In some industries, sex sells. In the science journalism industry, however, potentially killer asteroids sell even more. Due to a quirk of how NASA refers to the many asteroids it tracks, countless headlines such as "Massive and Potentially Dangerous Asteroid Will Approach Earth Tonight" and "'Potentially Hazardous' Asteroid to Pass by Earth on Super Bowl Sunday" fill Google News every month. Those aren't tabloids - they're from Newsweek and New York Magazine, respectively. The problem is, NASA's definition of "potentially hazardous" isn't the same as the general public's.
Image: NASA Goddard Spaceflight Center/Flickr
There are countless asteroids in this solar system, and astronomers try to keep track of as many as they can, including those that may cause harm in the future. But the term "Potentially Hazardous Asteroid", a label that NASA routinely gives to various space rocks, doesn't mean that Earth is in danger - or even potentially in danger, at least not any time soon. It just means that scientists should continue tracking that rock, and let us know if they do become a concern later.
"It really has to do with what 'potentially' means," Matthew Holman, director of the International Astronomical Union's Minor Planet Center, told Gizmodo. "It's something that in the distant future could possibly impact the Earth but doesn't necessarily mean something about what's happening today."
Most recently, folks have latched onto a large asteroid called 2002 AJ129. It won't pass especially close - at its nearest, the space between it and Earth will still be about 10 times the distance from us to the moon. But it's big, maybe a kilometre in diameter. It is of no impact concern and will not hit Earth. In fact, in the past month there have been several smaller asteroids that have passed between the Earth and the moon that you have not heard about. But still, people have written about AJ129 with concerning headlines.
"Potentially hazardous" may be useful for scientists, but is a bad term for everyone else. These asteroids are not hazardous to you at the time that they are covered by the news. The term doesn't adequately explain what is occurring: NASA defines these objects as asteroids whose orbits come within around 20 lunar distances of the Earth's orbit and that are around 140m in diameter or larger.
"[The public] naturally thinks that the potential hazard is associated with the flyby," said Holman, instead of what it really means, which is that "this thing in the distant future at another encounter can impact the Earth."
Maybe one day, in the distant future, one of these objects will hit us - which is why scientists keep track of them and consider them potentially hazardous. But for us regular folks, we should really be calling these "Noteworthy Asteroids" or "Future Potential Impactors" or something like that.
The real issue is that articles calling these asteroids "potentially dangerous" miss the point: The asteroids that caused the Chelyabinsk or Michigan events weren't labelled "potentially hazardous" because they weren't tracked at all. Something the size of the Chelyabinsk meteor (15-20m) hitting an urban centre could still be devastating.
"The things that are in that size range, we really know much much less about those," said Holman. "I think those would be the next priority." Perhaps with more funding, astronomers would be able to predict these events with a few days warning.
If you really want to be concerned about impacts, a better place to look would be the Torino or Palermo scales, which measure likelihoods that asteroids will hit and how much energy they have - their potential to do damage.
There is only a single asteroid with a Torino rating above zero (absolutely no chance to cause harm on Earth), called 2017 XO2. It has a rating of one, meaning "A routine discovery in which a pass near Earth is predicted, that poses no unusual level of danger. Current calculations show the chance of collision is extremely unlikely with no cause for public attention or public concern. New telescopic observations very likely will lead to reassignment to Level 0." The potentially worrisome flyby will be in 2057, and more data will determine just how concerning the asteroid is. It will more than likely miss us completely.
So next time you see a headline that mentions a "potentially hazardous" or "potentially dangerous" asteroid, it's worth getting upset. Not at the asteroid, of course, since it definitely won't hit Earth. Get upset at the writer for not doing their research, and get upset at the government for not having a better asteroid monitoring system for asteroids that may actually be hazardous.