New GIADA paper

The Rosetta blog has some details of a new paper published by the GIADA team. The last update I had on the Grain Impact Analyser and Duset Accumulator was here, Thread: Rosetta Comet Orbiter -** 120 Icy Patches ** Philae Phones Home ** Ceres Fly Over Video ** New Coma Discovery ** Picture MOTHER LODE ! (Page 58) . The paper concerns the properties of the "fluffy particles" picked up in the detector. You may remember that the COSIMA team talked about the different populations of dust particles Thread: Rosetta Comet Orbiter -** 120 Icy Patches ** Philae Phones Home ** Ceres Fly Over Video ** New Coma Discovery ** Picture MOTHER LODE ! (Page 58) and Thread: Rosetta Comet Orbiter -** 120 Icy Patches ** Philae Phones Home ** Ceres Fly Over Video ** New Coma Discovery ** Picture MOTHER LODE ! (Page 59)

Marco Fulle, the GIADA PI has been outlining some of the key differences in the two main populations of dust. The compact solid grains are believed to be material that has been processed in the hot environment around the newly ignited Sun. Their densities range from 0.8 to 3 g.cm-3, consistent with the ices and minerals which make up the comet. The fluffy particles on the other hand are believed to originate in the interstellar cloud and pre-date our solar system. Their densities are as low as 0.001 g.cm-3 which Fulle likens to a dandelion seed head.

This low density has an effect on the velocity of the grains in GIADA. Currently Rosetta's potential varies between -5V and -10V depending where it is in it's orbit. The photo-ionisation of water in the jets creates clouds of electrons which impact Rosetta and drive it's potential into negative values. There is an opposing process of the photo-electric effect from solar UV driving Rosetta's potential to small positive values but at the moment the electrons are winning the battle, it is particularly noticeable how the potential is driven to more negative values when the COPS pressure sensor detects the densest jets. The negative spacecraft potential and similar negative potential on the fluffy grains has the effect of slowing them in the GIADA instrument ( and may even be repelling some ) skewing the results.

blog entry here.
[link to blogs.esa.int]

K