Three years ago, we stared with awe at the first close-up images of
Pluto captured by NASA's New Horizons spacecraft. Now the spacecraft is
set to make history again as it flies by an even more distant world.
Ultima Thule lies 1.6 billion kilometres beyond Pluto in the Kuiper Belt — a cosmic doughnut of small primitive objects.
No spacecraft has ever explored a world this far away from the Sun, said Alan Stern, the mission's principal investigator of the Southwest Research Institute in Boulder, Colorado.
"It took us almost 13 years to get there travelling at this amazing speed of more than a million kilometres per day," Dr Stern said.
At that rate, New Horizons is expected to zoom past Ultima around 4.30pm (AEDT) on New Year's Day.
"Ultima is quite mysterious. We don't know much about its size and shape," Dr Stern said.
What we know so far is from its silhouette captured by telescopes such as the Hubble Space Telescope as the icy object passed in front of stars.
"From that we determine it's about 30 kilometres long, and it's shaped a little bit like a figure eight," Dr Stern said.
This suggests Ultima Thule could be two objects connected together, what's known as a contact binary, or two objects orbiting each other.
"We've been taking images of Ultima Thule ever since August from onboard the spacecraft, but it's just a dot in the distance that grows brighter and brighter and brighter."
As New Horizons homed in on its target in early December, extensive searches revealed no signs of moons, rings or any hazards near the object.
The first hints of what Ultima Thule really looks like will begin to emerge on December 31, when the piano-sized spacecraft is still further away from the object than the Earth is to the moon.
Then on January 1, New Horizons will zoom over Ultima at around 50,000 kilometres per hour, just 3,500 kilometres above the surface — three times closer than its approach to Pluto.
On board the spacecraft is a suite of seven instruments that will map the composition and topography of the object, take its temperature, search for signs of an atmosphere.
Travelling at the speed of light, messages from the spacecraft take more than six hours to make the 6 billion or so kilometres back to us.
"By January 2 we'll have detailed images that we can make maps from on the ground so this is all going to happen very fast," Dr Stern said.
The Kuiper Belt extends from around 4.5 billion kilometres from the Sun (Neptune) to 7.5 billion kilometres from the Sun.
The first small object — 1992 QB1 dubbed "Smiley" — was discovered in this region in 1992. Since then more than 2,000 objects have been discovered.
Some of these worlds rival Pluto in size, but most of the worlds are only tens to hundreds of kilometres across.
"We have a pretty good understanding of these worlds. Many of them have … moons, some even have rings," Dr Stern said.
These small worlds are ancient time capsules left over from the birth of the solar system 4.6 billion years ago.
"We know that Ultima Thule was born at this very great distance from the Sun and has always been in that region of the solar system."
At that distance temperatures are freezing — almost absolute zero or -273 degrees C.
"Those temperatures should preserve the record of the formation of Ultima Thule very faithfully over all those billions of years," Dr Stern said.
Flying through the densest part of the region, New Horizons will pick up details never imagined by previous missions such as Voyager.
The Voyager spacecraft made their way above and below the Kuiper Belt in the 1990s, but were "blissfully unaware" of its existence.
"The Voyagers didn't even look at the Kuiper Belt because they didn't know there was a Kuiper Belt to look at," Dr Stern said.
"And of course they had, by today's standards, very primitive instrumentation based on 1970s technology."
"But never before New Horizons have any of these objects been studied up close with cameras and spectrometers and any of the gear we're taking along."
But New Horizons' journey doesn't stop at Ultima Thule. The team already has other Kuiper Belt objects in its sights.
"The spacecraft is very healthy, it's not using any of its back-up systems and it has power and fuel to operate for close to 20 more years," Dr Stern said.
"There's a lot of future exploration ahead for New Horizons."
For now, though, the team is waiting to celebrate on New Year's Day.
Ultima Thule lies 1.6 billion kilometres beyond Pluto in the Kuiper Belt — a cosmic doughnut of small primitive objects.
No spacecraft has ever explored a world this far away from the Sun, said Alan Stern, the mission's principal investigator of the Southwest Research Institute in Boulder, Colorado.
"It took us almost 13 years to get there travelling at this amazing speed of more than a million kilometres per day," Dr Stern said.
At that rate, New Horizons is expected to zoom past Ultima around 4.30pm (AEDT) on New Year's Day.
What do we know about Ultima Thule?
Ultima Thule (ultima thoo-lee) — or 2014 MU69 as it is officially known — was discovered in 2014."Ultima is quite mysterious. We don't know much about its size and shape," Dr Stern said.
What we know so far is from its silhouette captured by telescopes such as the Hubble Space Telescope as the icy object passed in front of stars.
"From that we determine it's about 30 kilometres long, and it's shaped a little bit like a figure eight," Dr Stern said.
This suggests Ultima Thule could be two objects connected together, what's known as a contact binary, or two objects orbiting each other.
"We've been taking images of Ultima Thule ever since August from onboard the spacecraft, but it's just a dot in the distance that grows brighter and brighter and brighter."
As New Horizons homed in on its target in early December, extensive searches revealed no signs of moons, rings or any hazards near the object.
The first hints of what Ultima Thule really looks like will begin to emerge on December 31, when the piano-sized spacecraft is still further away from the object than the Earth is to the moon.
Then on January 1, New Horizons will zoom over Ultima at around 50,000 kilometres per hour, just 3,500 kilometres above the surface — three times closer than its approach to Pluto.
On board the spacecraft is a suite of seven instruments that will map the composition and topography of the object, take its temperature, search for signs of an atmosphere.
Travelling at the speed of light, messages from the spacecraft take more than six hours to make the 6 billion or so kilometres back to us.
"By January 2 we'll have detailed images that we can make maps from on the ground so this is all going to happen very fast," Dr Stern said.
Why explore the Kuiper Belt?
Ultima Thule is just one of thousands of objects that call the Kuiper Belt home, ranging from dwarf planets to comets.The Kuiper Belt extends from around 4.5 billion kilometres from the Sun (Neptune) to 7.5 billion kilometres from the Sun.
The first small object — 1992 QB1 dubbed "Smiley" — was discovered in this region in 1992. Since then more than 2,000 objects have been discovered.
Some of these worlds rival Pluto in size, but most of the worlds are only tens to hundreds of kilometres across.
"We have a pretty good understanding of these worlds. Many of them have … moons, some even have rings," Dr Stern said.
These small worlds are ancient time capsules left over from the birth of the solar system 4.6 billion years ago.
"We know that Ultima Thule was born at this very great distance from the Sun and has always been in that region of the solar system."
At that distance temperatures are freezing — almost absolute zero or -273 degrees C.
"Those temperatures should preserve the record of the formation of Ultima Thule very faithfully over all those billions of years," Dr Stern said.
Flying through the densest part of the region, New Horizons will pick up details never imagined by previous missions such as Voyager.
The Voyager spacecraft made their way above and below the Kuiper Belt in the 1990s, but were "blissfully unaware" of its existence.
"The Voyagers didn't even look at the Kuiper Belt because they didn't know there was a Kuiper Belt to look at," Dr Stern said.
"And of course they had, by today's standards, very primitive instrumentation based on 1970s technology."
"But never before New Horizons have any of these objects been studied up close with cameras and spectrometers and any of the gear we're taking along."
What happens beyond Ultima Thule?
The fly-by will produce enough data that will keep scientists busy for the next one and half years, Dr Stern said.But New Horizons' journey doesn't stop at Ultima Thule. The team already has other Kuiper Belt objects in its sights.
"The spacecraft is very healthy, it's not using any of its back-up systems and it has power and fuel to operate for close to 20 more years," Dr Stern said.
"There's a lot of future exploration ahead for New Horizons."
For now, though, the team is waiting to celebrate on New Year's Day.
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