by Tim Reyes
New Horizons’ January 1st flyby target could be considered an unborn comet — just waiting for a gravitational nudge to hurl it into the inner Solar System. It likely holds all the volatile ingredients — water, carbon monoxide, methane to form a cometary tail of debris if it were heated by the Sun’s rays. However, the gravitational tug of war that has created the present configuration of the Solar System tells a different story.
New Horizons’ flyby target- 2014 MU69, provisionally named by the mission as Ultima Thule, is classified as a cold classical Kuiper Belt Object (KBO), even more so, as a member of the kernel of that group. Our present understanding of cold classical KBOs says that Ultima Thule has been waiting in its present orbit for a very long time. Present Solar System models indicate that it has been in its present location for billions of years, possibly since the birth of the Solar System. This is in stark contrast to Pluto and Charon, also KBOs, that were gravitationally thrashed about by Neptune and are also the products of a cataclysmic collision. In further contrast, Ultima Thule appears to have orbited the Sun undisturbed.
Ultima Thule is New Horizons second flyby objective in the Kuiper Belt and already we know that this second flyby target could not be more different than its first. In July 2015, the space probe flew past Pluto and its moons, the once ninth planet of our Solar System but now considered the first Kuiper Belt objects discovered and first to be seen up close. Pluto defines a class of KBOs called Plutinos. The Cold Classical KBOs are represented by the second KBO ever found- 1992 QB1, Albion. And 1992 is considered the discovery year of the Kuiper Belt and thereafter thousands of KBOs have been discovered. The Kuiper Belt is far larger in both volume and more massive than the Asteroid Belt that resides between Mars and Jupiter.
The Aside: There is no contesting that Pluto and Charon are Kuiper Belt Objects. The unveiling of many more KBOs of similar size triggered the International Astronomical Union (IAU) to redefine planet and define a second category called a dwarf planet. According to the IAU, a planet must be in gravitational control of its local environ. Neptune has that property, Pluto does not. Asteroid Ceres does not. It’s this writer’s opinion that the definition of planet will someday be revised to not include gravitational influence and that a subclass will be defined called primary planet that defines the present eight and also other subclasses that define the likes of Pluto and Ceres, ever more distant planets and planets not bound to any star at all.
Everything we know about 2014 MU69 indicates that this is a classical Kuiper Belt Object. And astronomers do not know much about them. Their understanding of KBOs is inferred from the best models of the Solar System and the Kuiper Belt and then from more direct observations of comets, the Trojan asteroids, the moon Triton and now also Pluto and Charon. All the Kuiper Belt Objects are so distant that none are resolved as bodies except for a few image pixels by the largest ground telescopes and Hubble. Furthermore, the vast majority are too faint to be studied spectroscopically. This means that their light is too faint to be broken down by spectrographic devices to reveal the absorption lines of the chemicals residing on their surface. This is where the instruments of New Horizons are about to step in. This is just one of many questions that New Horizons’ observations will begin to tackle within a matter of hours. What is Ultima Thule, a cold classical KBO, made of?
You might think that if KBOs are representative of comets, then we have flown by several and measured their physical properties up close and personal. This can only be a half-truth and the truth remains to be discovered by New Horizons’ flyby of MU69 and many more detailed observations to come. In actuality, Ultima Thule is unlikely to ever become a comet. It’s already spent upwards of 4 billion years at 4 billion miles from the Sun. A question that may be answered is whether it is of the same physical makeup as the population of KBOs (not cold classicals) and more distant objects of the Scattered Disc and Oort Cloud that are gravitationally influenced and become comets.
Ultima Thule is primordial. It appears to be extremely old. Its orbit is nearly circular and is not in gravitational resonance with Neptune. It appears to have avoided the shifting around the Solar System that many other objects experienced by way of the gravity of Jupiter, Saturn, Uranus and Neptune. Furthermore, observations already carried out by New Horizons team reveals that it has no light curve. The light reflected off its surface is extremely steady. This means that it is either not rotating or its rotation axis is pointing right at the Sun and consequently right at the Earth and right at the space probe New Horizons.
It is also known that many cold classical KBOs are binary objects — two small bodies that orbit each other, gravitationally bound together. The curious case of Ultima Thule is that its silhouette which was revealed by a stellar occultation appears to show a binary pair but its light curve shows no sign of two objects orbiting each other — causing variations in brightness as one eclipses the other. Possibly, within hours, New Horizons will reveal two separated pixels of tiny rocks orbiting each other like Pluto and Charon presently do — their orbits tilted completely towards us, their circular dance facing clearly at us. On the other hand, Ultima Thule could simply be a single object without a rotation; a bowling ball pin or peanut shaped object like several asteroids and comets observed. It could be two ancient objects that merged in a slow speed collision that imparted no net rotation. Speculatively, it may be possible that any rotation that it did have was taken away by early interactions with the smallest constituents of the solar nebula from which it was born or also by the rush of the young Sun’s solar wind and radiation over billions of years.
The classical cold KBO 2014 MU69, Ultima Thule, is not a very distant, tiny, dark inconsequential rock. It is an early puzzle piece that will help tie together our fragmented understanding of the third region of the solar system, the Kuiper Belt. New Horizons encountered Pluto and Charon and the moonlets at the inner edge of the Belt. Ultima Thule resides at near the middle. The population of KBOs is so great that despite the vastness of the region, New Horizons will likely be able to flyby another. And even after Ultima Thule, there is the even more vast region called the Scattered Disc which could hold another potential flyby object sometime in the 2020s or even 2030s. Even more mysterious than Pluto was, the flyby of Ultima Thule is about to tell us more about our Solar System.
