When the New Horizons probe was launched in 2006, it had a straightforward mission: go into the night and take pictures of our outer solar system and what lies beyond, so we can see what’s out there. Demoting Pluto didn’t reroute the spacecraft nor strike it from the itinerary, though, and after taking a whole bunch of great vanity shots of Pluto the probe is still hurtling outward for… well, you know. New horizons. Now the spacecraft has finally caught sight of its next scientific quarry: the most distant planetary body humanity has ever explored, 2014 MU69, more recently known as Ultima Thule.
In classical and medieval times, poets and scholars wrote of a mythical, icebound, far-northern island described as being “at the back of the north wind” — that is, farther north than Boreas, the north wind, itself. This place was called Thule, the place farthest north, and like Atlantis, it’s been identified as all sorts of geographical spots: well-meaning cartographers and authors identified it as the British Isles, Iceland, Greenland, Norway, or even a sort of frozeniferous ether so far beyond man’s experience that there was “no longer any proper land nor sea nor air, but a sort of mixture of all three of the consistency of a jellyfish in which one can neither walk nor sail.”
Ultima Thule means “beyond Thule” — that is to say, a “farthest place” beyond the borders of the known world — and so the name alludes to the exploration of the frozen and far-distant Kuiper Belt and Kuiper Belt objects that New Horizons is performing, boldly going where no one has gone before.
New Horizons is still 107 million miles away from Ultima Thule, but what’s a couple AUs between friends? When the probe zooms past Ultima on January 1 of 2019, it’ll be a cool four billion miles or so away from the sun. The Ultima flyby will be the first-ever close-up exploration of a small Kuiper Belt object, and the farthest exploration of any planetary body in history, shattering its own record by about a billion miles. These images are also the most distant images from the Sun ever taken. Until now, the record for most distant image was the “Pale Blue Dot” image taken by Voyager 1 in 1990.
“MU69 is humanity’s next Ultima Thule,” said Alan Stern, principal investigator for the New Horizons project, in a statement . “Our spacecraft is heading beyond the limits of the known worlds, to what will be this mission’s next achievement. Since this will be the farthest exploration of any object in space in history, I like to call our flyby target Ultima, for short, symbolizing this ultimate exploration by NASA and our team.”
Ultima Thule the KBO, as opposed to Ultima Thule the Platonic ideal, is a lumpy, highly irregular, two-lobed hunk of rock covered in methane ice. Its lobes are so pronounced, in fact, that it may be a contact binary. But it’s no larger than about 25 miles in its largest dimension. We can’t yet resolve any of its surface features with any instrument available to humankind, but we’ll know more when we buzz it in a few months. We do know that Ultima is what’s called a “cold” KBO — it sails in a placid orbit of low eccentricity and inclination, around 44 AU out. We also know its brightness varies by less than 20% all told. This suggests that it’s spinning with an axis pointed directly at Earth.
But 20% variation in brightness, in an object nearly too dim for anything but Hubble to perceive, just doesn’t make much difference. To find it in the sky, the project team has to subtract out photos of known background stars taken by New Horizons’ LORRI visual-spectrum camera from the (noise-reduced) background blackness of deep space. Even then it’s frankly pretty potato. The panel on the right in the image above represents a differential composite where they did just that, yielding a precious few pixels of welcome scientific reassurance that we’re pointing this spacecraft properly. NASA notes that the many artifacts in the star-subtracted image are caused either by small mis-registrations between the new LORRI images and the template, or by intrinsic brightness variations of the stars.
“The image field is extremely rich with background stars, which makes it difficult to detect faint objects,” said Hal Weaver, New Horizons project scientist and LORRI principal investigator. “It really is like finding a needle in a haystack. In these first images, Ultima appears only as a bump on the side of a background star that’s roughly 17 times brighter, but Ultima will be getting brighter – and easier to see – as the spacecraft gets closer.”
The poet and polemicist Claudius wrote in the 5th century CE of “Thule lying icebound beneath the pole-star.” What might this poet to emperors have thought, had he known that one day we would send a spaceship there?
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