Impact Events

Abstract

Dinosaurs died out because of a cosmic collision: an asteroid hit Earth and caused havoc. Could a similar fate be in store for humanity? Could death come from the skies? H.G. Wells considered the possibility more than a century ago. We now know this particular scenario is unlikely, but other equally destructive events are almost certain to happen. The question is: when? And: what can we do about it?

Appendices

Commentary

H.G. Wells pioneered many science fiction themes: time travel, invisibility, malevolent aliens—and, in this story, impact events. The science in “The Star” has dated but the story’s core message—that a celestial body could cause catastrophe here on Earth—is even more plausible now than it was when Wells wrote his tale in 1897.

In terms of the astronomical knowledge of his day, the overall setting Wells describes in “The Star” is essentially correct. Consider, for example, his description of the outer solar system. He was right in saying the outermost planet is Neptune: the planet had been discovered half a century earlier, in 1846. Furthermore, just 17 days after Neptune was first identified, astronomers discovered it had a satellite. We now call the satellite Triton, but that name didn’t come into common parlance until the 1930s. Up until then scientists referred to it in the same way Wells refers to it: “the satellite of Neptune”. There are now 14 known satellites of Neptune, but it wasn’t until 1949 that astronomers found a second Neptunian moon. Wells was faithful to the science as it then stood.

Or consider the description Wells gives of stellar distances. By 1897, the distance to about sixty stars had been determined using the method of parallactic shift. So Wells, in his story, was thus giving an accurate feel for the immense distances separating the stars.

Wells was less convincing in his description of the effects of the trespassing star on the orbital dynamics of the solar system. He was also far from convincing about the nature of the star itself. Indeed, in some places in the story it’s not clear whether Wells thinks of the interloper as a star or a planet. Whatever Wells imagined, though, we now understand that no star is going to barrel in from outer space and disrupt the solar system. Most stars move through space with a velocity, relative to the Sun, of a few kilometres per second. At this speed, even if the nearest star happened to be on a collision course with the Sun (and it isn’t) then we’d be waiting for a million years for the crash to happen. We wouldn’t be taken by surprise. It’s true that astronomers have identified a few so-called hypervelocity stars, which move at speeds of up to 1000 km/s, but even if the nearest star were a hypervelocity star (which it isn’t) and heading straight for us (which it isn’t) there’d still be a delay of one thousand years before it reached us. The notion that a star could appear seemingly from nowhere and then, within a few days, disrupt the solar system is not credible.

Nevertheless, although his description of a wandering star disturbing the clockwork mechanism of our solar system was wrong, Wells was right to highlight the possibility of life on Earth being disrupted by celestial bodies. And in this he proved himself to be farsighted. The science of his day, which was coming to terms with the implications of Darwin’s theory of evolution, was emphasising gradual—almost imperceptible—change. The notion of catastrophic change was frowned upon. But we now know catastrophe can indeed come from the skies.

About ten years after Wells published “The Star”, a mysterious event occurred near the Stony Tunguska River in Siberia. An explosion flattened more than two thousand square kilometres of forest (see Fig. 12.1). Few people lived in Tunguska, so no casualties were reported; had the same event occurred in London or New York, millions of people might have died. The explosion was an impact event—the air burst of a meteoroid. The meteoroid disintegrated before it hit Earth’s surface, so it’s difficult to know for sure what the object was. The best guesses, however, are either that it was an asteroid about 60 metres in diameter or a comet about 190 metres in diameter.

Fig. 12.1

A photograph taken by a member of the 1929 expedition to investigate the Tunguska event. The explosion flattened vast swathes of forest: it’s estimated that 80 million trees were toppled. The energy release involved in this event was much greater than when the first atom bomb exploded over Hiroshima (Credit: Public domain)

The Tunguska event itself might not have been a catastrophic occurrence, but over recent decades scientists have come to appreciate that similar, much larger, events have influenced the development of life on Earth.

Buried beneath Mexico’s Yucatán Peninsula is a crater called Chicxulub, which gets its name from a nearby town. The crater was the result of an impact event, a collision involving an asteroid or comet about 10 to 15 kilometres in diameter. The Chicxulub impactor was much bigger than the Tunguska impactor, and the effects were bigger too. As with Tunguska, no people died when the meteoroid hit Chicxulub—but only because no humans were around at the time of the impact. The collision happened about 66 million years ago, long before humans were on the scene. The dominant animals back then were dinosaurs, and the worldwide climate disruption caused by the impact helped them on their way to extinction. Indeed, Chicxulub caused a mass extinction event: about three quarters of all plant and animal species died out. This was a clear case of destruction raining down from the heavens. If a similar sized meteoroid struck Earth today, humans would almost certainly suffer the same fate as the creatures who were around when the Chicxulub impactor struck.

We now know that the history of life on Earth is not just the story of natural selection operating gradually and infinitesimally slowly over long aeons. Our planet gets peppered with space rocks—most of them small; a few of them large; an occasional monster—and these impacts have the capacity to alter the course of evolution. Indeed, in 1984 an influential paper claimed that, if one examines the fossil record in detail, it’s possible to discern a periodicity in mass extinction events: they seem to occur at intervals of about 26 million years. Two groups of astronomers then postulated a mechanism that could explain such a cycle. Suppose the Sun had a companion—a small red dwarf or brown dwarf star, orbiting the Sun at a distance of about 1.5 light years. Such a companion would exert a gravitational tug on the Oort Cloud of comets, disrupting orbits and increasing the number of comets falling into the inner Solar System—and thus increasing the likelihood of an impact event on Earth. This hypothetical companion star was given the name Nemesis.

Numerous surveys have failed to detect the existence of any Nemesis-like object orbiting the Sun, and more recent analyses of the fossil record suggest the claimed periodicity in mass extinction events was a statistical artefact. But the matter isn’t entirely settled—and so it’s interesting to speculate that a star might well be the cause of death and destruction here on Earth; not a wandering star, as Wells envisaged, but a constant companion star to the Sun.

Science has updated another element of the “The Star”. Wells, when discussing the vastness of the cosmos, stated that “no matter had ever to human knowledge crossed this gulf of space”. Well, that was true when Wells was writing. But in 1911 Victor Hess began a series of experiments that led to the discovery of cosmic rays—particles from space that bombard Earth’s atmosphere. We now know that some cosmic rays can originate from far outside the solar system; indeed, some of the highest energy cosmic rays originate from outside our Milky Way galaxy. And in November 2017, as I began to re-read Wells’ story for this volume, astronomers provided details of a visitor from another solar system—not a mere subatomic particle but a piece of rock. A rock that had indeed crossed the “gulf of space”.

The International Astronomical Union called the object 1 I/2017 U1; you can also refer to it as ’Oumuamua, which means “scout” in Hawaiian. No object like ’Oumuamua had ever been seen before in the solar system. It is relatively small, extremely dark, and unremarkable except for its highly elongated shape (it’s a spear of rock about 400 metres long, very much along the lines of how I visualise the mysterious craft in Arthur Clarke’s Rendezvous with Rama; see Fig. 12.2). What sets ’Oumuamua apart is its velocity. It’s moving so fast, relative to the Sun, that it can’t have originated in the solar system and it won’t be captured by the solar system. ’Oumuamua came from a distant planetary system (it’s not known which) and it will soon leave our solar system. Although ’Oumuamua is the first such interloper to be found, some estimates suggest that on average three such objects enter our solar system each day and three leave each day. Fortunately, ’Oumuamua is not on a collision course with Earth. But if a similar object were to strike Earth, the damage to our civilisation might be worse than anything Wells described. And we would have much less time to prepare for our end than the characters did in “The Star”: the dark red surface of ’Oumuamua absorbs 96% of the light that falls upon it, and if that is typical of these interstellar interlopers then we’d have a hard time spotting the object before it struck.

Fig. 12.2

An artist’s impression of the first detected interstellar asteroid, which was discovered on 19 October 2017 by astronomers using the Pan-STARRS 1 telescope in Hawaii. The spear-shaped object is dark, about 400 metres long, and was wandering through space for millions of years before its chance encounter with our Solar System (Credit: ESO/M. Kornmesser)

We shouldn’t be surprised that Earth is subject to impacts: look up at the Moon and you see a surface pocked by innumerable craters, the result of countless impacts that have happened over the past four billion years. The Moon isn’t some sort of magnet for incoming meteoroids. The reason we see so many craters on the Moon is because our satellite has no weather systems to erode them, no system of plate tectonics to erase them. Earth is struck by meteoroids with the same frequency as the Moon (indeed, we believe the Moon itself was created when Earth was hit by a Mars-sized object) but Earth’s various systems tend to expunge the resulting impact craters.

Meteoroids have struck Earth in the past and they’ll strike Earth in the future. Indeed, if you consider the statistics, you’re more likely to die from a meteor strike than you are from a lightning strike. There isn’t much chance of a large asteroid dropping on us any time soon—but if one did impact then all seven billion of us would die. Multiply the small chance of a strike happening with the total devastation caused by such an impact and you get a result that is far from negligible. We insure against many events that are unlikely to happen; surely our civilisation could develop a better guard against asteroid strikes? It would be money well spent.

To end on a truly depressing note: the cosmos offers existential threats against which we have no control. Consider, for example, a supernova—the final explosion of a high-mass star. A supernova releases vast amounts of radiation. If a nearby star went supernova then many life forms here on Earth would be threatened. Indeed, some scientists have suggested that supernovae may have been responsible for at least some of Earth’s past mass extinctions. Or consider a gamma-ray burst—the most powerful sort of explosions in the universe, which can result from either a neutron star merger or the collapse of a rapidly rotating, high-mass star. If Earth found itself in the jet of radiation coming from a nearby gamma-ray burst then the planet would be toast. If you are particularly neurotic then you might also want to worry about the possibility of false vacuum collapse. Our universe might be stuck in a situation in which the vacuum is not in its lowest energy state. If that’s the case the universe could tunnel from the false vacuum to a state of lower energy. But there’s little point in fretting about this particular threat. If the false vacuum did collapse then it’s entirely possible that all matter would be destroyed instantaneously and without warning. You wouldn’t feel a thing.

Notes and Further Reading

• the outermost planet is Neptune—For details of the discovery of Neptune see, for example, Standage (2000).

• the method of parallactic shift—Measuring a stellar parallax involves carefully recording the position of the star, relative to the background of distant fixed stars. If the star’s position is observed in January and July, say, then that six-month separation means the star is being observed from vantage points separated by the diameter of Earth’s orbit around the Sun. If the star is nearby then it’s apparent position will shift, just as a pencil held at arm’s length shifts its position relative to the background when you observe it first with only your left eye closed and then with only your right eye closed. From the size of the parallactic displacement of an object you can determine the distance to that object. See Webb (1999) for further details of this and other methods for measuring astronomical distances.

• a mysterious event occurred near the Stony Tunguska River—I first learned of the Tunguska event from Baxter and Atkins (1975). This book is still worth reading, but Rubtsov (2009) gives a much more in-depth of the explosion and what might have caused it.

• a crater called Chicxulub—For further details of the Chicxulub impact, and other impact events, see for example Verschuur (1997).

• a periodicity in mass extinction events—The periodicity was claimed by Raup and Sepkoski (1984), who examined the fossil record and identified 12 extinction events over the past 250 million years.

• a mechanism that could explain such a cycle—The Nemesis hypothesis was proposed independently by Whitmire and Jackson (1984) and Davis, Hut, and Muller (1984).

• a statistical artefact—Bailer-Jones (2011) found no evidence for periodicity in Earth’s impact record, and thus removed the need to hypothesise the existence of Nemesis. Furthermore, no sign of a companion star to the Sun as surfaced in any astronomical survey.

• the object 1 I/2017 U1—For details of the initial discovery, see Meech et al. (2017). The resemblance of ’Oumuamua to the spaceship Rama, as described by Clarke (1973), made it inevitable that astronomers listened for possible alien signals! But nothing was heard.

• a truly depressing note—For a fun guide to some possible routes to Armageddon, see Darling and Schulze-Makuch (2012).