Lesson Plan

The Metal Asteroid

1852 to 1936, Naples, Heidelberg and Copenhagen • NASA Psyche spacecraft Mars flyby, 15 May 2026: gravity-assist slingshot at 12,333 mph, 2,800 miles above Mars, redirecting Lindy Elkins-Tanton's mission toward late-2029 arrival at the largest M-type asteroid in the main belt. • May 13, 2026

Before using this lesson plan: Read the full story or listen to the 5-minute narration (works as a bell-ringer).

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Lesson Plan: The Metal Asteroid

Subject area

STEM: astronomy, planetary science, geophysics, history of science.

Recommended audience

High-school physics and earth science (Years 11-13 / Grades 10-12)

First-year undergraduate astronomy, planetary science, or geophysics

General STEM enrichment for ages 14+

Lesson length

60-75 minutes (one period plus optional follow-up).

Learning objectives

By the end of the lesson, students will be able to:

Describe the discovery of asteroid 16 Psyche by Annibale de Gasparis in 1852 and place it in the historical context of nineteenth-century asteroid astronomy.

Identify the principal techniques (reflectance spectroscopy, radar, infrared spectroscopy, bulk density estimates) by which astronomers infer the composition of distant asteroids without visiting them.

Explain the M-type asteroid classification and the hypothesis that 16 Psyche may be an exposed metallic core of a destroyed protoplanet.

Describe Inge Lehmann's 1936 discovery of the Earth's solid inner core and explain the indirect (seismic) method by which planetary interiors are normally studied.

Outline the trajectory of the NASA Psyche mission, including the 2026 Mars gravity-assist flyby and the planned 2029 asteroid arrival.

Articulate why the Psyche mission is the first attempt to study a planetary metallic core by direct observation rather than indirect inference.

Key terms

Asteroid, main belt, minor planet

Reflectance spectroscopy, M-type / S-type / C-type

Radar reflectivity, bulk density

Differentiated body, protoplanet, planetary core

Iron-nickel meteorite, hydrated mineral

Seismic wave (P-wave, S-wave), shadow zone

Inge Lehmann, Beno Gutenberg

Gravity assist, solar-electric propulsion, Hall-effect ion thruster

Discovery-class mission, principal investigator

Pre-class preparation

A periodic table (for the chemistry of metals).

A printed schematic of Earth's interior layers (crust, mantle, outer core, inner core).

A photograph of the Capodimonte Observatory in Naples and a photograph of the Psyche spacecraft in the JPL cleanroom.

Lesson outline

1. Hook (5 min): the point of light on 17 March 1852

Describe the cold open: de Gasparis at the brass eyepiece in the Capodimonte tower, the faint moving point of light in Sagittarius. Pose: he had no way to know what the point of light was made of. Today's spacecraft is about to find out. What changed in 174 years?

2. De Gasparis and 19th-century asteroid astronomy (10 min)

Annibale de Gasparis (1819-1892), Capodimonte Observatory, Naples.

Discovered nine asteroids between 1849 and 1865, more than any other European astronomer of his era.

The Cauchoix-Lerebours refractor: a French-built achromatic telescope, the standard observatory instrument of the 1840s-50s.

Method: photographic plates had not yet been adopted for astrometry; de Gasparis measured asteroid positions visually against background stars using a micrometer eyepiece, observing on consecutive nights.

Why so many asteroids in this era? After the discovery of Ceres in 1801 by Piazzi and Pallas in 1802 by Olbers, the catalogue grew slowly. By 1850 only 13 asteroids were known. De Gasparis's contributions and those of his Italian, German and American colleagues doubled the catalogue within a decade.

3. The first hint of composition: spectroscopy (10 min)

1859-1860: Kirchhoff and Bunsen at Heidelberg invent the prism spectroscope and establish that the spectral lines of a heated element are diagnostic of its identity.

1864-1872: astronomical spectroscopy begins. William Huggins demonstrates that stars are made of the same chemical elements as the Sun.

1960s-1970s: with better detectors, reflectance spectroscopy of asteroids becomes possible. Asteroids are classified into spectral types: S (silicaceous), C (carbonaceous), M (metallic), and many subtypes.

16 Psyche is identified as the largest M-type asteroid in the main belt. Its visible-light spectrum looks like the spectrum of iron-nickel meteorites that occasionally fall to Earth.

4. Inge Lehmann and the indirect view of planetary cores (10 min)

Inge Lehmann (1888-1993), Danish seismologist.

1936 paper "P prime," published in Bureau Central Séismologique International, demonstrated that the Earth has a solid inner core inside its liquid outer core.

Her method: seismic waves from a 1929 New Zealand earthquake recorded at distant seismographs around the world. The P-wave shadow zone caused by the liquid outer core was already known (Beno Gutenberg, 1913). Lehmann noticed that within the shadow zone, faint P-waves still arrived; the only way to explain them was a solid inner core deflecting the waves back outward.

Discussion: every planet's metal core is normally invisible. We sense the Earth's core through seismic waves; we sense Mars's core through gravity and (since 2018) the InSight seismometer; we sense Mercury's core through its magnetic field. None of these is direct.

16 Psyche, if it really is an exposed core, would let us look at one directly. The Psyche mission is, in this sense, an attempt to make the indirect method redundant.

5. Building the case for 16 Psyche as a metal core (10 min)

Reflectance spectroscopy (1970s onward): consistent with iron-nickel.

Radar reflectivity (Arecibo and Goldstone, 1990s-2000s): high reflectivity consistent with metal-rich surface.

Bulk density (2010s, from satellite orbits): ~4.0 g/cm³, well below pure iron-nickel (~8 g/cm³). Either Psyche has very large internal voids (rubble pile of metal blocks) or it is a mixture of metal and silicate.

Mid-infrared spectroscopy (JWST, 2024): detected hydrated minerals on the surface, suggesting the surface is not pure metal but a more complicated mixture.

The picture as of 2026: Psyche is probably metal-rich but not pure metal. Whether it is best described as an exposed core, a partial core, a rubble-pile composite, or something else, will require direct measurement.

6. The Psyche mission (10 min)

Proposed by Lindy Elkins-Tanton at Arizona State University in 2015 as a Discovery-class mission (NASA's medium-cost planetary science programme).

Selected by NASA on 4 January 2017.

Spacecraft design: solar-electric propulsion (Hall-effect ion thrusters), twin extended solar arrays, multispectral imager, gamma-ray and neutron spectrometer, magnetometer.

Launched 13 October 2023 on a SpaceX Falcon Heavy from Kennedy Space Center.

15 May 2026: Mars flyby for gravity assist. Spacecraft passes 2,800 miles above Mars at 12,333 mph. Multispectral imager will be used to image Mars as a calibration exercise.

Late 2029: arrival at 16 Psyche. Two years of observation in successively closer orbits.

Mission cost: approximately $1 billion (Discovery-class cap was $850M plus instrument provisions).

7. What we are about to learn (5 min)

Is 16 Psyche an exposed core? If yes, the magnetometer should detect a remnant magnetic field characteristic of a once-active metal core.

What is the composition? The gamma-ray and neutron spectrometer will measure surface elemental abundances directly.

What is the morphology? High-resolution imaging will reveal the surface geology, including any features (cliffs, faults, impact basins) that show structure beneath the surface.

What does it tell us about planetary cores in general? If Psyche really is an exposed core, the answer will inform our understanding of every metal core, including Earth's.

8. Wrap and assignment (5 min)

Pose: "If the Psyche mission finds the asteroid is not pure metal but a composite, what would be the most likely explanation? Identify three alternative hypotheses for the body's origin."

Optional homework: write a 500-word memo from the perspective of a 2030 graduate student looking back at the Psyche mission's first year of asteroid observations, describing what has been confirmed and what has been complicated.

Discussion questions

Annibale de Gasparis discovered 16 Psyche in 1852. Why was the composition of asteroids essentially unknown at that time? Identify the technology that would be required to determine asteroid composition without visiting them.

Reflectance spectroscopy works by comparing the spectrum of sunlight reflected from an asteroid surface with laboratory spectra of meteorite samples. Why is this method indirect? What can go wrong in the inference?

Inge Lehmann discovered the Earth's solid inner core in 1936 using a single 1929 earthquake recorded at distant seismograph stations. What does her method tell you about the kinds of natural experiments that can substitute for laboratory experiments in planetary science?

The Psyche mission's principal investigator, Lindy Elkins-Tanton, proposed the mission in 2015 and will not see the asteroid until late 2029, a 14-year wait. What does this say about the timescales of planetary science research compared to other scientific disciplines?

The 2024 JWST infrared spectroscopy detected hydrated minerals on Psyche's surface. How does this complicate the "exposed metal core" hypothesis? Propose two alternative explanations.

Suppose the Psyche mission arrives in 2029 and finds the asteroid is not metal at all but a rocky body with metal-like surface reflectance. What does the field need to do? What does this tell you about scientific predictions?

Extension activities

Visualisation: plot the orbits of Psyche and the other large M-type asteroids in the main belt using a free planetarium application (e.g., Stellarium). Compare with the orbits of the principal S-type and C-type asteroids.

Reading exercise: read the abstract of de Gasparis's 1852 letter to the Astronomische Nachrichten and the abstract of the most recent Psyche-team paper (available via NASA's mission page). Identify what each communicates and what the gap of 174 years allows.

Calculation: at a closest approach of 4,500 km and a speed of 19,848 km/h, calculate the deflection angle of the Psyche spacecraft's trajectory caused by the Mars flyby. Use simple geometry and the gravitational parameter of Mars.

Writing assignment: write a 1000-word essay on the relationship between indirect observation and direct measurement in planetary science. Use Inge Lehmann's 1936 paper and the upcoming 2029 Psyche-arrival as the central case studies.

Time Rhymes