The Size of Psyche

In 2022, NASA and Arizona State University will send a spacecraft to investigate the unique asteroid (16) Psyche, an assumed 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, or planetesimal, or whatever we are supposed to call it now, a hit and run remnant of something 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 the asteroid 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. If the spacecraft detects a magnetic field, it will indicate that Psyche is a metal core.

Here are the links to the Psyche mission websites at NASA and ASU.

Psyche is the twelfth 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.

It is thought to have the shape of a flattened ellipsoid (tri-axial ellipsoid) and a recently published best fit suggests dimensions of 277 × 238 × 168 km. Its sphere-derived diameter is something like 226 km (or thereabouts).

The Shape of Psyche - Images of Psyche from the HARISSA survey
Images of asteroid (16) Psyche from the HARISSA survey. Credit: ESO/LAM

Psyche’s density is estimated to be in the region of 3.4 to 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 any collisions it has 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 and crust of a disrupted planetary body, and pallasites are thought to be core and 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 planet’s 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 simply observing at closer and closer altitudes (700 km, 290 km, 170 km, 85 km) during 1,256 orbits of the asteroid, measuring and characterising the gravity, topography, composition, and magnetisation (if any).

But back to the size of Psyche…

In 2020, I saw somewhere on the internet (on a website that shall remain anonymous) the size of Psyche being compared to the size of Texas — being that well-used size analogy first applied to the fictitious metal asteroid in the 1998 movie Armageddon.

When I read that size comparison for Psyche it caused me severe angst. But it appeared somewhere that I wouldn’t immediately think to question, so I presumed I must be missing something big.

But still, I worked it out for myself. And then again. And again. And once I’d convinced myself that what I was reading was wrong (in terms of cross-sectional area, surface area, or any other way you might want to compare it), I sent off an email…and the glitch (let’s call it) on the website was speedily acknowledged and corrected.

So what is the best comparison for the size of Psyche?

Below is a graphic that compares the size of Psyche (pink blobs and pink text) to the size of a few US states and a country across the pond. It compares both surface area and cross-sectional area, just so we can be clear what we are comparing when we do these types of land area comparisons for asteroids.

A reminder: surface area is wrapped around the asteroid (4πr2) whereas cross-sectional area is just the face (πr2).

The size of Psyche
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 (1) 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 (16) Psyche is much smaller than that. It has an estimated diameter of 226 km (that’s the diameter it would have if its irregular ellipsoid volume were remodelled into a sphere). And that 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.

In fact, 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 (Massachusetts + Connecticut combined). All shown in the graphic above.

The moral of the calculation is a simple one: always check the dimension you are plugging in (for example, radius not diameter) otherwise you may get an erroneously inflated size! We’ve all done it at some stage.

You can read an entertaining analysis I did for the enormous fictitious metal asteroid in Armageddon in the article It’s the size of Texas.

The size of Psyche compared to other asteroids

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

The “16” in the number designation of (16) Psyche refers to the fact that Psyche was the sixteenth object to be discovered. The fact that Psyche is also the sixteenth largest object in the asteroid belt in terms of size is just coincidental. In the early days, it was simply because bigger ones were spotted before smaller ones, reflecting more sunlight (although that’s not always the case either, since it also depends on reflectance, or spectral type, and of course distance).

The dwarf planet (1) Ceres was the first ‘asteroid’ to be discovered because it is so large…it really is the size of Texas. But (4) Vesta is the second largest and (704) Interamnia is the fifth largest. You get the point.

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 irregular shapes and the diameter given is a geometric mean (the size it would have if its volume were remodelled as a sphere).

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 the twelfth largest asteroid in the asteroid belt in terms of mass and the largest so-called M-type asteroid. It is classified as an M-type asteroid in the Tholen taxonomy, or X-type in the later Bus-DeMeo classification scheme. Historically, 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 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).

Of the nine or so known M-type asteroids, the five largest 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 above, 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.

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

Planetary Scientists Yearn for the Cleverest Handles for all their Equipment

But not the Psyche scientists.

Something I particularly like about the Psyche mission is that none of the names are acronyms — neither the name of the mission nor the names of the ensemble of onboard scientific instruments being sent to characterise the asteroid.

So there will be no ALICE, VIRTIS, OSIRIS, ROSINA, MIMI or GRaND, for example, the likes of which we have heard about on other missions to asteroids…and then had to go off and look up the meaning because the acronym itself tells us absolutely nothing about what the instrument does.

No. We will all know immediately we hear the name of the instrument what the instrument does when the scientist talks about it at the briefings. A set of two fluxgate magnetometers. A gamma-ray and two neutron spectrometers. A pair of multispectral imagers. And determination of the asteroid’s gravity using the spacecraft’s X‐band radio telecommunications system and its own mass.

It might, of course, mean the briefings will be longer though.

Mysteries and wonders

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