All you need to know about Pluto as it gets stranger by the minute

Artist’s illustration of the New Horizons spacecraft as it approaches Pluto and its largest moon, Charon. (NASA)

The New Horizons spacecraft is now just a few days from reaching Pluto. After traveling for nine years, it is already sending back amazing close-ups of that tiny world. It will zoom past that “dwarf planet” on Tuesday morning, July 14, with its closest approach a mere one Earth-width above the freezing surface.

Pluto is tiny: only 1,400 miles wide – much smaller than our Moon. And Pluto is really a double object: Pluto and Charon, with a mere two-to-one size difference. (It’s pronounced Karen, like the feminine name.) The pair orbit around an empty piece of space between them, once a week. Several other even-smaller moons are there too, with names like Nyx.

On Tuesday morning, the New Horizons craft cannot stop and orbit. Instead it will skim closely by, traveling at eight miles per second, giving its cameras just a half-hour window of close approach before it continues onward, never to return.

On Pluto’s hemisphere that won’t be facing those close-up cameras, the approach pictures already show a bizarre series of four giant, evenly spaced circular black spots, each a few hundred miles wide. They resemble nothing else in the known universe. So, fortunately, unlike the smudgy, blurry lack of details that Voyager 2 showed on Neptune in 1989, or the blank aqua overcast seen on Uranus in 1986, here finally is an outer planet where our fly-by has lots of detail to observe.

As we all know, in 2006 the International Astronomical Union (IAU) demoted Pluto to a dwarf planet because it really does not match the other eight. It’s far smaller, with only four percent the mass of even tiny Mercury. And it has a very unplanetlike orbit from every angle. But the clincher was finding more Plutos out there. Eris is bigger than Pluto, while Makemake, Quaoar, Sedna and a few others are almost as large. If Pluto’s a planet, then those others must be planets, too. It became clear that there’s a Kuiper Belt out there with thousands of small, icy, unplanetlike bodies, and Pluto’s one of them: a whole different ballgame from the “original eight.” So if you’re one of those who’d like to see Pluto called a major planet again, be aware that you’re opening the door to lots more “major planets” that will be tiny ice-balls with odd names, all of which will be smaller than our Moon.

Anyway, what’s in a name? Until about a century ago, rabbits were classified as rodents. Then their order was abruptly changed, so that now they’re lagomorphs. That’s mostly because they have four incisor teeth instead of two. But hey, they still hop around. So Pluto is Pluto, regardless of which mental box we try to make it fit.

Its widespread appeal is helped by its popular name, though most people don’t know that it was originally suggested by a schoolgirl, or that its first two letters, PL, honor the initials of Percival Lowell, who tirelessly hunted for it, and at whose observatory it was finally found. As for the cartoon dog, it was originally named Rover. In 1931, a year after Pluto’s discovery, the Walt Disney folks decided to exploit the newly found world’s publicity, and changed the character’s name to that of the planet.

Anyway, it’s a whole new world – especially starting on Wednesday, which is when the New Horizons craft sends us all the photos that it frantically took on Tuesday as it whizzed past: the first-ever spacecraft to visit that strange tiny world.


Want to know more? To read Bob’s previous “Night Sky” columns, visit our Almanac Weekly website at



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