As Pluto disappears into New Horizons' rear view mirror, the little space probe that could is veering towards its next target in the Kuiper belt. 2014 MU69 is small enough that it could nestle into the crook of Cape Cod, and currently is about as far from New Horizons as Saturn is from the Sun.

That's not even squinting distance. And yet, New Horizons' earthbound navigators need to steer the craft to a near collision course with the object. Through a series of four thruster burns—the last completed today—New Horizons will close a nearly three and a half million mile gap between its post-Pluto trajectory and MU69's orbit.

Frederic Pelletier is the person in charge of getting New Horizons to MU69. An interplanetary navigator at KinetX Aerospace, he's a master of orbital propagation: the discipline of guiding a hunk of metal through space from one rock to another. "It involves a lot of things, like Newtonian equations to understand the orbits of both MU69 and New Horizons, to determine their position at encounter time," says Pelletier.

When probes are just a few months out from a close approach—like New Horizons was in April before its rendezvous with Pluto—engineers can calculate a trajectory based on imagery from cameras on the craft itself. If the target's in sight, it's easier to steer toward it. But MU69 is way too far away for that. So at astronomical distances, engineers instead rely on calculations of velocity and trajectory derived from Earth-orbiting telescopes and radio tracking.

"We're using astronomy from the Hubble Space Telescope to determine where the asteroid is," says Pelletier. The background stars give him and his co-navigators a reference frame to determine how fast and in what direction MU69 is moving—basically, the same kind of dead reckoning sailors used before GPS.

NASA, ESA, SwRI, JHU/APL, and the New Horizons KBO Search Team

But imaging the Kuiper belt target isn't the only problem for Pelletier's team. New Horizons itself is too tiny to track using Hubble. Instead, the scientists are using radio communications between the probe and huge earthbound antennas—called the Deep Space Network—to triangulate its position relative to the Earth and Sun. New Horizons' engineers shore up those estimates whenever the probe passes other known space objects, like the famous dwarf planet it just happened to pass in July. "We have a very good understanding of where New Horizons is, especially after Pluto," says Pelletier.

In the three months since passing through the Pluto system, New Horizons has traveled almost one astronomical unit—93 million miles, or the average distance of the Earth to the Sun. In the course of traveling all that way, it's now completed four burns, correcting a disparity of 3,500,000 miles between its original trajectory and MU69, which is still about 15 AUs away.

Engineers split that total correction roughly equally between the four burns, which each lasted about 25 minutes—longer than anything the team had performed so far. They probably would have been OK doing the whole correction at once, but "when you open up the thrusters like that there is always the risk that something will break," says Pelletier. "We didn't want to risk anything going wrong on board." Since they are still so far away from MU69 they had the luxury of taking the time to get this correction, well, correct.

Between each burn segment New Horizons sent telemetry information back to Earth, which Pelletier & Co. used to check their math. The data also goes to other navigators at NASA, who independently check of how well the correction was executed. About three days passed between each of the first three burns, during which time New Horizons traveled about 2,300,000 miles. Between the third and fourth burn, the team waited a bit longer, about a week and 5,600,000 miles.

"We allowed more time between three and four to be able to assess the performance and readjust as necessary," says Pelletier. But on November 2 he and his team determined that everything went so well with the first three burns that they decided to leave the parameters untouched from the original calculations. "This is quite amazing," he says.

Even with that success, Pelletier estimates the team may be more than 300,000 miles off target. Sometime between summer and winter 2016, he expects to do more corrections based on Hubble observations of MU69.

And the latter half of 2018, New Horizons will spy MU69 for the first time with its own little eye. The combined imagery from New Horizons and Hubble will give the ground navigation team a stereoscopic view of the faraway world, which they'll use to make their final calculations. "Think of it as we are looking at it from Earth right now, that's one eye," says Pelletier. "With New Horizons' view in 2018, we’ll get the second eye and we will know much better where this thing truly is."

Then they'll switch to optical navigation, and New Horizons can swoop in for its intimate view of the furthest world humanity and its robots have ever explored.