In 1908 something from space came into the atmosphere over Tunguska, Siberia, and while still high above the ground, it exploded.
Trees were flattened for many kilometres and glasses rattled on bar shelves in Paris, thousands of kilometres away. It has been suggested that the object was a comet, a lump of dusty ice a kilometre or two in diameter.
If that explosion had happened over Western Europe the result would have been a colossal disaster. This story fits the medieval idea of comets being harbingers of disaster. In Shakespeare's play Julius Caesar, a comet appeared in the sky as a herald of the assassination. "When beggars die there are no comets seen, but the heavens themselves blaze forth the death of princes".
However, although cometary impacts can be bad, they can be good. Without them the Earth could arid and lifeless.
When the Earth formed some 4.5 billion years ago, from the accretion of lots of smaller lumps of material, it was hot and molten. This was inevitable, since the energy from the collisions was released as heat. If water were present, it would have been in the form of superheated steam. This would rise to high altitudes and be stripped off by the vigorous wind of the young Sun. The temperatures were far too high for the complex organic chemicals forming the chemical building blocks of life to exist. So scientists have suggested that the water and the complex, organic chemicals had to arrive after the Earth had cooled enough for them to accumulate on our world's surface without breaking down. One possibility is that the water and the chemicals arrived later, by comet. These objects are means of delivering ice and other materials from the outer Solar System to the Sun's near neighbourhood, where we live.
The big, dark, cold clouds between the stars contain a mixture of gases, ice, dust and organic chemicals. These are the result of the reaction over millions of years of the waste products from the energy production of long-dead stars. These chemicals include water, methane, hydrogen cyanide, alcohol, ammonia and so on. If you put a mixture of such materials in a bottle, and pass an electric discharge through it, simulating what might be happening in the atmosphere of a young planet, you obtain aminoacids, the building blocks of life.
In the outer Solar System out beyond Pluto, there is still a lot of left-over construction material from the construction of the Sun and planets. Far from the Sun, it has been preserved, deep-frozen for billions of years. Occasionally, something disturbs a lump of it, throwing it on a course towards the Sun, where it could ultimately collide with the Earth or another planet. So repeated comet impacts may have brought to the young Earth water and the raw materials for getting life started. We know life appeared possibly as soon as the Earth's surface had cooled enough. We find remains of primitive living things in rocks 3.5 billion years old, only a billion years or so after the Earth formed. These creatures changed little until about 500 million years ago, when there was an almost explosive appearance of countless new and more sophisticated living creatures. This important role for comets is why we are so interested in them, and have sent spacecraft for increasingly close looks at any of these objects coming into our neighbourhood. The Rosetta spacecraft, currently exploring Comet 67P, has sampled the water evaporating from it. The results so far suggest that the Earth may not have got most of its water from comets, but we may well have acquired our organic chemicals from them. However, although we may have gained much from cometary impacts in the past, we are currently not looking forward to any more of them any time soon.
Saturn lies low in the sunset glow. Venus. Mars and Jupiter will be low in the east by 4 a.m. Mars and Jupiter lie very close together. The Moon will reach First Quarter on the 20th.
Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton.