Square Kilometre Array key science: a progressive retrospective
  I summarize the science drivers presented at the workshop for Phase I of the Square Kilometre Array: 'Advancing Astrophysics with the Square Kilometre Array'. I build from the historical perspective of the original Key Science programs: 'Science with a Square Kilometre Array', and consider progress in astrophysics since 2004. I then present my 'score card' of the primary science drivers proposed by the Science Working Groups, and further developed in the white papers and presentations at the meeting, assuming a conservative high frequency of 3GHz. The science case for the SKA phase I is compelling, with the right mix of killer applications (eg. pulsars and gravity, 21cm cosmology), foundational radio astronomy (eg. cosmic magnetism, baryon cycle, high energy phenomena), and high risk-high return 'game-changing' programs (eg. fast radio bursts, BAO intensity mapping, SETI). A strong case was made at the conference for band 5 (4 to 15GHz), in particular in the area of planet formation and exobiology. Such a capability engages the rapidly growing exoplanet community, and enables fundamental breakthroughs in most of the key science areas. The case for real-time data spigots that allow for commensal observing is also strong. Ultimately, the greatest discoveries that will come from the SKA are likely even richer still, and beyond prognostication. 
Christopher L. Carilli
astro-ph.IM