In 1802, the German astronomer Heinrich Olbers observed what he thought was a planet in the Main Asteroid Belt. Over time, astronomers will call this cosmic body Pallas, an alternative name for the Greek warrior goddess Athena.
The subsequent discovery of many more asteroids in the Main Belt would result in Pallas being reclassified as a large asteroid, the third largest in the belt after Ceres and Vesta.
Over the centuries, astronomers have sought to get a better look at Pallas to learn more about its size, shape, and composition. At the turn of the century, astronomers concluded that it was a compressed spheroid (elongated sphere).
Thanks to new research by an international team, the first detailed images of Pallas have finally been taken, showing that its shape is more like a 'golf ball' – meaning it is heavily dimpled.
Pierre Vernazza of Laboratoire d'Astrophyisque de Marseille in France was the principal investigator for a team that included members from 21 research institutes around the world.
Mikael Marsset, a fellow in the Department of Earth, Atmospheric and Planetary Sciences at MIT, was the lead author of the study (recently appearing in Nature Astronomy).
For centuries, astronomers have known that Pallas rotates in a highly inclined orbit compared to most objects in the Main Asteroid Belt. While most of these objects follow the same approximately elliptical path around the Sun and have orbital inclinations of less than 30 °, Pallas's orbit is tilted 34.837 ° relative to the plane of the Sun (for reasons that still remain a mystery).
For their research, Vernazza and his team took 11 images of the asteroid using the Spectro-Polarimetric High Contrast Exoplanet Exploration Instrument (SPHERE) on ESO's Very Large Telescope (VLT).
These images were taken in 2017 and 2019, when the team reserved one of the four telescopes that make up the VLT to capture images of Pallas while it was at its closest point in its orbit to Earth.
Thanks to the extreme adaptive optical system of the SPHERE instrument, the team observed a surface that was completely covered with crater dimples, resembling a golf ball.
Turning to the question of why this is so, the team considered the possibility that Pallas's slanted orbit would cause it to take multiple impacts over the more than four and a half years (1,686 days) required to complete one orbit around the Sun.
They calculated that these impacts would be four times more destructive than collisions of two asteroids in the same orbit.
'Pallas's orbit implies very high velocity strikes. From these images, we can now say that Pallas is the most cratered object we know of in the asteroid belt. It's like opening a new world. '
Sources: Photo: Marsset et al., Nature Astronomy, 2020
