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A very special encounter is set to take place in the Kuiper Belt, six and a half billion kilometres from Earth, right at the beginning of the New Year. NASA’s New Horizons spacecraft will visit object 2014 MU69, better known as Ultima Thule. At 06:33 CET, New Horizons will fly past Ultima Thule and use its measuring equipment to examine the object from a distance of just 3500 kilometres. New Horizons was launched to space approximately 13 years ago to investigate the dwarf planet Pluto. This is the first close-up exploration of a body beyond Pluto.

During the relatively brief Ultima Thule flyby phase, at a speed of 14 kilometres per second, seven scientific experiments will record images, spectra and physical measurement values. On board the spacecraft are two plasma instruments (PEPSSI and SWAP), one dust detector (Venetia), a radio experiment (REX) and three optical devices: the UV spectrometer Alice and the LORRI and Ralph high-resolution camera systems. The German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) Space Administration has supported Martin Pätzold’s participation in the REX experiment with funds provided by the German Federal Ministry for Economic Affairs and Energy (Bundesministerium für Wirtschaft und Energie; BMWi). REX will use radio waves to determine the thermal radiation and mass of Ultima Thule. It is the only instrument on board New Horizons that involves a contribution by German planetary researchers. During the flyby, Pätzold, Deputy Director of the Rhenish Institute for Environmental Research at the University of Cologne (EURAD), together with Michael Bird will perform two measuring procedures. REX will determine the surface temperature, mass and density of Ultima Thule. These values will provide an indication of the object’s internal composition and how it formed over four and a half billion years ago at the edge of the solar nebula.

First impression of the unknown object

In the run-up to the forthcoming rendezvous, planetary researchers managed to gain a ‘glimpse’ of Ultima Thule. In mid-August 2018, the Long Range Reconnaissance Imager (LORRI) spotted the relatively small object from a distance of 172 million kilometres as a tiny pinprick of light surrounded by a multitude of background stars. Although no surface structures were discernible from this distance, the position confirmed that New Horizons was on the right course to pass Ultima Thule as predicted on the morning of 1 January 2019 – a premiere for all solar system researchers. By the end of New Year Day, when the first images arrive, scientists are likely to know whether it is a round or oblong object, whether there are one or more objects, and what the surface is like. Researchers assume that Ultima Thule will be between 20 and 30 kilometres in size.

Ultima Thule – mystery at the edge of the Solar System

Ultima Thule was discovered on 26 June 2014 with the Hubble Space Telescope during a search for possible objects in the Kuiper Belt that appeared to be potential targets for NASA’s New Horizons spacecraft during the period following its flyby of the dwarf planet Pluto and its moon Charon. Ultima Thule orbits the Sun at distances of between 6.4 and almost seven billion kilometres.

The new object was initially labelled 1110113Y. In May 2015, when its orbit was determined with sufficient accuracy, it was given the official designation 2014 MU69. In March 2018, the New Horizons team selected the nickname, Ultima Thule, from the suggestions submitted. According to Celtic-German legend Ultima Thule represents a fictional, far-flung place in the far north, beyond the mythical island of Thule at the end of the world – a symbolic name for the exploration of the unknown. Ultima Thule will only be given its definitive name after the coming flyby, once it is clear what the object actually looks like.

In 2017 and 2018, the observations of stellar occultation by Ultima Thule indicated that the object may consist of two bodies orbiting each other. The DLR- and NASA- operated SOFIA airborne observatory played a decisive role in these observations. The upcoming observations will therefore be extremely exciting and enlightening for the planetary researchers.

The Kuiper Belt

The Kuiper Belt, sometimes also referred to as the Kuiper-Edgeworth Belt, is the cosmic home of Ultima Thule. It is a donut-shaped region of icy, sometimes extremely primitive, bodies ranging from a few kilometres to several thousand kilometres in diameter, such as the dwarf planets Pluto, Eris, Makemake and Haumea. The Kuiper Belt is immediately adjacent to Neptune and probably extends to a distance of 18 billion kilometres from the Sun. It is also the source of most short-period comets. Together, all the objects in the Kuiper Belt have only a fraction of the Earth’s mass.