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A stochasticly updated blog about interesting topics in Physics & Astronomy

How it’s All Going to End: Comets (Part 2)

Edit: As pointed out to me, my use of the words comet, meteor and asteroid in the below article are appalling. There are differences between them, which I might write about someday, but for the moment, treat them the same (although they’re not)

Last week, I spoke about how in about 4.6 billion years, the Sun will expand and consume the Earth. Thankfully, this is very, very far into the future, and also, we know (roughly) when it will happen. Today, I’m going to talk about something that quite literally could happen at any moment – the impact of a comet.

Comets are clumps of rock, gas and ice that can vary in size from a couple of hundred meters across to 10’s of km across. A comet which comes close to the Earth is called a NEO – Near Earth Object. The smaller ones pose little threat to us. A small meteor will crash into the Earth’s upper atmosphere, and will burn up in a glorious fireball (known as a shooting star, even though shooting stars can also be tiny bits of space debris and waste from the International Space Station burning up as well). However, if a very large comet, say a couple of kilometers, were head towards the Earth, it would be far too big to burn up in the atmosphere, and would strike the surface of the Earth with unimaginable force. The impact would trigger earthquakes across the globe, create super tsunami’s and raise a cloud of dust that would cover the Earth, blocking out the sun and killing all plants, which would devastate the ecosystem.

That's what a 10-12 km meteor impact would look like from space. Click for source

That’s what a 10-12 km meteor impact would look like from space. Click for source, and also for an interesting article on how the dinosaurs may have died

Scientists currently believe that this type of impact was probably what doomed the dinosaurs. In fact, scientists believe they have found the crater from where this meteor hit – Chicxulub Crater. Now, time for some statistics – there are quite a lot of meteors in the solar  system, mostly small objects that wouldn’t affect us too much if they hit, some that would cause nationwide catastrophes, and a few that would end us. So, to put everything into a logical scale, we invented the Torino Scale. This relates the size (and speed) and probability of a meteor hitting us on a scale from 1 to 10.  A 5km meteor that has absolutely no chance of hitting us would have a Torino value of 0, meaning it’s no threat, while a 100m meteor that is definitely going to hit us has a Torino value of 8, meaning it’ll hit and cause “localized destruction for an impact over land or possibly a tsunami if close offshore”. Also, by taking the amount of comets we know are out there, and doing some statistical magic on them, we can figure out how often they can happen. The 3 worst scenarios are

  • Torino Value: 8 – A collision is certain, capable of causing localized destruction for an impact over land or possibly a tsunami if close offshore. Such events occur on average between once per 50 years and once per several 1000 years.
  • Torino Value: 9 – A collision is certain, capable of causing unprecedented regional devastation for a land impact or the threat of a major tsunami for an ocean impact. Such events occur on average between once per 10,000 years and once per 100,000 years.
  • Torino Value: 10 – A collision is certain, capable of causing global climatic catastrophe that may threaten the future of civilization as we know it, whether impacting land or ocean. Such events occur on average once per 100,000 years, or less often.

So, according to above, a Torino 10 comet should strike the Earth every 100,000 years or so. But the last major-major impact known was the extinction of the dinosaurs nearly 66 million years ago, and the highest rated meteor on the scale at the moment is a 1. Actually, no object has ever been rated above a 4 (which means “A close encounter, meriting attention by astronomers. Current calculations give a 1% or greater chance of collision capable of regional devastation. Most likely, new telescopic observations will lead to re-assignment to Level 0. Attention by public and by public officials is merited if the encounter is less than a decade away.”) since the invention of the scale. So either our statistics are wrong, or else we’re missing something. I’ll give you a hint. We are missing something. Something quite big.

Jupiter, the bi-polar giant (via the Cassini spacecraft)

Jupiter, the bi-polar giant (via the Cassini spacecraft)

The thing we’re missing is Jupiter, the largest planet in our Solar System. You see, a majority of the meteors that could potentially strike the Earth come from the very far reaches of the Solar System, or from the Asteroid belt out beyond Mars. The asteroids in the belt are usually well behaved, and follow orbits that we can predict. The objects from the farthest parts of the Solar System are a completely different story. It’s from these comets that our statistics get messed with. So what’s going on with them? Well, below there are 3 images. The first is a setup of Jupiter and a comet approaching Jupiter at roughly 10 km/s.  The second image shows the path that the comet took around Jupiter, and it’s velocity jumped up to around 20km/s.  The third image shows what happened to the comets path as it approached Jupiter. What happened here is exactly the same thing that the astronauts of Apollo 13 used to get home – the comet underwent a gravitational slingshot around Jupiter.

Comet approaching Jupiter

Comet approaching Jupiter

Comet passing Jupiter

Comet passing Jupiter

Change in Orbit

Change in Orbit

Jupiter does this trick to a lot of comets that try to make their way into the inner solar system (the inner solar system being Mercury, Venus, Earth & Mars). Jupiter takes these comets, and fires them pretty much straight back out to the far reaches of the solar system, at a higher velocity. And this is what ruins our statistics – Jupiter is protecting us. However, there is a slight problem, as every once and a while, Jupiter sheds its cloak of protection and will take a comet and fire towards us using the same trick, meaning it’s got a higher velocity, meaning it goes higher on the Torino scale. But this happens so rarely that we can forgive Jupiter its occasional outbursts and be thankful for its protection.

Now, another problem, and I’ve been avoiding talking about this one because, quite frankly, it’s scary. How do we detect comets? Well, when a comet comes into the inner solar system, and the sun begins heating it up, the ice on it will begin  to melt, and the gas will be blown outwards by the solar winds, and we’ll see a nice big tail on the comet. But what if the comet is just solid rock, and has no ice or gas to make it visible? Or what if it doesn’t get close enough to the sun to melt the ice? Will we see it?

No. No, we will not (bar a few exceptions).

A close encounter with a comet is when the comet comes within the orbit of the moon (the moon’s orbit is roughly 385,000 km, and is tiny in astrophysical scales). So imagine how terrified astronomers are when the find that a comet has come within less than 1/3 of this distance, and we don’t know until after the object had passed us! Admittedly, this is a rare occurrence, and the few times that it has happened the comets have been no bigger than a 10’s of meters (eg 2012 XE54 which passed by late December, and was only discovered a few days beforehand) but the idea that comets can get so close to us without us knowing is still terrifying.

So, now, again, what do we do? WHAT CAN WE DO?

I’ll tell you. We can get all “Armageddon” on these comets. Literally.

There are plans by both NASA and the ESA (European Space Agency) that if a comet were discovered that was going to impact the Earth and cause local or global catastrophe, then we target it with missiles and blow it up Bruce-Willis-Style. If the comet is mostly made of ice and gas, the explosion should melt these components and whatever’s left over will either break up upon entering our atmosphere, or be deflected off course. And if the comet is made entirely of rock? Well, we can just attach an engine to the side of it and nudge it off course.  That is assuming we have enough notice of course.


3 movies to watch to get a feeling for how a comet would affect society –

  1. Armageddon
  2. Deep Impact
  3. Seeking a Friend for the End of the World

Also, there is an entire division of the ESA devoted to finding these comets and coming up with plans to deal with them (the SSA).

So, as usual, comment with questions, share and subscribe. See you Thursday for a talk on Andromeda and the Great Collision.

Finally, if there are any mistakes above, let me know. My bring isn’t functioning so well today.


About irishphysicist

I'm a PhD student with the Departments of Physics in University College Cork, Ireland and University of Notre Dame, Indiana. I want to try and bring astrophysics to the public, and also would like world domination. But that's a story for another day.

4 comments on “How it’s All Going to End: Comets (Part 2)

  1. fahrusha
    February 11, 2013

    I enjoyed reading your post. One thing bothered me a bit. You seemed to interchange the terms meteors, asteroids and comets rather too freely. Meteors and asteroids are both primarily rocky objects, while comets are like dirty snowballs, that is mostly frozen liquid which in turn is mostly water.
    Are you aware of the large object DA-14 that is passing under our satellites over the Indian Ocean this Friday? I hope the astrophysicists at NASA are correct in their calculations or it could be “bye, bye Mumbai” (sorry I couldn’t resist, but that was tasteless on my part). I recently posted about DA-14 and three visible to the naked eye comets that are on the astronomical menu in 2013.

    • irishphysicist
      February 11, 2013

      Hey Fahrusha,

      You’re absolutely right, I should have been a lot more strict with my usage of meteor, asteroid and comet. Might end up doing another post about the differences later on.

      And yes, when researching stuff for the above article, nearly every google result returned information on DA-14 as first result. Should be a hell of a show.


  2. Brian Pettit
    February 12, 2013

    Mark – why do we never sees pictures of the Sun being hit by these beasties? Also – if Jupiter has the second biggest gravitational pull of the solar system – does it get hit a lot by comments/meteors etc? Also – when Earth gets hit – it this due mostly to our gravity or just to the bad luck of a shitty Jupiter slingshot?

    • irishphysicist
      February 12, 2013

      Hi Brian,

      1. Good question about the Sun. Yeah, this actually happens quite a bit (maybe not any asteroids, but most certainly quite a few comets impact the Sun). Infact, NASA’s Solar & Heliospheric Observatory (SOHO) have a few videos of these events – http://sohowww.nascom.nasa.gov/gallery/Movies/comets.html . The only thing is they aren’t sure if the impact of the comets are related to the massive explosions on the surface of the sun.

      2. Yup, Jupiter gets hit by quite a lot of comets. But, due to its strong gravitational field, it pulls quite a lot of them apart. The most famous of these is Shoemaker-Levy 9 (http://en.wikipedia.org/wiki/Comet_Shoemaker%E2%80%93Levy_9)

      3. Earth getting hit actually tends to be just unlucky – our path just happens to cross that of a comet. Our gravitational pull isn’t quite strong enough to cause a serious build of comets that target us.

      Thanks for the q’s!


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This entry was posted on February 11, 2013 by in Astrophysics and tagged , , , , , , , , , , .
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