Because “potentially hazardous” means something different when you work for NASA.
The Daily Fail, in its ongoing quest to be the most sensationalist it can be, seems to have a policy of threatening asteroid armageddon every few months. The most recent was last week, with the flyby of 2015 BN509, a near-Earth object (or NEO).
The headlines were, well, you can see above. But the Daily Fail’s was kind of amazing: “Astronomers spot asteroid bigger than the Empire State Building that they say could crash into the Earth” (I’m linking, but don’t actually go). It’s all technically true, but boy does it mean something different when it comes out of the mouth of a NASA scientist compared with when it’s in the tabloids.
A “Potentially Hazardous Asteroid” (or PHA) is a very specific technical term for NASA:
“All asteroids with an Earth Minimum Orbit Intersection Distance (MOID) of 0.05 AU or less and an absolute magnitude (H) of 22.0 or less are considered PHAs. In other words, asteroids that can’t get any closer to the Earth (i.e. MOID) than 0.05 AU (roughly 7,480,000 km or 4,650,000 mi) or are smaller than about 150 m (500 ft) in diameter (i.e. H = 22.0 with assumed albedo of 13%) are not considered PHAs.”
There are currently 1773 known PHAs, though the numbers go up and down as we discover more and track them better.
But please, don’t lose any sleep over 2015 BN509.
NASA and ESA both track the orbits of every single NEO they know about — and maybe they don’t know about all of them, and maybe that’s something you should be concerned about instead — but if we know about it, chances are we’re good. Take this little chunk of space debris: the very closest it gets to us — it’s MOID (minimum orbit intersection distance) — is 0.0246445 AU (1 AU is the distance from Earth to the Sun, just FYI, and is a Very. Large. Distance.). Note, by the way, the specificity of that number — scientists don’t include that many digits unless they’re pretty darn sure about them. Now convert that to miles.
0.0246445 AU = ~2,290,849 miles.
For reference, the moon is 238,900 miles away.
Say it with me: the very closest 2015 BN509 can come to Earth is almost ten times further away than the moon.
And for a little more perspective, the distance to the moon is astronomical. You could fit thirty Earths between Earth and the moon.
Furthermore, if you run the numbers out a hundred years from now, the closest it comes is next year in early February, when it’ll be 0.0329833 AU away, or ~3,065,989 miles. That’s nearly thirteen times as far away as the moon. So don’t worry about it.
Scientists keep a couple lists of NEOs. There’s the one I linked to above, which is ESA’s NEODyS-2 list, and then there’s the (nicer GUI) NASA Sentry list, which has some helpful features. My favourite is that you can list all the potential threats to Earth by various factors, including different risk calculations, and it’ll give you helpful information.
The absolutely very most likely to hit us asteroid is known as 2010 RF12, which we noticed back in 2010 when it whizzed past Earth at less than a quarter of the distance to the moon. But it’s not exactly “Empire State Building Sized” (seriously, why do we use this as a measurement? Might as well be giving its mass in Badgers). There’s also (29075) 1950 DA, which could do some serious damage, being 1.3km wide. It’s got one of the higher probabilities of hitting us, which NASA helpfully explains is “0.012% chance of Earth impact” or a “99.988% chance the asteroid will miss the Earth.” And if that worries you, again, chill: you’ll be long dead by then. It’s not likely to pose a threat (if 0.012% is a threat) until 2880.
All of which is to say that next time you read one of these insane little “oh god oh god we’re all gonna die” posts about asteroids, maybe take a minute and look it up over at NEODyS-2 or Sentry. You’ll be glad you did.
Have a great day.
Thanks for reading! Except for the very *very* occasional tip (we take Venmo now!), we only get paid in our own (and your) enthusiasm, so please like This Week In Tomorrow on Facebook, follow us on Twitter @TWITomorrow, and tell your friends about the site!
Richard Ford Burley is a human, writer, and doctoral candidate at Boston College, as well as Deputy Managing Editor at Ledger, the first academic journal devoted to Bitcoin and other cryptocurrencies. In his spare time he writes about science, skepticism, feminism, and futurism here at This Week In Tomorrow.