‘The heavens themselves blaze forth the death of princes’ (H6003; Julius Caesar, Act II Scene 2). In my book New Worlds for Old, it seemed appropriate to juxtapose that with Johannes Kepler’s, ‘There are as many comets in the heavens as there as fish in the sea’. Princes, take note.
It’s believed that the Solar System has a vast retinue of comets, occupying the ‘Oort Cloud’ at distances out to two light-years from the Sun. Oort believed that these comets were debris from the break-up of the planet which formed the Asteroids, but we now know that no such planet ever existed. Most astronomers believe that the cloud has been with us since the origin of the Solar System, though there are sceptics who insist that if so it would long since have been disrupted by passing stars. Various means have been suggested whereby the Sun can capture comets from interstellar space or from denser interstellar clouds. Isotope analyses from Halley’s Comet support those theorists, indicating a carbon 12/carbon 13 ratio very different from those found in the rest of the Solar System.
Wherever comets come from, every year a number of them swing past the Sun and recede again on very long elliptical orbits. Some, however, pass close to major planets and have their speed reduced. (Others are expelled from the Solar System altogether.) These form ‘families’ of short-period comets: Halley’s is one of the few members of Neptune’s. If it was one of Jupiter’s many hostages, coming back every three to five years, it would have lost its ices and become fainter much more quickly.
The Oort cloud of comets appears to be a sphere up to two light-years in radius (1.9 light-years out in the direction of Alpha Centauri), ranging from 13 billion km. from the Sun out to at least 135 billion, and containing untold numbers of icy, dusty bodies whose origins and detailed compositions remain mysterious. Within the Oort Cloud the Kuiper Belt circles the Solar System beyond the orbit of Neptune; it was the source of Kohoutek’s Comet in 1973, and is probably also the source of the ‘Centaurs’, Chiron and Hidalgo, in erratic orbits among the outer planets, and of Phoebe, the captured moon of Saturn. We don’t know if it has an outer edge or if it merges into the Oort Cloud.
Phoebe, Saturn’s outermost moon, is a very long way out, in a retrograde orbit, with high orbital eccentricity, but in the plane of the Ecliptic, whereas all the other moons are in the plane of Saturn’s equator. All this indicates that Phoebe is a captured object, but remarkably, although it’s less than 100 km in diameter, in the far-encounter Voyager pictures it appeared spherical, and dark, with a blotchy surface. It led Drs. Victor Clube and Bill Napier to suggest that Phoebe may be a captured ‘super-comet’, like the one which they suggest broke up in the inner Solar System around 3000 BC (The Cosmic Serpent, Faber, 1982).
In June 2004 Cassini passed Phoebe on its approach to Saturn. Photos showed a most irregular surface which is far from spherical, with water, methane and carbon dioxide ice, huge craters, extraordinary changes of level and a smooth crust which seems to show evidence of melting. Jay Tate of Spaceguard UK took to calling it ‘The Little Prince’s asteroid’. The presence of carbon dioxide ice strongly suggests an origin in the outer Solar System (Stephen Battersby, ‘Saturn Special: Motley Crew’, New Scientist, 14th January 2006).
At least we can be reasonably certain that the comet-like objects in the Kuiper Belt are left over from the formation of the Solar System. Indeed, F.L. Whipple suggested that the outer planets Uranus and Neptune had formed by accretion of comets, 800 million years after the rest of the Solar System had taken shape. It may even have been triggered by a passing brown dwarf star, before the cluster in which the Solar System formed was broken up (‘Did a Passing Dwarf Star Form the Ice Giant Planets?’, Astronomy Now, December 2005.)
During the history of the Solar System, other stars may have passed within three quarters of a light-year of the Sun every eleven million years, on average (Journal of the British Interplanetary Society, December 1972, p.467). The Soviet astronomer S.K. Vsekhsvyatskiy calculated that as many as 10,000 stars may have passed within 0.6 psc of the Sun, around the orbital distance of Pluto, during its history, and as they would have grazed the fringe of the Oort cloud of comets at that distance, it would be completely disrupted unless it was frequently or continually replenished. (S.K. Vsekhsvyatskiy, The Nature and Origin of Comets and Meteors, ‘Prosveshcheniye’ Press, Moscow, 1967, NASA Technical Translation TT F-608, US Government Printing Office, April 1970). The last star to enter the Oort Cloud was the red dwarf/brown dwarf pair Scholz’s Star, 70,000 years ago, and the next is expected to be the red dwarf Gliese 710, 1.4 million years from now. (Tim Haines & Christopher Riley, Space Odyssey, BBC Books, 2004.)
It’s hard to see how our Kuiper Belt and Oort Cloud of comets could have remained stable during all that, unless we exchange comets with passing stars, or pick up new ones as we pass through the central plane of the Galaxy (Victor Clube & Bill Napier, The Cosmic Serpent, op cit). Remarkably enough, the answer to that may already be in our hands, because NASA’s Stardust probe successfully flew past Comet Wild 2, capturing dust samples in aerogel material, and those were returned to Earth for analysis in January 2006. First results were even more surprising, showing that the comet contained material which had come from near the Sun (or some other star?)
From the invention of the telescope until 2017, there had never been a passing object which was unequivocally from interstellar space. My most successful short story, ‘The Comet, the Cairn and the Capsule’, was set on a mission to a comet which was a priority for interception because it was the first to be detected on an unquestionably hyperbolic trajectory, i.e. it was travelling at over the escape velocity of the Solar System and had definitely come from outside. My characters were to deposit an instrument package on the nucleus before returning with samples, but when they reached it, through large shoals of floating rock and ice, they found a ‘cairn’ of similar packages deposited by other civilisations whose planetary systems the comet had passed through. In addition to adding their package to the cairn, my astronauts carved a set of pictures on to the payload fairings to tell any future finders who we were and where we were.
I lost the chance to be hailed as a prophet for that, because Eric Burgess and Carl Sagan proposed their famous plaque for Pioneer 10 before the story was published. But the first interstellar object to pass through, named ’Oumuamua after the Polynesian word for ‘Scout’, turned out indeed to be so strange that Prof. Avi Loeb of Harvard has suggested in all seriousness that it may have been artificial, specifically a solar sail, and further analysis suggests that it was under active control and on a definite exploratory mission, going on to Ross 248, which will be the nearest star to us in 29,000 years’ time.
Avi Loeb has explored his idea in detail in his book Extraterrestrial, The First Sign of Intelligent Life Beyond Earth, John Murray, 2021. I’ve gone into the spaceflight aspects of the question in a paper for the online journal Concatenation, which can be found at http://www.concatenation.org/science/lunan-oumuamua-visitor.html. My story ‘The Comet, the Cairn and the Capsule’ was reprinted in my collection of spaceflight stories, From the Moon to the Stars, Other Side Books, 2019, along with my first published story, ‘Derelict’, which was about an abandoned starship entering the Solar System. It was quite remarkable that they both became topical again at the same time.
The second interstellar object to turn up, Comet Borisov in 2019, was unquestionably a comet, and turned out to have similarities to Comet Hale-Bopp (1996), because both appear to have formed in the outer reaches of their systems, but incorporating material originating from much further in, and have seldom if ever passed close to a star before. But it’s enough to disprove the argument above that all the comets we see are from interstellar space, because Hale-Bopp is definitely ‘one of ours’.
One common misconception is that tails follow comets through space. In fact, they always point away from the Sun, so comets draw away from the Sun moving tail-first. But it’s also not true that comets’ tails are pushed away from the Sun by the Solar Wind: although that outflow from the ‘coronal holes’ in the Sun’s outer atmosphere has a major effect on the Earth’s magnetic field, the physical pressure it exerts on gas molecules (or on solar sails) is much less than the pressure of sunlight.
By the 1970s it was known that viruses could survive in vacuum and could resist heavy doses of radiation. It gave new life to the concept of panspermia, popular in the early 20th century, which suggested life might have been brought to Earth on meteorites. In the abstract near-final sequence of 2001, a Space Odyssey, there’s a horizontal split-screen shot, with the lower half returning to the data flow which began the sequence, but hovering over it are diamond shapes, constantly mutating – viruses, at the interface between living and non-living matter, and panspermia in action, travelling between the stars in crystal form to bring life to new worlds. The late Sir Fred Hoyle and Prof. Chandra Wickramasinghe argued that both bacteria and viruses are widespread in the Universe and are transferred to planets by cometary impacts and even by escaping from comets and falling into the atmosphere. [Lifecloud, J.M. Dent, 1978, followed by “Diseases from Space” (1979), Evolution from Space (1981), and Sir Fred’s own The Intelligent Universe (Michael Joseph, 1983)]. Biologists have not accepted their arguments, but Prof. Wickramasinghe is continuing the campaign.
Prof. Graham Cairns-Smith suggested in his book Genetic Takeover (Cambridge University Press, 1982) that the first living creatures on Earth had been highly evolved clays, a very different form of ‘silicon-based life’ from the normal high-temperature science fiction concept; as the late John Macvey pointed out, such creatures would exhale solid quartz and that might be a touch painful (Whispers from Space, Macmillan, 1974). Cairns-Smith suggested instead that his living carpets of clays evolved to a level of complexity at which they began to use our kind of organic molecules for specialised purposes – and then we stole the planet from them.
There have been signs of clays in meteorites, suggesting that such life could even have evolved in the comets, and similar materials were detected when NASA’s Deep Impact probe struck Comet Tempel 1 in July 2005 (Alan Longstaff, ‘Liquid Water in Tempel 1’, Astronomy Now, November 2005), while the amino acid glycine was found in samples returned to Earth from Comet Wild 2 by the Stardust probe in 2006 (Nancy Atkinson, ‘Amino Acid Found in Stardust Comet Sample’, Universe Today, August 17th, 2009). So far, no two comets which have been visited by spacecraft have been found to be alike, and that may mean that their origins are equally diverse.
(To be continued)
See also the archived astronomy articles by Duncan Lunan including The Sky Above You – December 2021