We present the results of an imaging survey of Mercury's Hill sphere in search for objects dynamically bound to the planet, motivated by the existence of hermeocentric orbits that have been shown to be stable over 5 Myr or more. A six-day survey of Mercury's apparent vicinity from 6 to 140 Mercury radii, with full coverage between 19 and 73 Mercury radii, was performed with the Nordic Optical Telescope on La Palma using ALFOSC in the R-band. The deepest limiting magnitude of 18.6 at a signal-to-noise-level of 3 corresponds to a hermeocentric object size of 0.5 km, while the brightest limiting magnitude corresponds to a size of 1.6 km. While two suspected sources were found, no hermeocentric objects could be confidently identified.
Our survey significantly improves on the results obtained from Mariner 10 data both in terms of aerial coverage and smallest detectable object size, but still no hermeocentric satellite has been identified. This result is however not unexpected for two reasons. Firstly, the survey size limit is 1.6 km, and as any objects likely to be in orbit are of impact debris or captured Inner Earth Object origin, existing natural satellites are probably significantly smaller. Secondly, though the dynamical lifetime of close hermeocentric objects (mean semimajor axes smaller than 30 Mercury radii) are of the order of at least 5 Myr, major impacts capable of ejecting substantial debris fragments are not very likely to have occurred during the past several Myr. This survey is not able to make predictions on the probability of existence of small hermeocentric objects (in the size range less than decameters), which remain undetected. These have to await possible discovery in close-range searches from the MESSENGER and BepiColombo spacecraft.