（Communications Research Laboratory, Kashima）
Based on a talk given to the Japan Spaceguard Association on 98／02／22
Significant impact events on the Earth’s environment have occurred within living memory. The Tunguska object of 1908, perhaps 50 metres in size, entered the Earth’s atmosphere over central Siberia, producing a fireball as bright as the Sun and finally exploding a few kilometres above the ground over the region of the Tunguska river. The blast from the explosion flattened trees over an area of more than 2000 square kilometres.The energy released was equivalent to the energy contained in approximately 20 million tons TNT explosive, a thousand times the energy of the Hiroshima bomb. An iron object that exploded over the Sikhote-Alin region in the far east of Russia in 1947 was the second biggest known impactor of this century, over a hundred craters from 1 to 14 metres in diameter being formed.
Astronomers have calculated how often, on average, impacts of various sizes can be expected to occur. For example, a Tunguska-sized impact is expected every century or so, on average, whereas an object large enough to produce the crater detected on the Yucatan peninsula of Mexico, and (it is believed) kill all the dinosaurs, comes perhaps every hundred million years.
For simplicity, astronomers often assume that these impacts are completely random (independent) in time. We may ask whether this assumption is true or whether there are times when there are many impacts, and times when there are fewer impacts. Variations during human history could be interesting.
Anyone who watches meteors knows that they do not arrive randomly in time. On some nights every year, the Earth passes through a stream of meteoroids and a meteor shower occurs, with larger numbers of meteors than on average nights. Occasionally, the Earth passes through an unusually dense concentration of particles and there is a meteor storm. The meteors that we can observe are generally small particles (a centimetre, for example) burning up in the atmosphere.
There is reason to believe that larger objects (of Tunguska size and above) are, like meteoroids, concentrated in space, so that the Earth will occasionally pass through a swarm of these objects. The argument is based onthe idea of large, disintegrating comets.
The collision of the fragments of periodic comet Shoemaker-Levy 9 with Jupiter in 1994 not only gave everyone a direct illustration that comets can impact planets, but also showed that the splitting of comets can occur, and lead to several collisions in a short space of time. Cometary splitting can happen for example if a comet passes extremely close to a planet: Shoemaker-Levy 9 was torn apart by Jupiter’s gravity when it passed near Jupiter in 1992. There are dozens of known examples of comet splits, including the near-Earth comet Machholz 2, discovered soon after the Shoemaker-Levy 9 impacts, and found within a few weeks to have several large fragments. Comet Biela, discovered last century by an Austrian army officer, was notable because firstly, it was seen to split into two large fragments, and secondly, it later, after disintegrating completely, gave rise to spectacular meteor storms (the Andromedids). Split comets can be a danger, because even if the original comet’s orbit misses the Earth, a large fragment can be on a different orbit and hit the Earth.
Although comets of average size are more numerous than really large comets, one giant comet contains more mass than all the smaller ones combined. Therefore it is the largest comets (above 100 km, say) that are most significant as regards perturbing the Earth’s environment. Overall, such a giant comet is captured (by the planets’ gravity) into the inner solar systemwith an average interval of about 100 thousand years (although this interval is highly variable). Thereafter the giant comet will split and fragment over many thousands of years, during that time depositing much material on Earth.
It seems that there is a giant comet whose disintegration products have been present over human history, and are still present in near-Earth space. There is a big meteoroid stream, the Taurids, producing meteors in October and November. Large numbers of meteors can be detected using radar, and these meteors do not arrive randomly in time or space. Instead, it seems that half of all meteors may come from a broad stream surrounding the Taurids. This suggests that the Taurid stream is debris from an extremely large comet.
Comet Encke is one of the most well known comets, orbiting the Sun in only 3.3 years (the shortest time for any known comet), and having been one of the earliest successful predictions (by the German astronomer and mathematician Encke about 200 years ago) of the return of a comet. Encke, another comet (called Helfenzrieder, observed in 1766), and several asteroids (which may be extinct comets), appear to be part of the Taurids. The direction in the sky that the Tunguska object came from suggests that it also could be related to Comet Encke and the Taurids.
In addition, dust in the inner solar system is gradually lost because dust particles collide with each other, and when they are small enough, they are blown away by the pressure from solar radiation. However, the dust that is observed must be supplied from somewhere: probably much of it comes from the giant comet.
Thus an originally giant comet (which has now lost most of its original mass) is the parent of the Taurid stream. The parent object is expected to release both small pieces (of the size that produce most of the meteors we see) and larger fragments (up to Tunguska size and larger). The fragments will be concentrated near the orbit of the parent before they gradually spread out as time passes. Concentrations of material near the parent’s orbit are seen in other meteoroid streams, and the Infra-Red Astronomical Satellite found a bright trail of dust concentrated close to the orbit of Comet Encke. Tunguska-sized objects could easily be present in the dust trail.
Therefore, although the Taurid stream is very broad, at the core of the stream we expect a concentration of meteoroids and cometary fragments, including objects of Tunguska size. As a result of Jupiter’s gravity, the orbit of this central concentration changes over thousands of years, sometimes being brought into intersection with the Earth’s orbit (Figure 1). That is, in 3 dimensions, the central concentration usually misses the Earth’s orbit, but there will be times when it intersects the Earth’s orbit. At these times, the impact frequency on the Earth is much higher. Assuming the parent object is closely related to Comet Encke, it seems that the last intersections were around AD 300 to 500, and before that somewhat earlier than 2000 BC (Figure 1), although there is some uncertainty in these calculations. The most recent intersections may have been a couple of centuries earlier, and the intersections before that could have been as early as 3000 BC. Despite the uncertainty, the calculations always show that there are `dangerous times’ lasting a couple of centuries, when there is a greatly increased impact frequency, followed by a few thousand years when the Earth’s orbit is not intersected by the Taurid core. The next intersection in the future will be around AD 3000.
The fear of comets that many cultures have had in history may have had a basis in occasional fragmentation events or Tunguska bombardments. In various cultures there are legends of rocks falling from the sky and being associated with fires and deaths. A British priest named Gildas recorded what was apparently a catastrophe in Britain in the 5th century AD (cf. Figure 1), after which there was a significant migration of people to northern France. This time also marked the decline of the Roman Empire and the start of the European Dark Ages.
The timings of the Earth intersections before that are uncertain, but are around the 3rd millennium BC. The astronomer Dr Duncan Steel has suggested that the building of many ancient monuments, including the well known Stonehenge in Britain and the pyramids in Egypt, was related to the existence of an unusually active sky at the time. He has shown how the past orbit of the Taurid core (different from the present orbit) could relate to the original orientation of Stonehenge.
Dr Ichiro Hasegawa’s research into fireballs recorded in China and Japan shows certain times of greatly increased fireball activity over the past 2000 years. For example, a fireball enhancement is found at the time of the Dark Ages. There are further enhancements, some of which may result from occasional large fragmentations in the Taurid stream (there is evidence for this based on variations between months). Dr Hasegawa has noted that a late 18th century fireball enhancement corresponds to the time when Comet Encke became active. Maybe Encke fragmented from the Taurid parent then. Dr Victor Clube has identified events signifying difficult times for European society, corresponding to all the times of increased fireball activity.
Astronomers are not yet agreed on whether the impacts of Tunguska and larger objects are completely random in time. However, the idea that there are concentrations in space due to fragmented comets, leading to episodes of Tunguska bombardment on Earth, has a basis in astronomical observations and calculations. The importance of the Taurids now and in the astronomically recent past may be supported by historical evidence, although such evidence is necessarily speculative. There may also be evidence from studies of climate, since the input of enough material from space can affect the Earth’s temperature, as some dust particles take years to descend through the atmosphere, and atmospheric dust can reflect sunlight back into space. Overall, it seems likely that the sky was different a few thousand years ago compared to how it is today, and it can be expected to become different again in the future.
Further reading:
V. Clube & B. Napier, The Cosmic Serpent (Faber & Faber, London, 1982); Japanese translation by S. Yabushita
J. Gribbin & M. Gribbin, Fire on Earth (Simon & Schuster, London, 1996); Japanese translation by S. Isobe, H. Yano & M. Yoshikawa
Figure 1
Changing orbit of the core of the Taurid stream, showing when the Earth’s orbit is intersected. The parts of the orbit respectiely above and below the ecliptic are shown as thick and thin lines. The timescale is AD. In these diagrams, Earth intersection epochs occur around AD 300-500 and2000--2200 BC.