Mohawk Valley Astronomical Society

Return to Newsletter Index

Heavy Metal

by Perry Pezzolanella

This is not about rock music, but rock is almost a correct word. Asteroids are a vast collection of rocks as small as boulders to as large as several hundred miles across orbiting mainly between Mars and Jupiter. Most are fractured and battered rocks made of a mixture of carbon compounds, silicates, and metals. One asteroid stands out among them as being unusually rich in metals and begging for exploration. Fortunately, NASA is answering the call.

Psyche is the largest of the rare (type M) metallic asteroids. Telescopic observations reveal Psyche to be 90% metallic and 10% silicate rock. It could be the unique remnant of a class of asteroids that formed so close to the sun that only metals could condense out of the early solar nebula and were later flung into the asteroid belt. It could also be the inner, metallic core of a once larger protoplanet that had its overlying layers of rock violently blasted off by impacts with other asteroids. The surface will not be unchanged as there will be impact craters and it will be littered with hydrated rocks from other asteroids. Psyche may actually look like a giant rubble pile. It is a large asteroid about 130 miles across far out in the asteroid belt about 300 million miles from the Sun. Reaching it will not be too difficult since Dawn has already proven it can be done by not only flying to, but orbiting, Vesta and then continuing on to do the same at Ceres.

A spacecraft also called Psyche was approved in early 2017 with the goal of launching sometime during August 6-26, 2022 and arriving at Psyche during January 2026 after a gravity assist from Mars in 2023. It will be powered by solar electric propulsion to slowly and carefully reach its namesake world. It will gradually settle into orbit and spiral into lower orbits getting as close as 25 miles from the surface allowing for resolution as good as 15 feet. The primary mission is for it to orbit Psyche for at least 21 months, with hopes of an extended mission. The cameras will be similar to the Mars 2020 rover and will allow geologists to reconstruct Psyche’s history. The cameras will have filters tuned to specific wavelengths of visible and near infrared light. It will orbit Psyche at various distances with each having a goal. At the higher orbits, around 400-500 miles above the surface, the spin axis and rotation period of Psyche will be nailed down. Global color mapping will be achieved at lower orbits around 100-200 miles above the surface along with gravity science and magnetic field mapping, and at the lowest orbit, hardly 25-35 miles above the surface, it will map the mineral composition, determine crater ages, and do high resolution mapping. If the spacecraft determines that the silicate material on the surface is primarily high-magnesium pyroxene or olivine, then these could be remnants of a crystallizing magma ocean, which means Psyche differentiated and had its mantle stripped - a remnant metallic core.

Asteroids may seem to be nothing more than a bunch of rocks or a debris pile, but Dawn proved that Vesta and Ceres are unique in themselves. A metal-rich world as unusual as Psyche is sure to surprise everyone when seen close up and will hopefully add a critical piece to the jigsaw puzzle of the creation and evolution of the Solar System.