In the early hours of January 1st 2019, as people all around the world were celebrating the new year, a small spacecraft named New Horizons was making history at the edge of our Solar System. It was flying by a small object nick-named Ultima Thule, Latin for ‘beyond the known world’, located within the Kuiper Belt. This is a region of space that lies beyond the orbit of Neptune, some 4.5 billion kilometres away from Earth. It contains icy relics that are the leftovers of our Solar System’s formation, dwarf planets, and comets. The Kuiper belt is a somewhat unexplored region, due to the sheer distance and time required for any spacecraft to reach it. This encounter made Ultima Thule the most distant object ever explored by humans, at over 6.4 billion kilometres away at the time of the fly-by.

You may have heard of New Horizons before. It was the spacecraft that famously explored the Pluto system back in 2015, and it has scientists and astronomers alike buzzing with excitement. It revealed large mountains of water ice, the largest glacier in our Solar System, and a thin blue atmosphere clinging to the dwarf planet. New Horizons was the fastest spacecraft ever launched and is still travelling at over 14 kilometres per second. Even at this speed, it took almost ten years before reaching Pluto after it was launched in 2006. The spacecraft was going too fast to slow down to orbit Pluto, so the exploration was confined to a few hours of observation as it whizzed past, known as a planetary ‘fly-by’. The sheer speed of this spacecraft placed it on an escape trajectory, meaning it will leave the Solar System and continue out into the galaxy, potentially forever. Knowing that New Horizons would continue its journey through the Kuiper Belt, the science team at NASA had planned to visit a Kuiper Belt object (known as KBOs) after the Pluto encounter. The existence of Ultima Thule was not yet known, until its discovery in 2014 by the Hubble Space Telescope and was given the provisional designation (486958) 2014 MU69. Hubble could not resolve any features as it was too distant and small. Observations by ground in the lead up to the fly-by indicated that the object in question was around 30 kilometres long, and likely had a very elongated shape, or even a possible binary system. New Horizons made its first observation on Ultima Thule in August 2018 and made a few course corrections that would bring it within 3,500 kilometres from the surface. The one-way travel time for signals to the spacecraft is over six hours, so all observations had to be planned and programmed in advance as New horizons would not be able to communicate with Earth in real time during the encounter.

New Horizons made its closest approach to Ultima Thule at 13:33 EST on January 1st 2019. The whole encounter was over within a few hours, and New Horizons used its suite of scientific instruments to scan the object in detail. Around 10 hours later, New Horizons ‘I’m okay!’ signal was received back on Earth, meaning it had survived its fly-by and had completed its exploration of the most distant object ever encountered. It began to immediately beam back its data results, although at a slow one to two kilobits per second (much slower than your old dial-up speeds). The first images of Ultima Thule arrived soon after. It revealed a fascinating double-lobed object, which was aptly nick-named by NASA to appear like a snowman. It is thought that two icy objects orbited each other billions of years ago and slowly fell towards each other until they became one in a slow collision. The surface has a red hue similar to that of Pluto, suggesting it is covered in organic compounds known as tholins. Ultima Thule also appears to spin like a propeller blade due to its two-lobed shape, taking over 15 hours to spin on its axis. As data continued to beam back, astronomers discovered that a large portion of Ultima Thule was not actually round as the initial images showed, but rather flat. Its shape is closer to resembling a lumpy pancake, as opposed to a round spherical snowman. Ultima Thule formed during the same time as the Sun and planets around 4.5 billion years ago, but it has remained almost unchanged since then. This gives astronomers a well-preserved specimen of what the Solar System was like during that time, and it is believed to be the most well-preserved primitive body ever explored. An encounter like this helps astronomers better understand how our Solar System formed, and what conditions were like billions of years ago.

New Horizons has only sent back a fraction of its data, and it will take over 20 months for this to be complete due to the slow downlink rate and distance. It will continue to traverse the Kuiper Belt region and will eventually leave the Solar System to explore interstellar space, joining other NASA probes like the Voyager and Pioneer missions. The onboard power source is expected to last well into the 2030s, and it is very likely that it will encounter one or two more KBOs during its journey, further expanding our knowledge of deep space. Missions like New Horizons demonstrates our ability to push the limits of space exploration, further understand the universe around us, and our place in it. Even after New Horizons emits its last signal and its batteries eventually die, its mission is far from over. It will continue to wander amongst the stars for eternity and may very well be found one day by someone.

Josh Kirkley, Astronomy Educator

GIF Above: The propeller-like motion of Ultima Thule, seen over 7 hours during the lead-up to its closest approach. 

Above: Ultima Thule is flatter and lumpier that astronomers initially thought.

Front blog image: The surface of Ultima Thule has a muddy brown hue due to tholins and organic compounds, much like Pluto and its moon Charon.