It's been a busy few weeks for Alan Stern, Principal Investigator of the New Horizons mission and the man who has advocated for, developed, and overseen the operation of a Pluto mission for the better part of three decades. Having spent a good portion of his life incubating New Horizons, and the last ten years flying it to its destination, the crowning moment of his effort will be over in the space of a few hours on Tuesday. He and his teams of scientists and engineers will anxiously await the signal, due at 8:53 pm EDT on that night, that will indicate the close encounter is proceeding nominally. Having endured the nerve-wracking anomaly on 4 July that Stern called the mission's "Apollo 13 moment," he now anticipates their "Eagle has landed."

"There's so much energy in the room, and everywhere you go on this project," he told gathered media on Sunday evening. "I imagine it was like this on Voyager. Crossing the entire solar system, and then watching it all happen in a matter of weeks, it's like stepping off an escalator and on to a supersonic transport."

After weeks spent poring over each image from the spacecraft's LORRI imager as they arrive, and on a day full of technical briefings from mission engineers and scientists, Stern was asked to take a step back and address a simple question: What are we seeing?

"We are seeing a spectacular system, a spectacular binary planet, in which the two members are completely dichotomous," he said, referring to Pluto and its large moon Charon. They're often referred to as a "binary" because of their closeness in size, and their common rotation around a point in space that's known as their "barycenter" -- where their two masses balance.

"They look as if they are completely different worlds that just as well could've been raised billions of miles apart, but they weren't." Both worlds, Stern said, "have very complex and nuanced stories to tell us. In both cases we see evidence that these are worlds that have, at different points in their history, been active."

Principal Investigator for the New Horizons mission, Alan Stern, speaks with gathered press on Sunday night.

Unraveling the details of these stories is New Horizons' mission, and in doing so the team aims to find out, on behalf of the Solar System's only known residents, how our planetary system came to be.

"We really want to understand the process of planetary accretion," Stern says, "how planets are built."

It's now generally accepted that, in broad strokes, our system of 8 major planets and thousands of smaller bodies rotating around a central star was born from a rotating cloud of gas and dust around 5 billion years ago. As the material at the center of this rotating protoplanetary disk became more and more dense, it became hot and dense enough for nuclear fusion to begin, giving birth to our sun. Further out in the disk, particles of dust collided with one another to form larger particles, which collided with other particles to form still larger particles, and so on, until large enough bodies were formed that swept out all the remaining dust in their orbit, and for whom gravity was strong enough to shape them into spheres.

A protoplanetary disk 450 light years distant, surrounding the young star HL Tau at center, imaged by the Atacama Large Millimeter Array. The dark bands may be orbits in which planets have already formed and have swept up the remaining dust in their path. Our solar system is thought to have formed from a similarly configured disk. Credit: ALMA (ESO/NAOJ/NRAO)

At some point in this chain reaction of collisions, Stern pointed out, the body that would become Earth was Pluto-sized. The body that would become Pluto, at a point during this process, was the size of the rest of the population of smaller Kuiper Belt Objects. And those smaller Kuiper Belt Objects were the size of objects like 67P/Churyumov–Gerasimenko, the comet currently being visited by ESA's Rosetta spacecraft. 67P itself is likely a Kuiper Belt Object that was kicked into the inner Solar System early in its formation. By characterizing Pluto and Charon, Stern says, the New Horizons team hopes to help "connect the dots" between these disparate stages of planetary evolution. With the flyby less than 48 hours away, that effort is about to ramp up dramatically.

"The afterburner is about to kick in," said Stern.

Understandably the majority of the public focus has been on the spectacular images returned from New Horizons' LORRI and MVIC imagers, but Stern laid out some deeper inferences that can be made from those images, as well as what other instruments will be doing. The occultation studies of the atmosphere I've described in a previous post. LEISA, a spectrometer that operates in the infrared regime, has been observing Charon and Pluto for weeks, and is just now at the point where the spectra it is recording match the textbooks -- that is data, developed from observations by Earth-based telescopes. LEISA has detected water ice on Charon, which was first discovered from Earth in the late 1980s, as well as methane and the much harder-to-detect nitrogen of Pluto's atmosphere, which was also previously known. As LEISA gets closer and closer looks at the bodies in the next few days, the spectra will begin to show, they hope, never-before-seen details about the composition of their surfaces, with some compounds they predicted they'd have seen, and perhaps some they have not.

"Our geology and geophysics team is very excited by what they are seeing," Stern said. "It is a story that looks to my eye like a very complex convolution of geology and volatile transport." The presence of an active transport system that shuttles materials from the upper atmosphere down to the surface, and possibly other materials up from the subsurface, makes bare bones geology a bit more difficult for Pluto, since the surface rock and ice will be, to an extent, covered over with other materials. This makes Pluto a highly complex and likely diverse object, somewhat unique in its size class, and it will require a synthesis in investigation on the part of Randy Gladstone's atmosphere team and Jeff Moore's geology and geophysics team. Each will have a mountain of data to mine. Gladstone's team will have the results of their occultation experiments, and Moore's team will have LORRI images that can pick out features as small as football fields at closest approach, MVIC color maps of the surface, and topographic maps created by making a stereo image of most of the close approach hemisphere (photographing the same region from two different perspectives to create a sort of 3D map).

Annotated image of Charon taken by New Horizons on 11 July. Credit: NASA/JHUAPL/SWRI

Not that Charon won't have its own scientific depths to plumb. Its ancient, exposed surface, unsullied and unrenewed by the products of atmospheric transport like Pluto's, will likely have a bevvy of craters to be imaged, counted, and characterized by the New Horizons team. Determining the frequency and size distribution of impactors colliding with Charon (i.e. how often do they hit and how big are they) will allow scientists to compare this data with existing models of the Kuiper belt, and refine estimates about the numbers and sizes of objects that reside within it.

The impact craters will also tell us about the mechanical properties of Charon's surface. Observing the ratio of their depth to their diameter will tell us how deep certain-sized impactors are able to penetrate into Charon, and hence about the density and properties of the top layer. Studying the ejecta blankets (sheets of material kicked up by the impact that have settled back on the surface) will also reveal surface properties. If the ejecta blankets are approximately the same color as the surface, it may tell us that the surface itself extends fairly deep. If they are instead bright sheets of material, the team may conclude that impacts kick up fresh ice just below the surface.

All of the information we've been getting from New Horizons is, in sum, going to get a lot better very quickly. From mid-April, when New Horizons surpassed the resolution of the Hubble telescope with respect to Pluto, until now, the data has improved in resolution by a factor of about 30. The data at closest approach will be better by a factor of 180. With Stern, the man who has advocated for this tiny, much-maligned icy body since the 1980s, and two children of the man who discovered it in 1930 on hand, Pluto is about to unfold secrets and surprises before our very eyes.