Scientists for a long while believed that most of the impact craters and other damage visible on Mars must have been inflicted billions of years ago, that the solar system today is a far quieter and far safer place than it was in primordial times, and that the chances of Earth colliding with an asteroid or comet are so small as to be insignificant.
We now know that they were wrong about Earth—and new evidence, which we will review in the next chapter, has forced the abandonment of the formerly dominant uniformitarian view. Could they also have been wrong about Mars? And could there indeed be some kind of mysterious connection between the two planets, as so many ancient sources seem to suggest?
21
Earth Cross
EVERYTHING is moving. Nothing stays still.
The Moon moves around its own axis and orbits Earth. Earth moves around its own axis and orbits the Sun. The Sun moves around its own axis and orbits the center of the galaxy. And the galaxy too is in motion through the expanding universe.
Earth is our home, and our immediate concern. But we will see that it is subject to mysterious and violent tides that perturb the entire solar system and that are governed by the galaxy. If we wish to have a clear picture of what it means to live on this planet, we are obliged to take account of the galaxy and the solar system, and we would be wise to pay attention to any lessons that neighboring planets have to teach. After all, we share their cosmic environment so closely that whatever happens to them can reasonably be expected to happen to us.
Mercury, Venus, the Moon, Mars, and Jupiter all tell us one thing, very simply and very clearly. In Gene Shoemaker’s words: “Comets really do hit planets.”1
And, as we shall see, not only comets hit planets (although comets are by far the most deadly danger), but also vast swarms of meteoroids and asteroids, ranging in size from a meter up to 1,000 kilometers, tear through the solar system at furious speeds.
Such objects, in all possible size ranges, can and frequently do hit planets. Earth has not encountered a very big one—say in the 200-kilometer-plus range—for billions of years. But we now know that it has encountered several in the 10-kilometer range in just the last 500 million years, and that each of these collisions has resulted in the near total extinction of life.
To find out what Earth would look like if it had taken direct hits from a barrage of much bigger objects, we only need to look at the ravaged face of Mars. Curiously, when we do so, we find a “face” staring back at us from the plains of Cydonia.
CROSSING THE LANES
If we envisage the orbits of the planets as a series of flat circular lanes laid out concentrically around the Sun, little Mercury turns in the inner circle. Outside it is Venus, then the Earth, then Mars, then Jupiter. Beyond Jupiter, far from warmth and light, are four farther planets—Saturn, Uranus, Neptune, and Pluto, respectively. Circulating among them all, crisscrossing the lanes in which the planets move, are the turbulent swarms of orbiting rocks and iron we have discussed, loosely classified and graded according to size as either meteoroids or asteroids.
Exactly what these objects are, where they came from, and why some are stony and some metallic (almost like the melted and fused components of gigantic iron machines), are not matters that scientists have settled yet, and there is no consensus. One school of thought is that they are the leftover debris of the iron core and stony mantle of an exploded planet.2 However, no convincing mechanism has yet been suggested to explain how a planet-sized body could explode. Another idea is that they are remnants from the early days of the solar system—the surplus matter not used up in the formation of the planets. A third theory, the one that we ourselves favor, is that they are closely related to comets, particularly to giant interstellar comets that periodically enter the solar system. The argument is that many of the asteroids and the smaller meteoroids may be the fragmented remains of these dead comets.
BIG UNSTABLE OBJECTS
Fully 95 percent of all known asteroids lie in the “main belt” between the orbits of Mars and Jupiter. But several other populous groups of asteroids circulate between the orbits of Mars and Venus—straddling Earths orbit. These are thought to have been “the principal producers of craters larger than 5 kilometers in diameter on Earth, Moon, Venus, and Mars.”3
There are also large asteroidal objects that lie permanently outside the orbit of Jupiter and others, with highly elliptical orbits, that cross Jupiters path as they climb toward aphelion (farthest point from the Sun) but that swing into the domain of the inner planets as they fall toward perihelion (closest to the Sun).
Among these asteroidal objects is 944 Hidalgo, which has an orbit of 14 years and a diameter in the range of 200 kilometers. On each turn that it takes around the solar system it swings out far beyond Jupiter—almost as far as Saturn—and then swings back in again approaching the orbit of Mars.4
Another more distant and probably slightly bigger object (estimates vary from 200 kilometers to 350 kilometers5) is 2060 Chiron, which presently orbits between Saturn and Uranus but has exhibited highly unstable behavior in recent years.6 Astronomers studying its trajectory have concluded that it is very likely in due course to fall into the inner solar system and perhaps to become an Earth crosser.7 If that were to happen, says Duncan Steel, it
would spell disaster for humankind even if Earth did not receive an impact by Chiron itself, or even any large lumps, because the amount of dust in the atmosphere would lead to a significant cooling of our environment.8
A third 200-kilometer-plus asteroid is 5145 Pholus.9 Its steeply elliptical orbit takes it across the paths of Saturn, Uranus, and Neptune.10 Like Chiron, it has been described by astronomers as “inherently unstable” and is thought likely to “plunge into an Earth-crossing orbit”—although probably not soon.11
There is a frightening object called 5335 Damocles, estimated to be 30 kilometers in diameter, which crosses the orbit of Mars at perihelion and then swings out as far as Uranus before returning to the inner solar system again in an orbit of forty-two years. According to Duncan Steel of Spaceguard Australia:
This asteroid has an elongated, high-inclination orbit which would classify it as an intermediate-period comet, except that it shows no signs of outgassing, seeming to be totally inert. Its name was chosen to remind us of the Sword of Damocles, because its future orbit has a good chance of evolving into an Earth-crossing one.12
MAIN BELT
Since the discovery of Hidalgo, Chiron, Pholus, and Damocles, other large unstable asteroids have been found with the same ability to cross from the outer solar system into the inner solar system—and even to threaten Earth.13 But there are also vast armies of asteroids that revolve around the Sun in stable orbits and present no threat to us at all.
These include the members of the Trojan group that share the orbit of Jupiter, some following the planet, some leading it. Photographic surveys have so far identified 900 individual objects with diameters exceeding 15 kilometers.14
All the “main-belt” asteroids orbiting between Jupiter and Mars also appear, for the moment, to be in secure orbits. Their total number is thought to exceed half a million, including such true giants as Ceres.15 Really a mini-planet in its own right, this country-sized sphere of rock has a diameter of 940 kilometers, revolves around its own axis in 9 hours 5 minutes, and orbits the Sun once every 4.61 years.16
Ceres is very dark and reflects only about 10 percent of the sunlight falling on it.17 To date it is the largest asteroid identified. Next down in size are Pallas (535 kilometers), Vesta (500 kilometers), and Hygeia (430 kilometers). Davida and Interamina are both around 400 kilometers in diameter. Juno is about 250 kilometers in diameter. All in all more than 30 main-belt asteroids with diameters greater than 200 kilometers have now been positively identified and catalogued—with significant new discoveries being made every year.18
AMORS
Moving in from the main belt we begin to encounter the first swarms of “near-Earth asteroids,” a broad category that
includes all asteroids capable of passing inside the orbit of Mars.19 The most distant of these do not extend as far as the orbit of Earth. But a little closer in there is another family of Mars crossers, the Amors, of much more immediate interest. A characteristic of the Amors (more than 130 had been catalogued by March 199520) is that they are easily perturbed by Jupiter and by our own planet’s powerful gravity, with the result that several of them have now changed their orbits to become “part-time Earth crossers.”21 Many others in the same family do not presently approach Earth but, in theory, may be “unpredictably redirected” at any time.22
Astronomers from the Observatoire de la Cote d’Azur in France and mathematicians from the University of Pisa in Italy have for some years been paying particular attention to an Amor called 233 Eros, which is 22 kilometers long and 7 kilometers wide—dimensions that make it a substantially bigger and more lethal projectile than the K/T object that killed off the dinosaurs.23 Although Eros does not currently cross Earths orbit it does undergo “relatively frequent close encounters with Mars and long-range perturbations by the outer planets.”24 These have altered its course to such an extent that in 1931 it “swished to within 17 million miles of Earth—much closer than any planet.”25 Computer simulations indicate that Eros is very likely to become a true Earth crosser within the next million years and that in the longer term “a collision is likely.”26
So far about 15 other Amors on Eros-like trajectories have been found, and all of them could one day hit the Earth.27 None are as massive as Eros, but both 1627 Ivar and 1580 Betulia have diameters approaching 9 kilometers.28
APOLLOS
Moving in again from the zone of the Amors we come to the Apollo asteroids (named after 1862 Apollo, a 1-kilometer object, the first in this class, discovered in 1932 by the German astronomer Karl Willhelm Reinmuth).29 The chief characteristic of the Apollos is that they “deeply cross the Earths orbit on an almost continuous basis.”30
Since the early 1990s a number of observatories have mounted aggressive searches to establish the true extent of the “Apollo problem.” The conclusions that they have come to are that these Earth-crossing projectiles are extremely numerous, that there are likely to be more than 1,000 of them with diameters exceeding one kilometer,31 and that some may exceed 50 kilometers in diameter.32
Known large Apollos (of which more than 170 had been catalogued by March 1995) include the frightful world killer 2212 Hephaistos, which has a diameter of 10 kilometers.33 Although smaller, another deep Earth crosser, Toutatis, looks almost equally unpleasant. It is what is known as a contact binary—“two fragments either welded together or held in place by a very feeble gravity.”34 The larger element has a diameter of 4.5 kilometers, while the smaller element is 2.5 kilometers wide.35 The composite object behaves in an unbalanced and unpredictable manner as it tumbles through space.36 All that is certain is that it has already crossed Earths orbital path at a distance from us of just over 3 million kilometers37—a distance that our planet covers in about 30 hours—and that the effects of a collision with such a rapidly rotating and unstable object would be devastating.
The existence of Toutatis proves that there are still giant rocks out there that can be doomsday asteroids and that they come close to us.38
Several Apollos with diameters in the 5-kilometer range have been found during the 1990s,39 and, as we saw in chapter 19, a number of smaller Apollos, such as Asclepius (0.5 kilometers), Hermes (approximately 2 kilometers), and Icarus (2 kilometers), have made extremely close fly-bys of Earth. There are also large and mysterious Apollo objects such as Oljato and Phaeton that behave much more like comets than asteroids, and which we will have reason to investigate in later chapters.40
A tiny fragment of Phaeton hit Earth on 13 December 1997. It landed in politically troubled Northern Ireland, close to its border with the Irish Republic, creating an explosion that was initially thought to be a terrorist bomb. Examination of the crater by scientists from the Royal Armagh Observatory and from Belfast’s Queens University showed that it was in fact a meteorite and that the parent body was Phaeton.41
It is worth repeating that all of the Apollos are permanently locked in Earth-crossing orbits and that they are accompanied by an unknown number—probably thousands—of as yet undetected and perhaps massive companions. There are no traffic lights at the intersections where they cross the great circle in the sky around which Earth orbits and, over very long periods of time, the laws of chance make collisions inevitable.42
Is a collision between Earth and an Apollo object likely at any time in the near future?
The only honest answer to this question is nobody knows—because nobody has the faintest idea how many of these projectiles there really are out there. Apollos are notoriously invisible to telescopes and are indeed so elusive that even those that have been catalogued frequently “disappear.” The 1862 Apollo, for example, after which the whole swarm is named, was lost to telescopes soon after it was discovered in 1932 and was not spotted again until 1973.43 Hermes, which passed so close to Earth in 1937,44 vanished and has not been seen since. For this reason, says Brian Marsden of the Harvard-Smithsonian Center for Astrophysics, it is “one of the most dangerous near-Earth objects.”45 Hephaistos, the biggest Apollo of all, successfully managed to evade detection—despite its 10-kilometer girth—until 1978.46
ARJUNAS, ATENS, AND OTHERS
Tom Gehrels, professor of planetary sciences at the University of Arizona at Tucson, and the principal investigator of the Spacewatch program at Kitt Peak Observatory, has identified a special subgroup of Earth-crossing Apollos that he has named the Arjunas. With diameters of up to 100 meters, they follow the orbit of Earth very closely. This means that they are unusually susceptible to our planet’s gravitational attraction and have “very short expected orbital lifetimes before colliding with Earth.”47
Moving in from the Arjunas, the next significant belt of asteroids that we encounter have been named the Atens. Astronomers estimate—although once again it is really just a guess—that at least one hundred of them exceed one kilometer in diameter. They have highly elliptical orbits that put many of them on repeated Earth-crossing paths.48
Still further in toward the Sun are other objects following even more steeply elliptical orbits. A typical example is 1995 CR, discovered by Robert Jedicke of Spacewatch in 1995. This 200-meter inner-solar-system wanderer follows
a highly eccentric path that crosses the orbits of Mercury, Venus, Earth, and Mars. This type of orbit is highly unstable (chaotic) and before long, at an unpredictable time in the future, 1995 CR will smash into one of these four planets, or the Sun, or will be thrown out of the solar system.49
Just as scientists cannot give us accurate estimates of when particular asteroids will collide with Earth, or of the absolute numbers of asteroids in any of the subfamilies, so also there can be no firm and final estimate of the total number of potential impactors. A broad consensus has nevertheless been reached by astronomers that there are likely to be at least 2,000 asteroids of a kilometer or more in diameter distributed among the main Earth-crossing families50 together with somewhere between 5,000 and 10,000 objects of half-kilometer size and perhaps as many as 200,000 objects of quarter-kilometer size.51 Confirmation of these estimates can only come from close observations of the sky and, indeed, the rate of discovery of Earth-crossing asteroids showed dramatic increases during the 1990s. In 1989 only 49 such objects had been discovered (4 Atens, 30 Apollos, and 15 Amors), but by 1992 this number had increased to 159, an increment of 110 in just two years. Three years later, in 1995, the grand total had risen to over 350, a further increment of 200—making an average for 1989 to 1995 of more than 50 new discoveries a year.
“Although many of these are small objects,” commented Duncan Steel in 1995,
it is true that we have now found many more of the 1-kilometer-plus asteroids that threaten a global catastrophe than we had catalogued only five years ago. However, we still know of only a
small fragment of the total population of such objects: few scientists involved in this area believe that we have to date discovered more than 5 percent of that total. Although none of the known asteroids is going to hit Earth in the foreseeable future (the next century or two) this is not a particularly comforting fact, because if there were an asteroid due to strike home soon, then there is a greater than 95 percent chance that we would not have found it yet.52
TIME TO SAVE THE WORLD?
Humanity’s fundamental ignorance about the true extent of the threat posed by Earth-crossing asteroids is unlikely to be lifted soon—despite the fact that many scientists seriously believe it would be possible to use controlled nuclear explosions and other techniques to deflect potential impactors if they could be identified in time. It is not our purpose here to explore the various strategies that have been proposed to achieve this objective. Nor would we be in any position to assess their relative merits. Our impression is that many of them are very close to the limits of modern technology. Nevertheless, we have no doubt that the prospect of an imminent collision with a 10-kilometer Apollo would focus the minds of politicians and galvanize global industry and science into action.
But would there be time to save the world?
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
The Mars Mystery Page 23