Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

What a visit to the curious metal object might look like (Image: JPL/Corby Waste)

Object: asteroid 16 Psyche

Alter ego: naked metal core

If Jules Verne were alive, he’d raise a toast. In a twist on his notion to journey to the centre of the Earth, a proposed spacecraft may get to visit the core of a proto-planet that was long ago stripped of its rocky outsides and cast adrift in the solar system.


Asteroid 16 Psyche was discovered in 1852, but it was not until the 1980s that it was recognised as an oddball. Radar observations made from Earth revealed that Psyche is about 200 kilometres across and is made of 90 per cent iron and nickel, with 10 per cent silicate rock.

This composition is strikingly similar to that of Earth’s metal core. That means Psyche could have started life as a small rocky world with a metal core and a silicate mantle, similar to the large asteroid Vesta. And astronomers think larger planets like Earth and Venus could have formed when such nascent worlds collided and merged.

But other times, incoming asteroids might have stripped a proto-planet of its soft outer layers. Psyche could have fallen victim to a series of hit-and-runs that robbed it of its mantle, leaving just the metallic core behind. If that core had been liquid at some point, it would have given the object a strong magnetic field. In fact, Psyche could still have a remnant field almost as strong as the Earth’s.

Unique core sample

“It could be like a little refrigerator magnet in space,” says Linda Elkins-Tanton of the Carnegie Institution in Washington DC, who presented an idea for a mission to Psyche at a meeting of the American Geophysical Union in San Francisco last week.

“A mission there is the only way that humankind will ever visit the core of any body,” says Elkins-Tanton. “We can learn about the building blocks of the planets in the first million years of the solar system in a way that we can’t do any other way.” Her team’s proposed spacecraft would orbit the asteroid for about six months, making measurements of the metal world’s gravity, composition and topography.

Physically, Psyche could look quite different from any other space object we’ve yet seen. Physicists have run laboratory experiments on how impact craters form on metal surfaces, and they suggest that Psyche’s craters could have dramatic rims that froze in splash-like patterns.

“But here’s the thing: we don’t know what we’re going to see,” says Elkins-Tanton. “We’ve seen rock worlds and ice worlds and gas worlds, but we’ve never visited a metal world. We have no idea what it will look like. We only know we’re going to be surprised.”