Home > Arthure C. Clarke, Clement's Game, Larry Niven, When Worlds Collide > SciFi Round Nine: Asteroid Impact

SciFi Round Nine: Asteroid Impact


The near-Earth asteroid 4179 Toutatis is flying by Earth right now.  Closest approach was 0.046 astronomical units (just under 7 million kilometers) on 2012 Dec 11.

This is not unusual.  Toutatis’ orbital period is just over 4 years, so it flies by the Earth every 4 years for 24-28 years when the objects are in phase with each other.  Toutatis was briefly observed in the 1930s, during the last series of approaches, but formally discovered by Christian Pollas in 1989, just after the 1988 flyby.  It has been observed with radar imaging during every flyby since – 1992, 1996, 2000, 2004, 2008, and I’m part of the team observing it right now.  There is also a series of optical images from the Chinese Chang’e 2 spacecraft, which flew by Toutatis on Dec 12.  So we know quite a bit about the asteroid’s shape, spin, and internal structure.  There’s a lot of interesting science there.

We also know Toutatis’ trajectory in space, down to a few hundred meters over the last twenty years.  Running the orbit forward, we can say that it will not hit the Earth anytime in the next several hundred years and almost certainly not within the next several thousand (it can’t get closer than ~700,000 km away until the orbit has drifted from the current ellipse).

But despite all of this, there is still considerable nonsense associated with people claiming that Toutatis will hit.  This willful ignorance of reality annoys me, so I will defuse my annoyance by discussing asteroid and comet impacts in fiction.

The Truth

Asteroids do hit Earth.  Objects the size of a car fireball in the upper atmosphere every few months.  In one case, 2008 TC3, the object doing the fireballing was discovered approximately 30 hours before impact, and a careful search of the predicted impact zone duly found appropriate meteorites.  Objects several tens of meters wide hit the Earth every century or so.  The most recent one flattened a big expanse of forest in Siberia a bit over a hundred years ago.  Objects several kilometers wide hit on a tens-of-millions-of-years timescale.  The most famous of those is the Chixulub impactor, which triggered the mass extinction of most of the currently-living dinosaurs (the birds were the exception).

So there is an asteroid impact hazard.  It is very well-characterized, because we know the rate of past impacts in the geological record.  As the potential effects of a large asteroid hitting the Earth became well known in the late 1980s and early 1990s, the US Congress was persuaded to order and fund – through NASA – efforts to locate at least 90% of all near-Earth objects larger than 1 km in diameter (there are about 1000 of them).  That project was known as Spaceguard, and included a number of groups focused on discovering objects, better measuring the orbits of known objects, and understanding the physical properties of asteroids in general.  There were relatively few people working on this full-time in the beginning – the comparison was to the staff of a McDonald’s franchise.  But Spaceguard did its jobWe can now say that no near-Earth asteroid larger than 1 km in diameter will hit the Earth in the next hundred years.

The "Burger Bar That Saved The World", courtesy Nature (http://www.nature.com/news/2008/080625/full/4531164a.html).  Steve Ostro, who was my thesis advisor, is depicted in the lower right.

The “Burger Bar That Saved The World”, courtesy Nature. Steve Ostro, who was my thesis advisor, is depicted in the lower right.

More extensive survey programs now aim to push their completion limits for the near-Earth asteroids down to ~140 m or so.  That point is pragmatically defined: for smaller objects, the cost of finding them decades to centuries before they are going to hit is higher than the cost of finding any impactor a few days or weeks before impact and simply evacuating the blast radius.  The unknown near-Earth objects are not civilization-ending.  We will not all die from asteroid impact.

Should a few-hundred-meter object be found to be on a collision trajectory with decades of warning, there are well-developed plans for deflection.  Nukes are not necessary.  Kinetic impactors are useful for objects where slower and better-controlled approaches would take too long.  If you have enough time, you can just coat the object with a very thin metallic layer (paint or film) and let radiation pressure do the work for you.  But the favored method right now is something called a gravity tractor: put a spacecraft next to the asteroid and hover.  If you angle the rocket exhaust to miss the asteroid, then momentum goes from the exhaust to the spacecraft to the asteroid and the trajectory is very slowly and precisely adjusted.

So we will not be hit by a large asteroid anytime soon, and we can deal with the smaller ones.  There remains a slight impact hazard from long-period comets, which can’t be found more than a couple of years before any potential impact because they are too far away.  But that’s a once-per-hundred-million-years event, and there are ways to largely mitigate even an impact like that.

Works That Get Some Things Right

Although it is tangential to the plot, special mention goes to Arthur C. Clarke’s Rendezvous With Rama. In the backstory to the book, an object smashes into the Mediterranean in 2077 and ruins large sections of eastern Italy.  There is no such object in reality, but since Clarke was writing in 1973, that’s pardonable.  One part of the response to this impact is to set up a survey program called “Spaceguard” to find any other potentially hazardous objects.  Several decades later, the survey program discovers the alien spacecraft that the humans label Rama when it is still many months from closest approach to the Sun.  Clarke’s Spaceguard is of course the namesake for the real one.  His prediction was just 85 years or so behind the times.

As far as impacts themselves go:

In Larry Niven’s fiction, the impact effects are generally fairly well done.  In Lucifier’s Hammer, the impactor is a comet rather than an asteroid and a close approach turns into an impact when the comet outgasses unexpectedly.  The one problem with that is that that’s a known effect, so any comet coming close enough for outgassing to possibly cause an impact would be dealt with.  In Footfall, aliens dropped the snowball as part of an effort to terraform Earth to their liking.  In both cases, people and civilization do survive, although the details are unpleasant.  The Ringworld has suffered impacts too, punching holes through even its incredibly durable construction.  Niven was sneaky enough to specify the failure properties of the ring material so that it would bend enough that impacts breaking through from the outside would deflect the ring surface enough locally that the air would not all leak out.  Sufficiently large impacts onto the inside of the ring would still be a problem.

Unfortunately, these are the exceptions.

Those That Get Many Things Wrong

For the earliest impact stories, egregious failures to understand the effects of asteroid impacts can be partially pardoned by the knowledge at the time.  But some things can’t use that excuse.  In Jules Verne’s 1877 Hector Servadac, the eponymous hero and several dozen others are picked up by a passing comet.  Verne clearly didn’t understand what happens to material that goes from zero to roughly 20 km/s nearly instantly.  Of course, Verne was the man who proposed launching people to the Moon using a giant cannon with 20,000 g acceleration, so math was not his strong point.  By the 1930’s, the public had started to understand the principle of reaction engines.  So Balmer & Wylie wrote When Worlds Collide, in which spacecraft at least have rockets.  But they got the physics of impacts entirely wrong, as well as requiring various impossibilities in terms of the rogue planet that is doing the impacting.

When Worlds Collide, 1933.  Earth-Shattering Kabooms do not look like this.

When Worlds Collide, 1933. Earth-Shattering Kabooms do not look like this.

When Worlds Collide also illustrates one of the two ways that impacts get misinterpreted.  Since it was written, there has been a pattern of making impacts far worse than they would plausibly be, either by making the impacting object far too large or making it far too destructive for its size.  For example, Balmer and Wylie wanted to destroy the Earth and have the humans evacuate to one of the rogue planets that came in.  To do that, they invoke an impactor the size of Neptune.  That’s nonsensical overkill – we should refer to Earth hitting it rather than it hitting the Earth.  In reality, for a rogue body hitting the Earth, it would only need to have a diameter of ~4000 km (smaller than Mars) to disintegrate the planet entirely.  I quote The Impact Effects Calculator.  There are also few enough rogue planets that the expected rate of such objects hitting the Earth is far longer than the age of the universe.

Books may often be bad, but movies are the most egregious offenders when it comes to doing impacts wrong.  Most particularly are the two impact films of the late 1990s: Armageddon and Deep Impact (no relation to the spacecraft of the same name).  Both are horrifically bad in terms of the science.

Armageddon is the worse of the two, since it has an asteroid as large as Ceres be pushed by a comet onto an impact trajectory that is moving about 50 times too fast for anything gravitationally bound to the Sun.  That is equivalent to shooting a rhinoceros with a handgun and having the rhino go into Earth orbit It doesn’t workArmageddon gets still worse in that a space shuttle is somehow scrambled to launch within two weeks and match that impossible speed, that somebody built a bomb with a yield per mass ten thousand times higher than total conversion, and that Bruce Willis could bury that bomb four hundred kilometers underground.  I could go on, but it is So Bad It’s Painful.

Deep Impact has a more reasonably sized comet doing the impacting, and it is discovered with over a year of warning.  Good enough.  But it still overestimates the effects of the impact.  An 10-km comet hitting the ground would cause a mass extinction.  But it wouldn’t kill everything.  You wouldn’t want to be within 1500-km or so of ground zero, or anywhere on an adjacent coast if it hit in the ocean.  But everyone outside of that zone would be relatively okay.  A year is time enough to evacuate the zone and stockpile food for the impact winter, if everyone in the world is preparing for it.

That is the problem with Deep Impact: an amateur astronomer finds the comet.  That does happen in reality.  But then no-one else finds it, and there is an attempted coverup and Masquerade.  That’s even more nonsensical than the other examples we gave in our Masquerade post.  The comet is quite literally shining in the sky.  Anyone can see it.  And there is no reason to attempt a coverup and every reason to make it public, so that people can prepare.  So the science is better but the sociology is equally absurd.

Thinking through all of this has made it clear to me that the public misunderstanding of the impact hazard is closely tied to popular misrepresentations of it.  My normal approach is to try and explain things correctly.  But how can I work around the public perception of Bruce Willis?

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  1. 2013/03/17 at 7:36 am

    Thanks for that well written discussions of the impact hazard as described in science fiction.
    The newly discovered comet C/2013 A1 scheduled to make a close flyby of Mars in 2014 is worrying because it should be *extremely* rare for such a large comet to get that close to a terrestrial planet.
    Coupled with the rare close meteor and asteroid encounters that happened to Earth on the same day this year, leads one to suspect there is an underlying cause for these close encounters occurring within such a short time frame.

    Bob Clark

    • michaelbusch
      2013/03/17 at 8:15 am

      Neither the flyby of Mars by C/2013 A1 next year nor the recent flyby of 2012 DA14 happening within a couple of days of the Chelyabinsk meteorite flashing in the sky are worrying, or give any reason to suspect an “underlying cause” – they are simply coincidences that are expected to happen every so often.

      First, consider Chelyabinsk and DA14. Something the size of DA14 flies within a million kilometers of Earth on a weekly basis. So given that the Chelyabinsk meteorite hit Earth, it is unsurprising that there was a 20-40 m object flying by Earth a little bit afterwards (30%-50% chance of that happening). DA14’s flyby was relatively close to Earth, but their orbits show that the two asteroids are indeed entirely unrelated to each other: they were traveling at almost right angles to each other, and their orbits never got closer than 500,000 km to each other. They also are dramatically different in composition. The relative timing of the flyby and the impact is simply a relatively high-probability coincidence.

      C/2013 A1 is currently predicted to pass 100,000 km from Mars on 2014 Oct 19, although that distance is still uncertain because of the short time since Siding Spring discovered it and because comets jet around a bit from outgassing. It may be as far away as 350,000 km. Mars experiences a flyby of an object of that size or larger at 350,000 km or closer every few decades, although most of those are asteroids rather than comets. Something like the C/2013 A1 flyby is expected to happen every few centuries. Mars has been observed intensively for well over 100 years, so the probability of seeing something like this flyby is again quite high. The probability of impact is quite low, 0.043% as of the last assessment, and will most likely go to zero in the next few months as we get better data on its trajectory.

      • michaelbusch
        2013/04/03 at 1:49 am

        Two weeks more of data on C/2013 A1 has allowed us to reduce the uncertainties in its motion. The impact probability is now assessed at 0.01%.

  2. michaelbusch
    2013/10/15 at 6:12 pm

    Addendum: something I did not note when I wrote this post, but should have:

    Many media dealing with impacts are variously racist, sexist, or otherwise bigoted. “When Worlds Collide” is also incredibly xenophobic (especially in the sequel). And consider the demographics of the mission crew in Armageddon – notice the excess of white men?

    Basically, the “save the world from rocks falling from the sky” idea gets co-opted to promote the far-too-common narrative of “the hero has to be a white male”. That more general pattern in fiction is a very serious problem.

  1. 2013/02/26 at 7:13 pm
  2. 2016/03/25 at 10:51 pm

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