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NASA’s New Horizons brought the darkened realm beyond Neptune to light. This region, also known as the Kuiper Belt, is full of objects left over from the formation of the Solar System. These frozen time capsules could well be hiding the answers to what happened in its genesis. After NASA’s incredible flyby of Pluto, the New Horizons spacecraft made history once again when it sought out Kuiper Belt Object (KBO) (486958) 2014 MU69, nicknamed Ultima Thule. The flyby of Ultima Thule on 1 January 2019 had astronomers marvelling at its appearance, and they have been attempting to apply any new discoveries to any gaps in our knowledge – in this case, comets.
When Ultima Thule came into shape, the first thought that came to mind for some was its stunning resemblance to Comet 67P/Churyumov– Gerasimenko. This particular comet was the subject of an intense examination period when it was paid a visit by the European Space Agency’s Rosetta spacecraft, along with the Philae lander, on 6
August 2014. This mission caught the comet at its closest point to the Sun, known as a perihelion, 186 million kilometres (116 million miles) from the Sun.
This 6.5-year orbit takes it outwards to the orbit of Jupiter, but when comets get close to perihelion they undergo maximum sublimation as material is stripped away by the Sun’s heating effect. A comet’s nucleus contains the constituent material
that accreted in the early Solar
System, but sublimation occurs at a closer distance to the Sun. This creates a tiny atmosphere known as a coma, the solar winds sweeping away material to form an ion tail, while normally larger pieces of material form a separate dust tail which travels at a 45 degree angle between the comet’s path and the ion tail.
The Rosetta mission brought back a host of data that has led to some remarkable discoveries, including that a surprising amount of water vapour is released from the surface, along with measurements of deuterium – hydrogen with one proton and one neutron – nitrogen, oxygen and argon. Due to this, astronomers deduced this comet is much more ancient than first thought. Overall analysis has estimated that Comet 67P formed from cold components of the interstellar cloud of gas and dust that populated the area around the Sun
4.6 billion years ago. This is the same sort of time period astronomers predict that Ultima Thule was conceived.
Alas, saying Ultima Thule and Comet 67P are two of the same is completely wrong. “Comets come from a part of the Kuiper Belt millions of miles away from where Ultima Thule is, which is called the ‘scattered disc’. Ultima Thule is from the cold ‘classical region’ [in the Kuiper Belt] which is completely different,” Dr Alan Stern, principal investigator of the New Horizons mission, tells All About Space. “It is as different as North America versus Europe.
“Secondly, Ultima Thule is more than 100times as massive as Comet 67P, so it's rather like comparing something the size of a lunchbox to something the size of a human being. In fact, it would be even more extreme than that.” While
67P has a length of 4.1 kilometres (2.5 miles) at its longest, Ultima Thule is roughly 30 kilometres (19 miles). However, other comets have been seen to be bigger than 67P: Halley’s Comet, for example, which
“Comets come from a part of the Kuiper Belt millions of miles away from where Ultima Thule is”
Dr Alan Stern
is approximately half the size of Ultima Thule, and even Comet Hale-Bopp is estimated to be between 30 and 40 kilometres (19 to 25 miles) across.
Even the bi-lobe feature, the striking visual similarity between the two objects, was not formed in the same way. “In the case of Ultima Thule, you can see from the imagery that this is the result of two distinct objects that formed independently, and came to be resting on one another. They underwent a very gentle merger,” explains Stern. “In the case of comets, we really don’t know that this is a primordial signature because the comets we have observed have evolved by being near the Sun. It’s quite likely their shapes and other properties have been very heavily affected by that.” Differences in origin, size and formation of the bi-lobial feature means that they are less like brothers but, if you investigate further, they are more like cousins.
The Solar System is not the same as it was over 4.5 billion years ago. The area around the Sun was a hectic field of clumps of rock and ice known as planetesimals, which were continuously bumping into each other like you would if you were trying to walk through a crowd of people. Each bump meant the planetesimals could collect more mass, and the ones that gathered a substantial amount became the planets we see today. The ones that weren't able to accrete as much became the
Solar System’s leftovers, including dwarf planets, asteroids, comets, KBOs and others. Although it seems that KBOs and comets could have been similar once upon a time, due to the dynamic and tempestuous nature of the early Solar System they were cast into different orbits, and therefore different lives.
For Ultima Thule, its orbit has been a stable one as it follows the same route around the Sun beyond that of Neptune, and it will continue this path until it returns to the same spot in almost 300 years’ time. It has been doing this for billions of years, frozen in the icy realm of the Kuiper Belt where the light from the Sun and surface activity are at a minimal. Comets tend to follow a more elliptical orbit, one that takes them from the Kuiper Belt – and even further – and swoops into the inner Solar System, crossing the path of Jupiter while doing so. Could it be that a change of direction could transform Ultima Thule into a comet?
“If you moved it into the inner Solar System, the heat from the Sun would go deep into Ultima Thule”
Dr Joel Parker
Dr Joel Parker, director of the Southwest Research Institute in the United States who has worked on both the Rosetta and New Horizons missions, seems to think so, as he tells All About Space:
“Ultima Thule has been in the distant outer Solar System, in cold storage, for its entire life. It likely has lots of ice not too far below the surface and throughout,” Parker explains. “If you moved it into the inner Solar System, the heat from the Sun would go deep into Ultima Thule, evaporating those ices, and they would escape as gas, dragging dust with the gas – which is exactly what a comet is.”
It’s possible that, with a push, Ultima Thule could one day become a comet, and it’s not as farfetched as some may think. It isn’t far off scientists’ understanding of the dynamical evolution of comets. Comets are essentially objects formed in the cold classical region, or were placed there thanks to the evolution of the Solar System’s juggernauts – for example Jupiter and Saturn – and were then knocked into the inner Solar System because of some other event.
What would have to happen to send Ultima
Thule hurtling into the Solar System? “Passing stars and galactic tides likely have changed the orbit of much more distant objects in the Oort Cloud, but for something closer in, and in that area of the Kuiper Belt, it would have to be a crazily close passing star, which is very unlikely,” says Parker. “This is unlikely because Ultima Thule is obviously in an extremely stable orbit that has kept it in that region of the
Solar System for the last 4.5 billion years.”
A passing star is a scenario that is extremely unlikely to happen anytime soon, as astronomers would probably notice a huge star bombing towards the Solar System. But if it were, and Ultima Thule was knocked inwards, then it would undergo some drastic changes as soon as it started moving closer. “[Sublimation] could happen at any distance closer than its current orbit. If it were moved closer to the Sun the thermal wave would go deeper, heating ices that have never been heated before. So it could become ‘active’ – although only slightly – even before it reached the orbit of Neptune,” says Parker. “As it got closer it would pass distances where the temperature – based on distance from the Sun – would make any volatile ices near the surface sublimate, for example nitrogen, carbon monoxide, carbon dioxide, and water.”
After passing Neptune it would have to weave past the gas giants, but wouldn’t be able to avoid the ‘King of the Solar System’, Jupiter. Depending on the course of future Ultima Thule, the powerful gravity of Jupiter could affect it in one of three ways: firstly, it could enter the planet’s orbit – a similar story to how Neptune got its moon Triton. Secondly, it could avoid being captured and simply be redirected, whether that’s into the area where our terrestrial planets reside or slingshotted elsewhere. Or finally, it could be directed into a collision course with Jupiter, just like Comet Shoemaker-Levy 9’s demise in 1992. This event saw Comet Shoemaker-Levy 9 torn apart into over 20 pieces due to Jupiter’s tidal forces before smashing into the planet with the force of 300 million atomic bombs. Scientists estimated that Comet Shoemaker-Levy 9 was no bigger than two kilometres (1.2 miles) in size.
Nonetheless, the comet version of Ultima Thule would undergo rapid changes as the influence of the Sun became more and more apparent. If it were to enter an orbit similar to 67P, then it would most likely lose material at a similar rate, meaning that approximately ten metres (33 feet) of surface material would be stripped off the object per orbit. With an object the size of Ultima Thule, it would definitely make a great sight for observational astronomers who love photographing comets. Then again, there would also have be a huge star flying past the Solar System at the same time!