4 Vesta - Coordinate Controversy

Coordinate Controversy

Olbers Regio (dark area), which defines the prime meridian in the IAU coordinate system, in a Hubble shot of Vesta

There are two longitudinal coordinate systems in use for Vesta, with prime meridians separated by 155°. The IAU established a coordinate system in 1997 based on Hubble photos, with the prime meridian running through the center of Olbers Regio, a dark feature 200 km across. When Dawn arrived at Vesta, mission scientists found that the location of the pole assumed by the IAU was off by 10°, and also that Olbers Regio was not discernible from up close, and so was not adequate to define the prime meridian with the precision they needed. They corrected the pole, but also established a new prime meridian relative to the center of Claudia, a sharply defined crater 700 meters across, which they say results in a more logical set of mapping quadrangles. All NASA publications, including images and maps of Vesta, use the Claudian meridian, which is unacceptable to the IAU. Data published by the Gazetteer of Planetary Nomenclature, a branch of the IAU, uses the Claudian coordinate system as well, and notes that it has not been approved by the IAU. Michael A’Hearn, a member of the IAU working group in charge of the NASA database that will make Dawn data available to the public, insists that the data use an IAU-sanctioned coordinate system. He has suggested that a third system, correcting the pole and rotating the Claudian longitude by 155° to coincide with Olbers Regio, would be acceptable to the IAU.