit and that second encounter comes off, New Horizons still might not be done. “We’re going to keep looking for other KBO’s even farther out,” says Stern. If they’re close enough to New Horizons’ path, and if there’s enough fuel left for another trajectory adjustment, next July’s Pluto flyby could be just the start of an extraordinary series of close encounters with the most remote colonies in the Sun’s cosmic empire.