The universe is big. Really big. Possibly infinite. We don’t know, and we can’t know, because our perception of its size is limited to the observable universe: the portion of it that we can observe with our technologies. The observable universe is roughly spherical, and its radius from earth is around 46.6 billion light-years. (One light-year is 5,878,625,373,183.6 miles.)
Our solar system is also big. Not big like the universe, but big to us. Probably bigger than you think. The way we’re taught in school, our solar system reaches from the Sun to Pluto (or, if you learned from a curriculum designed after August 2006, Neptune). We learn about the Sun, the planets, and the asteroid belt.
There’s more: the Kuiper Belt. Just beyond Neptune is a huge disk of asteroids, volatiles, and dwarf planets—including Pluto. It’s twenty times as wide, and at least twenty times as massive, as the asteroid belt. Outside the Kuiper Belt is the Oort cloud, a spherical halo of comets that could span as far as 3.2 light-years into the interstellar space surrounding our system.
But I’m getting ahead of myself, because the Oort cloud is a little like the sky beyond the observable universe. We are confident that it’s there, but we haven’t directly observed it. It’s a bit mind-boggling: we can observe planets around other stars, stars in another galaxy, and galaxies almost as old as the observable universe, but Oort cloud objects—far closer but far smaller—have so far evaded our investigations.
The edge of our observable solar system, then, is back in the Kuiper Belt. And the furthest object a non-astronomer can observe in the Kuiper Belt is Arrokoth.
You might remember a flurry of excitement in 2015 when the New Horizons probe sent back its first image of Pluto: a tannish circle, muddied near the base, with the distinct shape of a heart on its lower right side. The stunning high-res photos and data that reached earth a year and a half later never managed to garner the omnipresent cultural hype of that first glimpse, our first clear look at a distant, controversial, beloved little rock.
So if the coolest images of Pluto barely made a splash, you can imagine how Arrokoth, which New Horizons explored three and a half years later, has struggled to earn recognition.
Compared to Pluto, of course, Arrokoth is at almost every disadvantage. Hardly anyone has heard of it—it was only discovered in 2014, as part of a search for potential post-Pluto New Horizons targets. It’s lumpy, formed from two lobes of different-sized rocks that end up looking more like a Venus figurine than a planet. It’s smaller than New York City.
Like Pluto, the initial image sent from the flyby—an intimate portrait of the most distant object we’ve observed in our own galaxy—was low-res. New Horizons passed Arrokoth around 43.4 AU (one AU is 92,955,807.3 miles) from the Sun and was still travelling away from us as it transmitted the more complex data to Earth. It would take twenty months for that data to drip through the solar system. So I waited.
And waited.
At some point, the highest resolution photo (above) showed up, and I thought: that can’t be all. It looks too soft, too reminiscent of Pluto’s first pale heart. Unintentionally, I came to think of it as a placeholder, as something to taste while we waited for the rest of the data, the rest of the images.
But that is all. That’s it; that’s Arrokoth. That’s all we’ll ever see of it. Future missions may explore other Kuiper belt objects, likely en route to an exploration of interstellar space, but nothing about Arrokoth is interesting enough to revisit.
Arrokoth’s data will inform future missions; its legacy will live and fizzle through the explorations that supersede it. We will eventually glimpse something farther and lesser known and lumpier and smaller, and Arrokoth will be little more than a footnote in the average person’s observable solar system.
