And as predicted, the jets are most definitively there, turning on and off, shooting enormous long firehose skews of material into nearby interplanetary space, forming the cometary coma. Apart from the jets the ground is dark—very dark. You see where the jets emanate mainly from discrete caverns on the surface—probably where a particular kind of ice is turning into gas. This self-contained volatile little world, with its varying landscape and composition, confirms the deduction of generations of earthbound astronomers on the nature of the cometary nucleus.
*Peter Gay, Education of the Senses (Oxford University Press, New York, 1984). Gay, professor of history at Yale University, does not mention this balloon ascent; Mabel Loomis Todd interests him chiefly because of the light her candid diaries shed on Victorian sexual behavior.
*Tasks successfully carried out in June, 1985.
*The French/Soviet balloons released into the atmosphere of Venus were also tracked by American radio telescopes. They lasted for about two days before they were destroyed by the fierce Venus environment.
CHAPTER 19
Stars of the Great Captains
Whatever opinions we may adopt as to the physical constitution of comets, we must admit that they serve some grand and important purpose in the economy of the universe; for we cannot suppose that the Almighty has created such an immense number of bodies, and set them in rapid motion according to established lows, without an end worthy of his perfections, and, on the whole, beneficial to the inhabitants of the system through which they move.
—THOMAS DICK, THE SIDEREAL HEAVENS AND OTHER SUBJECTS
CONNECTED WITH ASTRONOMY, AS ILLUSTRATIVE OF THE CHARACTER
OF THE DEITY AND OF AN INFINITY OF WORLDS, PHILADELPHIA (1850)
We have seen that comets very likely brought some of the stuff of life to the early Earth and to countless other planets in the vast Milky Way Galaxy. We have also seen that where the comets giveth, they also taketh away.…
The Cretaceous Tertiary impact of 65 million years ago wiped out most of the species of life on Earth (as luck would have it, good for our ancestors, bad for almost everybody else). Even much less energetic impacts, down to an equivalent explosive energy of ten thousand megatons of TNT, can have absolutely devastating consequences. Because human life is dependent on agriculture, which in turn depends on a few carefully cultivated species, our food supply is in general more vulnerable to catastrophe than that of other animals. But of course, the more catastrophic the impact, the less likely it is to occur—or, equivalently, the longer we have to wait before it is likely to transpire.
And yet, as the July 1993 collision of over twenty fragments of Comet Shoemaker-Levy 9 with Jupiter reminds us, such catastrophes happen in our time. And the fact that the resulting impact blemishes in the clouds of Jupiter were Earth-sized reminds us that the consequences of such a collision can be global.
What an unexpected finding! How well it dovetails with the speculations of Halley and Laplace and such science fiction writers as Ignatius Donnelly, as well as with legions of lesser scientists and more journeyman writers. For this very reason we have a tendency to dismiss cometary catastrophe, to put it out of our minds. There is nothing in recorded human history that argues for such collisions* —and naturally a part of our minds think, so if it never happened, it never will happen. But this is a dangerous and inappropriate extrapolation from very limited experience. Except for Hiroshima and Nagasaki there never has been a nuclear war. It does not follow that nuclear war is impossible. Likewise, the human species has never before been able to deplete the protective ozone layer. That does not mean that we are not capable of doing it. And so on, for global warming, the accidental generation of horrendous disease, the destruction of an acre of forest every second until there is almost none left … We may never have to face such calamities. We may take all the necessary precautions. But it would be suicidally foolish to ignore the possibilities.
It’s like insurance: Insurance companies decide the premium you must pay based both on the likelihood of a calamity and its calamitousness.
What precautions could be taken about a comet or asteroid hitting the Earth, and what degree of effort should be invested?
A handful of dedicated astronomers, some amateur, some professional, have been searching for these interplanetary interlopers. They include Carolyn and Eugene Shoemaker, David Levy, Eleanor “Glo” Helin, a team at the University of Arizona, and a smattering of others. Helin has joined forces with a special Air Force camera system on the island of Maui in Hawaii. The rate of discovery has picked up slightly. Previously classified cometary bursts in the upper atmosphere havebeen declassified. The number of comets in the tens and hundreds of meters diameter size range looks to be larger than we had thought. They are collectively called Near Earth Objects (NEOs). From the number of NEOs that we have detected we can make a rough estimate of the total that could work real havoc on Earth. The answer is in the thousands. None of them are known to have orbits that will impact the Earth in the next few centuries—but tomorrow one might be discovered that will.
In this same very partial inventory we can calculate the probability that such an asteroid or comet will hit the Earth in the next century. The answer is a little less than one in a thousand. To me, a tenth of a percent chance of a global cataclysm in the lifetime of my children and grandchildren seems very serious. Consider how much we worry about, and how much effort we put into preventing, say, airline catastrophes—where the chance of a single passenger dying on a regularly scheduled commercial flight is only one in a million. One passenger.
At the very least, we should be mustering a serious global search for near-Earth asteroids and comets. We know how to do it, the astronomical instruments all exist, there are professional scientists ready to go to work, and the thing could be pretty much completed in a decade or two. The total cost? At the outside, one stealth bomber. Even if all we buy for this money is a little peace of mind, isn’t it a bargain?
And what if we’re unlucky? What if a newly discovered NEO turns out to have a predicted orbit that fifty years from now crosses the orbit of the Earth with the Earth in the way—a comet with our name on it? There are ways of giving the offending comet a little push near perihelion, which changes its orbit sufficiently so that it does not impact the Earth. Some methods involve nuclear weapons, others do not. A solid, cohesive cometary nucleus might require merely a set of small shoves, but something loosely cohesive—the equivalent of a few hundred much smaller comets sharing the same orbit and very weakly attached—would pose a more complex challenge. Clearly to safeguard the Earth in this way we must know our asteroids and comets. And the only way to know this reliably is by sending spacecraft to them. The foregoing program—endorsed by astronomers worldwide (and by China occasionally when it was seeking an argument not to have to stop testing low-yield nuclear weapons)—would constitute enormous progress.
A Memo to the Administrator
In 1995, the Administrator of NASA, Daniel Goldin, asked Carl Sagan to provide a summary of reasons for sending spacecraft to the near-Earth objects (comets and asteroids that come very close to the Earth). Such missions are expensive, they must compete with other worthy projects within NASA and without, and—especially because the catastrophes from space are so easily lampooned or dismissed (what is sometimes called the “giggle factor”)—it might be a good idea to stack up the reasons and see if they make sense.
A population of some 2,000 small worlds more than 1 kilometer (0.6 mile) in diameter orbits the Sun very near Earth. Some have arisen from the main asteroid belt; others are dead or dying comets deriving ultimately from the outer solar system. Most NEOs are gravitationally perturbed into and out of near-Earth orbits on timescales of 10 million to 100 million years.
NEOs, therefore, represent a convenient opportunity to study small worlds at much less cost than missions beyond the orbit of Mars.
NEOs represent fairly pristine samples of the early planetesimals that built the pla
nets—with clues to conditions in the solar nebula from which the solar system formed. Some NEOs have undergone more physical and chemical processing than others.
The dumbbell shape of some NEOs (e.g., Toutatis) raises the intriguing possibility that we are seeing here remnants of the accretion of planetesimals, and therefore the processes that led to the building of Earth and of the planets.
Some NEOs are carbonaceous. Organic molecules of cometary and/or asteroidal origin played an important role in the origin of life on Earth around 4 billion years ago. Comets entering Earth’s atmosphere today have their organic molecules largely destroyed or modified thermally. Microscopic particles of cometary debris collected in the stratosphere represent so little cumulative mass that it is very difficult to determine their organic chemistry. The study of carbonaceous NEOs may, therefore, provide important clues to the origin of life.
While many meteorites have been collected on Earth, we are unable to relate these meteorites to the physics, chemistry, and history of any specific solar system asteroids or comets. We do not even clearly understand how meteorites are connected to the different asteroidal and cometary populations. It is possible that a weakly coherent class of NEOs exists that never survives entry into Earth’s atmosphere and is unknown to us.
The dinosaurs and 75 percent of the species then on Earth seem to have been rendered extinct by the impact of a 10-kilometer-diameter (6-mile) NEO some 65 million years ago. The Tunguska event of 1908 in Siberia, the recently declassified United States Air Force data on small objects entering Earth’s atmosphere, and particularly the impact of some two dozen fragments of Comet Shoemaker-Levy 9 with Jupiter all underscore that dangerous impacts may occur today. The best estimate of the probability of a civilization-threatening collision in the next century is nearly one in a thousand.
A program to inventory all large NEOs is needed to determine not only orbital elements but also something of the physical and chemical properties of the inventoried NEOs. Ground truth on the NEO is needed to calibrate remote observations. Transponders may have to be emplaced on potentially dangerous NEOs.
The population of NEOs probably ranges from highly coherent objects to weakly cohesive fluffballs. If the time ever comes when it is necessary to deflect or disintegrate a NEO on an Earth-impact trajectory, it will be essential to have experience with the various populations of NEOs. For example, the coupling constant in the use of standoff chemical or nuclear weapons may be highly variable from NEO to NEO.
The dangers presented by NEOs over the next century probably provide another coherent justification for the development of long-duration human spaceflight capability. Some NEOs are easier to get to than the Moon. Some round-trip missions to interesting NEOs with as much as a 30-day stay time on the NEO surface take less than a year. In general, missions to NEOs are intermediate in difficulty and risk between lunar and Martian missions.
As for Mars, robotic missions and human missions to NEOs provide an opportunity for the development of rover, telepresence, return-sample, and virtual-reality space technologies and provide a justification for space-station and other means of investigating long-term human survival in space. Likewise, observations and missions to NEOs are intrinsically international because everyone on Earth is equally at risk from the impact of a large NEO. (The language of the House Science and Space Committee’s proposed legislation for a program like Spaceguard mandates it be done internationally.)
NEOs represent a new and unknown environment for exploration—in many respects, more tantalizing than the partially explored Moon.
There are other issues that would have to be resolved at a later time. There is, for example, the possibility of a misuse of deflection technology, so that a comet or asteroid that would not otherwise have hit the Earth could be made to hit the Earth. (There are, of course, countermeasures that can be taken.) There is also the remote possibility of a long-period comet soaring out of the Kuiper Belt or the Oort Cloud on a beeline for the Earth with only a few months’ notice to us. But if we are equal to the challenge of the near-Earth objects, we think it likely that our descendants will have the means to deal with these challenges as well.
Spacecraft exploration of NEOs is beginning, although slowly. In 1996 an American spacecraft called NEAR (Near Earth Asteroid Rendezvous) was launched. It is intended to match orbits with the large NEO called Eros, named after the god of love. It will follow Eros in its trajectory around the Sun, mapping its surface, determining its composition and average density, and seeing what changes happen during the seasons of its year. It will come within 15 kilometers of parts of the asteroid. The European Space Agency has tentative plans to launch Project Rosetta in 2003, to visit Comet Wirtanen in 2011, and deposit one or more landers on its surface. NASA has approved development of the Stardust mission, which would collect dust from a comet in 2004 and then return the samples to Earth in 2006. Stardust will collect grains of interstellar dust in the solar system before flying within about 100 kilometers of the nucleus of Comet Wild-2.
The pace is slow. We are not yet sending humans to NEOs, we are not moving NEOs around, we are not even counting the bulk of NEOs yet, but we are making progress.
A comet passed quite near … so they sprang upon it, together with their servants and their instruments.
—VOLTAIRE, MICROMEGAS, 1752
Once we have considered rerouting asteroids and comets to prevent them from impacting the Earth, we can also contemplate rearranging their orbits for other purposes. Before humans invented mining, the only ready source of iron for metallurgy was from meteorites. This fact is still commemorated in many languages, where the word for iron is connected with the word for the sky. (The Latin word siderus comes from a word for star or constellation, as in the English word sidereal; and the Greek word for iron is sideros.) Billions of years ago an asteroid melts, and forms a nickel-iron core under a rocky mantle; a violent collision with another asteroid strips off the mantle, leaving a bare metallic core, and a subsequent collision sends a shard of iron careening through space. Millions of years later, it falls on the Earth, startling some itinerant tribesperson and providing another inducement toward technology and civilization. We humans are already experienced in working with extraterrestrial resources.
Intrepid but involuntary visitors to a comet. From the Jules Verne novel Hector Servadac.
Since the platinum group of metals (Chapter 17) is relatively more abundant in asteroids (and probably comets) than on the Earth, there may be real economic motivations for mining small nearby bodies. An asteroid 100 meters across has a mass of a million tons. On a planet with dwindling supplies of accessible metals—among them, nickel and the platinum group—this may provide a significant incentive for commercial utilization of the asteroids. A rocket motor is towed to the asteroid, attached to it, ignited, and used to change its trajectory. Perhaps there will be a gravity swingby of the Moon or a nearby planet to bring it to a convenient orbit about the Earth; there it would be dismantled, and great cargo ships would carry the valuable metals to the Earth.
The great danger in such maneuvers would be that an error is made and the captured NEO crashes down into the Earth below. But the technology we are talking about is at least a century or two into the future, and we can imagine adequate safeguards present by then.
From a purely economic standpoint, the question is whether the money spent to capture, dismantle, and transport asteroidal material to Earth would be better spent on extracting ores already on Earth, but in less tractable deposits or at greater depths. Where the commercial benefit of space mining becomes most apparent is in building large structures in Earth orbit—were such constructions otherwise justified, for the manufacture of pharmaceuticals and exotic alloys, for example; for scientific investigations; as a way-station to the planets; or—if we are so foolish as to permit it to happen—for the introduction of weapons systems into space. It is very expensive to carry material up from the surface of the Earth against the pull of gravity, a
nd far cheaper to utilize materials that are up there already. The Moon is already in orbit about the Earth, but it seems woefully depleted in a number of key substances, especially water and organics. If we humans begin large orbital constructions in the next century, it will make sense to use asteroids and comets as raw materials. In the process, we will gain considerable practice in living on small worlds, and moving them about the inner solar system at will. This capability then seems likely to open a startling vista:
Crossing the Earth’s orbit, as we have said, are thousands of worldlets. Many of them are of cometary origin. The total area of all these worlds is ten thousand square kilometers, a sizable chunk of territory—although, individually, each is only as big as a square block in an urban center. If there are a hundred trillion cometary nuclei in the inner plus outer Oort Clouds, then the total surface area of the comets is equivalent to hundreds of millions of planets each the size of the Earth. A hundred million Earths parked conveniently in our backyard is a heady prospect.
The comets are traveling so slowly that it would be possible, even with current technology, to overtake one. A million years or more must pass for a comet to get here from the outer Oort Cloud; the Voyager spacecraft will traverse the same distance in ten thousand, and likely future technologies will lower the transit time to less than a human life span. If we are planning where humans might live in the far future, the comets offer by far the greatest range of possibilities. But since a few square kilometers will not accommodate many people, we must imagine a large number of small worlds, each sparsely populated. But in what sense can a comet be considered habitable?