Eric Christensen is the director of the NASA-funded Catalina Sky Survey (CSS) and an Asteroid Day Expert. He answers a user submitted question. Do you have an asteroid related question? Submit it here.
User submitted Question from Joey:
With events like Chelyabinsk, and the Indian incident with the first recorded death and the Pacific Ocean explosion, would you say the odds of an incident are ever increasing? If so. What was the older statistic compared to whatever it may be now? Thank you for this wonderful opportunity to ask an expert.
Answer by Eric Christensen:
Great question, Joey. I’ll lead off with the answer: No, the overall odds of an asteroid impact haven’t changed in recent years, and they are not increasing. But I have a few thoughts about why it might seem that they are.
Over the last several years, increasing attention is being paid to asteroid flybys, asteroid (or meteoroid*) impacts, and meteorite falls. There are several reasons for this. First, our ability to detect asteroids is steadily improving, so there are more opportunities for news stories. You can see the plots at the bottom of this page that the rate of NEO discovery has increased nearly every year for the last 20 years. We in the asteroid survey business have been fortunate that NASA has maintained a long-term commitment to finding and tracking NEOs, and this commitment has paid off as new survey facilities have been added and existing survey facilities are improved. Just within the last month, asteroid surveys have detected about half a dozen small NEOs that have approached the Earth to a distance closer than the Moon. Ten years ago, the discovery of an asteroid that made a flyby closer than the moon was a fairly rare event, perhaps a few times per year. So the fact that we’re seeing more of these flybys today just means that we’re getting better at spotting them, and not because the number of close flybys is actually increasing.
The second reason I think it might seem that asteroid events are happening more frequently than in the past is because news media landscape has also changed fairly dramatically in the last ten years or so. We live in an increasingly globally connected world, and news comes to us in new and non-traditional formats. So it is not uncommon to hear about odd happenings in distant places, but sometimes it can be difficult to separate trusted sources from untrustworthy ones. The Indian incident you mention is an example of this. It was widely reported at the time that a meteorite had struck and killed an employee at an engineering college, and even “official” sources were quoted as confirming this. But over the next few days it was determined that this was simply a tragic accident having nothing to do with meteorites. Of course, very few “news” sources that originally reported the incident bothered to correct the false claim.
Other recent examples of spectacular events being blamed on asteroid impacts include a 2011 gas explosion in Argentina and a 2014 explosion near a Nicaraguan military installation or here. It is interesting to note that the mis-attribution of the Nicaragua event to a meteorite strike was made close in time to the predicted (and widely-reported) flyby of a small NEO, 2014 RC. This is a pretty clear case of media attention on an asteroid story influencing people’s perception of the hazard posed by asteroid impacts.
So, while the odds of an asteroid impact are not increasing with time, the odds of an asteroid-related story being published are indeed increasing!
So what are the real odds of an asteroid impact event? That depends on the size of the asteroid. Impacts from large asteroids are fortunately infrequent events. Dinosaur-killing NEOs 10 kilometers in diameter impact the Earth only about every 100 million years. The impact of a NEO >140 meters in diameter could cause significant regional damage, on the scale of a country or ocean basin, but this statistically happens only about once every 10,000 years. Tunguska-class impacts (~40 meter objects) may happen on roughly 1,000-year intervals, and Chelyabinsk-class objects (~20 meters) may impact a few times per century. The damage from these smaller objects will be highly localized, only affecting people or infrastructure immediately under or very close to the impact zone. Since humans only occupy a few percent of the total surface of the Earth, the odds of another Chelyabinsk or larger impact adversely affecting a populated area is statistically small, on the order of 0.1% per year. Of course, as the people of Chelyabinsk can attest, a low statistical probability of a damaging impact from a small asteroid doesn’t mean that it will not happen!
Sub-Chelyabinsk asteroids/meteoroids impact with even greater frequency, but rarely do damage to human populations, aside from the occasional hole in the roof from a small meteorite (unless you’re very, very unlucky, like poor Mrs. Ann Hodges in 1954). Objects the size of the one that impacted over the Pacific Ocean on February 6 (about 5 meters in diameter) tend to hit every few years, and can impact without anyone witnessing the event, if it happens over an unpopulated area. Smaller, car-sized asteroids probably impact several times per year (See here for a list of recent small impacts over the last decade), Rarely, these smallest and most frequent of impactors are detected in advance, and the impact location and time can be predicted. But the only likely consequences of these smallest impacts are a nice light show, and if we’re lucky, a few meteorites that can help advance our knowledge of the Solar System.
* A meteoroid is simply a small asteroid. Though near-Earth asteroids come in a continuum of sizes from multi-kilometer behemoths all the way down to dust-size particles, some scientists draw a rather arbitrary dividing line between “asteroid” and “meteoroid” at 1 meter in diameter. It’s a bit like calling a small rock a pebble instead of a boulder.