Psyche, exploring a metal world

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In our solar system, we have 8 bodies revolving around the sun which are currently called planets. They are grouped into 3 categories. The four closest to the sun -- Mercury, Venus, Earth, and Mars -- are rocky planets. The next two further out --Jupiter and Saturn -- are called gas giants. They have a super dense solid core surrounded by a thick layer of helium and hydrogen in gas form. The furthest two from the sun --Uranus and Neptune-- are called ice giants. They have helium and hydrogen in their atmosphere, too but also ammonia, water, and methane which freeze into solid "ice". Between Mars and Jupiter is the asteroid belt made up of rocks from 33 feet (10 m) wide to 329 miles (530 kilometers) in diameter.

One of these asteroids is called Psyche (pronounced "sigh-key"), and NASA has its eye on sending a probe to it this year to arrive by 2029. It's 210 km (130 miles) wide (about the length of Massachusetts), and it is either all metal or a combination of metal and rocks. Why is any of that that so interesting?

To begin, Italian astronomer Annibale de Gasparis discovered the asteroid on March 17, 1852 and named it after the Greek goddess of the soul who was born mortal and married Eros, the god of Love. He also discovered 8 other asteroids and wrote his first research paper in 1845 on the largest asteroid Vesta. which was discovered in 1807 by Heinrich Olbers.

We know its general shape already, something like an oblong potato (most asteroids seem to have that shape), but radar images have also shown some of its topography. Red color below represents peaks, and blue represents depressions  and craters.

Radar images from all sides of Psyche

Psyche rotates every 4 hours for a short day cycle, and it travels around the sun every 5 years. It also rotates on its side, that is, perpendicular to its route around the sun similar to an airplane propeller spinning sideways to the direction the plane flies, which is unlike most objects in the solar system. Again, what makes Psyche so special that NASA is building a specific explorer to send there? From measurements scientists have made so far, they think it could be different from other asteroids. They think it might actually be the core of a planet that formed when the other planets did, about the size of Mars, but that may have been hit by something which ripped away the outer layers. See the animation video below for a demonstration.

Clips from psych.asu.edu website video

After the outer layer was removed, the remaining core cooled and hardened, forming a thinner layer that had been hit by other smaller asteroids over time to leave craters. But, most of Psyche is its original core, which is very different than its thicker original crust but similar to what we know about Earth's core. Scientists want to study Psyche because they can't drill deep enough into the Earth to get samples of its own core (too far, too much pressure, too hot). Learning more about Psyche might give us more information about our own planet's layers.

By watching how Psyche affects the movement of other objects, scientists have estimated its mass and density. Right now, they have calculated it is one of the densest asteroids studied so far, twice as dense as most, about 3,400-4,100 kg per cubic meter. By measuring the type of light coming from Psyche, scientists have also estimated it is composed of an iron-nickel metal, maybe kamacite, not just rocks like iron silicates. Kamacite is an alloy of 90-95% iron and 5-10% nickel, only in meteorites, so it is reasonable to think that Psyche may have the same composition. Data gathered about its surface show metallic material in part of the craters, so scientists think this may mean a thin layer of the original crust was hit by other asteroids deep enough to cause eruptions of metallic lava to rise to the surface before the hot core cooled.

Kamacite meteorite from China

So, what is the NASA Psyche mission? Despite earlier delays, the mission to Psyche is now expected to launch in October, 2023 with a spacecraft explorer of the same name, and arrive in August of 2029. It will not land on the asteroid, but it will spend about 2 years orbiting and recording various data. What data?

The Psyche spacecraft will carry several instruments. One will create high-resolution images using special filters to distinguish between metallic and silicate (rocky) material. Another two will analyze the elements on the asteroid's surface. Others will measure and map out the magnetic field and gravity field to learn more about its center.

Psyche spacecraft under construction (NASA)

The spacecraft will also use a unique type of propulsion system called SPT (stationary plasma thruster). Energy from the sun will be collected by the solar panels and converted into electricity. That will be fed into a supply of xenon gas to make electrically charged ions (plasma) that shoot out the back to propel it. Gas is lighter than the usual chemical liquids used in other spacecraft, and the SPT system is expected to take less time for the spacecraft to reach the asteroid.

SPT thruster in use, from psyche.asu.edu

For some explanations of what we know about Psyche's surface and how we learned it, check out this article from MIT. 

For another blogger's excellent recap of what Psyche it, go to vissiniti.com.

For an hour-long scientific description of the asteroid and project, checkout this YouTube video lecture by Dr. Lindy Elkins-Tanton of the National Academy of Science.