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Prof Hans Rickman

Hans Rickman took a PhD in astronomy at Stockholm University in 1977 and spent one year as an ESA external fellow at Nice Observatory, after which he was hired at Uppsala Observatory. This was his main affiliation until retirement in 2014, but he also worked at PAS Space Research Center in Warsaw between 2007 and 2015. His activities within the IAU included the Presidency of Comm 20 in 1997-2000, and terms as General Secretary in 2000-2003 and advisor to the Executive in 2003-2006. In connection with this, he also organized an interdisciplinary study, sponsored by ICSU, about "Comet/Asteroid Impacts and Human Society". A Springer book with the same title, based on this project and co-edited by Rickman and Peter Bobrowsky of the IUGS, was issued in 2007.
Rickman's research has focused on comets and in particular their dynamical and physical evolution. In the later part of his career, he turned to the role of comets in early solar system evolution as predicted by the Nice Model. This concerned their contribution to the Late Heavy Bombardment and their formation as icy planetesimals beyond the planetary orbits.

Monte Carlo methods to calculate impact probabilities
The new methods for impact probability calculation developed by Rickman, Wisniowski and co-authors will be presented. The two basic elements are (1) the calculation of the MOID by a geometric/numeric approach (Wisniowski & Rickman, 2013);1 and (2) the exploration of the timing conditions for an impact using the MOID concept in the two variants called MOID-chord and MOID-track (Rickman et al. 2014).2 Two applications of these methods will be described: first, a survey of averaged impact probabilities corresponding to the Opik or Wetherill approaches with special attention to the behavior near singularities; and then, new MC simulations of the time-integrated impact rates for Jupiter Family comets using their actual orbital evolutions and non-averaged impact probabilities.

  1. Wisniowski, T. and Rickman, H. Fast Geometric Method for Calculating Accurate Minimum Orbit Intersection Distances (MOIDs). Acta Astronomica 63, 293–307 June (2013).
  2. Rickman, H., Wisniowski, T., Wajer, P., Gabryszewski, R., and Valsecchi, G. B. Monte Carlo methods to calculate impact probabilities. Astronomy and Astrophysics 569, A47 September (2014).

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