It's the solar system's most mysterious former planet.
And a group of scientists are gathering together in the Northern Territory outback on a mission to unlock its secrets: A mission that could all fall apart.
As the sun sets, an unlikely band of amateur astronomers gather in a paddock.
With them is a team from NASA, considered world-leading scientists in their field.
They're assembling 12 special, large, portable telescopes, shipped in pieces to Australia by NASA.
As they finish — and the four-wheel drives lining the paddock cut their engines — the field descends into total darkness. The only lights now are from headlamps and the glow of phones and laptops.
This operation on the outskirts of Katherine, nearly 300km south of Darwin, is the first night of three well-orchestrated practice-runs for a fourth and final night as they prepare for a rare event that these professional and amateur scientists hope will reveal secrets of the solar system's most mysterious dwarf planet.
A body five billion kilometres from Earth: The dwarf planet we call Pluto.
An outback mission
Professor Leslie Young is one of the world's leading Pluto scientists.
She was the deputy science lead on NASA's 2015 New Horizons mission, which sent a spacecraft past the distant world.
And she's the science lead on this NASA-funded mission. It has been in planning for six months, with Professor Young and a team of US scientists travelling all the way to the Northern Territory specifically for this event.
"We didn't know Pluto had an atmosphere until 1988, when we did an experiment very similar to what we're trying to do here," she says.
Now astronomers such as Professor Young are desperate to find out how Pluto's atmosphere works.
And that's what this mission is all about.
"What's the pressure and temperature in the atmosphere? Are there any winds?"
According to scientists, there are clues that the atmosphere on Pluto is not behaving as expected. It's been getting thicker even though models suggest it should be getting thinner as the planet moves further and further from the sun.
The scientists hope to solve those mysteries through this project, and even — possibly — reveal what lies beneath Pluto's frozen surface.
According to Professor Young, this Earth-based mission — which is probing a different part of the atmosphere to New Horizons — could reveal even more secrets than the spacecraft itself.
The central flash
For the scientists, the equation is relatively simple: In four days time, Pluto will pass directly in front of a star.
When it does that, it will temporarily block most of the light coming from the star — an event called a "stellar occultation".
Crucially for these astronomers, as that happens, the starlight will shine straight through Pluto's thin atmosphere, bending and colouring the starlight.
Exactly how it is bent and coloured will help the scientists create the weather report for the dwarf planet.
In effect, Pluto will cast a kind of fuzzy shadow in the starlight on the Earth's surface — a shadow the size of Pluto.
And, if one of the 12 telescopes is in the very centre of that small shadow, then something surprising will happen during the occultation.
Just as the star is nearly completely blocked out by Pluto, suddenly, it will flash very brightly — perhaps becoming more bright than it is normally.
That flash, called a "central flash", happens because Pluto's atmosphere bends the starlight, focusing it on one spot like a magnifying glass.
And, according to detailed calculations, the best place on Earth to observe the occultation is in the Northern Territory: Katherine, to be exact.
Months of preparation, involving nearly 40 astronomers, have all been focused on this one "central flash" moment.
It's a flash that will last perhaps a second.
But it's an event so significant that astronomers hope it will reveal a treasure trove of information about the very deep layers of Pluto's atmosphere.
The practice run
Among the group in Katherine is retired engineer, Dave Gault.
He and his wife drove an RV from Sydney to take part in the event, his fourth occultation of Pluto.
Dave is one of several amateur "shadow chasers" who travel around the country, and the world, capturing these rare occultations.
However, there's one thing Mr Gault worries will ruin the whole mission: A word he's scared to mention.
Each telescope needs to be run by a team of two or three people. And, within a couple of hours of this practice run, almost all of them are up and running.
By the end of the night, all but two of the 12 telescopes are operating correctly — and these are problems they hope to iron out in the coming days.
With another three nights to get it right, Professor Mark Buie from the US-based Southwest Research Institute — who is in charge of logistics — is relatively happy with the outcome.
Professor Buie, who has discovered nearly 1,000 dwarf planets, tells everyone it only gets better from here.
"In my experience, the first night is always the worst," he says.
Space in the family
Professor Young's interest in science and astronomy was piqued by her father, Larry Young, a well-known biomedical engineer in the US.
Her father even trained to fly the NASA space shuttle, although never made it to orbit. Instead, his job was to figure out why astronauts got space sick.
"{What] we learnt [from dad] is that, if you play your cards right, you can really enjoy what you do for a living," Professor Young says.
And it seems those lessons had the same effect on her brother, Eliot Young.
Three years her senior, Eliot is also a colleague of Leslie's at the Southwest Research Institute. They both worked on the New Horizons mission and are considered two of the world's leading Pluto specialists.
And they're not the only major firepower converging on the Aussie outback.
In addition to Professor Buie, they're also joined by husband and wife professorial team Anne Verbiscer and Mike Skrutskie, both astronomers at the University of Virginia.
Professor Skrutskie is a seasoned occultation observer, and does events like this a couple of times a week at home, "but mostly asteroids".
"I do that as a hobby," he says. "But Pluto rarely passes in front of a star."
After two more practice nights, the team is ready to go, but not everything is going entirely to plan.
The two telescopes that didn't work properly on the first night couldn't be fixed.
"You never know what it's going to be, but you've just got to stay flexible and adjust to circumstances," Professor Buie says.
"And I think we got through all of the problems."
It's with the remaining telescopes distributed in 10 locations around Katherine that the team head into the crucial final night, praying the weather stays clear.
The big show
Having the telescopes distributed across multiple locations serves a few key purposes for the scientists.
First, if the calculations are at all wrong, they hope at least one will be close enough to the centre of the shadow to see the "central flash".
Then, with the combined imagery and data from these small-ish telescopes, what is collected will be comparable to that from one, much-larger telescope.
But the weather is beyond their control, and the team knows it could wreak havoc on the mission.
"If there's too much cloud, you don't get any data," Professor Buie says.
And, as the sun sets on the final night, clouds roll into Katherine, a thick blanket spanning the horizon.
The cloud cover is so unusual, it makes the local news that day.
However, the team ploughs on in the hope it will lift.
"We've already practised, we know what we're doing," he says.
Going home with no data was always a possibility, but not one Professor Young thought likely.
"We've been preparing for six months, hoping that five minutes go well," she says.
"It doesn't have to be good weather all night long. It just has to be good weather at quarter of two in the morning."
But, at midnight, that's looking all but impossible as thick clouds block virtually every star in the sky.
And then, about an hour before the occultation is due to begin, the heavens play ball.
The thick cloud doesn't just ease, but completely disappears …
revealing the most-spectacular view of the Milky Way, right across from one side of the horizon to the other.
Five minutes before the occultation is set to begin, the teams start recording data.
"Watching the occultation … you have to be very still," Professor Young says, as she prepares herself for the moment.
Right on time, at 1.46am, the star begins to fade as Pluto passes in front of it.
The observers stay virtually silent.
Suddenly, two minutes later, what looks like an explosion appears on the screen of a laptop as the star flashes incredibly brightly.
And, just as quickly as it began, it's gone again.
"Yes!" shouts Eliot Young. "Oh, my God. That was spectacular."
"That was the brightest central flash that anyone has ever seen," says Professor Skrutskie.
"My occultation life is complete."
Their calculations were incredibly accurate. Over a distance of more than 5 billion kilometres, Pluto cast a tiny shadow on the Earth, and Eliot's telescope was directly in the middle of it.
The precise way the light from the star fades and flashes will reveal crucial facts about whether the atmosphere is still — counterintuitively — getting thicker, even as the dwarf planet moves further and further from the sun.
That, in turn, will provide clues about how much heat is trapped in its sub-surface — and, therefore, what might lie below the frozen nitrogen and methane that coats the dwarf planet.
But those clues will take months to pull from the data.
In the meantime, this grainy image of a distant frozen world, blocking out an even-more-distant star, is the most exciting thing these scientists have seen since a spacecraft flew by Pluto in 2015.
It's a result Leslie Young says made the journey — and the six months' planning — all worth it.
"We got luckier than we had any right to [be, and] we have in hand that data set to tell us everything that we came here to get."
Credits
Reporting and additional photography: Michael Slezak
Photography: Brendan Esposito
Editing and digital production: Nick Sas
