2 Pallas

• 1. Introduction

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  2 Pallas sits in the asteroid belt as the third-largest asteroid in the Solar System, and it has been hiding in plain sight since the night of the 5th of April 1779. That evening, the astronomer Charles Messier recorded it on a star chart while tracking a comet. He assumed it was just a star and moved on. It would be more than two decades before anyone looked closer.

  When German astronomer Heinrich Wilhelm Matthias Olbers finally identified Pallas as something new on the 28th of March 1802, he was not even looking for it. He was searching for Ceres, which had been discovered only a year before. Pallas happened to be drifting past Ceres at the time, a chance alignment that would scramble everything astronomers thought they knew about the gap between Mars and Jupiter.

  What Olbers had found was not simply a rock. Pallas may be a relic of the Solar System's earliest construction, a body that survived the planetary formation era intact while almost everything else of its kind was either swallowed by growing planets or shattered in collisions. Its orbit is strange, its surface is ancient, and no spacecraft has ever reached it. The questions Pallas raises about where planets come from, and why so few of these early bodies endured, are still being worked out today.

• 2. A Relic of Planet Formation

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  During the era when the Solar System's planets were assembling, objects grew through a process called accretion, clumping together until they reached roughly the size of Pallas. Most of those bodies did not survive. They were either absorbed into the growing planets, flung out of the inner Solar System, or destroyed in collisions with one another.

  Pallas, Vesta, and Ceres appear to be the only intact survivors of that early stage still orbiting within the path of Neptune. That makes Pallas not just an asteroid but something closer to a frozen record of what the raw material of planets looked like before the sorting process began.

  Its composition points toward great age. Pallas is a B-type asteroid whose surface spectrum corresponds closely to CM chondrite meteorites, the same broad category linked to Ceres. More precisely, the surface composition resembles the Renazzo carbonaceous chondrite meteorites, which are even lower in hydrated minerals than the CM type. The Renazzo meteorite was found in Italy in 1824 and is considered one of the most primitive known. Pallas's interior probably never reached the roughly 820 Kelvin threshold needed to dehydrate silicates, which means it never differentiated into distinct layers the way Earth did. Its interior is likely quite homogeneous, a single continuous body of ancient material rather than a layered world.

• 3. The Peculiar Orbit

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  Pallas sits in the main asteroid belt in terms of its distance from the Sun, but its orbit behaves like nothing else in the neighborhood. Its orbital inclination of 34.8 degrees tilts it sharply away from the flat plane that most belt objects follow, and its orbital eccentricity approaches that of Pluto.

  That combination of inclination and eccentricity means objects striking Pallas hit it on average twice as fast as they would hit Vesta or Ceres. Smaller impactors, which are far more common, can gouge craters as large as those cut by much bigger objects elsewhere. The result is a surface saturated with craters. As of 2020, researchers had identified 36 craters, 34 of which are larger than 40 km in diameter, and craters above that size cover at least 9% of the surface.

  The inclination also produces extreme seasons. With an axial tilt of 84 degrees, Pallas leans so far on its side that during each Palladian summer and winter, large sections of the surface spend a period on the order of an Earth year in either constant daylight or unbroken darkness. Near the poles, that stretch of continuous sunlight can last as long as two years.

  The steep orbit has one further consequence that illustrates just how unusual Pallas's path truly is. On the 9th of October 2022, Pallas passed only 8.5 arcminutes south of Sirius, the brightest star in the night sky. No planet can approach Sirius within 30 degrees. Pallas managed it because its inclination allows close approaches to stars that other Solar System bodies cannot reach.

• 4. Craters Named for Ancient Weapons

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  Pallas carries the name of an epithet of the Greek goddess Athena. In one version of the myth, Athena accidentally killed Pallas, daughter of the sea god Triton, and then adopted her friend's name in mourning. The goddess's association with warfare proved fitting when the time came to name the features on its surface.

  All craters on Pallas are named after ancient weapons. The southern hemisphere holds features with names drawn from Greek: Akontia, meaning a dart; Doru, a spear; Kopis, a chopping sword; Sarissa, a pike; Sfendonai, a slingstone; Xiphos, a sword; and Xyston, a lance. The northern hemisphere follows suit with Latin and pre-Roman names including Pilum for a Roman javelin, Scutum for a Roman leather-covered shield, Falcata for a pre-Roman Iberian sword, and Sica for a dagger.

  The asteroid's astronomical symbol, a spear or lance, was historically depicted with a lozenge-shaped blade, though graphic variants appeared over the years including leaf shapes and triangles. A generic symbol showing a disk with the discovery number was introduced in 1852 and became standard. The lozenge-blade symbol was revived for astrological use in 1973.

  Not every association with the name Pallas traces back to Athena. The pallasite class of stony-iron meteorites carries a different origin entirely; it honors the German naturalist Peter Simon Pallas, not the asteroid. The chemical element palladium, however, was named directly after 2 Pallas, which had been discovered just before the element was identified.

• 5. The Problem of Getting There

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  No spacecraft has visited Pallas. The same orbital inclination that makes Pallas scientifically compelling also makes it difficult to reach. A mission must climb sharply out of the plane most inner Solar System trajectories follow, requiring a significant fuel penalty.

  When the Dawn spacecraft was being planned to visit Vesta and Ceres, mission designers considered adding a Pallas flyby. The high inclination made it impossible to route the probe there without abandoning the other targets. Dawn went to Vesta and Ceres; Pallas remained unvisited.

  A later proposal, the Athena SmallSat mission, aimed to fly past Pallas as a secondary payload aboard the Psyche mission, with a planned launch in 2022. It was not funded, outcompeted by other mission concepts including the TransOrbital Trailblazer Lunar Orbiter. The scientists behind the Athena proposal described Pallas as the largest unexplored main-belt protoplanet. That description remains accurate.

  Science has had to work around the access problem. Radio signals from spacecraft orbiting Mars and from landers on its surface have provided estimates of Pallas's mass by detecting the tiny gravitational nudge Pallas exerts on the motion of Mars. In September 2007, the Dawn team used a once-in-twenty-years opportunity, when Pallas passed closest to Earth, to observe it with the Hubble Space Telescope and gather comparative data alongside measurements from Ceres and Vesta.

• 6. A Family and a Possible Connection

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  In 1917, the Japanese astronomer Kiyotsugu Hirayama was plotting asteroid motions when he noticed that certain groups shared nearly identical orbital parameters. One of those groupings, three asteroids with similar inclinations and orbital shapes, clustered around Pallas and became known as the Pallas family. Since 1994, more than ten members have been identified, with semi-major axes ranging between 2.50 and 2.82 AU and inclinations of 33-38 degrees. Spectroscopic comparisons in 2002 confirmed the family was real.

  The leading explanation for how the family formed is a major impact. A suspected large basin at Pallas's south pole may have blasted away a volume equal to roughly 6% of Pallas itself, twice the volume of the Rheasilvia basin on Vesta, which is one of the largest known impact basins in the Solar System. That collision may also have increased Pallas's current inclination and slowed its rotation. Calculations suggest that without the basin, Pallas's shape would be close to an equilibrium form matching a rotation period of about 6.2 hours.

  A smaller crater near the equator may be the source of the Pallas family members more broadly. Some researchers have proposed a further connection: if the near-Earth asteroid 3200 Phaethon is an ejected fragment of Pallas, then a Palladian surface enriched in salt deposits could explain the unusual sodium abundance in the Geminid meteor shower that Phaethon produces each year. One bright spot on Pallas's surface, similar to bright spots observed on Ceres, could be consistent with salt left behind by water that migrated upward through the otherwise homogeneous interior.