An asteroid with a platinum core has been targeted by large mining firms after flying close by to Earth.

Asteroid 2011 UW-158, which whizzed 1.5m miles past Earth on 19 July, spans just under a kilometre in width.

Its core is made of platinum and experts claim that it is worth around £3.5tn for its resources, which has piqued the interest of large space mining firms.

Astronomer Bob Berman told Sky News there is a loophole in the 1967 Space Treaty that corporations are looking to exploit.

He said: "There's the little matter of the 1967 Space Treaty and almost all countries have signed that saying no nations can claim a celestial body but it doesn't say anything about companies, corporations, firms, private entities.

"Nations cannot claim a celestial object but a private company can so if they can get there and they can mine it with all that platinum then good luck to them, it's all theirs."

Planetary Resources has been touted as one of the companies looking to mine the asteroid after recently sending a rocket from the International Space Station (ISS): "At first they were hoping for the platinum, which this asteroid contains but nowadays they're thinking of water, which can be even more valuable to the International Space Station or other astronauts," said Berman.

The asteroid will not come as close to Earth as it did recently for another three years.

Nasa expects us to be able to mine asteroids by 2025 and said that the materials could "be used in developing the space structures and in generating the rocket fuel that will be required to explore and colonise our solar system in the 21st century".