The metal world
Psyche
Psyche is unlike any other body visited so far: it is made not just of rock or ice, but largely of metal. It may be what is left of the core of a planet that never formed, laid bare by collisions. To find out, NASA has sent a spacecraft of the same name to meet it, launched in 2023 and due to arrive in 2029.
- Size
- 278 × 238 × 171 km
- Distance from Sun
- 438 M km
- Orbital period
- 5.01 years
- Rotation
- 4.2 hours
- Discovered
- 1852
- Mission
- Psyche (arrival in 2029)
Physical characteristics
Psyche measures 278 × 238 × 171 km: it is shaped like a flattened potato and is the largest of the M-type asteroids, those that reflect light and radar waves the way a metal would. Its density, between 3.4 and 4.1 grams per cubic centimetre, is higher than that of almost all other asteroids, but far from that of pure iron, which is close to 8.
According to the most widely cited estimates, metal, mostly iron and nickel, makes up between 30 and 60% of its volume, and the rest is rock. For this reason the classic hypothesis, the bare core of a protoplanet that lost its crust and mantle, is no longer taken for granted: Psyche could be a mixture of metal and rock, or a body that formed already rich in iron. The spacecraft will decide.
Orbit and rotation
Psyche orbits in the outer part of the main belt, 438 million km from the Sun on average, or 2.93 astronomical units, and takes almost exactly 5 years to complete one lap. Its orbit is tilted 3.1° to the ecliptic.
It spins once every 4.2 hours, and does so almost on its side: its rotation axis lies close to the plane of its orbit, rather like that of Uranus. As a result each hemisphere goes through long seasons of continuous light and darkness, which the spacecraft will have to take into account in order to photograph the whole surface.
Discovery and exploration
Annibale de Gasparis discovered it on 17 March 1852 from the Capodimonte Observatory in Naples, three years after Hygiea. It was the sixteenth asteroid known and was named after the maiden of Greek myth loved by Eros. For more than a century little was known about it beyond its orbit: its metallic nature emerged from radar observations, starting in the 1980s.
NASA's Psyche spacecraft lifted off on 13 October 2023 on a Falcon Heavy rocket. On 15 May 2026 it flew past Mars 4,609 km above the surface, using the planet's gravity to gain speed, and arrival at the asteroid is planned for August 2029. It will stay in orbit for about two years to map its surface, composition, gravity and any magnetic field, which would be evidence of an ancient molten core.
Frequently asked questions
Is Psyche made of metal?
Partly. Radar and density measurements indicate that iron and nickel make up between 30 and 60% of its volume; the rest is rock. It is still the most metal-rich of the large asteroids, but not the block of pure iron imagined until a few years ago.
When will the Psyche spacecraft arrive?
Arrival is planned for August 2029. The spacecraft launched in October 2023 and flew past Mars in May 2026 for a gravity assist. Once there it will enter orbit around the asteroid and stay for about two years.
How much is asteroid Psyche worth?
A figure does the rounds, 10 quintillion dollars, but it is a back-of-the-envelope calculation made by multiplying the estimated mass of iron by its market price. It makes no economic sense: no one knows how to bring that metal to Earth, and if it got here the price would collapse. The mission has no mining aims: its purpose is to understand how planetary cores form.
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