Asteroid Pallas: A Giant of the Main Belt

Pallas Asteroid: the third largest asteroid — **Pallas**, with a mean diameter of about 512 km, is the third largest asteroid in the main belt, after Ceres and Vesta. Pallas has a surface dotted with large craters, distributed across both hemispheres, giving it a characteristic appearance reminiscent of a **golf ball**. The asteroid was observed in detail thanks to the SPHERE instrument with adaptive optics on the Very Large Telescope (VLT).
Image credit: ESO/M. Marsset et al./MISTRAL algorithm (ONERA/CNRS) (new window)

Scientific Summary

(2) Pallas is the second asteroid discovered (1802) and the third largest in the main belt, with a mean diameter of about 512 km. Its irregular shape (estimated dimensions around 550 × 516 × 476 km) and its highly inclined (34.8°) and eccentric orbit (e ≈ 0.23) make it a unique object, subjected to particularly violent impacts. Classified as a protoplanet, it is the prototype of B-type asteroids, primitive and rich in hydrated silicates. Its dark surface (albedo of about 0.16) and its mean density of about 2.9 g/cm³ — close to that of the most hydrated carbonaceous chondrites — suggest a composition rich in hydrated minerals, with modest porosity, offering valuable clues about the formation of the solar system.

Why is the asteroid Pallas a major scientific object and what makes it unique?

Asteroid Pallas is much more than a simple rock wandering in the main belt. Its official designation, (2) Pallas, and its discovery on March 28, 1802, by the German astronomer Heinrich Wilhelm Olbers (1758-1840), mark the beginnings of the exploration of our inner solar system. But its true importance lies in its extraordinary characteristics which make it a natural laboratory for planetologists. Unlike the majority of large asteroids, Pallas is not spherical, displaying an oblong shape that betrays a history of violent collisions. Its orbit, inclined by nearly 35° relative to the plane of the planets, is a dynamic anomaly that intrigues scientists: this inclination, combined with its eccentricity, results in average impact velocities of about 11.5 km/s, almost double the average in the main belt, which explains the exceptionally cratered surface observed since 2020.

As a protoplanet, Pallas is a frozen remnant from the early ages of the solar system, 4.5 billion years ago. Its primitive composition (hydrated minerals, clays, silicates) defines it as a B-type asteroid, a rare subclass of carbonaceous asteroids. Understanding Pallas means reading an essential chapter in the book of planetary formation, a process that led to the birth of Earth and the other planets. Its future exploration, although no mission is funded today, remains considered a scientific priority: a light probe concept (Athena) was proposed to NASA in 2018 for a flyby, but was not selected, leaving Pallas as the largest body in the inner solar system never yet visited by a probe.

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Remarkable physical characteristics of Pallas

Irregular shape: Unlike many large asteroids, Pallas is not spherical but has an oblong shape, with dimensions estimated at about 550 × 516 × 476 km according to the 3D model established from VLT/SPHERE images.
High orbital inclination: Its orbit is particularly inclined (34.8° relative to the ecliptic plane) and eccentric (e ≈ 0.23), much more than most main-belt asteroids.
Dark surface: Its albedo (reflectivity) is low, around 0.15 to 0.16 according to IRAS and polarimetric measurements, suggesting a composition rich in hydrated silicates.
Not quite an asteroid: Like Ceres, Pallas is considered a protoplanet, a relic of the formation of the solar system that could not accrete into a planet.
No known satellite: unlike some large asteroids, no moon has been detected orbiting Pallas.

A crater-scarred surface: the VLT/SPHERE revelation

In 2020, an international team led by Pierre Vernazza (Laboratoire d'Astrophysique de Marseille) delivered the first very high angular resolution images of Pallas, obtained with the SPHERE instrument with extreme adaptive optics on the ESO Very Large Telescope. Eleven series of images, acquired in 2017 and 2019, allowed the reconstruction of a 3D model of the asteroid and mapping of its craters.

The result is spectacular: Pallas's surface turned out to be the most heavily cratered ever observed in the main belt, with at least 36 craters over 30 km in diameter, some reaching a hundred kilometers, covering about 10% of the asteroid's total surface. This collisional record is directly explained by Pallas's inclined and eccentric orbit, subjecting it to impact velocities nearly twice the average in the belt. Simulations suggest that a major impact, caused by a 20 to 40 km body about 1.7 billion years ago, could be at the origin of both a large impact basin and the family of fragments that accompany Pallas on its orbit today.

These same observations also allowed refining Pallas's density to about 2,890 kg/m³, a value that contrasts with previous very divergent estimates (ranging from Ceres's density, 2,160 kg/m³, to that, much higher, of Vesta, 3,460 kg/m³) and that brings Pallas closer to the most hydrated carbonaceous chondrites. A bright spot, detected in the southern hemisphere, could correspond to a salt deposit exposed by an impact, by analogy with the spots observed on Ceres by the Dawn probe.

N.B.:
More recent infrared observations made with the NIRSpec spectrograph on the James Webb Space Telescope have completed this picture: they confirm the presence of an absorption band linked to hydrated minerals (Mg-OH) around 2.72 µm on Pallas's surface, comparable to that observed on Ceres and Hygiea, but without trace of the ammoniated minerals detected on these other two bodies — an indication that Pallas's aqueous alteration, although real, remained less advanced than that of Ceres.

Composition and structure

Spectroscopic observations suggest that Pallas is mainly composed of hydrated silicates, with a possible contribution of organic matter on the surface. Its mean density, now estimated at about 2.9 g/cm³ thanks to the shape model derived from VLT/SPHERE images, remains lower than that of typical stony meteorites, which could indicate a moderate internal porosity rather than a massive and compact structure.

Place in the asteroid classification

Pallas is the prototype of the B-type asteroids, a rare subcategory of C-type carbonaceous asteroids, representing only about 4% of the known asteroid population. These objects are among the most primitive in the solar system, having undergone few modifications since their formation 4.5 billion years ago, apart from impact alterations. Pallas holds a special place in the history of astronomy because its discovery helped establish the existence of the asteroid belt as a distinct region of the solar system.

References

Marsset, M. et al. (2020). The violent collisional history of aqueously evolved (2) Pallas. Nature Astronomy, 4, 569–576. DOI: 10.1038/s41550-019-1007-5 (new window)
Rivkin, A. S. et al. (2025). Observations and Quantitative Compositional Analysis of Ceres, Pallas, and Hygiea Using JWST/NIRSpec. The Planetary Science Journal, 6, 9. DOI: 10.3847/PSJ/ad944c (new window)
Vernazza, P. et al. (2021). VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis. Astronomy & Astrophysics, 654, A56. A&A 2021 aa41781-21 (new window)
Tedesco, E. F., Noah, P. V., Noah, M., & Price, S. D. (2002). The Supplemental IRAS Minor Planet Survey. The Astronomical Journal, 123, 1056–1085. ADS 2002AJ....123.1056T (new window)
NASA JPL Small-Body Database. 2 Pallas. ssd.jpl.nasa.gov (new window)
ESO. Dimpled like a golf ball, the Very Large Telescope reveals the cratered surface of Pallas. eso.org (new window)

FAQ: All you need to know about asteroid Pallas

What is the size of Pallas and how does it rank among other asteroids?

With a mean diameter of about 512 km, Pallas is the third largest object in the main asteroid belt, after Ceres and Vesta. It represents about 7% of the total mass of the belt. Its shape is irregular, with dimensions estimated at about 550 × 516 × 476 km according to the 3D model established from VLT/SPHERE images.

What makes Pallas's orbit so special?

The most striking orbital characteristic of Pallas is its high inclination of 34.8° relative to the ecliptic plane (the plane in which most planets orbit), combined with a notable eccentricity (e ≈ 0.23). This combination results in some of the highest impact velocities in the main belt, making it a unique and intriguing dynamic case for astronomers.

Why is Pallas called a "protoplanet"?

Pallas is considered a protoplanet, that is, a celestial body that began the process of forming a planet by accumulating mass, but was interrupted before becoming a fully-fledged planet. Like Ceres, it is a remnant from the early stages of the formation of the solar system, which could not accrete enough to reach planetary size, probably because of gravitational perturbations from Jupiter.

What is the composition of Pallas's surface?

Pallas's surface is dark with an albedo (reflectivity) of about 0.15 to 0.16. Spectroscopic observations, including recent data from the James Webb Space Telescope, indicate a composition dominated by hydrated silicates, with a characteristic absorption band around 2.72 µm, but without the ammoniated minerals observed on Ceres or Hygiea. Its mean density, now estimated at about 2.9 g/cm³, suggests a moderately porous structure.

Has Pallas ever been visited by a space probe?

No, no space mission has yet visited Pallas to date, making it the largest body in the inner solar system still unexplored. A light probe concept, Athena, was proposed to NASA in 2018 for a flyby shared with the launch of the Psyche mission, but it was not selected for funding. Because of its size and unique characteristics, Pallas remains considered a priority target for future exploration missions, which could uncover the secrets of its internal composition and its role as a preserved witness of the early solar system.