The Jerusalem Post had an interesting article on 2018 AH, an asteroid that will pass earth on December 27, 2021. In astronomical terms, it is not huge. The asteroid is about 100 meters in size, the equivalent of the Washington Monument. And there is little chance of it hitting the earth, because it will cruise by at a safe distance of about 4.5 million kilometres. But this is not the first time 2018 AH made an appearance. As the name suggests, it zoomed by earth in 2018. But there were a couple of interesting things about that earlier visit that caught my attention. First it was only 296,758 kilometres away when it slipped by earth, which means it was nearly 100,000 kilometres closer to earth than the moon (384,000 kilometres). The second thing that concerned astronomers is that they had no idea it was coming. They only saw it two days after it had already zoomed by. Apparently, it had approached on a trajectory from the sun, making it impossible to see. So what would …