The Enemy Above
I met a traveller from an antique land
-Percy Bysshe Shelley
In 1998, the movies Deep Impact and Armageddon made millions of viewers aware of the danger of extra-terrestrial bombardment. Every 300 thousand years, a comet or asteroid large enough to cause global ecological damage hits the Earth. The most famous such impact occurred about 65 million years ago, and played some role in the extinction of the dinosaurs. Smaller objects hit the Earth more frequently, the most notable North American example being the object that created the meteor crater in Arizona, USA, around fifty thousand years ago. Asteroids and comets of various sizes have hit the Earth periodically throughout its existence, and paleogeologists have identified impacts that have caused much greater ecological disasters than the famous K-T extinction (1). The Earth will be hit again, and the results will be just as catastrophic. Can we prevent such a catastrophe?
The Origin and Composition of Planetesimals
Planetesimals are extra-terrestrial bodies that are not considered planets or permanent planetary satellites, and fall into two categories: comets and asteroids.
Since the 1986 return of comet Halley, the popular media has described comets as "dirty snowballs," which is quite accurate. Comets are composed mainly of light elements in the form of ices, with smaller amounts of heavier materials. As a comet enters the inner solar system, the sun begins to vaporize its surface. The released gases stream out behind it in a long tail, making new comets quite easy to find (2).
Comets are located in two major areas, the Oort cloud and the Kuiper belt. In 1950, Jan Oort's calculations of certain comets' orbits seemed to indicate that they originated at least fifty thousand Astronomical Units away (3). He proposed that comets formed with the solar system, then were flung out of it by gravitational interaction with the giant planets. Periodically, some sort of gravitational interaction will deflect an Oort cloud comet into the inner solar system, where it can be observed. The Oort cloud probably accounts for the long period comets (those with a period of two hundred years or more) that enter the inner solar system. (Arnett 2)
The Kuiper Belt, which was proposed in 1992, is probably much older than the Oort cloud. Astronomers in the 1990s have mapped the orbits of about forty large comets beyond Neptune (4), and estimate that there are around thirty-five thousand large objects in the Kuiper Belt. (Arnett 2) Scientists propose that, as the solar system formed, comets remained among and beyond the planets. Gravitational interaction with the giant planets "cleaned out" the inner solar system; some comets were destroyed in collisions, a few were shepherded into various belts, and many more were flung outward to form the Oort Cloud. The comets of the Kuiper Belt, just beyond range of the giant planets' gravitational influence, remained as remnants of the primordial solar system.
Astronomers recognize three types of asteroids: carbonaceous chondrites, silicaceous asteroids, and metallic asteroids. Carbonaceous chondrites consist mainly of carbon compounds. As a result, they have a very low albedo (reflectiveness), making them difficult to find. Carbonaceous chondrites make up about three-quarters of all asteroids. Silicaceous asteroids, which make up fifteen percent of all asteroids, are composed mainly of silicates, and are therefore similar in overall composition to the Earth. Metallic asteroids are composed mainly of iron, although they contain a significant concentration of precious metals. (Arnett) Although carbonaceous chondrites have the lowest albedo, the layer of pulverized material on their surface makes all asteroids very dark. Generally speaking, silicaceous and metallic asteroids are about as reflective as the mountainous areas of the moon, while carbonaceous chondrites are as reflective as the moon's maria (5). The moon may appear bright in our night sky, but that is only because of its proximity. An asteroid, one-hundredth the diameter of the moon, and a hundred times farther away, would be invisible to the unaided eye, and very difficult to spot even with a powerful telescope.
Most asteroids orbit in a wide belt between the orbits of Mars and Jupiter. Other notable groupings include the Trojan asteroids, which orbit in two clusters that form an equilateral triangle with Jupiter at the third vertex, and the Apollo, Amor, and Aten asteroids. Apollo asteroids approach Earth's orbit, and generally orbit the sun between the orbits of Earth and Mars. Aten asteroids also approach Earth's orbit, but from closer to the sun. (Arnett) The Amor asteroids, whose orbits cross that of the Earth, pose the greatest potential threat. (See Fig 1)
The Effects of an Impact
In 1908, something exploded in the sky over the Tunguska region of Siberia with a force of around forty megatons of TNT (6). An eyewitness at Vanavara, 90 kilometers from the explosion, gave this account:
The sky split apart and a great fire appeared. It became so hot that one couldn't stand it. There was a deafening explosion [and my friend] S. Semenov was blown over the ground across a distance of three sazhens [six meters]. As the hot wind passed by, the ground and the huts trembled. Sod was shaken loose from our ceilings and glass was splintered out of the window frames. (clarification added by Gallant)
Due to the harsh environment of Tunguska, the first expedition did not reach the site of the explosion until 1927. When Leonid Kulik's party reached the site, they found a huge area of trees that had been felled in a radial pattern, and evidence that a large area of forest had burned. (Gallant) Kulik, a Russian asteroid research pioneer, suspected that a large meteorite had caused 0the explosion, but he found no evidence of a crater. For years, scientists have been returning to the explosion site, but no one has conclusively identified the object that was responsible for the event. Different groups have alternately proposed comets and asteroids, and a Japanese group actually spent some time promoting a UFO hypothesis (7). Current theories suggest that a small asteroid, around 100 meters wide, exploded into tiny fragments shortly before it hit the ground.
The Tunguska explosion was caused by a relatively small object, and devastated a large but unpopulated area. What would happen if a large object struck near a major city? In 1998, David Crawford of Sandia National Laboratories performed a computer simulation of a 1.4 kilometer asteroid striking the Earth at a low angle off the New England coast with a force of three hundred gigatons of TNT (8) (Crawford). The simulation was performed on Sandia's teraflops (9) computer, and tracked 100 million data points. The following summary comes from Sandia's press release on the simulation: (German)
According to the simulation, the impact would vaporize the asteroid, deform the ocean floor, and eject hundreds of cubic miles of superheated water vapor, melted rock, and other debris into the upper atmosphere and back into space. Much of the debris would then rain down over the world for the next several hours and also form a high global cloud, says David Crawford of Sandia's Computational Physics and Mechanics Department. The shock wave from the impact would level much of the New England region. The heat would incinerate cities and forests there instantaneously. The global cloud would then lower temperatures worldwide, and a global snowstorm likely would ensue and last several days to several weeks, initiating a "nuclear winter" that would create more hardships for earth's inhabitants.
The New England region of the United States is one of the most densely populated areas of the western world. Crawford speaks almost casually about much of it being "leveled," but in more human terms, tens of millions of people would die in seconds, and tens of millions more would perish as an enormous tsunami (10) swept around the world, inundating coastal cities. And billions more would die as the debris thrown high into the atmosphere by the impact enshrouded the globe, causing a winter that would last for several years. Around the world, crops and livestock would freeze; agricultural production and distribution would stop. Only countries with significant food reserves would have any chance of significant survival rates.
Perhaps one reason that extra-terrestrial impacts are not an important public issue is that it is virtually impossible to relate directly to the scale of the event. A simulation performed by Crawford in 1997 demonstrated that a comet one kilometer wide striking the ocean would instantly vaporize three hundred to five hundred cubic kilometers of seawater. (Frazier) The human mind thinks in terms of boiling a few liters of water, and simply cannot relate to the amount of energy required to evaporate five hundred cubic kilometers of water within a fraction of a second. Humanity has never seen devastation on the scale that a major impact event would cause.
Perhaps the one event in human history that compares to a fairly large impact event was the Krakatoa explosion. On August 27, 1883, after several months of eruptions, the volcanic island of Krakatoa exploded with a force of one hundred megatons of TNT. The explosion was heard three thousand kilometers away in Madagascar, and raised forty-meter tsunamis that killed 36 thousand people as they swept away villages on the surrounding islands. (NASA)
What Can Be Done?
The producers of the movies Deep Impact and Armageddon both used the idea of landing on the incoming object and placing a nuclear bomb beneath its surface. In Deep Impact, the strategy was to break up the relatively light comet into fragments small enough that they would burn up in the atmosphere. In Armageddon, the astronauts made use of an existing fault line to send two halves of the heavy, metallic asteroid flying by either side of the Earth.
Due to the limits of a motion picture both films were riddled with inaccuracies, but the overall strategy employed in both of them is sound. The idea of using a nuclear explosion to destroy an incoming asteroid has been considered for years. But a nuclear explosion is not the only process that can destroy an asteroid. According to an article in the April 1997 Popular Mechanics, a group of scientists at the 1997 Lawrence Livermore national conference proposed that a lattice of several million tungsten bullets could pulverize a small asteroid into harmless fragments. The bullets, which would be connected by thin fibers, could be launched by a heavy-lift booster such as the Russian Energia or the American Titan IV. (Alpert)
Another strategy suggested in the article is deflection by mass ejection. If the offending asteroid is discovered far in advance, a rocket could be crashed into it, pushing it off course. Solar energy could even be used for this purpose.
Perhaps the tool with the greatest potential for warding off threatening planetesimals is the free electron laser (FEL). The FEL produces a beam by accelerating electrons in a cyclotron, then sending them through a "wiggler," which contains a series of electromagnets. As the electrons pass through the wiggler, the electromagnets force them to change direction rapidly, making them release photons, the particles of light. Since the electromagnets in the wiggler are evenly spaced, the light emerges at one frequency, comletely in phase; it is a laser beam. By varying the spacing of the electromagnets, the beam can be tuned to any frequency. (Savage, 111) If the apparatus is constructed in such a way that any combination of electromagnets can be turned on or off, the beam can be tuned to multiples of the wavelength created when all of the electromagnets are on, without a bulky mechanism to move them. Unlike chemical lasers, FELs do not need to be refueled. The only thing they need in abundance is electricity.
A FEL with a radar tracking system could be placed in orbit, powered by solar panels. It would require routine maintenance from shuttle astronauts, but would be otherwise autonomous. A separate orbiting radar satellite would scan the heavens for dangerous objects, and report to a ground station. If a comet or asteroid that threatened the Earth were detected, the Earth defense team would first analyze its composition by means of a spectroscope (such analysis is routinely performed on planetesimals). The analysis would determine what frequency to use on that particular object.
With the analysis complete, the orbiting laser would fire a test burst, actually two pulses several microseconds apart, at the near side of the offending object (11). If the team has chosen the right combination of frequency and power, the first pulse will vaporize a small amount of the object, and the second pulse will detonate it. The exploding material will expand at up to ten thousand meters per second, giving a large push for the amount of material expended. After the team makes any necessary adjustments, they will let the laser fire automatically, firing closely timed double pulses at the comet or asteroid, pushing it off its original course.
This strategy has several advantages. Since the FEL can be tuned to any frequency and set to any power level, a beam can be suited exactly to the material that makes up the asteroid. The evaporation-detonation double pulse spreads the impulse over a larger area than an impacting body would, giving a stronger push that has less chance of breaking up the object (12). An existing system in orbit would not require a heavy-lift launch every time an object threatened the Earth. And the FEL strategy reduces the possibility of developing even more powerful nuclear weapons specifically for asteroid defense, weapons that could threaten that which they propose to defend.
The FEL system could be used to deal with a much more immediate threat, one that has so far received much less public attention. Since 1957, when the Soviet Union launched Sputnik, the world’s space programs have sent hundreds of objects into orbit. A few have returned to Earth as their orbits have decayed, but many more remain in a vast, orbiting junkyard. In 1984, NASA launched the Long-Duration-Exposure Facility (LDEF), a huge satellite covered with panels of different materials, which was to remain in orbit for ten months. (Pasachoff, 424) Because of the space shuttle Challenger disaster and the rescheduling of subsequent shuttle missions, LDEF was not recovered until January of 1990 (Dardano & See). After nearly six years in low Earth orbit, LDEF's panels were riddled with craters from micrometeorites and orbital debris. Of the impactors that could be identified, most were natural, but a significant number were man-made, consisting of specks of paint, aluminum, stainless steel, and the remains of electrical components. (Dardano & See, see Fig 2)
The increasing amount of debris in orbit will eventually lead to a situation in which all man-made satellites will have a very high statistical probability of being destroyed by orbiting debris. With a certain amount of debris, the destruction of one satellite could set off a chain reaction, destroying all man-made satellites, and creating an impenetrable cloud around the Earth, a situation known as the Kessler syndrome. Seventy thousand objects one centimeter or larger could be enough to start such a chain reaction. Some estimate that Earth orbit could have that many such objects within twenty years. (Savage, 151)
As Marshall Savage suggests in The Millennial Project, FELs could destroy small pieces of space junk, or push them into the atmosphere where they will burn up. Thus FELs will not only protect Earth from natural events, but also save humanity from its own shortsightedness.
Scientists estimate that there are nearly two thousand near-Earth asteroids over a kilometer in diameter. (Blindsiding) The odds of one striking the Earth in a particular year are astronomically small, but the damage that will result is astronomically great. When such odds can be calculated numerically, mathematical consultants can calculate the acceptability of a given risk. For instance, air shows rarely take place over cities because even though the chance of a crash may be tiny, the potential effects of a crash are serious enough to make the small risk unacceptable. (Gonick) Given the unfathomable potential effects of a comet or asteroid impact, it is easy to see why policy makers are becoming interested in protecting Earth from an event that is likely to take place once every 300 thousand years.
At some point in the future, a large comet or asteroid will strike the Earth. Will we be ready when it does?
Alpert, Mark. Killing Asteroids. Popular Mechanics. Hearst, April 1997.
Armageddon. Motion Picture. Produced by Jerry Bruckheimer. Released by Touchstone Pictures, 1998.
Arnett, Bill. Asteroids. Tuscon: Lunar and Planetary Library, University of Arizona, 1997.
Arnett, Bill. The Kuiper Belt and The Oort Cloud. Tuscon: Lunar and Planetary Library, University of Arizona, 1996.
Blindsiding Earth. Astronomy. November 1998.
Crawford, David. Personal communication, 24 November 1998.
Deep Impact. Motion Picture. Produced by David Brown and Richard D. Zanuck. Released by DreamWorks Pictures, 1998.
Dardano, Claire and Thomas H. See. Long Duration Exposure Facility (LDEF) Archive System. Hampton: NASA Langley Research Center, 1998.
Frazier, Ken. Comet Crash: Teraflops computer simulates powerful impact into ocean. Albuquerque: Sandia National Laboratories, 1997.
Gallant, Roy A. "The Sky Has Split Apart!" The Cosmic Mystery of the Century. Southworth: University of Southern Maine, 1992.
German, John. Real (not reel) Deep Impacts. Albuquerque: Sandia National Laboratories, 1998.
Gonick, Larry. Calamity Counters. Discover. Disney Publishing, May 1996.
Krakatoa. NASA Windows to the Universe. University of Michigan, 1998.
Pasachoff, Jay. Astronomy. Toronto: Saunders College Publishing, 1987.
Savage, Marshall.The Millennial Project. Toronto: Little, Brown and Company, 1994.
1. K-T stands for Cretaceous-Tertiary, the boundary between the Cretaceous period, the end of the dinosaur era, and the Tertiary period, the first of the modern era.
2. Comets actually have two tails, a bright one made of vaporized surface materials, and a dimmer ion tail, made of charged particles.
3. An Astronomical Unit, abbreviated AU, is equal to the mean distance from the Earth to the sun, about 150 million kilometers.
4. Many scientists have begun to accept the idea that Pluto and its satellite, Charon, are not really planets, but are the closest, and possibly largest, of the Kuiper Belt comets.
5. Maria - Latin for seas, the maria are the dark areas of the moon, a result of lava flows.
6. The energy released by a powerful explosion is typically given as the equivalent of a certain amount of trinitrotoluene, or TNT. A forty-megaton explosion is as powerful as an explosion of forty million tons of TNT, or about two thousand times the force of the world's first nuclear explosion, over Hiroshima in World War Two.
7. Reference lost. The Japanese team, in the 1930s, was convinced that a nuclear explosion had occurred. Since no one had developed a nuclear bomb yet, they argued that it could only be the result of an exploding alien spaceship.
8. "Giga" denotes one billion.
9. One "flop" denotes one arithmetic operation per second. "Tera" is the prefix for one trillion, so a teraflops computer is capable of at least one trillion computations per second.
10. Often incorrectly called a "tidal wave"
11. In The Millennial Project, Marshall Savage advocates using FELs in this manner to propel a spacecraft into orbit by firing double pulses at a block of ice on the back of the capsule.
12. Breaking up a large object would probably create more problems than it would solve; multiple objects striking a larger area of the Earth would create more ejecta than one large object.
The Enemy Above, copyright 1998 by George Beckingham