Pluto, as seen by New Horizons on July 8, superimposed on one of the starfields used for navigation. This is the approximate size and location of Pluto, relative to the background stars, from New Horizons' perspective.
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Stephen Gwyn/NRC Canada/CFHT/NASA/JHUAPL/SwRI
Pluto, as seen by New Horizons on July 8, superimposed on one of the starfields used for navigation. This is the approximate size and location of Pluto, relative to the background stars, from New Horizons' perspective.

Guided By Starlight, New Horizons Speeds Toward Pluto

“Second star to the right, and straight on ‘til morning,” Peter Pan told Wendy as they sailed toward Neverland. Though the story is a fairytale, navigating by starlight is a tried-and-true method for crossing oceans on Earth as well as the vast cosmic sea.

Like Peter Pan and millennia of humans, NASA’s New Horizons spacecraft is flying by the light of the stars. As it sails toward its July 14 rendezvous with Pluto, the spacecraft is keeping a close eye on thousands of pinpricks of light. Not only do the stars help tell the spacecraft exactly where it needs to go, they tell the spacecraft where to point its instruments as it meets up with Pluto.

“The stars are always the key factor in determining that,” says Gabe Rogers, New Horizons’ lead guidance and control engineer.

Seeing Stars

Riding aboard New Horizons are two star cameras, each aimed in a different direction.

“They’re used for determining which direction the spacecraft is pointed in, or where all the cameras and instruments are oriented,” Rogers says.

Ten times each second, the cameras snap images of their starfields. They compare those images with an onboard map of more than 10,000 stars. Based on that information, the spacecraft can figure out if it’s tipped up or down, or slightly swiveled.

“All you need is about three or four stars,” Rogers says.

If something is amiss, the spacecraft will automatically adjust its pointing. This piece of the puzzle is crucial for Tuesday’s Pluto encounter, when the spacecraft will have one chance to make hundreds of observations as it speeds through the Pluto system. Of course, none of that can happen if it ends up looking in the wrong place. (Learn more about the historic mission to Pluto on the National Geographic Channel.)

Starflight

There’s another way stars are helping New Horizons plot its course, and it has to do with finding Pluto itself. It might seem as though this should be simple, but it’s not. Pluto was discovered in 1930 – just 85 years ago. A full Pluto-year is the equivalent of 248 Earth-years, so we haven’t actually seen it complete a full orbit around the sun. As such, we’re not exactly sure how that path unfolds.

“We don’t really know its distance very well,” Rogers says.

Up until today, the spacecraft’s Long-Range Reconnaissance Imager (LORRI) was snapping photos used for optical navigation. These images include the starfield behind Pluto and its moons. Two teams, one at California-based KinetX Aerospace and the other The Johns Hopkins University’s Applied Physics Lab, have been reviewing the navigation images and using them to adjust New Horizons’ course heading.

“They look at those images and compare them to the known starfields, and come up with where we are in space,” Rogers says. “I can happily say that as of Saturday morning, in the last set of optical navigation images, Pluto is right where it’s supposed to be.”

But just how far away is Pluto? We know it’s on the order of 3 billion miles away. Yet New Horizons is traveling at more than 30,000 miles per hour, and at that speed, flying by Pluto will only take around three minutes. In other words, the spacecraft needs to deploy its instruments at exactly the right times to get all the data it’s tasked with collecting. So, New Horizons needs to know how far from Pluto it is, with much greater precision than simply a few thousand miles, give or take.

Figuring out that precise distance is something the team is constantly working on. In fact, there have been daily updates over the last few weeks – and there will be two more updates today (Sunday). The second of these, scheduled for late tonight, will offer the last opportunity to perform a trajectory correction, if needed, Rogers says. “Then we’re just sort of cruising to Pluto.”

One-Hour Special Mission Pluto hosted by Jason Silva premieres Tuesday, July 14 at 9/8c on National Geographic Channel.