Spacecraft closes in on new Ultima Thule
On January 1, the New Horizons probe will begin transmitting data from Ultima Thule, 6.4 billion kilometres from Earth in the Kuiper belt. What will it find wonders Robin McKie , of The Observer.
NEARLY six and ahalf billion km from Earth, a swarm of little worlds circles the dark edge of our solar system. The sun is so remote from this place that it appears no brighter than a star. This is the Kuiper belt, a doughnutshaped ring of icy objects that is one of the most mysterious and scientifically intriguing regions of space around our sun.
The belt is made up of fragments left over from the formation of the sun’s planets billions of years ago. For decades, researchers have dreamed of getting a closeup look at one but have been thwarted by the belt’s utter remoteness.
But this sad state of scientific ignorance is about to come to an end. On January 1, the US probe New Horizons, which has been hurtling away from the sun for the past 13 years, will sweep past Kuiper belt object 2014 MU69 and, for the next 24 hours, use its cameras, detectors and scanners to scrutinise it in detail. By the end of the probe’s encounter, an object that is currently no more than a dot in astronomers’ telescopes should be transformed into a world rich in astronomical and geological detail.
‘‘We are going to find out what this object is made of,’’ Alan Stern, the principal investigator of the New Horizons mission, said.
‘‘How was it was constructed? Does it have an atmosphere? Does it have rings? Who knows what we might find.’’
In the process, MU69 will become the most distant object that has ever been explored remotely by human beings.
It should be a remarkable encounter — a point recognised by mission control staff, who recentlygave the little world a title that is more memorable than its drab astronomical catalogue number. They have renamed it Ultima Thule, after the region that ancient geographers believed was the remotest in the inhabited world. (Both
Greenland and Iceland are candidates for the original.)
It will take less than 24 hours for New Horizons to whizz past Ultima Thule and survey it with its instruments. Then it will start to beam its findings back to Earth to provide scientists with that precious data about the early history of our solar system. Then, having completed its task, it will plunge further into deep space, in the rough direction of the constellation Sagittarius.
‘‘The crucial point is that everything is now going perfectly with New Horizons,’’ Stern said.
‘‘We have just made one correction to its current course and will have the opportunity to make three more before January 1. However, I don’t know if these will be necessary — we are already very close to the correct trajectory that we need to follow.’’
New Horizons was launched from Cape Canaveral in January 2006 and used a flyby of Jupiter in February 2007 to increase its velocity to more than 48,000kmh. For the next eight years it hurtled towards its primary target: the dwarf planet Pluto and its five moons, Charon, Styx, Nix, Kerberos and Hydra.
The robot spacecraft completed its 4.8 billion km journey to Pluto and its moons on July 14, 2015, and began beaming back data to Earth, messages that took more than four hours to reach mission control. And the information they revealed when they eventually reached Earth surprised astronomers.
Many had expected Pluto and its moons would be revealed to be icy, dead places. Instead, New Horizons uncovered many signs of geological activity, as well as evidence that ice has been flowing recently across Pluto’s surface.
The planet had towering waterice mountains and vast plains of frozen nitrogen, and a reddishbrown cap of material on Charon was found to be composed of organic molecules that could be important ingredients of life.
Pluto was also discovered to have a thin, blue atmosphere, rich in nitrogen. For a world in such a remote orbit around the sun, it proved to be surprisingly energetic.
‘‘We expected to find that Pluto was a simple inactive world but in fact it turns out to be no less complex than Earth or Mars,’’ Stern said.
‘‘We also found that it was geologically active and again we didn’t expect that. We thought Pluto would have lost its geothermal energy long ago, but that is not the case.’’
However, it was the response of the public that most excited him.
‘‘The popular reaction to the probe’s discoveries — that is what got me. It was the first flyby of a new planet to have taken place since the ’80s and people were really enthusiastic about it.
‘They were really gripped by the discoveries we made.
‘‘The public is really interested in space exploration. And that has encouraged us.’’
New Horizons then headed into the Kuiper belt, an additional trip that would add a further
1.6 billion km to its itinerary.
Studying the little worlds of the belt will be a very different affair from scrutinising Pluto or Charon, however. The former is about 2365km in diameter, the latter about 1206km. By contrast, Ultima Thule is probably only about 32km in diameter, which is typical for an object in the Kuiper belt. (The belt is named after the DutchAmerican astronomer Gerard Kuiper, who proposed its existence in a paper in 1951.)
Ultima Thule was discovered in 2014 by astronomers who had been using the Hubble space telescope to sweep the Kuiper belt for suitable targets for New Horizons. Five candidates were pinpointed and Ultima Thule emerged as the clear favourite.
‘‘It was the easiest Kuiper belt object for us to get to,’’ Stern said. ‘‘It is the one we can approach using the least amount of fuel and so maximise our reserves, which could be crucial in dealing with any unforeseen manoeuvres in the future.’’
Intriguingly, the selection of Ultima Thule adds another first to New Horizons’ achievements. It will be the first object in astronomical history that will be studied by a spaceship that was launched long before that object was discovered.
Since Ultima Thule’s discovery, scientists have been trying to find out more about it. The Hubble space telescope has continued its surveys and astronomers have tried to exploit stellar occultations, which occur when an object like Ultima Thule passes in front of a star and disturbs the light received from it on Earth.
From these observations, scientists believe Ultima Thule is either a single object about 32km in diameter or is made up of two objects swirling around each other, each measuring 14km to 19km in diameter. It is even possible Ultima Thule has a moon, or possibly several.
‘‘Ultima Thule could have an atmosphere, rings or several moons,’’ Stern said. ‘‘We are certainly hoping for a lot because we uncovered so many surprises when we got to Pluto.
‘‘However, we will have to wait until 1 January to discover the truth.’’
Many key features of Ultima Thule should become clear when the first data reaches Earth next month. However, they will have a longer wait before they get the total picture, for it will take months to transmit all the information sent back by New Horizons after sweeps to within 3540km of Ultima Thule in January.
Because New Horizons is so far away, its radio signals are extremely weak. And a weak signal means low data rates.
‘‘As a result, New Horizons can only transmit data at about 1 kilobit per second and at that rate, it will take more than a year to learn exactly what it discovers about Ultima Thule.
‘‘We will gather about 50GB of information from New Horizons after its instruments have studied Ultima Thule,’’ says Stern. ‘‘That is roughly the same amount that we gathered from Pluto and its moons — and that took more than 16 months to be beamed back to Earth.’’
However, New Horizons is now 1.6 billion km further from Earth than it was when it passed Pluto three and ahalf years ago, so it will take even longer to beam back its data about Ultima Thule.
‘‘I expect it will take around 20 months to get our data back this time,’’ Stern said.
‘‘That means that in September 2020 we should know all that we are going to learn about Ultima Thule. That might seem a bit of a wait. On the other hand, Ultima Thule is a fossil record from the earliest days of the solar system so I think we can wait just a little bit longer to find out about it.’’
Ultima Thule is likely to remain the remotest object visited by a space probe for a very long time, unless Stern and his colleagues go on to explore yet another Kuiper belt object in the mid2020s.
‘‘Once we get all the data back from Ultima Thule we can start thinking about the next goal,’’ he said.
‘‘New Horizons has a nuclear battery that will provide the craft with another 20 years of electricity, so we could easily go on to another object. We will have the power to do that.
‘It remains to be seen if there is popular backing for another encounter, however. In a way, it depends what we find at Ultima Thule.’’ — Guardian News and Media
❛ We are certainly hoping for a lot because we uncovered so many surprises when we got to Pluto. However, we will have to wait until 1 January to discover the
truth