We set a record. Never before has a spacecraft explored anything so far away. Think of it. We’re a billion miles farther than Pluto [and] Just like with Pluto, we could not be happier. What you’re seeing is the first contact binary ever explored by spacecraft: two completely separate objects that are now joined together.
– Alan Stern, New Horizons principal investigator
The astronomical year got off to a flying start on January 1st, 2019 when NASA’ New Horizons vehicle – the same craft that flew by Pluto and Charon and their attendant moons in 2015 – shot past (486958) 2014 MU69, a trans-Neptunian Object (TNO) residing in the Kuiper belt. A relatively tiny object, and dubbed Ultima Thule, it wasn’t even known about when the New Horizons mission launched in January 2006.
As I noted in my previous Space Sunday report, Kuiper Belt objects are of particular interest to planetary astronomers and scientists as they represent the oldest near-pristine material in the solar system, and so could contain many secrets, from how rocky planets formed through to the origins of life. Ultima Thule itself has been of particular interest because data gathered from the Hubble Space Telescope (HST) suggested it might be a binary object due to its apparent brightness fluctuating, suggesting two bodies orbiting one another. However, as New Horizons slipped into the final days leading up to the fly-by, it seemed to report no variance’s in the light reflected by the object.
The space craft reached its point of closest approach to Ultima Thule at 05:33 UT on the morning of January 1st, 2019. However, the nature of the approach, coupled with the huge distance between Earth and the vehicle meant that the first images and data wouldn’t be received for several hours after the probe has passed the object (it takes over 6 hours for radio signals to reach Earth from the vehicle), so at the time of closest approach, scientists and the public had to make do with the images received in the 24 hours preceding it.
These images, captured while New Horizons was still more than 1 million kilometres (635,000 mi) from Ultima Thule, were enough to confirm that, rather than being either a single elongated object (as suggested by the lack of variance in brightness the probe was recording) or two objects orbiting one another, Ultima Thule is in fact a “contact binary” – objects conjoined after gently colliding with one another, to form a shape initially referred to as a “bowling pin” (this latter changed to “dirty snowman” as clearer images were received). They also revealed why New Horizons wasn’t seeing any brightness variations: whereas Hubble was seeing Ultima Thule from more of an “end on” angle (like a bottle tumbling through the air towards you), New Horizons was approach it more-or-less along its axis of rotation (like standing in front of a slowly turning propeller), so it was always reflecting the same amount of light.
The initial images led members of the New Horizons mission team to call Ultima Thule the “first ever” contact binary object to be explored. However, this might be disputed; the nucleus of comet 67P/Churyumov–Gerasimenko, as seen by ESA’s Rosetta mission, as has two lobes connected by a narrow “neck” region which could mark it as a contact binary.
Nevertheless, there is still something magical about the way the two lobes came together – as a member of the New Horizons team put it, the bump of them joining would have been so gentle, had it been caused by a car bumping your own, it wouldn’t result in any real damage. The lobes themselves are of unequal size; at 19 km (12 mi) across, the larger has been dubbed “Ultima”, while the smaller lobe has been dubbed “Thule”, and is 14 km (9 mi) across. Combined, these give the object an overall length of some 33 km (21 mi). That they came together so gently has already been seen as a confirmation of the pebble accretion theory of planetary formation.
The exterior of both lobes is probably a mix of water, methane and nitrogen ices, doubtless mixed with other elements / minerals, and the reddish hue revealed in the colour images thus far returned is likely the result of the irradiation of ices on its surface – a process witnessed on Pluto. However, it will not until photographs taken much closer to the object – notably those at closest approach, a mere 3,500 km (2,200 mi) – are received in mid-February, that we’ll have a clear view of the object’s topography.
Following the fly-by, the images received by mission control were taken at distances between 137,000 km (85,000 mi) and 28,000 km (18,000 mi) from the object, and part of the initial data transfer. In all, some 7 Gb of data was gathered, but due to the complexities involved, it will take 20 months for all of it to be received on Earth. In fact, at the time this article was written, and due to the passage of the Sun between the spacecraft and Earth, data transfer has been suspended for five days (January 5th through 10th, 2019) to prevent data loss due to solar interference. Even so, the images that have been received have been enough to not only reveal some of Ultima Thule’s secrets, but to also create new mysteries about it.
One of these mysteries is that computer modelling suggests that given the way the two lobes came together, Ultima Thule should have a rate of spin to complete one revolution every 3 or 4 hours. However, data from New Horizons indicates it is spinning far slower: one revolution every 15 hours. So something must have slowed it down – the question is, what?
The most obvious explanation would be the gravitational influence of nearby objects – say two or three small moons orbiting Ultima Thule. However, due to the risk of collision, the space around Ultima Thule was surveyed well ahead of the fly-by, and astronomers are convinced there is nothing orbiting it either beyond 800 km (500 mi) or closer than 160 km (100 mi) – although that does leave a fairly large sphere of space between the two which may yet reveal one or more objects. More will be known on this in late January, when data on New Horizons’ own studies of the space around Ultima Thule should be received by mission control.
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