Ultima Thule

11 March 2019

You may remember that the New Horizons spacecraft flew past a strange-shaped Kuiper Belt object in early January. This rock was eventually named “Ultima Thule”. Ultima Thule is a seriously long way away from the Earth, being about 45 times further out from the Sun - it's past Neptune.

What’s it doing out there? Well, the short answer is not a lot. Its orbit is boringly round. Other Kuiper Belt objects have a tendency to cross orbits and "encounter" each other now and again. When this happens, one tends to speed up and the other tends to slow down. This affects the eccentricity (or roundness) of both orbits, which can mean one of them gets knocked out of the Kuiper Belt entirely, coming way closer to the Sun. (I don’t want to worry you, but this means “closer to the Earth” as well).

Ultima Thule is not ever likely to be this exciting. Phew.

But back to its strange shape.

From a long time before the New Horizons flyby, NASA thought that the object was made up of two potato-shaped rocks (“Ultima” being the larger one and “Thule” being the smaller one) joined together. The idea was that at some time in the past there was the gentlest of collisions which led to the two rocks joined without shattering each other.

This idea came from the front-on views that New Horizons was getting as it approached the object from the sun side.

But, as the craft zipped past, it got a different perspective. Looking back, the photos had high contrast because they were taken into the sun. They showed some fine outlines of the very edge of the rock.

Studying these outlines, the boffins at NASA changed their minds about what shape they thought Ultima Thule was. They have described Ultima as a “pancake” and Thule as a “dented walnut”.

But "pancake" and "dented walnut"? Such creativity in descriptions is definitely to be encouraged. Personally, I’m waiting for the next description, which I’m hoping will be that it looks like Alfred Hitchcock.