Never saw the asteroid coming
PremiumThere are over 2,000 known PHAs, of which about 160 may be as large or larger than 1994 PC1
The dinosaurs never saw it coming. Life for them had been good for many million years, then it pretty much went up in smoke. That’s what happened about 65 million years ago, when an asteroid smashed into the planet in what is today Mexico’s Yucatan Peninsula. Though the crater it left is buried under the Peninsula, it is still detectable. About half is offshore, half on land. Its diameter is about 180km, and it is estimated to be 20km deep. Those numbers give you a sense of the mind-numbing scale of the impact when this asteroid crashed into our earth. There is, of course, another measure of that. This colossal collision is now understood to have set off what is now known as the Cretaceous-Paleogene extinction event, in which three of every four living species on earth suddenly went extinct. Among them, the dinosaurs.
Yet consider this: the asteroid itself was only 10km in diameter. Of course, “only" is relative here—that’s a rock larger than Mt Everest, after all. But when you understand that the collision created a crater twice as deep as the asteroid itself and 18 times as wide, you start to understand the force with which this relatively small object smashed into earth. One estimate is that the explosion was the equivalent of more than a billion atom bombs of the kind that the US dropped on Hiroshima in 1945.
How did this asteroid pack such a punch? Because of its size and because it was hurtling through space at about 30km per second. That speed is not unusual for celestial objects. In fact, our earth orbits the sun at just about 30km/s. So when two objects that are moving at that speed collide, even if one is a thousand times larger than the other, you can start to imagine the results.
To some degree at any rate, all of this must have been on some human minds over the last few weeks. That’s because we have recently had a close call with an asteroid.
Hold on to that thought. For we should get used to the idea that close calls with asteroids are actually a daily affair. The National Aeronautics and Space Administration (Nasa) has an “Asteroid Fast Facts" page with nuggets like these:
* “Every day, earth is bombarded with more than 100 tonnes of dust and sand-sized particles."
* “Space rocks smaller than about 25 metres will most likely burn up as they enter earth’s atmosphere and cause little or no damage." This accounts for most of the 100 tonnes mentioned above.
* “About once a year, an automobile-sized asteroid hits earth’s atmosphere, creates an impressive fireball, and burns up before reaching the surface."
Now it’s unlikely you’ve seen one of these space objects actually strike earth. But that’s really because most burn up on their way down, from friction as they travel through our atmosphere. For that reason, the atmosphere really acts as our protection against such calamities.
Still, some asteroids do reach the surface of the planet—the count is estimated at about 17 a day. But most of them touch down in areas where there are no humans around. After all, 71% of the surface is water, and there are also stretches of land that are uninhabited. Even so, there are occasional strikes we do notice. Like the one in 2019, for example. On 1 February that year, an asteroid thought to have been about the size of a van broke up over Cuba, and the debris fell on several houses in the city of Viñales.
But if you weren’t in Viñales then and haven’t witnessed any other such impact, it’s also fairly likely you have seen one of these objects as it tries to strike earth. Think of a “shooting star", or meteor. That’s really just an asteroid, or more likely a fragment from one, burning up as it hurtles through our atmosphere.
So yes, there are close calls every day. So why was one in particular in the news of late? Because it was big, because we can track its path, and because it passed relatively close to earth.
We’re speaking of an asteroid that astronomers have labelled (7482) 1994 PC1. From the name, you might guess, and rightly, that we’ve known about this rock since 1994. That’s when an Australian astronomer called Robert McNaught discovered it. We now know it is a little over 1km in diameter. After working out its path through the skies, we learned more things about 1994 PC1:
* It has appeared in astronomical photographs since early 1974.
* Its closest-known approach happened in January 1933.
* It made another close approach to earth on 18 January.
That’s right. Just over a month ago as you read this, a rock 1km wide zoomed past earth. Should you have been worried? Maybe so. After all, 1994 PC1 is hundreds of times larger than the asteroid that exploded over Viñales. If it had entered our atmosphere, it’s more than likely it would not have burned up entirely, and would indeed slammed into our planet. The same Nasa page suggests that such an impact “could have [had] worldwide effects."
Take those laconic words as you wish. But be grateful 1994 PC1 only brushed by our planet. Flying at about 35km/s, the closest it got to earth was about 2 million km, over five times the distance to moon.
If you’re a little underwhelmed by that distance, unable to see what the fuss is about, it’s worth remembering that even 2 million km is minuscule by celestial standards. 1994 PC1 orbits the Sun once in about 19 months, generally at a similar distance from the Sun as we are and only getting as close as in January once in many years. But that path can change—say, by a collision with another rock, or simply because paths evolve over time. And even a tiny change would be enough to wipe out that 2 million km. This is why 1994 PC1 is classified as a “potentially hazardous asteroid" (PHA): an object that gets closer to earth than about 20 times the distance to moon, and is large enough to cause significant damage if it does hit earth.
Now you know why 1994 PC1 made the news. There are over 2,000 known PHAs, of which about 160 are likely to be as large or larger than 1994 PC1. Of those, only 19 are considered as possible threats to earth over the next century. Again, that may not seem worth worrying about. But remember that these are only the known PHAs, meaning ones we have actually detected and whose paths we have worked out. There are almost certainly others we have not yet detected. Also remember that the spectre of these potential hazards is why we are experimenting with altering an asteroid’s path ourselves, by deliberately flying a spacecraft into it. The idea, of course, is to nudge the asteroid enough off course that it will miss us (bit.ly/3BUjfTd). Of course, too, all this depends on detecting a potentially destructive asteroid early enough that we can launch a mission to alter its course. The dinosaurs couldn’t do that. Look what happened to them.
Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.
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