New Horizons is racing to Pluto so quickly, we’re literally learning new things every single day. Exploration is a true planet-wide “Today I learned...” moment: we now know what makes up Pluto’s atmosphere, what makes up its ice cap, and exactly how big it is.

With data analyzed within the last day, researchers on the New Horizons team have processed enough Pluto flyby data to start nailing down all-new details. In the very first announcement of scientific discoveries from the New Horizons mission, we learned Pluto is the largest object in the Kuiper Belt, has gas escaping its atmosphere, and that the light-coloured polar cap is an ice cap.

The latest, greatest image of Pluto captured on July 12, 2015 from a distance of 2.5 million kilometers (1.6 million miles). The heart rotating into view will be imaged in greater detail on July 14; the bullseye rotating out of view will not. Image credits: NASA/JHUAPL/SWRI


Pluto is 2,370 kilometers (1,473 miles) in diameter, give or take 20 kilometers. This makes it undisputedly larger than Eris, the second largest object in the Kuiper Belt at 2,336 kilometers with a potential error of +/- 12 kilometers, and ends a decade-long debate over which object is larger. It’s been difficult to measure Pluto’s size because it has an atmosphere that acts as a mirage, blurring the boundaries of just how big the dwarf planet is. This new measurement sets off a whole train of new conclusions: it’s slightly larger than we thought it was, which paired with the mass that we already knew very well, means it’s lower density. A lower-density Pluto indicates it has a higher proportion of ice than we previously thought. That Pluto has more ice layered on its rocks might mean that its troposphere is lower than we thought (which has to-be-determined implications for atmospheric models), but also sets it compositionally apart from the smaller-but-heavier Eris.

Pluto’s largest moon Charon was far easier to pin down. Its lack of substantial atmosphere made it easy to determine the 1,208 kilometer (751 mile) diameter even using ground-based telescopes, although those numbers have now been confirmed. Today, New Horizon’s LORRI camera is peeking in on two of the smaller moons, Nix and Hydra. The wee moons are an estimated 35 kilometers (20 miles) and 45 kilometers (30 miles) across respectively, but we won’t be able to confirm those sizes until after processing today’s data. The tiniest moons Kerberos and Styx are even more wee and not nearly as bright, making them difficult to measure but we should be able to tease out their dimensions (and what’s happening with their weird orbits!) from later observations.


Idealized Nix and Hydra dimensions, although we’re fairly certain the small moons will be far more irregular in shape. Image credit: JHUAPL/Google

Meanwhile, we’ve started sniffing nitrogen escaping from Pluto. Our models anticipated we’d start detecting traces of the atmosphere about a day out from closest approach, but instead we started picking up traces a full five days away. That time difference equates to much farther away: the probe started picking up ionized nitrogen at around 6 million kilometers away from the dwarf planet instead of the predicted 1 to 2.5 million kilometers.

The early detection of nitrogen could mean anything from the source being stronger than we thought to the atmosphere being stripped from the dwarf planet more rapidly than we’ve modelled. It could also means something more exotic, like a yet-to-be-determined process concentrating the escaped gas and our probe just coincidentally intercepting the stream. Distinguishing between the options is going to take a lot more data, during which we’ll also be learning what else is in Pluto’s atmosphere, and if Charon and Pluto actually share an atmosphere within their odd little system.

Finally, those alleged ice caps on Pluto? They’re definitely made of ice — methane and nitrogen ice, specifically. If only the rest of the initial geological interpretations were so easy to confirm!

Charon as seen on July 12, 2015 from a distance of 2.5 million kilometers (1.6 million miles). The potential chasms, craters, and ejecta rays are coming into focus. Image credits: NASA/JHUAPL/SWRI

The New Horizons mission is going so incredibly well so far, with no new glitches to interrupt the collection of glorious data. In the next 24 hours, the probe will reach closest-approach, taking 150 photographs as it soars through the Pluto-Charon system and out into deeper space. Photograph resolution will improve by two orders of magnitude to 100 meters per pixel, putting all the current “best ever” images to shame. We’ll be live-streaming the closest approach celebration at 7:30am Eastern Time for everyone who wants to gleefully cheer on the probe in real time. The probe will be out of communication most of the day as it’ll be busy collecting data, but we’ve asked it to phone home by 8:30pm on Tuesday night.

In the meantime, we’re left with the words of New Horizon’s Principal Investigator Alan Stern, exalting in our collective excitement over one little probe’s venture into the unknown:

Top image: Colour composites of Pluto and Charon of black-and-white LORRI images collected on July 11, 2015, colourized with Ralph data. Credits: NASA/JHUAPL/SWRI

io9 will be hosting live coverage during New Horizon’s close approach starting at 7:30am ET/4:30am PT on Tuesday morning, and during its first call home after the flyby starting at 8:30pm ET/5:30pm PT on Tuesday evening.