Mining asteroids, eh? That’s the latest idea to come out of the west coast of the US.

Back in 1995 we were apparently poised to conquer the last frontier – the ocean floor.

The Americans were planning to launch Deep Flight 1, a propelled vehicle that could explore the deepest parts of the world’s oceans, with a launch near Monterey, California. Not too fantastical, one might have thought then; the Japanese had earlier that year sent a robot down the Mariana Trench.

A bit further north from Monterey, in the suburbs of Seattle, Washington, are the offices of Planetary Resources, a company which aims to send craft to (relatively) nearby asteroids and extract valuable minerals.

To judge by the progress so far with seabed mining, the space thing may be a way off.

For years now there has been the lure of the seemingly vast amounts of minerals lying in the seabed – nickel, iron ore, manganese, copper and cobalt being among the most frequently mentioned, with rare earths becoming a late entrant.

Several commercial enterprises have been formed to pursue this wealth of the oceans, of which Canada’s Nautilus Mining is possibly the best known. But, of course, not everyone is happy with the notion. Nautilus has run into trouble with opposition in Papua New Guinea, Kiwis Against Seabed Mining is trying to stop offshore ironsand extraction off the Taranaki coast, and the Northern Territory government has imposed a moratorium on anything outside the high tide mark.

Setting aside the obvious point - that is, why would you spend billions extracting nickel, iron ore, manganese etc from the seabed when prices (or demand) don’t make such propositions economically feasible? - there’s the cost factor itself. For crying out loud, many land-based mining companies are wrestling with rampaging costs as it is. (There are exceptions: the numbers for dredging phosphate off Namibia are said to stack up.)

The Japanese are possibly a case apart. They produce only 0.6% of the oil they use, they import their LNG and China has got its hands tightly around Tokyo’s throat on rare earths supplies vital to Japanese technology manufacturing.

So it makes some sense for them to, for example, investigate (along with methane gas and oil off their coastline) what seems a large rare earths resource lying under the seabed near the country’s easternmost point, a small atoll that is sitting on what a Tokyo university scientist believes is a 200-year supply. (Although, again, why bother? By the time the Japanese get their hands on those rare earths, Western production will be sufficient to replace all their supply from China; indeed, established deposits outside China now total 400 years of consumption at present rare earth usage rates.)

So back to Planetary Resources. You can check out all the proposed hardware on their impressive website, and even apply for jobs if paragliding has become too tame for you, and - at first blush - some people may even find their case plausible.

They claim there are more than 1500 asteroids which “are as easy to reach as the Moon and are in similar orbits as Earth. Asteroids are filled with precious resources, everything from water to platinum.”

Planetary argues that some near-Earth asteroids contain platinum group metals in much higher concentrations than the richest Earth mines.

“In space, a single platinum-rich 500m wide asteroid contains about 174 times the yearly world output of platinum, and 1.5 times the known world reserves of platinum group metals (ruthenium, rhodium, palladium, osmium, iridium and platinum).”

We’ll check back in 2015 and see how they’re going.

However, Planetary’s “president and chief asteroid miner” Chris Lewicki is not ruling out the oceans on Earth either. He just gave a talk at the University of Alberta which addressed the question of whether extracting precious metals from seawater would be easier than mining asteroids. (You think?)

So here was a calculation he ran past the audience.

As Lewicki explained, the approximate concentration of platinum in seawater is just 0.000000000234 grams per litre.

On the other hand, he said to consider how many litres of water there was in the ocean, which turns out to be 1,300,000,000,000,000,000,000 litres.

“Multiply this together and we end up with approximately 300,000 tonnes of platinum in the seawater. At today’s price for platinum (about $US1600 per ounce), this is $16 trillion … That is a kingly sum.”

As is next Tuesday’s $100 million Oz Lotto.

By comparison, dredging down a few thousand metres sounds just a tad easier than filtering every drop of water on Earth. And we're still trying to crack the dredging thing.