At 05:33 GMT on New Year’s Day 2019, NASA’s New Horizons made a flyby of the Kuiper Belt Object known as Ultima Thule.


Ahead of the encounter we spoke to Ramy El-Maarry from Birkbeck, University of London, who worked with ESA’s Rosetta team before joining New Horizons as a science team collaborator.

What is New Horizons doing at the moment?

New Horizons is now in its extended mission, looking at small bodies in the Kuiper Belt region.

The Kuiper Belt is the third zone of the Solar System.

If you think of the first zone as being the inner planets, the second zone is the giant planets.

The third zone, the Kuiper Belt zone, has lots of bodies that are potentially sources for comets.

My goal with Ultima Thule is to find out what comets look like before they enter the inner Solar System and become modified by activity.

This is our opportunity to look at a really primordial body that has been in deep freeze for 4.6 billion years and give us a glimpse of what the conditions were like in the early Solar System.

Why was Ultima Thule chosen as the probe’s second destination?

When New Horizons set off for Pluto we didn’t know Ultima Thule existed.

Then around 2014, there was approval for an extended mission and the scientists of the mission began to think what the next target would be.

Astronomers on the team started looking for potential bodies using the Hubble telescope and in 2014 discovered Ultima Thule, which at the time was just called 2014 MU69.

We looked for a name that’s easier to remember and Ultima Thule [the traditional name for places beyond the known world] was finally chosen.

It also had an orbit that had a very low inclination to the solar ecliptic.

This suggests that it wasn’t perturbed that much, so it was in that place in the Kuiper Belt since it formed – it didn’t form much closer to the Sun and then move outwards or vice versa.

It’s orbitally stable and has been in this zone since it formed.

It fits the bill about what we intended to do in terms of science, but the body was essentially chosen because it fit the trajectory.

What do we already know about Ultima Thule?

There wasn’t a lot known at the time it was discovered, however there was a chance for better characterisation when Ultima Thule passed in front of a star.

We were able to use this occultation to get a better understanding of Ultima Thule.

From this we understood that it was a body approximately 30km in diameter that’s quite irregular in shape.

It could either be what we call a binary system where we have two bodies that are orbiting each other in very close proximity, or it could even be a contact binary – a body that has merged, like Comet 67P.

We also know that it is likely reddish in colour.

That could be due to some organic material at the surface that has been altered by interaction with cosmic rays and solar winds.

What do we hope to learn about Ultima Thule?

We hope to learn about the surface geology.

We have some high-resolution cameras to get a better grasp of what the surface looks like.

We’ve also got an imaging spectrometer that will allow us to get information about what the surface is made of.

So together that will give us a lot of information on these bodies.

What will this tell us about the Solar System?

Essentially, we want to know about the conditions when the Solar System formed.

That would give us information about the other planetary systems we know about from exoplanets and so forth.

Looking at Ultima Thule, looking at Kuiper Belt objects, that gives us a better idea about comets in our Solar System.

It will also tell us lots the origin of water in our Solar System, and the origin of life.

What are you looking forward to on the flyby day?

I’m really looking forward to the first images. I think for me that’s the most exciting part.

New Horizons is one of the few missions where you still have an exploration flavour to it.

The flavour of the unknown.

This is a region that has never been explored by any other spacecraft – no one has ever explored a body in the Kuiper Belt at such close distance.

One of the real significances of this flyby is that New Horizons is going to be 3 times closer to Ultima Thule than it was to Pluto.

When will we start to see those images?

It’s a fly by so we’ll hopefully get a few images as we approach.

These will be followed by higher resolutions images as we get to closest approach, and we’ll get other images as we move further away.


The very large distances involved: it takes a long time to download these things, so we get some lower resolution images first, and the rest later at a higher resolution.


Elizabeth Pearson
Ezzy PearsonScience journalist

Ezzy Pearson is the Features Editor of BBC Sky at Night Magazine. Her first book about the history of robotic planetary landers is out now from The History Press.