Metal Asteroids: Dense Rarities

Illustration of 16 Psyche by Peter Rubin. Credit: NASA/JPL-Caltech/ASU

Most asteroids are rocky debris from the formation of the solar system. However, some asteroids contain large amounts of metal, though they are generally much more rare, with only 38 asteroids known to date. These asteroids are likely the parent bodies for many meteorites, as metal tends to survive the heat of atmospheric entry better than rock. 

These "M-type" asteroids will likely be important targets for future space mining opperations. Until then, let's take a look at some of the largest of these metal rich wonders.

16 Psyche

Animation stills of topological features on Psyche. Credit: Shepard et al/CC BY-SA 4.0

• Date Discovered: March 17, 1852
• Designation: 16 Psyche (A852 FA)
• Mass: 2.29x10^19 Kilograms
• Radius: 111.5 Kilometres
• Density: 3.977 g/cm^3
• Rotational Period: 4.2 Hours
• Orbital Period: 1825.95 Days (4.999 Years)
• Moons: None

16 Psyche is the only M-type asteroid that is getting its own dedicated mission. Launching this October, NASA's Psyche spacecraft will enter orbit around the asteroid in 2029. Psyche was discovered by Annibale de Gasparis in 1852, Making it one of the first asteroids discovered. In fact, the "16" in Psyche's designation refers to the fact that was the sixteenth asteroid discovered. It is quite massive, and makes up 5% of the Asteroid Belt's mass. It was once thought to be the exposed core of a protoplanet, but new studies seem to refute this. However, NASA coverage of the mission seems to still refer to Psyche as a possible core, so perhaps we just won't know for sure until the Psyche spacecraft gets there.

21 Lutetia

Lutetia as seen by Rosetta at closest approach. Credit: ESA

• Date Discovered: November 15, 1852
• Designation: 21 Lutetia (A852 VA)
• Mass: 1.7x10^18 Kilograms
• Radius: 47.88 Kilometres
• Density: 3.45 g/cm^3
• Rotational Period: 8.17 Hours
• Orbital Period: 1388.49 Days (3.8 Years)
• Moons: None

Lutetia is a large main belt asteroid that was imaged by the Rosetta spacecraft on its way to comet 67P. Lutetia remains the only M-type asteroid to be visited by a spacecraft, until Psyche reaches its target. Lutetia was named after the Latin name for Paris, by it's discoverer; Hermann Goldschmidt. Lutetia is a relatively sterotypical looking asteroid, with an irregular shape, and many impact craters. When Rosetta flew by the asteroid in 2010, it imaged half its surface at a resolution of around 60 meters per pixel. While Lutetia is an M-type asteroid, its metal content is more of a coating, and doesn't make up the bulk of the asteroid's mass.

22 Kalliope

Image of Kalliope from VLT. Credit: ESO/Vernazza et al/MISTRAL algorithm (ONERA/CNRS) CC BY 4.0

• Date Discovered: November 16, 1852
• Designation: 22 Kalliope (A852 WA)
• Mass: 2.29x10^19 Kilograms
• Radius: 83.77 Kilometres
• Density: 2.37 g/cm^3
• Rotational Period: -4.15 Hours*
• Orbital Period: 1814.29 Days (4.97 Years)
• Moons: 1 (Linus)

*Negative number indicates retrograde (clockwise) rotation

Kalliope was discovered in 1852 by John Russel Hind, and named after the Greek Muse Calliope. The asteroid has a small moon named Linus, after Calliope's son (the name is also intended to reference Linus Torvalds, and the Peanuts character Linas van Pelt. Kalliope is believed to be a rubble pile asteroid like Bennu and Ryugu, except with higher Iron-Nickel content. Spectroscopic analysis of the surface suggests that the outer rocks are mostly stony, while those in the interior likely have higher metal content.

69 Hesperia

Hesperia shape model. Credit: Astronomical Institute of the Charles University: Josef Durech, Vojtech Sidorin

• Date Discovered: April 29, 1861
• Designation: 69 Hesperia (A861 HC)
• Mass: 5.86x10^18 Kilograms
• Radius: 69.065 Kilometres
• Density: 4.38 g/cm^3
• Rotational Period: 5.66 Hours
• Orbital Period: 1875.1 Days (5.13 Years)
• Moons: None

Hesperia was discovered by Giovanni Schiaparelli in 1861, while he was looking for asteroid 63 Ausonia. The asteroid is named after Schiaparelli's home nation of Italy, with Hesperia meaning peninsula in Greek. The spectra of this asteroid resembles that of the famous Hoba meteorite, the largest meteorite found on Earth to date.

92 Undina

Undina's orbit. Credit: Solar System Dynamics

• Date Discovered: July 7, 1867
• Designation: 92 Undina (A867 NA)
• Mass: 4.43x10^18 Kilograms
• Radius: 63.21 Kilometres
• Density: 4.39 g/cm^3
• Rotational Period: -15.94 Hours*
• Orbital Period: 2076.14 Days (5.68 Years)
• Moons: None

*Undina is believed to possess retrograde rotation, but this has not been confirmned as of yet.

Undina takes its name from Undine, the heroine of a novella by Friedrich de la Motte Fouqué. Undina's surface is believed to contain hydrated silicates, based on spectroscopic analysis.

129 Antigone

Antigon shape model. Credit: Astronomical Institute of the Charles University: Josef Durech, Vojtech Sidorin

• Date Discovered: February 5, 1873
• Designation: 129 Antigone (A873 CA)
• Mass: 2.65x10^18 Kilograms
• Radius: 56.5 Kilometres
• Density: 2.96 g/cm^3
• Rotational Period: 4.96 Hours
• Orbital Period: 1773.5 Days (4.86 Years)
• Moons: Tentative Satellite

Unlike some of the other asteroids in this article, Antigon is believed to be much more pure in its metallic composition. In fact, it could be quite similar to Psyche, but on a smaller scale.

216 Kleopatra

Kleopatra model and faint images of its two moons. Credit: ESO/Vernazza, Marchis et al./MISTRAL algorithm (ONERA/CNRS)

• Date Discovered: April 10, 1880
• Designation: 216 Kleopatra (A880 GB)
• Mass: 2.65x10^18 Kilograms
• Radius: 61 Kilometres (Average)
• Density: 3.45 g/cm^3
• Rotational Period: 5.39 Hours
• Orbital Period: 1705.07 Days (4.67 Years)
• Moons: 2 (Alexhelios & Cleoselene)

Keopatra was named after the famous Egyptian queen, Cleopatra. As you may have noticed, many names beginning with "C," have the letter changed to "K." This is due to translation from Greek to English. Kleopatra is a highly intriguing asteroid, with a shape described as a "dog bone" or a "Dumbbell." Kleopatra also has two moons, allowing its mass to be measured with a high level of accuracy.