The asteroid 16 Psyche resides in the main asteroid belt between Mars and Jupiter. Discovered in 1852, this celestial body is famous for its unusual composition, which is unlike the rocky or icy makeup of most other asteroids. Its existence presents astronomers with a unique opportunity to study the building blocks of planets. This distinctive, massive object is the focus of a dedicated NASA mission designed to unlock the secrets held within its metallic structure.
Defining the Dimensions of 16 Psyche
The size of 16 Psyche is best described by its irregular, non-spherical shape, which is often compared to a lumpy potato. Rather than a single diameter, its dimensions are measured along three axes: 278 kilometers by 238 kilometers by 171 kilometers. This gives the asteroid an average diameter of approximately 226 kilometers, making it one of the largest objects in the main belt.
Determining the exact dimensions and mass from Earth requires advanced techniques like radar observations and stellar occultations. Scientists calculate its mass by measuring how the asteroid’s gravity minutely perturbs the orbits of smaller, nearby asteroids. Current estimates place 16 Psyche’s mass at \(2.3 \times 10^{19}\) kilograms.
Combining this mass with its volume allows researchers to determine its bulk density, estimated to be around 3.9 to 4.2 grams per cubic centimeter. This density is significantly higher than that of typical rocky asteroids, indicating a high concentration of metal within its structure.
Unique Composition and Structure
16 Psyche is classified as an M-type (metallic) asteroid due to its metal-rich composition. Observations using radar and spectroscopy indicate its surface and interior contain a mixture of metal, primarily nickel and iron. This composition distinguishes it from the more common C-type (carbonaceous) and S-type (silicaceous) asteroids.
The prevailing hypothesis suggests 16 Psyche is the exposed core of a failed planetesimal—a planetary building block that never fully formed. Early protoplanets became hot enough for materials to separate, causing heavy metals to sink and form a core. This theory posits that a catastrophic collision stripped away 16 Psyche’s outer rocky mantle and crust, leaving the metallic core exposed.
While the exposed core idea is compelling, the measured density presents a complication. Pure iron-nickel metal has a density of nearly 7.9 grams per cubic centimeter, almost double the 3.9 to 4.2 grams per cubic centimeter measured for 16 Psyche. This discrepancy suggests the asteroid is highly porous, perhaps a “rubble pile” of metallic fragments or a mixture of metal and less-dense silicate rock. Metal is estimated to make up between 30% and 60% of the asteroid’s volume.
Context and Comparison to Other Celestial Bodies
Psyche orbits the Sun in the main belt between Mars and Jupiter at an average distance of about 437 million kilometers. The asteroid holds approximately 1% of the total cumulative mass of the entire asteroid belt, demonstrating its dominance among the millions of objects there.
For perspective, the mean diameter of 226 kilometers is comparable to the length of the US state of Maryland. Its surface area is estimated at 165,800 square kilometers, roughly twice the size of the island of Ireland. Despite its size, 16 Psyche is significantly smaller than Earth’s Moon, which has a diameter of over 3,470 kilometers.
The asteroid completes one full rotation in a rapid 4.2 hours, and it takes approximately five Earth years to complete an orbit around the Sun. Its combination of size, mass, and metallic composition confirms its status as one of the most massive M-type asteroids known.
The NASA Psyche Mission
NASA launched the dedicated Psyche mission to explore this metal-rich world directly. The primary objective is to determine whether 16 Psyche is the stripped core of an early planetesimal. Studying the asteroid will provide an unprecedented opportunity to examine a planetary core, offering a unique window into the interiors of rocky planets like Earth, whose own core is inaccessible.
The spacecraft launched on October 13, 2023, aboard a SpaceX Falcon Heavy rocket. It is currently in an extended cruise phase, utilizing large solar arrays to power its solar electric propulsion system. This system uses Hall-effect thrusters that expel charged atoms of xenon gas, creating continuous thrust over the long journey.
The spacecraft is scheduled to perform a gravity assist flyby of Mars in 2026 to gain speed for its final approach. Arrival at the asteroid is planned for late July 2029, where the spacecraft will spend 21 months orbiting its target. It will map the surface and collect data on the asteroid’s composition, gravity, and magnetic properties from four successively closer orbits, descending from 700 kilometers to a final altitude of 85 kilometers.