The New Horizons spacecraft seems to have a penchant for holidays. On Bastille Day, 2015, it flew past the Pluto system, giving us the first close-up look at what had previously been hardly more than a dot of light on astronomical photographs and increasing our knowledge of the system by orders of magnitude.
Then on New Year’s Day, 2019, it flew past a Kuiper Belt object officially designated (486958) 2014 MU69. Since that designation doesn’t exactly sing on the evening news, it has been nicknamed Ultima Thule. Thule was the Roman name for an island in the far north of Europe, possibly one of the Shetlands, at the very edge of the Roman world. So, Ultima Thule has come to mean the farthermost place.
Discovered by the Hubble Space telescope only in 2014 and conveniently in the right place at the right time, it was chosen as the next destination for New Horizons. It lies a billion miles beyond Pluto and about 4 billion miles from earth. How far is that? It takes light (and, thus, radio waves) traveling at 186,000 miles per second over 8 hours to reach earth from there.
We knew almost nothing about it beyond its orbit. But we knew it was small, far too small to exhibit hydrostatic equilibrium (astro-speak for being shaped like a sphere). And, indeed, it turns out to be only 21 miles long and about half as wide. Interestingly, Ultima Thule is a “contact binary”—two objects, inevitably nicknamed Ultima and Thule that collided so gently at some point that their mutual gravity bound them together into something that looks a bit like a snowman. That mutual gravity doesn’t amount to much. Unencumbered by a space suit, a human might be able to reach escape velocity merely by jumping off it.
The Kuiper Belt was first hypothesized by Gerard Kuiper, a Dutch-American astronomer, in 1951. He thought that the celestial billiards played in the chaotic early years of the solar system would have thrown many objects into distant orbit. He was right, and Ultima Thule and its brethren are the remnants of the very earliest days, unchanged since then, and thus they have lots to tell us about those days.
It will be months before New Horizons can send all the data it has collected, but we will soon have much higher resolution photographs than we have now. It’s a great way to start a new year.