Would there be time to blow up or divert the incoming object, or would it be discovered too late?
Duncan Steel argues that at the present minuscule rate of public expenditure, “it would take perhaps 500 years to complete the search for all the Apollos larger than one kilometer, and longer for the Atens. Thus if one has ‘our number’ on it for the year 2025, we would most likely not find it ahead of time.”53
In an official document dated 19 February 1997, NASA notes that “cosmic impacts are the only known natural disaster that could be avoided entirely by the appropriate application of space technology.” In the same document NASA then goes on to admit:
The only technology we have today for defense against asteroids and comets is nuclear, and we would require years of warning in order to deflect or disrupt a threatening object…. The truth is that if we found an asteroid headed our way with less than several years’ warning, there is nothing we could do to protect ourselves except evacuate population from the impact site.54
What would it cost to get those “several years’ warning”?
According to a 1991–1992 NASA study, “All potential Earthimpactors down to one kilometer in size could be discovered and tracked in a program costing $300 million spread over five years.”55 A follow-up study, chaired by the late Eugene Shoemaker of Lowell Observatory and completed in 1995, concluded that advances in astronomical imaging systems could allow such a survey to be completed in ten years at a total cost of less than $50 million.56
The reader will recall that in 1994 Congress instructed NASA to identify and catalogue all Earth-crossing asteroids greater than one kilometer in diameter within ten years.57 We were baffled to discover that no such program had been launched by the beginning of 1998 and that NASA’s support for asteroid and comet search programs was at that point still limited to about $1 million a year.58
The “asteroid threat” remains an underresearched and largely unknown quantity. Assessments of it tend to be complacent—hence, we suppose, NASA’s lethargy—and yet such assessments are inevitably founded on the extremely narrow database of present knowledge about asteroids.
How can scientists and governments be sure that the little they have managed to learn so far is not hopelessly unrepresentative of the overall picture?
What level of real certainty is there that Earth is not about to share the dreadful fate of Mars?
In the next chapter we will consider comets, which the Chinese knew as “vile stars.”59 “Every time they appear,” wrote Li Ch’un Feng in the seventh century, “something happens to wipe out the old and establish the new.”60
22
Fishes in the Sea
JOHANNES Kepler, the seventeenth-century astronomer and mathematician, once exclaimed with perceptive wonder, “There are more comets in the sky than there are fishes in the sea!”1
We do not know how many fishes there are in the sea, but since the 1950s increasingly refined observations have led astronomers to a mind-boggling conclusion: there are at least 100 thousand million (100 billion) comets in the solar system at any one time, stored in two huge reservoirs that are known—after their discoverers—as the Oort cloud and the Kuiper belt.2
The Oort cloud, the more distant of the two, lies at the extreme limit of the Suns gravitational domain, a full light year out—50,000 times the distance between the Sun and Earth.3 Its form is that of a spherical “shell” entirely enveloping and surrounding the rest of the solar system. A number of astronomers are of the opinion that it may, on its own, contain 100 billion comet nuclei: “Most [are] between 1 and 10 kilometers in diameter, although some may be much larger.”4
Exactly how much larger, or how plentiful such objects really are, nobody is yet in a position to say; they are too far away from us to be seen by even the most powerful telescopes. It is entirely possible, however, that huge numbers of Oort cloud bodies could be more than 300 kilometers in diameter.
This has already been proven observationally to be the case with comets in the Kuiper belt—a flattened disk-shaped formation that lies beyond the orbit of Neptune. The Kuiper belt is very remote—its outer edge is almost fifty times farther than the distance from the Sun to Earth—yet it is still a thousand times closer to us than the Oort cloud.
Since the 1970s the astronomers Victor Clube and Bill Napier have been developing and refining a theory concerning the occasional penetration and destructive fragmentation within the inner solar system of what they call “giant comets,” which are hundreds of kilometers in diameter rather than a few tens of kilometers or less, such as those we usually see.5 While this theory was based on pure logic and calculation, it did not initially receive wide support from other astronomers. Today it is universally accepted. This is because Clube and Napier have been vindicated by telescope observations of the Kuiper belt, which has been proved to contain objects of exactly the sort they had predicted.
The first Kuiper belt object to be detected—1992 QB1—has a diameter of 250 kilometers.6 Other massive finds include 1993 FW, again about 250 kilometers,7 and 1994 VK8 and 1995 DC2, which both have diameters of about 360 kilometers.8 Recent observations have confirmed the impression that such objects may exist in very large numbers. By March 1996 more than thirty of them had been found,9 and in January 1998 Victor Clube told us that the Kuiper belt is literally “full of giant comets! They’re the only things we can see, actually—it’s so far away. They’re all a few hundred kilometers across.”10 Such discoveries have led to a widely accepted estimate that
there may be at least 35,000 objects larger than 100 kilometers in diameter orbiting in this region of the solar system just beyond the orbit of Neptune.11
Indeed it is a sign of how influential Clube and Napier’s work has become that a number of astronomers now consider Pluto, with its unusual elliptical orbit, to be nothing more than an extremely large Kuiper belt object—a former comet that has become a planet. Clyde Tombaugh, who discovered Pluto in 1930, is one of the supporters of this view and now calls it the “king of the Kuiper belt.”12
COMET-ASTEROID CROSSOVER
Another interesting possibility, which Victor Clube and others have investigated, is that certain large “asteroids” may also be Kuiper belt comets—perhaps in a temporarily “dormant” state—that are gradually falling into the inner solar system.13 “After about ten million years,” explains David Brez Carlisle, “the trajectory of anything orbiting in the Kuiper belt decays into chaos, generally into a quasi-elliptical orbit that [will ultimately bring] it into the zone of the stony planets.”14
Can comets be asteroids? Can asteroids be comets?
Like so many categories used by scientists, it turns out that the distinction between the two is not clear-cut. From various authorities the notion has entered popular culture that asteroids are formidable rocky obstacles whereas comets are “dirty snowballs.” The renowned British astronomer Sir Fred Hoyle strongly disagrees with the second part of this idea:
Comets are not just dirty snowballs. No dirty snowball at a temperature of minus 200 degrees centigrade ever exploded as comet Halley did in March 1991. Dirty snowballs are not blacker than jet black. On March 30–31, 1986, comet Halley ejected a million tons of fine particles, which on being warmed by the Sun emitted radiation characterized by organic materials, not dirt as one understands dirt.15
Whether it is a dirty snowball—or something more—an object is likely to be classified as a comet if astronomers observe that it has the following characteristics:
An extremely eccentric (as opposed to more or less circular) orbit, bringing it close to the Sun and then taking it far away again.
A volatile chemical composition that produces jets of gas, a large luminous cloud—“coma”—around the frozen central nucleus, and frequently a “tail” consisting of glowing particles blown away from the comet by the solar wind (with the result that the tail always points away from the Sun irrespective of the direction that the comet is traveling in).16
With
regard to the first characteristic—eccentricity of orbit—new discoveries have revealed a growing number of glaring exceptions to the rule. These include objects that are unmistakably comets in terms of their general appearance and volatility but that nevertheless move in near-circular orbits as asteroids do (the six comets of the Hilda group, for example).17 Conversely, we saw in chapter 21 that many asteroids have extremely eccentric orbits and that some, such as as Damocles, Oljato, and Phaeton, are already suspected as comets in disguise.
Damocles has “an elongated, high-inclination orbit that would classify it as an intermediate-period comet except that it shows no signs of outgassing, seeming to be totally inert.”18 Phaetons orbit also has curiously comet-like properties, and during the 1990s the previously dormant Oljato was observed to have become volatile—showing signs of “weak outgassing” and even a faint tail.19
Another likely case of mistaken identity among these Earth crossers and near Earth crossers is the 10-kilometer Apollo asteroid Hephaistos, now regarded by increasing numbers of astronomers as a “spent” fragment of a giant comet.20 Indeed, Victor Clube and Bill Napier maintain that many Apollo asteroids—perhaps most of them—are nothing more than the nuclei of degassed comets or fragments of degassed comets. A typical example is 1979 VA, which “has an orbit like a short-period comet with an aphelion close to Jupiter.”21
Looking outward to more distant reaches of the solar system, recent observations have demonstrated that the trans-Jovian “asteroid” Hidalgo also has a comet-like orbit.22 We saw in the last chapter that the trans-Uranian object Chiron has an orbit that is equally hard to label. Observations since the mid-1990s have shown that it is “slightly outgassing” and has begun to release volatiles in a manner that astronomers know is unlike any asteroid.23
Its icy nucleus of 350 kilometers would seem to suggest that it is a giant comet provisionally parked in a quasi-circular but unstable orbit.”24
For these reasons, says Professor Trevor Palmer, the view that some asteroids may be the remains of former comets is becoming widely held. “This could be the result of an icy nucleus being sealed off completely by the formation of an insulating crust, or by all the volatile material being boiled off, leaving behind a rocky core.”25
HALLEY’S COMET
The suggestion that 200-kilometers-plus objects like Chiron and Hidalgo could be former comets from the Kuiper belt gradually spiraling down into the inner solar system is supported by observations of smaller comets that have penetrated more deeply. For example, astronomers already agree that the present orbits of periodic comets Halley and Swift-Tuttle must have originated in just such a “spiraling down” after they had been “parked for a few million years in the Kuiper belt.”26 At the extremes of their steeply elliptical trajectories, before plunging back again toward the Sun, both objects still signal their origins by returning to the belt.27
“Periodic” comets—the term is a broad one that refers to all comets on orbits that will sooner or later bring them back through Earth’s skies—are subdivided by astronomers into three main groups: short-period, intermediate-period, and long-period. Short and intermediate-period comets have orbits varying from less than six years up to two hundred years; long-period comets have orbits of more than two hundred years rising, in some cases, to thousands and even hundreds of thousands of years.28
With an intermediate-period orbit of 76 years, Halley’s comet last passed by Earth in 1986 at which time it was intensively studied by space probes from several countries. It is a formidable body with an estimated mass of around 80 billion tons and dimensions of about 16 × 10 × 9 kilometers.29 Its potato-shaped nucleus is extremely black, reflecting only 4 percent of incidental sunlight, and slowly rotates around its axis once every 7.1 days.30
Recorded observations of Halley’s comet go back more than 2,200 years.31 Outgassing explosively on each approach to the Sun, it therefore has had the time to scatter immense swathes of debris in its ancient and well-trodden wake. Earth passes through this debris twice each year—in May and in the third week of October—at which times its skies light up with the Eta Aquarid and Orionid meteorite showers that descend from the comet.32
THE SWIFT-TUTTLE COLLISION HAZARD
Historical sources and modern observations record the existence of about 450 Earth-crossing comets. Most of these were of the long-period variety and have not yet returned either to menace us or to miss us. Out of the known short-and intermediate-period comets that revisit us more regularly, about 30 are locked on Earth-crossing orbits and could theoretically collide with our planet at some time in the future.33 Halley’s is one of these. Another is Swift-Tuttle, the parent body of the Perseid meteorite shower through which Earth passes each July and August.34 Astronomers studying Swift-Tuttle’s trajectory believe that this comet represents a serious and imminent hazard. As it approaches perihelion, its closest point to the sun, computer simulations show that its intersections with the path of Earth can, under certain circumstances, bring it perilously close to us. In particular, and well understood:
Near collision with Earth would take place if the comet were at perihelion in late July.35
For this reason Swift-Tuttle has been described by one authority as “the single most dangerous object known to humanity.”36 Calculations show that it will remain a threat for at least another 10,000 to 20,000 years,
after which its orbit is likely to deteriorate so that it will either fall into the Sun or be thrown out of the solar system, provided it doesn’t hit Earth before it does that.37
CAPE EFFECT
The Swift-Tuttle story begins with the first sighting of the comet in July 1862. Over the course of the next month, as it approached to within 50 million miles of Earth, it became a dazzling specter in the night sky with a tail 30 degrees long that was reportedly brighter than the brightest stars.38 For several weeks it pursued a serene and predictable course through the heavens—a course that was painstakingly tracked and logged by astronomers around the world. During the last few days that it was visible, however, it did something that no comet had hitherto been seen to do: It changed direction. As it disappeared from view, the Cape Observatory in South Africa noted with puzzlement that its trajectory had shifted by about 10 arc seconds during its transit of Earths skies.39
This so-called Cape effect is believed to have been caused by outgassing from the comet itself, outgassing so violent that Swift-Tuttle was literally jetted sideways.40
But was it a one-shot event, or something that happens regularly? In 1862, questions like these introduced an element of uncertainty into calculations of the likely date of Swift-Tuttle’s return—although it was generally felt that the period should be about 120 years.41 A similar projection was made in 1973 by Brian Marsden, the International Astronomical Unions (IAU) leading expert in the computation of orbits. After carefully rechecking and recalculating the 1862 data he concluded that the comet would return somewhere between 1979 and 1983.42
When it did not return on schedule Marsden widened the net of his calculations to include historical observations of comets that could be identified with Swift-Tuttle. He found a close match with sightings from 69 B.C., A.D. 188, and A.D. 1737, and on the basis of these came up with a new estimate that the comet would return in 1992 and would reach perihelion around 25 November of that year.43
Marsden’s prediction proved to be quite accurate, and the reappearance of Swift-Tuttle—on a trajectory that brought it to perihelion on 11 December 1992—was first observed by the Japanese astronomer Tsusuhiko Kiuchi on 26 September 1992.44
THE WARNING
Marsden now returned to his computers with refined orbital information in order to work out the date of Swift-Tuttle’s next approach to perihelion. He found that this would occur after a period of about 134 years, on 11 July 2126.45 Inevitably he began to wonder whether some recurrence of the Cape effect, or other orbital vagary, might cause him to be in error again.
The reader will recall that a near colli
sion between Earth and Swift-Tuttle is to be expected if the comet should ever reach perihelion in “late July”—indeed, it was Marsden who had been responsible for the original calculation that led to that prediction as far back as 1973.46 Looking at the problem again in 1992, his next step was to work out the exact date in late July 2126 on which a perihelion passage by Swift-Tuttle would be followed by collision with Earth. The computers highlighted 26 July 2126 and indicated that if the comet were to reach perihelion on that day, then it would crash into our planet a little less than 3 weeks later on 14 August 2126.47
So, the future of the human race seemed to hinge on the cosmically very small matter of the distance Earth would travel around its orbit in the 15 days between Marsdens calculated perihelion date for Swift-Tuttle of 11 July and the “black-spot” date of 26 July. He had to admit there was a chance he could have missed some vital factor. He therefore issued IAU circular 5636 (October 1992) in which he warned of the possibility that
periodic comet Swift-Tuttle may hit Earth on its next return.48
SAFE FOR THE NEXT MILLENNIUM?
A storm of publicity erupted after Marsdens announcement, and he was accused of sensationalism. Obliged to defend his position, he explained that the purpose of the circular had not been to scare anybody but to urge professional astronomers to pay special attention to the comet “during the next several years”:
The observations in 1862 showed that Swift-Tuttle behaved in a very peculiar fashion—something of the kind I have never seen before in nearly forty years of computing orbits…. The fact is that even if Swift-Tuttle doesn’t get us next time, it will have ample opportunity to do so in the more distant future.49