Pallas

Pallas is the third-largest object in the asteroid belt and one of the strangest big bodies in it. Some 512 km across — a fraction the size of the Moon, whose diameter is about seven times larger — it holds a little more than a fifth of the mass of the dwarf planet Ceres, yet it never rounded into a neat sphere. Instead it is a battered, lumpy world tilted almost onto its side, following an orbit so steeply inclined that it spends much of its year far above or below the crowded plane where most asteroids circle the Sun.

Discovered in 1802, only the second asteroid ever found, Pallas was a planet in the eyes of its finder and remained an oddity for the next two centuries. Only recently, with images sharp enough to resolve its surface, have astronomers begun to explain why it looks the way it does — pitted, dented, and, in one memorable description, dimpled like a golf ball.

Discovered by a doctor hunting a lost planet

Pallas was found on 28 March 1802 by Heinrich Wilhelm Olbers, a German physician who observed the sky from his home in Bremen. He was searching the region where Ceres had turned up the year before, and stumbled instead on a second moving point of light. It was only the second asteroid ever identified, and for a time it was counted among the planets.

Olbers named it for Pallas Athena, the Greek goddess of wisdom — fittingly, since so many of Pallas's traits read as a challenge. The discovery of two sizeable bodies sharing similar orbits led Olbers to propose that they were fragments of a shattered planet, an idea that seeded the modern study of the asteroid belt even though the belt is now understood as material that never coalesced into a planet at all.

A steeply tilted, eccentric orbit

Pallas circles the Sun once every 4.6 years at an average distance of about 2.77 astronomical units, placing it in the middle of the main belt near Ceres. What sets its path apart is its steep incline: Pallas's orbit is tilted roughly 34.8° to the plane of the solar system — far more than almost any other large asteroid — and it is noticeably stretched, with an eccentricity near 0.23, comparable to Pluto's.

That geometry keeps Pallas from being an easy target. Because it climbs so high above and dives so far below the belt's main plane, matching a spacecraft to its motion demands far more fuel than a visit to a body in a flatter, rounder orbit. It is a large asteroid that, so far, no probe has ever reached.

The golf-ball asteroid

In 2020, a team led by Michaël Marsset and Pierre Vernazza used the SPHERE instrument on the European Southern Observatory's Very Large Telescope in Chile to capture the sharpest images of Pallas yet. The surface they revealed was extraordinarily beaten up: at least 36 craters wider than about 30 km, scattered so densely that the researchers compared it to a golf ball. A giant impact basin roughly 400 km across scars the surface near the equator, and a curiously bright patch — possibly a salt deposit — stands out in the southern hemisphere.

The likely culprit is Pallas's own orbit. Cutting across the belt at a sharp angle, it slams into other asteroids at higher speeds and two to three times more often than flatter-orbiting giants like Ceres and Vesta. Billions of years of these violent, high-energy collisions have left Pallas more heavily cratered than either of its larger neighbours.

A primitive, water-altered world

Pallas is a dark, carbon-rich body — a B-type asteroid with a low reflectivity, meaning it returns only a small fraction of the sunlight that reaches it. Its composition appears closely related to a rare class of meteorites, the CR carbonaceous chondrites typified by the Renazzo fall, which preserve some of the oldest and least-processed material in the solar system.

Crucially, that material shows signs of having been altered by liquid water early in its history, when heat from radioactive decay could have driven chemistry between rock and ice inside the young body. Studying Pallas is therefore a way of reading a chapter from the solar system's infancy, before the planets had finished forming.

Its own family of fragments

Pallas does not travel alone. The high-speed impacts it has endured chipped off swarms of debris, and a cluster of smaller asteroids on similarly tilted orbits now forms the Pallas collisional family — the wreckage of blows that gouged out craters while leaving the bulk of Pallas intact. These fragments share their parent's dark, carbonaceous makeup, making the family a natural laboratory for tracing how a single body breaks apart over cosmic time.

Observing Pallas

Pallas is well within reach of a modest telescope or even binoculars. Near opposition, when Earth passes between it and the Sun, it typically shines at around magnitude 8, brightening to roughly magnitude 6.5 at its most favourable, near-perihelic oppositions — right at the edge of naked-eye visibility, but a comfortable binocular target from a dark site if you know exactly where to look. Through the eyepiece it appears as a faint, star-like point rather than a disc, so the pleasure is in the hunt: identify a nearby star field, sketch or photograph it, then return a night or two later to watch the point that has shifted. That slow drift against the fixed stars is the same telltale motion that betrayed Pallas to Olbers more than two centuries ago.