One in a million, but . . .

The latest addition to NASA-JPL's list of objects that might strike the Earth someday is an interesting case, though not a particularly threatening one.
Odds of impact are less than one in a million, so nobody should get worried. What's more, the object was just discovered on Thursday and has just over one day of observations, which makes the impact-odds calculation just this side of meaningless, and the whole thing will probably go away (the impact risk, that is, not the asteroid) within the next few days.
But here's what makes this one interesting: The asteroid, called 2006 HZ51, is the biggest object currently on the list, and one of the biggest ever, at an estimated 800 meters across (these estimates are hugely uncertain, so it's really more like somewhere between a half-kilometer and 1.5 km.). And, it is the object that has the nearest-term possible Earth-impact date of anything on the list: just over two years away, on the first day of summer (June 21) 2008.
Nothing to be afraid of, but an interesting case to think about. An object that big would be capable of devastating a continent, and wreaking havoc with the Earth's climate for years. So, what would we do if something like this came along that had a higher probability of impact? This certainly underscores, I think, the importance of being prepared and doing our homework. For example, the efforts of people like the B612 Foundation, which aims to demonstrate the technology for deflecting a threatening asteroid sometime before 2015.
It also demonstrates the degree of uncertainty that still exists about how to deal with such hazards. There is still no formal protocol, as far as I know, as to just how high the odds would have to be, and how soon the potential impact would have to be, before it triggered an all-out alert to governments about the possible threat, and worldwide efforts to figure out what to do about it. It would probably be too short a time for any meanigful effort to deflect the object, so mitigation in this case would probably consist of things like stockpiling food, and perhaps even evacuating certain areas considered most at risk. One of the peculiar things about asteroid impacts is that even when the probability of impact is very fuzzy, the exact time and the range of locations where the impact would happen can be quite sharply defined. That's both a blessing and a curse, because it makes the decisions even harder for the astronomers involved, who have to tread a fine line between the risk of causing a panic and the risk of being seen as covering up a serious hazard. Tough issues, and still largely unresolved, but it's examples like 2006 HZ51 that help to focus people's thinking about what should be done.
Astronomers tend to get very upset when reporters like me call public attention to objects like this, fearing that the public will see it as "crying wolf" and will not pay attention when a real hazard comes along. I don't agree with that point of view, and I may or may not write a news item about this object. But they're safe for now because hardly anybody ever looks at this blog, so this is just between us, ok?