To Ultima Thule and Beyond

Chris Lintott

In the early hours of New Year's Day, billions of miles from any Earthly celebrations, the New Horizons space probe swung by a small and extremely distant lump of ice and rock. It’s known to cataloguers as (486958) 2014 MU69, but the New Horizons team call it 'Ultima Thule' after the ancient expression for a place at the edge of the known world.

The nickname caused those of us gathered in mission control at the Applied Physics Laboratory outside Baltimore some anxiety in pronunciation; the team took its lead from Edgar Allan Poe, buried up the road, who in 'Dream-Land' helpfully rhymes ‘Thule’ with 'newly'.

The poem goes on to describe a ‘wild weird clime’, and Ultima Thule certainly inhabits a dark and eerie place at the distant edge of the Solar System. At noon the light is no brighter than a deep twilight dusk on Earth, and any human visitor would see the Sun reduced to the brightest star among many in a night sky that never sees day.

In 2015, after a voyage of three billion miles, New Horizons flew past Pluto, transforming what had appeared as little more than a point of light into an entire world, with nitrogen glaciers and mountains made of water ice. (There are souvenir posters for sale at mission control purporting to advertise a skiing trip to these crystalline peaks.) On its way out, the spacecraft turned to look back at Pluto, revealing the blue haze of the planet's tenuous atmosphere silhouetted against the dim sunlight.

Studying Pluto tells you about Pluto. Studying Ultima Thule might yield a greater prize in unlocking the secrets of how planets, in general, grow. Undisturbed since the very earliest days of the Solar System, it is a left-over brick, part of the rubble from a construction project which finished more than four billion years ago.

Finding a single brick in the vastness of space is not easy. Because New Horizons was sent first to Pluto, none of the few thousand other objects already known to exist beyond Neptune could be reached without exhausting the craft's reserves of fuel. Astronomers turned to the Hubble Space Telescope and found somewhere they could visit, a small rock visible only as a single point of light moving slowly against the background stars seen towards the centre of the Milky Way.

A few years after discovery, the orbit of Ultima Thule is not well established. With New Horizons dashing past at about eight miles per second, pointing its cameras in the right direction was a difficult task of choreography. Unlocking the secrets of the early Solar System requires thousands of things to go right, each of them taking place on a spacecraft so distant that instructions sent at the speed of light take more than six hours to reach it.

No wonder there were cheers and flag waving at mission control yesterday morning. New Horizons turned briefly towards Earth a few hours after closest approach to communicate with us, sending confirmation that the bounty of images and scientific measurements was safely on board. The last photograph taken before flyby was released, revealing a blurry image of something the rough shape of a skittle.

New Horizons has only a very weak transmitter on board, and it will take up to twenty months for all the information to be received back on Earth. For now, though, that brief signal saying all is well means everything to a celebrating team, some of whom have worked on the mission since the early 1990s. For others, the expected haul of data will provide work for the next decade. Meanwhile, four and a half billion miles away, Ultima Thule continues, undisturbed in the gloom, on its orbit around the Sun.


  • 2 January 2019 at 10:54pm
    UncleShoutingSmut says:
    I thought Pluto was no longer a planet.

  • 3 January 2019 at 12:17pm
    Rory Allen says:
    Technically, it's a "dwarf planet". But Alan Stern, who is in charge of the NASA mission, thinks the IAU's decision to downgrade Pluto from planet to dwarf planet was mistaken, and so keeps arguing that Pluto is a planet really.

    • 3 January 2019 at 6:48pm
      Russell Seitz says: @ Rory Allen
      Dwarf? at under 50 kilometers, Ultima Thule is a planetesmal peanut, and it certainly looks the part:

  • 5 January 2019 at 10:14am
    Rod Miller says:
    Pardon my vulgar ignorance, but I remember reading pretty recently published for-layman stuff stating that the Solar System began as a cloud of dust and gas.

    Yet here we have "a left-over brick (...) undisturbed since the very earliest days of the Solar System", and that brick consists of ice and ROCK.

    OK, hydrogen isn't a problem. Oxygen came from some ancient neighbourhood supernova. But what about the rock?

    • 5 January 2019 at 10:26am
      Russell Seitz says: @ Rod Miller
      Chill. Rod:
      "the Solar System began as a cloud of dust and gas.Yet here we have “a left-over brick ... and that brick consists of ice and ROCK."

      Dust+ water = clay
      Clay + solar heat= brick.
      Bricks are ceramics, and as mineralogist Cornelius Hurlebut often told his pupils, 'rocks are just ceramics that happen to have been made by God.'

    • 7 January 2019 at 12:23pm
      Rod Miller says: @ Russell Seitz

    • 9 January 2019 at 12:05pm
      sgt101 says: @ Rod Miller
      Mostly stars fuse hydrogen into helium to create energy (I think it's called PP fusion) once the star has a helium core things start to go wrong and a new process kicks off when the pressure of hydrogen fusion that keeps the star inflated vs. it's gravity fails. The helium gets squeezed so tight that it starts to fuse and this process continues for all the elements up to iron. I believe it is thought that this happens pretty fast - the star gets destabilised and in hours or days it goes from fusing helium to fusing iron, at which point it runs out of energy. Then a full on collapse happens and all the elements after Iron get made in the supernova that results.

      These elements are thought to pollute the gas clouds around the supernova and when some new condition causes a cloud to condense into a new stella system they are spun out and away from the newborn star into the disk of gas around it. They cool and condense many times, forming hundreds of thousands of planetiods, which clash, crash and smash each other into rubble and lava, which again mixes and and condenses. This is what creates rock, and ice. I think that you tend to get ice at the edges of the system and rock in the middle, but then the planetary dynamics of the stella system will throw rock out into the frozen wastes. It's thought that the positions of Jupiter and Saturn and the astroid belt betweeen Mars and Jupiter are explained by an event called "the grand tack" which would have ejected a lot of matter. The violence of the early solar system is witnessed by the fact that the Planet Uranus is tilted on it's side - causing folks to imagine it being smashed by a planet twice the size of the earth billions of years ago.

  • 8 January 2019 at 7:31pm
    Timothy Rogers says:
    And, part of the story, the mineral elements in the dust cloud were presumably manufactured by exploding stars of one sort or another at the end of their lives. Stars crank out these elements through fusion as part of their fuel cycle (I think I've got that right).

    • 8 January 2019 at 8:49pm
      Rod Miller says: @ Timothy Rogers
      Does our Sun produce anything heavier than helium?

      Doesn't it take an exploding star (or rather the Crunch that precedes the explosion) to produce the heavier elements? Of course, I guess it depends on the size of the star.

  • 8 January 2019 at 7:36pm
    Rachael Padman says:
    The solar system did indeed start as a cloud of dust and gas. But all what astronomers call "metals" -- elements other than hydrogen and helium -- were formed in stellar interiors, and/or in the case of elements heavier than iron, in supernovae. There is plenty of carbon, silicon, magnesium and iron out there to make most rock. The idea is that as the gas cools, grains of soot and silicates crystallise out, and then stick together as they collide and cool. In the meantime, lot of the light elements get blown out back into the interstellar medium. Over time, the grains coagulate into rocks, and if left undisturbed for long enough, into planets and other solar-system bodies. The proportions of rock, various ices (water, ammmonia, methane) and hydrogen helium in each depend on how far from the star it is, what sort of star it is, what else is going on in its solar neighbourhood, and what the initial composition of the cloud of gas and dust was...

    • 8 January 2019 at 10:00pm
      Rod Miller says: @ Rachael Padman
      If it's true that only elements heavier than iron are formed by supernovae, and anything lighter is forged in the normal course of things in stellar interiors, then you must be talking mighty big stars, I guess.

      Because our Sun creates nothing heavier than helium, correct?

    • 9 January 2019 at 6:58pm
      AndrewL says: @ Rod Miller
      It would be better to say that "elements heavier than iron are only formed by supernovae". Elements lighter than iron can be formed by burning (fusing together) the nuclei of lighter elements, as the process liberates energy. You just have to make the elements, and then spread them around.

      As the theory goes, a large amount of elementary baryons - protons and neutrons - will have been created in the early universe. Most of the free neutrons will have decayed to protons, and single protons will have combined with single electrons to make atoms of hydrogen-1 (this is all about finding the state with the lowest energy). A fraction of the neutrons, about a seventh, will have survived to combine with protons and they formed atoms of helium-4, and a very small amounts of other atoms (hydrogen-2, helium-3, lithium). So the starting composition of the baryonic matter (ignoring dark matter and dark energy) will be about 75% hydrogen and 25% helium by mass, and almost nothing else.

      Essentially all of the other elements were made inside stars. The most massive stars (say, 10 solar masses or more) very quickly (in a matter of millions of years) turn their hydrogen into helium, and then helium into carbon and oxygen, and then progressively as the core gets hotter and denser can combine those into heavier elements up to iron, getting through this process in perhaps hundreds or thousands of years, and ultimately explode as supernovae, spreading the debris around. Fusing nuclei above iron absorbs rather than liberates energy, but there is enough energy in a supernova to combine atomic nuclei and make heavier elements such as gold and lead and uranium (U-235 has a half life of about 4.5 billion years).

      Smaller stars fuse their hydrogen more slowly and steadily - the Sun should burn for about 10 billion years, and is about 5 billion years old - but stars of say three solar masses burn for only 3.5 billion years. In their late stages they will puff up as red giants and can blow off a substantial amount of mass from their outer edges as a solar wind, creating a planetary nebula, leaving behind the hot core as a white dwarf.

      So, in summary, stars a bit smaller than the Sun will sit around burning hydrogen for many billions of years. Stars around the size of the Sun up to several times the size of the Sun will puff up and blow relatively light elements around - carbon and oxygen and so on - and stars around ten times the size of the Sun or larger (and some "funnies", such as smaller stars in binary systems that accrete mass from their companion to such an extent that they also blow up as supernovae) created all of the heavy elements.

      Once you have enriched the interstellar medium with these heavier elements, you need to form a cloud of dust and gas than can collapse to form a new star (about 2% of the Sun's mass is elements heavier than helium, which you can detect through distinctive lines in the spectrum of light emitted by the Sun) with its solar system, ultimately including bodies such as the Earth, or Jupiter, or Thule.

    • 12 January 2019 at 2:08pm
      Rod Miller says: @ AndrewL
      Thanks! So you're saying that in the normal course of things, our sun produces at least some carbon and oxygen?

    • 14 January 2019 at 12:28pm
      AndrewL says: @ Rod Miller
      The Sun started with some carbon and oxygen in it, as well as hydrogen and helium. Until today, it has been turning hydrogen into helium (it is estimated to be about 60% helium at the core). It is not hot or dense enough for any appreciable amount of helium-4 to fuse into beryllium-8 and then carbon-12. It can happen, but just at very low rates, so the result is negligible. (No doubt someone has done the mathematics.)

      At some point the hydrogen at the Sun's core will run out, but it will keep burning hydrogen in a shell further out, increasing the size of the helium core. This is the stage at which it will puff up as a red giant.

      The core will get hotter and denser, and eventually the helium will be able to fuse into carbon via the triple alpha process (probably quite quickly, as a runaway process - look up the helium flash) leaving a core of carbon and oxygen. That hot core will become a white dwarf when the star sheds its outer layers as a planetary nebula.

      When those outer layers peel away, they will take some oxygen and carbon with them. That is all 6 or more billion years away. The Sun will eventually be a source of oxygen and carbon, but not until then. In the (very long) meantime, it is gradually turning its hydrogen into helium.

      The interesting point is that most of the Earth (and Thule, and the other rocky planets, and animals and plants, and the constructions of man) are built using material that must have come from nucleosynthesis in stars billions of years ago. As Carl Sagan had it, we are made of star stuff.

  • 9 January 2019 at 5:47am
    rumtytum says:
    I was surprised to see scientists waving US flags in celebration. Are they really so insecure that chauvinism seems an aporopriate response to a bit of good science?

    • 12 January 2019 at 1:57pm
      Rod Miller says: @ rumtytum
      Oh -- these are AMERICAN scientists. Chauvinism is drilled into Americans from Day One. It's like putting one foot in front of the other.
      And yes, Americans must be feeling a trifle insecure these days with every status except military status slipping.

      That said, there might also be an aspect of political theatre here. The presence of cameras y'know. If they are SEEN to wave American flags by the public, NASA may rise in public esteem -- and get funding. Far more cringeworthy things are done with great regularity.

    • 14 January 2019 at 5:34pm
      Timothy Rogers says: @ Rod Miller
      Rumtytum and Rod Miller are certainly leading the discussion away from its main point, which has to do with how unmanned missions can collect data that has some bearing on basic cosmological questions. So, the boys and the girls at mission control are celebrating, does one think they will break out paper hats and party razzers? Or, perhaps, start waving the UN flag? Give me a break – they’re waving their country’s flag because this was an American project funded with American tax dollars and part of a long-term national commitment to space exploration (not that some American politicians and professional soldiers are not interested in militarizing space as well). When Armstrong stepped onto the moon he said “ . . and one giant leap for mankind” (or something to that effect). But the lads did stake an American flag there because the mission represented a huge national effort. Is this chauvinism? Who knows, and in the case of the Ultima Thule project, who really cares? The USA can be rationally criticized for a broad array of its policies and actions, but this quibble? Come on, get serious. Underthought and overkilled.

  • 9 January 2019 at 2:43pm
    Timothy Rogers says:
    One other question about this. If I remember my popular science history correctly, some of the earthly elements were discovered by following up on light-spectographic indicators that these elements existed in the sun. I don't know if all of these elements are the product of specific "fusion chains" or were just gathered up from the dust and gas cloud that condensed into the sun. (I think helium itself was discovered this way, but my memory might be off).

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