By Duncan Steel 22/10/2020

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Disaster movies forever capture the public attention… but did you ever stop to think that the word disaster actually means bad star? That is, ‘dis’ implies a pejorative (as in disease, or disgust, or disrespect), while ‘aster’ comes from the Latin astrum, similarly the Greek astron. Obviously enough, this derives from old astrological beliefs. In modern science, asteroids are called that because they are star-like (points of light in a telescope), while comets get their name from an ancient Greek word meaning hair-like, referring to the tails exhibited as they cross the night sky.

The archetypal disaster movie really should involve an asteroid or comet striking the Earth, then, and there has been no shortage of them. To list a few:

The Wikipedia page for Armageddon states that “…astronomers noted that the similar disaster film Deep Impact was more scientifically accurate.” With that in mind, I take a look at the scientific accuracy of this latest offering, Greenland. No spoilers, I think, in what I write; the subject here is simply scientific veracity.

Comet Clarke

There is no comet called Clarke, though there is one named Clark (71P/Clark, the P implying ‘periodic’), for Michael Clark, a New Zealand astronomer who worked at Mt John Observatory (Lake Tekapo) for some years.

The fictitious interstellar Comet Clarke in the movie is, one supposes, named for the late Arthur C. Clarke, the science fiction author who was behind the movie 2001: A Space Odyssey. In his 1973 novel Rendezvous with Rama Clarke described an interstellar object that enters the solar system, initially thought to be an asteroid or comet but later realised to be a vast alien spaceship.

And here I need to say what is meant by ‘interstellar’ in this context. This implies an object that is not gravitationally bound to the Sun, or any other star. Such an object will pass through the solar system and depart, never to return, unless something drastic happens to it; something like running into a planet, or (much more likely) being diverted during a close approach to one of the planets – most probably Jupiter – into a smaller orbit in which it will circuit the Sun repeatedly.

Although they were theoretically predicted – we have witnessed comets being thrown out of the solar system by Jupiter, the inverse of the capture mechanism I just mentioned – no interstellar objects had been detected until just the last few years… and then two came along in quick succession, just like the proverbial London buses. 1/I ‘Oumuamua (‘I’ implying ‘interstellar’) was discovered almost exactly three years ago. Then 2/I Borisov was spotted in late August last year.

Comet Clarke – the imaginary comet in the movie – is described as being interstellar in origin. Obviously enough, the story-writers have tapped into this advance in our knowledge: interstellar objects (asteroids or comets) do exist, and they can sneak up on us; that’s a matter I will discuss more below.

There is, however, an asteroid named Clarke. This is (4923) Clarke, an asteroid (or minor planet) that circuits the Sun, but forever remaining in the main-belt between Mars and Jupiter. I should know about it because I was one of those who suggested the naming… I knew Clarke, and he provided the Foreword for one of my books, and the Afterword for another. Both were about, yes, the hazard posed to life on Earth by asteroids and comets. That was also the theme behind Clarke’s 1993 novel The Hammer of God, with which I helped him on the basic storyline. As a result there is a robot in that book named Steel.

Interstellar dust hitting Earth

Many astrophysicists study interstellar dust through examining the way in which it scatters and absorbs starlight. There has, however, been detections of interstellar dust reaching the Earth, and this was first achieved here in New Zealand. That is, the first identification of any solid material of interstellar origin hitting our planet was made in NZ.

The culprits were Dr Andrew Taylor (who now teaches physics at Christ’s College in Christchurch), Professor Jack Baggaley (recently retired after being on the faculty at the University of Canterbury for more than half a century), and myself. In a paper published in the journal Nature in 1996 we showed how the large radar system formerly located at Birdlings Flat just south of Christchurch had enabled us to detect the meteors (shooting stars) produced by exceptionally high-speed particles around 50-100 microns in size that are penetrating our atmosphere, and could be traced back to the solar system’s motion around the galaxy. Since then similar radar systems elsewhere, notably in Canada, have also rendered confirmatory data.

Warning times for comet impacts

In Greenland (the film) it is stated that the comet in question was discovered just a few weeks earlier. Is this valid?

The answer is a definite yes and no. Mostly no, though.

In 1994 the late Brian Marsden and I investigated the problem of how much notice we might expect that a comet coming from the depths of space (as opposed to a periodic comet, such as 1P/Halley, 2P/Encke, or 55P/Tempel-Tuttle) and so discovered on its only passage through the planetary region is actually going to hit the Earth. This we discussed in a 19-page book chapter, entitled Warning Times and Impact Probabilities for Long-Period Comets. Lest you think that these things you see in movies are not matters that scientists have thought about a great deal, I mention that the book in question (Hazards Due to Comets and Asteroids, published by the University of Arizona Press) had 120 contributing authors and 1,300 pages. Nowadays we have a biennial international conference to discuss progress and new understandings: the International Academy of Astronautics Planetary Defense Conference, which was in Washington DC last year, and next April will be in Vienna (if Covid-19 allows, else it will be online).

The answer that Marsden and I found was that a warning time of 250 to 500 days is feasible, if the necessary ongoing sky-search programme were conducted, in the best case. There are, however, a few types of ‘sneaky’ orbital path that would give us much less warning time. Paradoxically, we are more likely to get a long warning time for a comet that arrives and strikes the Earth on its inward passage towards perihelion (its closest approach to the Sun) rather than a comet that passes perihelion and then runs into our planet on its way outwards again, but these are rather technical points.

It’s important, though, to understand the broad distinction here between asteroids and comets, if we think of the former being mostly inert rocks in small orbits that circuit the Sun every few years, and the latter being lumps of ice and rock that mostly fly through the inner solar system once only. The asteroids we can discover and track, and predict their orbital paths forward for about a century into the future with sufficient precision that we can identify possible collisions with Earth decades ahead of time. That is, we are capable of knowing where they will be in, say, 20 or 30 years’ time despite the fact that they will have orbited the sun perhaps a dozen times in the interim. If you do not believe me, take a look at the possible future impacts tabulated here and here.  The comets – at least the ‘new’ comets we have not seen previously, unlike periodic comets such as 1P/Halley – are different, however. Those must be spotted on their one-and-only transit through the inner solar system. We cannot get decades of warning for those.

I mentioned warning times of 250-500 days above, and there are various considerations that lead to that time frame. One is this: whilst a comet is still far from the Sun, it is a quiescent ball of ice and rock, perhaps a few kilometres in size. It is difficult if not impossible for us to detect it, because of the tiny amount of sunlight it reflects back to our telescopes. As a comet intrudes closer to the Sun, though, it is warmed and then some of that surface ice starts to sublimate (go directly from solid to vapour), and that forms a cloud we term the ‘coma’ which may be 100,000 km or even a million km across. That makes a comet relatively easy to spot: it may have a cloud and tail that are larger than the Sun itself, and so become really rather bright, because it scatters a lot of sunlight back towards our telescopes. Nevertheless the coma is almost transparent, in that we can ‘see’ stars through it. (Think of this as like the sky during daytime: it is virtually transparent, and yet it is bright due to the atmospheric molecules scattering sunlight, and that light makes us less aware that the stars are there; regardless, one can see the stars during daytime with a telescope, and the Moon and Venus can be seen with the naked eye, if you know where to look.)

It is straightforward physical chemistry to work out when the ice will start to sublimate, and indeed cometary scientists talk about a ‘water line’: at 3 astronomical units (AU) from the Sun – the Earth orbits at 1 AU – water ice will start to sublimate. That is midway between Mars and Jupiter. It is easy then to do the sums regarding how long the suddenly-brightened and so easily-found comet might be in terms of time before it could slam into our planet.

That was thinking only of ‘ice’ being water ice. In fact planetary scientists speak of ‘ices’ more broadly, referring to solid carbon dioxide (or ‘dry ice’), ammonia, argon, carbon monoxide, methane, ethane, ethanol, and all manner of other organic molecules which we know exist in cometary nuclei. Each of these has a calculable ‘switch on’ solar distance, and because they are more volatile than water those solar distances indicate that a comet may brighten whilst out beyond Saturn, as its CO2, CO, CH4 and so on start to sublimate.

It is therefore conceivable that we could get more warning, perhaps as much as a few years, before a comet is due to impact Earth. What we would need to do is to scour the depths of space with more rigour and coverage than we are doing at present, with much of the search effort at present being directed towards near-Earth asteroids rather than comets.

How does the movie do in this regard? Near the end of Greenland there is mention of the main part of Comet Clarke being 9 miles across (call it 15 km). That’s a huge comet, about the same as 1P/Halley. Rather than being found a few weeks prior to an impact, one would anticipate that it would be spotted more than a year prior to approaching Earth, most likely by an amateur astronomer using a sophisticated (but small aperture) telescope. This is just what happened with 2I/Borisov last year (but note that comet does not cross Earth’s orbit, rather having perihelion just beyond 2 AU).

Are comet impacts predictable?

This is something the movie deals with correctly, if the viewer is paying attention. An inert object like an asteroid (all rock and metal) has an orbital path that is predictable, being governed only by the gravitational field of the Sun plus perturbations due to the planets. For completeness I mention that it is also feasible that for orbital predictions more than a century into the future we need to worry about radiative effects (the anisotropic re-radiation of solar heating by the asteroid), and also one might postulate that some asteroid might collide with another rock in space and so have its path altered somewhat. But, to a large degree, asteroid paths are predictable with the necessary precision to make evaluations of future collisions feasible: we can say with certitude that (99942) Apophis will fly really close past Earth in 2029, but it will miss; what we cannot yet say is whether it will come back and hit us on various possible dates between 2060 and 2105.

As it says in the movie, though, comets are not like this. They are not inert, they are active. The ices I described above jet from the cometary nucleus as they sublimate, and impose a non-gravitational force that has long been known to deviate cometary paths from those which would be followed if only gravity were acting on them. The difference may not be much, but it makes it almost impossible to determine for sure whether it is going to be a hit or a miss until quite late in the proceedings. That is, it may be only days beforehand that we are sure that a comet is going to miss our planet.

This is a major problem. With asteroids we can make secure predictions. Into the future we should anticipate that there will be cometary discoveries in which there are predictions at the one-in-a-thousand level (or less) that a comet will hit the Earth, without it being feasible to give a definitive “Phew, it will miss us!” until late on. In itself, this is going to lead to social problems.

How likely is a comet impact on Earth?

Comets from far beyond the planetary region – either from the Oort cloud (thousands seen over the centuries) or from interstellar space (two seen so far) – arrive in directions that we might take to be random, for present purposes. Imagine such a random comet coming in, with an orbit that will pass perihelion within 1 AU; that is, it has an orbit that will cause it to cross Earth’s orbit, before receding again into the depths of space. What is the probability that it will hit our planet?

That is actually an easy question to answer. The Earth has a certain cross-sectional area, just πr² where r is Earth’s radius. Now imagine a sphere centered on the Sun with radius R = 1 AU. That has a surface area 4πR². The ratio of those two areas is one to 2.2 billion. The comet gets two chances to collide with Earth, once on the way inwards, once on the way outwards. On top of that, Earth’s gravity would deviate a comet’s path such that a trajectory that would just slip past our planet if it were massless in fact is ‘sucked in’ by our gravity (this is called ‘gravitational focussing’). This enhances the collision probability slightly.

The bottom line is that a random comet that happens to have a path that comes closer to the Sun than Earth’s orbit has a collision probability very close to one-in-a-billion. Back in 1993 I did the necessary calculations in a more-refined way, but the basic answer is much the same: around one-in-a-billion.

That would be comfortingly-small, were it not for the fact that there is a continuous influx of comets, more being discovered every year, with many trillions thought to populate the Oort cloud.

How fast do comets move?

The Greenland movie was right in its content stating that comet collisions with Earth have a level of unpredictability due to the ices sublimating from the nucleus, causing a jetting force to act and deviate the cometary path by enough to make an actual impact (and impact location) uncertain. The script was incorrect, however, in having characters say (twice, I think) that comets travel faster than asteroids.

I can see how that thought might come about, however.

An interstellar object (like the comet in the film) is distinguished from other bodies orbiting the Sun by dint of it travelling faster than the parabolic limit (see later). That is, the speed v of an object in a heliocentric orbit is given by:

v² = GM [(2/r) – (1/a)]

Here G is the gravitational constant, M is the mass of the Sun, r is the heliocentric distance at any time, and a is the semi-major axis of the object’s orbit. For an elliptical/closed orbit, a is finite and positive; for example, if we imagine the Earth’s orbit to be circular at 1 AU then both r and a equal 1, and so v² = GM/r  and v=29.8 km/sec; in fact our orbit has a small eccentricity, resulting in our separation from the Sun varying between r=0.9833 AU at perihelion (in early January) and r=1.0167 AU at aphelion (in early July), Earth’s speeds at those junctures being 30.3 and 29.3 km/sec respectively.

For a more elongated orbit (higher eccentricity), a will be larger and so the body’s speed when it is at 1 AU from the Sun will be larger. 1P/Halley has a= 17.834 AU (raise that to the power 3/2 in accord with Kepler’s third law to get an orbital period of 75.3 years) and plugging that into the above equation renders a speed at 1 AU of near 41.5 km/sec, almost 1.4 times as high as that of the Earth.

If a were infinite then 1/a is zero. That is the situation with a parabolic orbit, and comets coming in from the Oort cloud have near-parabolic orbits. Then v² = 2GM/r and so the speed at any heliocentric distance is √2 = 1.4142 times the circular orbit speed; at r=1 AU, 42.1 km/sec rather than 1P/Halley’s 41.5 km/sec. The square root of 2 times the circular orbit speed is termed the parabolic limit.

What if the speed were higher still? That is, what if we tracked a comet travelling faster than the parabolic limit? Under that circumstance the orbit is not gravitationally bound to the Sun, the heliocentric orbit is hyperbolic, and the comet is not a member of the solar system. In the velocity equation above, this corresponds to a value for the semi-major axis a that is negative. As described earlier, so far we have spotted two macroscopic objects (in 2017 and 2019) that are hyperbolic/interstellar. Plus of course the tiny meteoroids/interstellar dust detected by meteor radar here in NZ.

The above equation governs the speeds for planets, asteroids, comets, meteoroids and everything else orbiting the Sun. The same basic equation we also employ for satellites orbiting the Earth: we just use the terrestrial mass rather than the solar mass in doing the calculations. Therefore the statement in the movie along the lines that comets travel faster than asteroids is technically wrong. As I wrote earlier, I can see how it came about, though.

First, comets tend to be in larger, more-elongated orbits than asteroids, and so their values of a are generally larger and so their speeds v are bigger, though not by much. We are also discovering more and more asteroids in orbits that we might say are ‘comet-like’, with an early discovery from 1991, in the asteroid search programme I directed in Australia, being a prime example: (5335) Damocles.

Second, because they arrive from semi-random directions, comets have much greater impact speeds on Earth than asteroids, in general. That is, asteroids are mostly in prograde orbits (they travel around the Sun in the same sense as does Earth) whereas about half of comets are in retrograde orbits. When you do the sums (which I have) it turns out that typical asteroid-Earth collision speeds are around 25 km/sec, whereas the average impact speed for a near-parabolic comet is close to 55 km/sec. It’s just like driving on the wrong side of the road: head-on crashes tend to be at higher speeds than rear-enders.

The movie script gets a “tut, tut”, then – but the fudging of scientific reality here is understandable.

Fragmenting comets

Without wishing to give anything away (and it’s shown in the movie adverts, anyhow), part of the storyline in Greenland involves Comet Clarke having fragmented into multiple chunks, with some of these striking Earth. Does this make sense?

Yes, it certainly does. This is just what we witness comets doing, with some regularity. Take a look at the photograph at the head of this blog piece. That’s a Hubble Space Telescope image of a disintegrating object known as C/2019 Y4 (ATLAS), in April this year. Images obtained a few days apart show that the cometary nucleus had shattered into dozens of pieces. In passing I note that the ‘Y4’ in the comet’s designation means that it was the fourth comet discovered in the second half of December last year; I wrote earlier that there are lots of comets out there, and so the one-in-a-billion chance is not so small as it can be disregarded.

The above comet, like that in the movie, came from well beyond the planetary region (though it is – just – gravitationally bound to the Sun, and would take about 17,000 years to complete an orbit if it had not broken apart), and has an Earth-crossing orbit, approaching closer to the Sun than the planet Mercury. C/2019 Y4 (ATLAS) is therefore another one-in-a-billion bullet that we dodged.

We have seen other comets do this in the past – shatter into pieces and often disappear entirely – one example that springs to mind being 73P/Schwassman-Wachmann 3. This was discovered in 1930, but then apparently disappeared, or went dormant; that is, ices were no longer sublimating, and so the dark comet nucleus was undetected by astronomers. Then in 1995 it re-appeared with a vengeance, breaking apart and thus exposing the ices to sunlight, making the comet much brighter and unmissable. The thing about this comet is that it is in a small orbit (we would even say ‘asteroid-like’) and passes close by Earth. It produces a meteor shower most years, meteoroids released by the comet that are around 0.1-10 cm in size entering our atmosphere and causing shooting stars. But there must be bigger lumps there, and sooner or later some of them must meet our planet in a fiery explosion.

Another short-period comet that has been seen to break apart is 3D/Biela, in the middle of the 19th century. (The ‘D’ means it has disappeared.) This produced various spectacular meteor storms, and it is by no means certain that we will not experience more.

A final note on fragmenting comets. 1P/Halley is the best-known comet to the general public. It also produces meteor showers, one in early May each year, and the other just about now (the end of the third week of October). The present meteor shower is called the Orionids, because it appears to emanate from near the raised arm of the warrior constellation Orion. If you go outside in a dark location after Orion has risen after midnight, you should see several shooting stars appearing to radiate from that location in the sky, between Orion and Gemini. Better is late into the morning hours, when Orion is highest, and after the Moon has set. What this means is: though you do not have a chance to see Halley’s Comet again until 2061, you can see bits of it tonight, if the sky is clear. Some tonnes of meteoroids derived from that comet are hitting Earth every day, as I write. (For context: the influx of meteoroids and interplanetary dust to our upper atmosphere is about 100 tonnes per day, averaged over the year.)

Movie graphics

When my sons were young, if I showed them ‘classic’ movies (for them, anything more than a decade old) their comments usually were not about the storyline or the acting, but rather the imagery. In short, the usual opinion was “The graphics are crap!”

So how does Greenland do?

Take a look at this image, apparently showing the effect of a large part of Comet Clarke having struck the Earth:

Most people would say that looks realistic, methinks. Imagine that the comet strikes at the average speed for near-parabolic orbits, that is 55 km/sec. It starts to ‘light up’ at an altitude of about 100 km. The mean, mode and median impact angle for asteroids and comets hitting the Earth (or any other planet) is 45 degrees. If the projectile comes in at that angle, then between crossing the 100 km altitude and hitting the surface the elapsed time is 2.6 seconds. Yes, tick, tick, tick, BANG!

The atmosphere ‘stops’ small objects. Meteoroids smaller than 1cm cause the typical shooting stars you see at night, and those are mostly between altitudes of 70 and 100 km. Something the size of a rugby ball may penetrate down to 50 km, producing a fireball or bolide (a very bright meteor). Meteorite-dropping events are rare: the meteoroid needs to come in at close to the minimum speed possible (11.2 km/sec, Earth’s escape speed), say below 15 km/sec, arrive at a grazing angle so the deceleration in the atmosphere is gradual, and also be strong: that’s why meteorites are disproportionately made of nickel-iron, or strong rock, rather than the friable carbonaceous chondrites which are more interesting because they are so primitive, formed early in solar system history.

Look at it this way. The surface pressure of our atmosphere is equivalent to a water depth of almost ten metres. One might expect it to ‘stop’ objects up to at least that size, therefore. In fact it does better: generally we do not expect even rocky objects as large as 50 metres across to reach the ground intact. How come, when surely the atmosphere is ‘soft’? Well, think of doing a belly flop off the top diving board at your local swimming pool… the water is also ‘soft’, but that’s going to hurt a great deal, and potentially be dangerous apart from the pain.

We’ve see directly what happens when a smaller asteroid (this one about 20 metres in size) hits the atmosphere, quite recently: the Chelyabinsk event in February 2013. Not much of the projectile remained intact down to below 20 km altitude. The initial mass was about 10,000 tonnes; only a few tonnes of solid remnants (i.e. meteorites) reached the ground.

Getting back to the graphic shown above, the artists seem to have based this more on photographs of atomic bomb tests than what we might expect a large comet impact to look like. (I am reluctant to criticise, however, because the cover illustrations on both my books on this topic are also a bit un-physical.) For example, the shock wave propagating outwards from the impact location can only move at the speed of sound in air (or perhaps somewhat faster through water, and faster still through crustal rock), and so for the shock front to have spread so far as is shown there would take far longer than the terrestrial rocks thrown out of the excavated crater at a much higher speed. One can have some fun thinking about this… so, a 15-km wide comet punches through the atmosphere at 55 km/sec; it would leave a hollow tube (a vacuum, almost) that would take 20 seconds or so for the atmosphere to re-fill; but in the meantime rock from the crater can have been ejected through that tube, unimpeded. I have often pondered whether this is how tektites form: rock is melted and flies out above the atmosphere through that empty tube, and then solidify into their well-known aerodynamic shapes as they re-enter at speeds of only a few kilometres per second, or less.

This is linked to the following movie poster:

South Korean poster for the film Greenland.

Cometary fragments arriving at hypervelocity (remember the 55 km/sec: over 150 times the speed of sound) simply would not look like that. They individually light up far, far brighter than the Sun. At Chelyabinsk, many people were blinded, temporarily at least. Rather than looking dark, an impact episode would be very bright! (This is, by the way, one reason that I like to tell people that in essence the dinosaurs were grilled to death.)

The following scene is also a bit hocus-pocus, if the intent were to show fragments of the comet hitting. As noted earlier, no matter how strong they are, one does not expect lumps of comet – even if solid metal or rock – to reach the ground intact. Even a lump above 100-m in size would explode at high altitude (above 20 km, twice the height of a jetliner) if it were travelling at 55 km/sec: in such a hypersonic entry the ram pressure on the front of the projectile is much bigger than its tensile strength, and so it is smashed to pieces; each of those pieces is rapidly decelerated, dumping its kinetic energy, which at 55 km/sec is about 360 times higher than the chemical energy of TNT. Bang!

Boulders not just falling from the sky, but arriving at hypervelocity: this just does not happen.

On the other hand, perhaps the episode shown above were intended to show rocks dropping from the skies after being ejected from a distant impact crater formed tens of seconds beforehand. If that were the case, well, they would not necessarily be dropping at such a high speed. They would be falling close to vertically at terminal velocity: the same speed as if they had been dropped from an aeroplane (perhaps as much as 500 kph).

Two more posters from the South Korean version of the movie:

South Korean poster for the film Greenland: Paris and the Eiffel Tower a little the worse for wear.
A bit closer to home: the Sydney Opera House falls prey to the comet fragments.

These show both Paris and Sydney in a shocking state, but there seem to be strange things still happening in the sky despite it being clear that the major impact and blast occurred some time beforehand. But let me not quibble; much.

Time-scale of the movie

The duration of the action in the film is two or three days. In the past there have been criticisms of movies such as Deep Impact and Armageddon for having impact warning times of ‘only’ about nine months, astronomers like me saying that in fact we can predict impacts by asteroids out to decades into the future. The rejoinder to that, of course, is that it does not make for a fast-paced storyline with human dramas portrayed.

The paradox here is that two or three days is too long for the phenomena depicted. We see fragments hitting the Earth, and then another day goes by, and then another. But the Earth moves about 2.58 million kilometres per day in its orbit around the Sun. The scenario depicted would require the cometary fragments to be spread transversely by more than five million km, several times the solar diameter. That doesn’t happen, except over long time-frames. Fragments of a comet spread mostly along the orbital path, not sideways.

The comet comes from the day-side

I would need to watch the film again to keep a tally, but my memory is that Comet Clarke is depicted at times in a night-sky, and at others in daylight; for example, the small boy in the movie is shown looking into a daytime sky for the comet, using a pair of binoculars (not something to allow any child to do: one glimpse of the Sun through those and there would be permanent eye damage).

Certainly, towards the culmination of the movie there is a statement that the main comet body (the piece 9 miles/15 km wide, as mentioned earlier) is going to hit western Europe at a time said to be “8:47 Eastern Time” (in the US). That means early afternoon in Europe, and so one deduces that the comet is coming from the day-side of the planet.

This we would term a ‘post-perihelion’ impact: the comet has passed perihelion, and is receding from the Sun.

A few points connected with that:

  • Marsden and I found that a reduced warning time (i.e. discovery time-lag before impact) is likely for post-perihelion impacts, like that which apparently occurs in this movie.
  • As the comet approaches Earth, we would be within its vast coma (cloud of tenuous gas surrounding the solid nucleus) and in effect there would be little difference between night and day in terms of sky brightness: there would be coma spread tens of thousands of kilometres around Earth, back-scattering sunlight; rather than darkness and gloom (as in the movie) the sky would tend to be bright, not just due to the huge light emission caused by the large fragments entering the atmosphere.
  • Whilst many people think that a comet’s tail trails behind it as it moves through space, in fact the gas tails of comets point in the anti-solar direction: the charged atoms (ions) are picked up by the solar wind, which at 1 AU travels typically at 400-500 km/sec outwards from the Sun, and that is far greater than the comet’s speed, so the gas tail (which looks bluish) points away from the Sun… and so will be intercepting the Earth as the comet approaches from the sunward direction, producing phenomenal aurorae.
  • Comets have another tail, this one looking pinkish, which tends to be a fan between the gas tail and the direction behind the comet’s motion, and that is made of dust and meteoroids; we would expect Earth to be bombarded by such solid particles as the comet approaches, lighting up the daytime sky with vast numbers of shooting stars. (I have only ever seen one daytime meteor/fireball: whilst sat on a beach in Abel Tasman National Park in 1983.)
  • It is feasible that a comet on a path like that under discussion here would present major challenges for astronomers to track: it might be discovered in the night-time sky, but then once it dips sunward (into the daytime sky) we would have great difficulty in tracking it from the ground, so our knowledge of it would be discontinuous. In the very late stages, the final week prior to a near-pass by the Earth (or maybe an actual impact), dependent on its orbital inclination (angle of tilt to Earth’s orbital plane) it could be very close to the direction of the Sun, and so unobservable – the sort of thing that gives people like me nightmares!

Lots of good things in this movie!

Not everything in the film is wrong, scientifically-speaking. Indeed I was impressed by the attempt at realism, within the constraints of making a gripping movie. Many of the points in the dialogue are viable, as I have indicated in places above. Another example is that the Tunguska event of 1908 is mentioned, and correctly-described (unlike in some past research papers where it has variously been ascribed to a mini-black hole passing through the Earth, an anti-matter lump from space, or an alien spaceship blowing up).

One of my rules in watching any science-fiction film is to try to suspend my critical faculties, based on knowing a bit about real science. For Greenland, though, because it concerns a topic on which I have worked for years, I deliberately paid attention to the science content.

There is another thing I might mention. Any heightened public awareness of the realities of the universe is good, I think. Many people argue that all education should be ‘relevant’, but I disagree. One could say that dinosaurs are not relevant to the modern world, but kids are fascinated by dinosaurs, and so they are a wonderful trope with which to encourage learning.

Shotgun blasts rather than cannonballs

In terms of my personal philosophy and scientific research work, I think Greenland is great, because it may raise awareness of fragmented comets being a major danger to life on Earth. Mostly, scientists working in this and related areas have talked about singular asteroids striking our planet and causing mass extinction events. I think of those as being like cannonballs, and in a review in Nature in 1991 I termed that view ‘Stochastic Catastrophism’.

Along with a small group of astronomers, all of us British it happens, I have developed a picture that I termed (in the same review) ‘Coherent Catastrophism‘. We see the major risk from space as being due to broken-up comets which cause epochs during which the interplanetary environment is much perturbed and then there are intervals lasting many millennia during which multiple substantial impacts occur (shotgun blasts – much more likely to hit the Earth than a singular cannonball), and also there are recurrent atmospheric dustings causing climatic downturns. Two of my more recent papers along these lines are here and here. It happens that there is growing evidence for such things occurring during the Holocene, but that is another story yet to be told in full.

A core point to make here, though, is this… Let me ask the rhetorical question of whether it is easier to hit a clay pigeon with a rifle (a single bullet) or a shotgun (a wide blast of pellets)? I don’t need to answer that. Similarly, a large (say 10 km) comet that has disintegrated into a thousand fragments each 1 km in size has a much, much greater chance of hitting Earth with at least one of those fragments. Look at the Hubble Space Telescope image of the broken-up comet at the head of this blog post, and think of the implications for life on Earth.

A hundred Tunguskas in a day

I mentioned earlier Arthur C. Clarke’s novel The Hammer of God, and how I had assisted him with the storyline.

As he was completing the manuscript we were in frequent contact, and then in 1992 comet 109P/Swift-Tuttle (which is the parent of the Perseid meteor shower seen each August) was re-discovered, having been missing for some time. At one stage it seemed possible that the comet itself would impact Earth on its next passage through the inner solar system, in 2126, but with accumulated observations it was possible to show that it will miss our planet by about two weeks. As I have explained earlier, it is between difficult and impossible to know precisely how a comet will move in its orbit, but we are sure that it will miss.

There is a problem, though. The Earth will move directly through the point on the ecliptic where the comet passed about 15 days earlier. What this means is that if the comet is fragmenting – and we know it spawns a great deal of small meteoroids, hence the Perseid shower – then the large chunks might be anticipated to be stretched out along its orbit, mostly trailing it. Just where the Earth will be in 106 years’ time.

The final text in Clarke’s book comprises a page headlined STOP PRESS. The very last words are as follows:

“…Dr Steel adds ominously that ‘fragments calving off the comet, as has been observed in several cases, may yet present a hazard. How do you fancy a hundred Tunguskas in a day?’ “

Keep that in mind if you go to watch Greenland.