Psyche is likely made almost entirely of nickel-iron metal. Its bulk appears to be metal but its surface appears to have small areas that are rocky. The composition of Psyche has been determined by radar albedo (radar albedo is the “ratio of a target’s radar cross section in a specified polarization to its projected area; hence, a measure of the target’s radar reflectivity”1) and by thermal inertia (thermal inertia “refers to the ability of a material to conduct and store heat, and in planetary science, its measure of the subsurface’s ability to store heat during the day and reradiate it during the night”2).
Scientists think Psyche is the exposed nickel-iron core of an early planet, one of the building blocks of our solar system. Psyche is most likely a survivor of multiple violent hit-and-run collisions, common when the solar system was forming. The asteroid Psyche may be able to tell us how Earth’s core and the cores of the other terrestrial (rocky) planets came to be. (The terrestrial planets are Mercury, Venus, Earth, and Mars).
The core of the Earth lies at a depth of 3,000 kilometers, or more than 1,800 miles. We have only drilled to 12 kilometers, or about 7.5 miles, so far because that’s the most our technology allows today.
Additionally, Earth’s core lies at about 3 million times the pressure of the atmosphere. The temperature of Earth’s core is about 5,000 Celsius (~9,000 Fahrenheit). Because we cannot currently see or measure Earth’s core directly, the Psyche asteroid offers a unique window into the violent history of collisions and accretion that created the terrestrial planets. It is the only known place in our solar system where we can examine directly what is almost certainly a metallic core of an early planet.
Psyche lies in the main asteroid belt between Mars and Jupiter. A day on Psyche is about 4 hours and 12 minutes. This is the sidereal rotation period, or the amount of time it takes Psyche to completely spin around and make one full rotation. A year on Psyche lasts about five Earth years (about 1,828 Earth days).
Psyche orbits the Sun at an average distance of 3 astronomical units (AU) (about 280 million miles); Earth orbits at 1 AU (about 93 million miles). Because Psyche and Earth orbit at different speeds, the distance from Earth to Psyche varies over a large range from less than 2 AU to greater than 4 AU.
Psyche is dense. Measurements are still being made, with estimates ranging from 4,000 kilograms per cubic meter (kg/m3) (a mix of rock and metal) to 7,000 kilograms per cubic meter (kg/m3) (similar to the density of a bar of steel). The surface gravity on Psyche is much less than Earth, and even less than the Moon. On Psyche, lifting a car would feel as light as lifting a dog or a 2nd grader!
As asteroids go, Psyche is relatively large and has an irregular shape. It is 279 x 232 x 189 kilometers (173 x 144 x 117 miles). If Psyche were a perfect sphere, it would have a diameter of 226 kilometers (140 miles). That is about the length of the state of Massachusetts (leaving out Cape Cod). If it were in Arizona it would stretch between Phoenix and Flagstaff.
The Psyche asteroid has a surface area of about 641,800 square kilometers (246,300 square miles), making it just smaller than the area of the state of Texas and quite a bit larger than the area of California.
Scientists have combined radar and optical observations to generate a 3D shape model of Psyche.This model shows evidence for two crater-like depressions. It suggests that there is significant variation in the metal content and color of the asteroid over the surface. But remember, no one has seen the Psyche asteroid yet, so we will not know what it actually looks like until the spacecraft arrives.
Psyche Science Goals
- Understand a previously unexplored building block of planet formation: iron cores.
- Look inside terrestrial planets, including Earth, by directly examining the interior of a differentiated body, which otherwise could not be seen.
- Explore a new type of world. For the first time, examine a world made not of rock and ice, but metal.
Psyche Science Objectives
- Determine whether Psyche is a core, or if it is unmelted material.
- Determine the relative ages of regions of Psyche’s surface.
- Determine whether small metal bodies incorporate the same light elements as are expected in the Earth’s high-pressure core.
- Determine whether Psyche was formed under conditions more oxidizing or more reducing than Earth’s core.
- Characterize Psyche’s topography.
1 Ostro, S. J. (1999). Planetary Radar. Encyclopedia of the Solar System. Available from http://echo.jpl.nasa.gov/asteroids/ostro_1998_encyc_ss.pdf