Asteroid collision confirmed by Rosetta/OSIRIS observations

Abstract The discovery of P/2010 A2 by the LINEAR surveyin January 2010 [2] revealed an object displaying alarge trail of material similar in shape to a cometarytail although no central condensation or coma couldbe detected. The appearance of this object in an aster-oidal orbit in the inner main belt attracted attention asapotentialnewmemberoftheMainBeltCometsclass(MBCs) [4] but the discovery of a nucleus [6], with anestimated diameter of 150-200 m [5], around 1500 kmaway from the trail implied that the extended objectwe were seeing could be the debris trail from a recentcollisionratherthanthetailofacomet. Duetothelowinclination of its orbit, it is difcult to conclude aboutthe nature of P/2010 A2 from Earth-based data only,as different scenarios lead to the same appearancein the orbital conguration at the times of observa-tions. We present here another set of images, acquiredfrom the unique viewing geometry provided by ESA'sRosetta spacecraft en route to comet 67P/Churyumov-Gerasimenko. Albeit faint (22 magnitude), the ob-ject could be observed by the high-resolution cameraOSIRIS. We used a Finson- Probstein model [3, 1] tosimulate the shape of the trail, and estimate the timeof emission and parameter (ratio between solar radi-ation pressure and gravity) for the dust grains. Sim-ulations were compared to the OSIRIS images andground based observations acquired at NTT and Palo-mar telescopes. Thanks to the different phase angleprovided by Rosetta, we could reduce the number ofsolutions to a unique model, leading to the conclu-sivedemonstrationthatthetrailisduetoasingleeventrather than a period of cometary activity.