The Size of Psyche

In 2022, NASA and Arizona State University will send a spacecraft to investigate the unique asteroid 16 Psyche, a metal-rich body about the size of Switzerland orbiting in the asteroid belt at an average distance of 2.9 AU from the Sun.

What is unique about asteroid Psyche is that it may, perhaps, be the exposed core of a protoplanet — a hit and run remnant, disrupted early in the history of the Solar System by one or more glancing blows from other bodies in the asteroid belt.

However, no-one knows for sure whether Psyche is an exposed core and we will have to wait until 2026 when the Psyche spacecraft arrives at the asteroid to find out more about its make-up and history. Here are the links to the Psyche mission websites at NASA and ASU.

Psyche is the 12th most massive object in the asteroid belt with an estimated mass of about 1% of the entire asteroid belt, or about 2.7 x 1019 kg, alternatively about 0.3% of the Moon.

Psyche is thought to have the shape of a flattened ellipsoid, specifically a tri-axial ellipsoid, with still uncertain dimensions of around 279 × 232 × 189 km. A recently published best ellipsoid fit suggests 277 × 238 × 168 km, with a sphere-derived diameter of 222 ± 4 km.

The Shape of Psyche - Images of Psyche from the HARISSA survey
Images of Psyche from the HARISSA survey. Credit: ESO/LAM

Psyche’s density in the region of 3.4 and 4.1 g/cm3 although estimates over the last decade have varied quite a bit, from 2.5 to 7.6 g/cm3, depending on how much metal the asteroid is assumed to contain and also on how much porosity it may have following the disruptive collisions it endured.

The assumed density is lower than that of stony-iron meteorites (mesosiderites and pallasites), the density of which are in the range 4.2 to 4.8 g/cm3. Mesosiderites are thought to be samples of the core+crust of a disrupted planetary body, and pallasites are thought to be core+mantle samples. From what I have read so far, the composition of Psyche may be more like mesosiderites than pallasites, as Psyche may have a distinct lack of olivine which is a major component of pallasites.

If Psyche is an exposed core, it’s the only exposed core we know about in the Solar System and will be a major leap forward for understanding more about planetary cores, since we are unable to visit our own, despite what Jules Verne and Hollywood have elaborately imagined.

The journey to the centre of the Earth is about 6,400 km and the deepest borehole drilled on Earth is just 12 km deep. With pressures up to 360 gigapascals (3.6 million times the pressure at Earth’s surface) and temperatures upwards of 6,000 degrees at the centre, we’ll have to look elsewhere if we want to investigate planetary cores in more detail.

At the moment we make do with characterising our own core remotely with laboratory modelling and with seismology (a positive application of earthquakes). And although this provides a wealth of information about the physical characteristics such as density, structure and composition, we all know that we really need to see something to understand it. It’s a bit like browsing online, convenient when there’s no store nearby, but it can’t replace the in-store experience to make sure we get what we thought we saw online.

So this is why Psyche is potentially going to help our understanding of planetary formation — and the prospect of being able to study an exposed planetary core must surely be the most exciting venture in asteroid science to date.

However, the Psyche spacecraft will not be deploying a lander or returning any samples to Earth, but will be observing at altitudes of 700 km, 290 km, 170 km and 85 km during 1,256 orbits of the asteroid at those four distances, measuring and characterising the magnetisation (if any), topography, gravity and composition.

But back to the size of Psyche…

I saw somewhere the size of Psyche being compared to the size of Texas (the famous size analogy for the fictitious metal asteroid in the 1998 movie Armageddon, of course). When I read that size, it caused me some angst, so I had to work it out for myself, which then prompted me to write this article.

Here is a graphical comparison of the estimated size of Psyche to U.S. states, and to Switzerland for those more familiar with Europe. The graphic compares the surface area and the cross-sectional area of Psyche, just so we can be clear what we’re comparing.

The size of Psyche
Credit: Vissiniti

In terms of surface area, Texas covers an area of 695,662 km2. Re-jigging that area to fit the cross-sectional area of a spherical asteroid requires the asteroid to have a diameter of 940 km, which is about the size of dwarf planet Ceres. Alternatively, wrapping Texas around the surface area of a spherical asteroid requires the asteroid to have a diameter of 470 km, which is between the size of asteroids 2 Pallas and 10 Hygiea.

But Psyche is much smaller than that, with an estimated diameter (if it were spherical) of roughly 226 km, which is equivalent to not quite the full length of Massachusetts. With that diameter, Psyche would have a surface area of 160,460 km2, which is about 23% of the size of Texas, and it would have a cross-sectional area of 40,114 km2, which is about 6% of the size of Texas. So the surface area of Psyche is more like the size of Georgia or Wisconsin, and its cross-sectional area is more like the size of Switzerland, or Massachusetts + Connecticut combined. This is all shown in the graphic above.

I did a similar analysis for the fictitious metal asteroid in Armageddon, which you can read here.

Using Psyche’s currently estimated dimensions, it is the 16th largest object in the asteroid belt by size, but an accurate size will be determined in 2026 once the spacecraft arrives.

The “16” in the name designation 16 Psyche refers to the fact that Psyche was the 16th object to be discovered (numbers have historically been assigned roughly in order of discovery). The fact that Psyche is also the 16th largest object in size in the asteroid belt is just a coincidence — these factors don’t always go hand in hand (almost never, in fact).

The following graphic shows where Psyche ranks in size compared to the twenty two largest objects in the asteroid belt. Most of these objects have very irregular shapes and the diameter given is a geometric mean.

The 22 largest objects in the asteroid belt (asteroids and a dwarf planet)
Graphic shows two images from orbit [1, 4], two images from Earth [3, 624], twelve constructed from adaptive-optics images and optical light curves [2, 7, 10, 15, 16, 31, 52, 65, 87, 324, 511, 704], and six shape models from light curves [19, 45, 48, 88, 451, 532]. Image Credits: [1, 4] NASA/JPL-Caltech/UCLA/MPS/DLR/IDA; [2] ESO/Marsset et al (2020)/MISTRAL (ONERA/CNRS); [3] Baliunas et al (2003); [7, 704] ESO/Hanuš et al (2019, 2020); [10] ESO/Vernazza et al (2020)/MISTRAL (ONERA/CNRS); [52] Merline et al (2013); [15, 65, 511] Viikinkoski et al (2018)/KeckII/AIDA/ADAM/DAMIT; [16, 324] ESO/LAM/Vernazza et al; [19, 45, 48, 88, 532] DAMIT/Ďurech et al (2010); [31] Yang et al (2020); [87] Carry et al (2019); [451] Michalowski et al (2005); [624] Marchis et al (2014).

Psyche is classified as an M-type asteroid in the Tholen taxonomy, or X-type in the later Bus-DeMeo classification scheme. A classification of M-type suggested the presence of surface metal when the classification system was devised, but radar albedo and thermal inertia measurements suggest that many M-type asteroids may not be solid or even rubble pile metallic bodies. So the M-types may be compositionally more diverse that originally assumed or not particularly metal rich at all — which makes Psyche unique. The M-type asteroids are a mystery still.

In the Bus-DeMeo taxonomy, which uses a larger asteroid dataset than the Tholen scheme, as well as a wider spectral range over which to classify asteroids and a different algorithm for grouping the principal features, the M class is no longer used and most M-types now fall into different sub-types of the larger X class.

The five largest M-type asteroids after Psyche are 22 Kalliope, 129 Antigone, 216 Kleopatra, 21 Lutetia and 135 Hertha. Their shape, size and spectral type compared to Psyche are shown in the graphic below (in which only 21 Lutetia is an actual image, the rest are constructed from adaptive-optics images, light curves and shape models, and the diameter given is a geometric mean).

The largest M-type asteroids (including 16 Psyche) fall into different X-class sub-types.
Image credits: [16] ESO/LAM/Vernazza et al; [22] Descamps et al (2008); [129, 135] Ďurech et al (2010)/DAMIT; [21] ESA 2010 MPS for OSIRIS Team/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA; [849] Hanuš et al (2017).

You can read more about asteroid classification schemes in my (very long) article on the history of asteroid classification here.

Just to wrap up, something I particularly like about the Psyche mission spacecraft is that the ensemble of onboard scientific instruments that will characterise the asteroid will not be given acronyms. There will be no ALICE, VIRTIS, OSIRIS, ROSINA, MIMI or GRaND, for example, the likes of which we have heard about on other missions and had to go off and look them up. No, we will all know what the instrument on the Psyche spacecraft does when the scientists talk about it, calling it what it is: a magnetometer, a gamma-ray and neutron spectrometer, a multispectral imager, and an instrument to measure gravity using the spacecraft’s X‐band radio communications system.

That actually makes me very happy as it means I won’t have to memorise the meaning or look up the acronym every time I hear or read about it. Although that last one in the list might become a bit of a mouthful.

A final thought. Psyche doesn’t appear to be part of an asteroid family (bodies with similar orbital characteristics) so where the remnants of the collisional material that left Psyche’s core so exposed? That remains a mystery too.

To quote Carl Sagan in the still unrivalled 1980 TV series Cosmos, when he speculated about the diversity of other bodies in our Solar System and beyond: “What wonders await?

Further Reading:

Mission to a Metal World: Psyche mission websites at NASA and Arizona State University (accessed 12 January 2021).

Elkins‐Tanton, L. T., Asphaug, E., Bell, J. F., Bercovici, H., Bills, B., Binzel, R., et al. (2020). Observations, meteorites, and models: A preflight assessment of the composition and formation of (16) Psyche. Journal of Geophysical Research: Planets, 125, e2019JE006296.

Ferrais, M., Vernazza, P., Jorda, L., Rambaux, N., Hanuš, J., Carry, B., Marchis, F., Marsset, M., Viikinkoski, M., Brož, M. and Fetick, R. (2020). Asteroid (16) Psyche’s primordial shape: A possible Jacobi ellipsoid. Astronomy & Astrophysics638, L15.