The giant on a tilted orbit
Pallas
Discovered in 1802, a year after Ceres, Pallas is the third most massive body in the main belt. But it follows an orbit so tilted that no spacecraft has ever reached it: everything we know about its surface comes from ground-based telescopes, which in 2020 showed it pockmarked with craters like a golf ball.
- Size
- 568 × 532 × 448 km
- Distance from Sun
- 414 M km
- Orbital period
- 4.61 years
- Rotation
- 7.81 hours
- Discovered
- 1802
- Orbital tilt
- 34.9°
Physical characteristics
Pallas measures 568 × 532 × 448 km, with a mean diameter of about 512 km: slightly smaller than Vesta, but with about a fifth less mass. Its density, 2.9 grams per cubic centimetre, and the colour of its surface point to a rock rich in hydrated minerals, similar to that of carbonaceous meteorites: material that met water in the early stages of the Solar System.
Images taken between 2017 and 2019 with the SPHERE instrument on the Very Large Telescope in Chile revealed a surface densely packed with craters: at least 36 more than 30 km wide, and an impact basin about 400 km across. The authors of the study nicknamed it the "golf ball asteroid". The reason lies in its orbit: Pallas cuts across the belt and strikes other bodies at about 11.5 km per second, twice the typical speed between asteroids.
Orbit and rotation
Pallas orbits 414 million km from the Sun on average, or 2.77 astronomical units, almost the same distance as Ceres, and completes one lap in 4.61 years. But its orbit is tilted 34.9° to the ecliptic, more than that of any other large asteroid, and it is also fairly eccentric: its distance from the Sun varies between 2.13 and 3.41 AU.
It is this tilt that keeps spacecraft away: reaching it and entering orbit would require a change of orbital plane, one of the most expensive manoeuvres in terms of propellant. Pallas spins once every 7.81 hours. Seen from Earth it never gets brighter than about magnitude 6.5: too faint for the naked eye, but within reach of binoculars.
Discovery and exploration
Heinrich Wilhelm Olbers discovered it from Bremen on 28 March 1802, by chance, while trying to recover Ceres, found the year before. It was the second body discovered between Mars and Jupiter, and its existence posed a problem: one planet was expected in that region, not two. It was William Herschel who proposed a new word for these objects, "asteroids", meaning "star-like".
No mission to Pallas is planned. What we know of it comes from ground-based telescopes with adaptive optics, which correct for atmospheric turbulence, and from occultations, the moments when the asteroid passes in front of a star and hides its light for a few seconds: timing the event from several places yields its outline. The 3D model on this page is reconstructed from the VLT images.
Frequently asked questions
Why has no spacecraft visited Pallas?
Because its orbit is tilted almost 35° to the plane in which Earth and the other planets move. To reach it and stay alongside, a spacecraft would have to change orbital plane, which takes a great deal of propellant. A fast flyby, at the point where Pallas crosses that plane, would be possible, but so far none has been funded.
Is Pallas bigger than Vesta?
No, though only just. Vesta has a mean diameter of about 525 km, Pallas about 512; the difference is clearer in mass, because Vesta is denser and weighs about a quarter more. Pallas is therefore the third body of the main belt, after Ceres and Vesta.
What does the name Pallas mean?
It is one of the names of Athena, the Greek goddess of wisdom, known as Pallas Athena. It was chosen by the discoverer, Heinrich Olbers. The asteroid in turn gave its name to a chemical element, palladium, isolated in 1803, the year after the discovery.
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