Working at the agency's Jet Propulsion Laboratory in Pasadena, California, the Massachusetts Institute of Technology graduate took a keen interest in the hyperspectral images beamed from satellites monitoring the world's oceans. These cameras are able to measure temperature and peer through the water to check on the health of the world's reefs.

What he saw over two years at NASA disturbed him. "Every year, the ocean temperatures were rising, the coral reefs were bleaching, things were getting worse and I was thinking, 'What are we actually doing to save this?' That was when I decided, back in May 2018, to leave the Jet Propulsion Lab and focus entirely on coral reefs," he says.

Rodan's love of the Great Barrier Reef was forged in childhood. The son of an Australian environmental scientist who had advised the Obama administration and an American mother who was a professor in public health, he regularly spent the Australian winter exploring the reef.

His hair is sun bleached, his easy trans-Pacific accent the legacy of a life spent between different hemispheres.

"I grew up in the States and I'd visit my family back in Australia and it was always winter in Melbourne. And my dad was always like, 'Stuff this, let's go visit the reef, let's go to the rainforest.' So I grew up almost every other year visiting the Daintree and going into the Whitsundays and I just loved the reef."

After leaving NASA, he travelled the world visiting marine research institutes and small reef restoration projects looking for a way to use the engineering skills he'd honed at MIT and NASA to help these stressed ecosystems.

"Over about eight to 10 months I discovered that no matter how much money they had as an institution or how small they were as an organisation, everyone was trying to grow coral but they were doing it by hand.

"They were using toothbrushes and PVC and zip ties and duct tape. And that's the solution they have for saving the reef. And I was looking at them like, 'Are you kidding? This is one of the biggest problems of our century.' I had just come from NASA so I was extremely spoiled with the tools we had at our disposal. We spent $5.2 billion putting a rover on Mars and we can barely scrape together a few thousand dollars to help save our reefs."

He set out designing an automated system that could monitor, clean and feed growing coral, a system he has called CHARM (Coral Husbandry Automated Raceway Machine).

It has a patent pending. In January 2020, he was in New York trying to source robotic parts for his first prototype after COVID shut down the supply chain from China.

As the pandemic swept across the US, Rodan relocated to Magnetic Island off Townsville.

"I lived in a shed. It was entirely solar powered. I had rainwater catchment, I was able to grow some veggies, that kind of stuff. I was able to go fishing. It was straight off Castaway, from New York City with 12 and a half million people to this tropical island with about five neighbours.

"But I really wanted to do that because if I could build this robot to grow coral off grid on a tropical island I felt like that could set a model for people all over the world."

So what does CHARM do? Equipped with a camera, the system's primary aim is to monitor the growing coral: "Monitoring is extremely important because you want to know if they are healthy, if they're feeding, if they're been knocked over, if they've got disease, if there's a pest. That monitoring takes a lot of oversight. People have to go one by one to every one of those coral."

Using artificial intelligence, CHARM can detect if there's a problem and either address it or alert a human to take action.

"It will go, 'Look, this task is pretty difficult' so it will alert the maintenance guy or whoever is working at the facility. 'Hey, clean-up on aisle 10.' The hope is that this robot can go do a lot of those easy tasks to allow for the more difficult ones to be done by the human," Rodan says.

CHARM is equipped with a rotor with a brush to clean the coral and a nozzle to ensure water is kept circulating. When reversed, the nozzle can also siphon water away, taking with it detritus that has built up around the infant coral.

"You want to be going around the edges, removing the algae that builds up through the day to provide a clean spot for the coral to keep growing with minimal competition from other sorts of bio flora. A light brush daily helps the coral grow significantly faster."

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Rodan says the aim is to be able to grow more coral more quickly.

"That growth factor is everything. So if we can grow it two times, 20 times, 50 times faster, that can significantly change how much coral you can grow and how much success you have in out-planting."

Professor Terry Hughes, of the Australian Research Council Centre of Excellence at Townsville's James Cook University, warns that the sheer scale of the reef would dwarf attempts at regrowing and planting coral.

"Eighty per cent of the Great Barrier Reef experienced severe coral bleaching at least once in 2016, 2017 and 2020, which killed tens of billions of corals," Professor Hughes says.

"Restoring billions of corals along the 2300km length of the Great Barrier Reef is not feasible or affordable.

"You would need to grow and deploy underwater approximately 250 million large adult corals, each larger than a dinner plate, to increase coral cover on the Great Barrier Reef by just 1 per cent.

"The key to saving the reef from escalating anthropogenic warming is to reduce greenhouse gas emissions and to protect the remaining billions of wild corals. Everything else is a distraction."

Rodan seems undaunted by the challenge he's set himself. Finding problems and solving them, no matter how large, is what drives engineers, he says.

"What I learnt at MIT was you might not be able to solve a problem on the first glance or even the 10th glance but you could at least have the courage and the confidence to give it a good crack. And you have a fun time doing it."