We’re down to the wire, folks. NASA’s live counter says we have 7.5 hours until New Horizons skims past Pluto. Unlike most other missions, there isn’t anything to see; the probe itself will go silent while it goes photo-crazy, taking 150 images in less than a day. Instead, you can settle for watching the live stream, virtually follow along with the Eyes on Pluto app, or failing all that, refresh the “Where’s New Horizons?” page. We won’t even learn what happened until over twelve hours later.
Until then, we at least have this.
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.