The Size of Psyche

NASA and Arizona State University, along with other partners, are working on a project to 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 the end of the decade* 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 would indicate that Psyche is a metal core.

*Launch update 24 June 2022: NASA announced that due to delays in testing software and equipment, the Psyche spacecraft will miss its launch window in late 2022. Launch will now be in 2023 or 2024, and because of the relative orbital positions of Earth and Psyche, this means the spacecraft will take longer to reach the asteroid, arriving in 2029 or 2030, depending on the actual launch date.

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 a currently estimated mass of about 1% of the entire asteroid belt, or about 2.7 x 1019 kg (which is just under 0.04% the mass of the Moon). The actual mass of Psyche will only be known once the Psyche spacecraft arrives at the asteroid, when measurements of the gravitational perturbation on the spacecraft’s orbit will enable the mass of the asteroid to be derived.

As for its size, asteroid Psyche 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 (the diameter the asteroid would have if the volume of material in its currently modelled shape were to be remodelled into a sphere) is estimated to be 226 km.

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 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. Current research suggests that the composition of Psyche may be more like mesosiderites than pallasites, since Psyche appears to have a distinct lack of olivine, which is a major component of pallasites.

If Psyche is an exposed core, it is the only exposed core we know about in the Solar System and will be a major leap forward for our understanding 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 will have to look elsewhere if we want to investigate planetary cores in more detail.

Currently, we make do with characterising our own planet’s core remotely with laboratory modelling and with seismology, the latter being a positive application of earthquakes. 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 is no store nearby, but it will never replace the in-store experience to make sure we get what we thought we saw online.

So this is why Psyche (asteroid and spacecraft) 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. That does not in any way mean that recent missions to asteroids (or comets) were not ground-breaking, with mini-rovers bouncing across the surface and spacecraft collecting up samples to return to Earth. Every mission has been an astonishing feat of complexity and achievement in its own way.

The Psyche spacecraft will not be deploying a lander or returning any samples to Earth, 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, and returning what will be some of the most eagerly anticipated images of an asteroid to date.

But back to the size of Psyche…

In 2020, I read on the internet the size of Psyche being compared to the size of Texas. The size of Texas is, of course, a now well-used size analogy that was first applied to visualise the enormity of the massive fictitious metal asteroid in the 1998 movie Armageddon.

When I read that size comparison for asteroid Psyche it caused me severe angst. But it appeared somewhere that I would not think to question (not immediately, anyway), so I presumed I must be missing something in what I was reading, or perhaps I was overly tired that day and couldn’t think straight, or had I simply slipped down a rabbit hole and ended up in a Wonderland where sizes were all out of kilter?

So I worked it out for myself…again and again and again. And once I’d convinced myself that what I had read on the internet was wrong (in terms of cross-sectional area, surface area or any other way I might want to compare it to the size of Texas), I sent off an email pointing out the anomaly.

The anomaly was speedily acknowledged and corrected on the website in question (although not on the app, which required a second email some time later).

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 surface area and cross-sectional area, just so we can be clear what we are comparing when we do these types of land area size comparisons for asteroids.

As a quick reminder, surface area is wrapped around the asteroid (4πr2) whereas cross-sectional area is just the flat face you see (πr2). In the diagram below, Psyche is represented by the pink blob.

The size of Psyche
The size of Psyche (credit: Vissiniti)

In terms of surface area, Texas covers an area of 695,662 km2. Rejigging 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 (the third and fourth largest asteroids, respectively).

But 16 Psyche is much smaller than that, being the 16th largest asteroid in the asteroid belt. It has an estimated diameter of 226 km (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 (or Massachusetts + Connecticut combined). All of that is shown in the graphic above.

The moral of this particular tale is a very simple one: always check the dimension that you are plugging into your calculation. In this case, ensure that you are using radius and not diameter, otherwise you will get an erroneously inflated size. In this case, using the diameter instead of radius, you will arrive at something the size of Texas, in fact.

And we’ve all done something similar at some time.

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

So 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 once the Psyche spacecraft arrives there.

The “16” in the number designation of 16 Psyche refers to the fact that Psyche was the 16th object to be discovered. The fact that Psyche is also the 16th largest object in the asteroid belt in terms of size is coincidental (but not entirely surprising). In the early days of planetary astronomy, it was simply a case of larger asteroids being spotted before smaller asteroids since they reflect more sunlight. Although that last statement is not always the case, since the brightness of an asteroid 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 — so large, in fact, that it really is the size of Texas (which is what caused the alarm bells to ring in my head when I first read Psyche being compared to that size).

Conversely, 4 Vesta is the second largest asteroid known but the fourth to be discovered after 2 Pallas and 10 Hygiea. Asteroid 704 Interamnia is the fifth largest but the 704th to be discovered. You get the point; there is no relationship between the number given to an asteroid and its relative size. The fact that 16 Psyche is the 16th largest asteroid and the 16th asteroid to be discovered is just a fluke.

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 classified as an M-type asteroid in the Tholen taxonomy, or X-type in the later Bus-DeMeo classification scheme — and it is the largest known M-type.

Historically, a classification of M-type suggested the presence of surface metal when the Tholen 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 even particularly metal rich at all — all of 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 a 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 16 Psyche are: 22 Kalliope, 129 Antigone, 216 Kleopatra, 21 Lutetia and 135 Hertha. Their shapes, sizes and spectral types compared to Psyche are shown in the graphic above, in which only 21 Lutetia is an actual image and the rest are constructed from adaptive optics images, light curves and shape models. The diameter given is a geometric mean.

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

The size of Psyche’s instrument names

Planetary Scientists Yearn for Clever Handles for their Equipment. But not Psyche scientists.

Something I particularly like about the Psyche mission is that none of the names associated with the mission are acronyms — neither the name of the spacecraft 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, the likes of which we have heard about with 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. When we hear the Psyche scientists talk about an instrument on the Psyche spacecraft in the many briefings that will no doubt take place, we will all know what it is that the instrument does. There are no acronyms, just two fluxgate magnetometers, one gamma-ray spectrometer, two neutron spectrometers and two multispectral imagers. And then there’s the way the asteroid’s gravity is determined using the spacecraft’s X‐band radio telecommunications system and its own mass.

No short names may cause the briefings to be longer than usual.

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?

I can’t wait to see the wonders on Psyche.

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.