The space rock – dubbed "16 Psyche" – is approximately 252km (156 miles) wide and is part of a belt of asteroids in the inner solar system.
The US space agency has now fast-forwarded its plans to send a rocket to the asteroid – bringing it forward to 2022.
Psyche is made up entirely of nickel and iron, which are the exact same materials at the Earth's core.
This – as well as its sheer size – had led experts to believe the asteroid is actually the remains of a planet.
But the value of just the iron alone if the asteroid was transported back to Earth, would be $10,000 quadrillion (£8,072 quadrillion).