David Block is Director of the Anglo American Cosmic Dust Laboratory and Professor of Computational and Applied Mathematics at the University of the Witwatersrand, Johannesburg, South Africa. He has been a visiting astronomer at the European Southern Observatory, the Institute of Astronomy (Hawaii) and the Center for Astrophysics at Harvard University. His research has twice been featured on the cover of the journal “Nature” and he was elected a Fellow of the Royal Astronomical Society of London at age 19. Ken Freeman FRS is Duffield Professor of Astronomy at the Australian National University. He was a fellow of Trinity College, Cambridge, Oort Professor at Leiden University, Blaauw Professorat the University of Groningen and Tinsley Professor at the University of Texas. He is one of the first astronomers to show that spiral galaxies are immersed in a vast halo of dark matter, and is one of Australia’s most highly cited scientists.