The storm of controversy that was created by Velikovsky's works, especially "Worlds in Collision", may have helped revive the Catastrophist movements in the last half of the 20th century; it is also held by some working in the field that progress has actually been retarded by the negative aspects of the so-called Velikovsky Affair. The assessment of Velikovsky's work by tree-ring expert Mike Baillie is instructive: "But fundamentally, Velikovsky did not understand anything about comets … As if to comfort his readers, at one point he says that no planet at present has a course which poses a danger to this planet: '…only a few asteroids—mere rocks, a few kilometres in diameter—have orbits which cross the path of the earth.' … He did not know about the hazard posed by relatively small objects, and, just in case there is any doubt about his mistake, he repeats the notion by noting that a possibility exists of some future collision between planets, 'not a mere encounter between a planet and an asteroid'. This failure to recognize the power of comets and asteroids means that it is reasonable to go back to Velikovsky and delete all the physically impossible text about Venus and Mars passing close to the earth."