Originally written November, 1994
There's a rock out there with your name on it.
Not you personally, of course, but you and every human being and the whole beautiful blue planet Earth. The rock will come from the realm of our own solar system, swirling among the nine major planets. It will smash through the atmosphere in a few seconds and explode with a violence that makes the most ferocious cold war nuclear weapon look like a cap gun. It will forge a crater the size of New Brunswick in the Earth's plastic crust, and a trillion tons of dust will be blown into the stratosphere to blot out the light of day for a hundred months.
It has happened before, and it will happen again. This is not a pilot for another TV series. This is serious science, and it's scary. Every now and then, a major planet takes a bang from one of the teeming millions of smaller objects, like rocky asteroids and icy comets, that are zooming through space along with us. Astronomers, geologists, even environmentalists, are studying every conceivable aspect of these collisions. Their initial conclusions are surprising and unsettling — the danger is more serious than previously thought.
Is there any defence against this calamity? Scientists are emphatically saying yes. Amid the hoopla of this summer's excitement over the crash of 21 hefty fragments of the shattered Comet Shoemaker-Levy 9 into Jupiter, the solar system's largest planet, important developments have taken place. The time has come to act.
Most of the more than 5000 known asteroids orbit safely in paths that keep them well away from the Earth. It wasn't until 1932 that an asteroid was first discovered in an orbit that might conceivably bring it to collide with Earth. Only a few dozen in such orbits were known just a generation ago. And the 1000 or so known comets seem few, fast and far between. The barrage that caused the dense population of craters on the Moon, and Mars and Mercury, was already known in the 1960s to belong to a distant time when the solar system was young. Certainly the heavy bombardment was over no less than 3.9 billion years ago, and no one considered either the earth-approaching asteroids or the comets to be a menace in the current era.
But recent observing programs — such as two that use older telescopes at Kitt Peak National Observatory near Tucson, Arizona, and on Palomar Mountain in California — have spotted a few hundred more Near-Earth Asteroids, or NEAs, and the number keeps going up. Robert Jedicke, a University of Western Ontario graduate who [in the 1990s] worked on the Spacewatch project at Kitt Peak, says he and his colleagues discover about two or three each month. "With any luck we might be able to double our discovery rate in the next year or so with various improvements to hardware, software and simple strategy changes," says Jedicke.
In the early 1980s, evidence mounted that the extinction of the dinosaurs 65 million years ago might be blamed on just such an impact. A thin layer rich in iridium, a chemical element known to be more concentrated in asteroids and comets than on earth, was identified all over the world in rocks that date from the time of the extinction of the dinosaurs. Eventually, the "smoking gun" was found: a buried meteorite crater in Mexico's Yucatan peninsula. This impact is known as the Cretaceous/Tertiary Event, and about 70% of all the species of life on earth at that time perished. Small mammals that likely were the ancestors of humanity were among the survivors.
In fact, mass extinctions have occurred repeatedly in the past, as often as every hundred million years or so, and some scientists even venture that they were all caused by colossal cosmic collisions. It now seems certain that the bombardment of Earth, though infrequent, is continuing to this very day. Erosion has wiped out any blemishes made on the Earth's surface more than a few billion years ago. Nevertheless, there are about 150 meteorite crater structures known on the Earth, says Eugene Shoemaker, a co-discoverer of Comet Shoemaker-Levy 9 who has spent his entire scientific career studying collisions and the objects that do the colliding. "The impact rate over the last 100 million years is a little higher than the average over the last 3.5 billion. Far from diminishing in recent geological time, the rate of cratering might actually have increased," says Shoemaker.
Now humans are on the scene. Never in the history of Earth have there been creatures who understand what has happened, and there is every reason to believe we can do something about it. In a 1992 NASA report, the U.S. space agency says "the stakes are so high for these very rare but very deadly events that it is clearly sensible for an enlightened technological society to develop means for detecting and deflecting or destroying Earth-threatening objects, with particular emphasis on the larger objects."
To begin with, programs like Spacewatch can be expanded. Right now, Robert Jedicke and his colleagues use a 90 centimetre diameter telescope — bigger than you can buy at a camera store, but hardly among the largest in the world. A telescope with a mirror 180 centimetres across is being built for Spacewatch, but on a shoestring budget. Robert Jedicke says the new telescope will be a major improvement, and it might discover asteroids ten times faster than the current system.
To go further, the NASA report suggests the construction of a global network of six fairly large telescopes, called Spaceguard, that would devote every clear night to a systematic search for asteroids. The search would be conducted to maximize the chances of discovering NEAs that might be on a collision course with the Earth. According to the report, Spaceguard could expect to discover 500 NEAs every month--more than ten a day.
It is estimated that the asteroid which knocked out the dinosaurs was 10 kilometres across, but an asteroid even one kilometre across could cause widespread damage. After 25 years of searching with a system like Spaceguard, the NASA report forecasts that 90% of all objects larger than one kilometre across would be found and catalogued. Their orbits would be studied, both to learn about their origin and to predict their future paths. Mathematician Paul Chodas, who was born and raised in London, calculates orbits at NASA's Jet Propulsion Laboratory in Pasadena, California. "We compile all the measurements from observatories around the world to come up with the best possible estimate of the object's path through space," Chodas says, "and we predict how close the object will come to the Earth."
"With only a few days' worth of measurements, we can only make short-term predictions of the object's path," Chodas explains. "If astronomers can catch an object on several passes by the Earth, over the course of years, our predictions will be reliable for decades or centuries into the future."
But suppose the Spaceguard survey — or even Spacewatch as it is currently operating — really does find an object that big headed directly toward Earth. What could we do about it? The NASA report suggests a variety of possible responses, all at the limit of our technology. The least difficult plan involves using a nuclear weapon. The most powerful rocket NASA currently has, the Titan III, could launch a major nuclear bomb to intercept an NEA.
The flight would take 18 months or so, and meet the asteroid millions of kilometres from the Earth. The explosion could be planned to either destroy or deflect the asteroid. Destroying it would carry the risk that the splinters might still be heading toward the Earth and could cause just as much damage. Deflecting it into a slightly different orbit is safer, but requires very accurate timing and a detailed knowledge of how the explosion would affect the asteroid.
Partly to learn what kind of asteroid it is and to predict these effects, Shoemaker and others argue that a prospecting mission should be sent to the asteroid ahead of time. Aside from the objective of defence, a robot spacecraft would return a wealth of scientific information that would help us understand the whole solar system. A reasonably inexpensive instrument package, like the Clementine mission that was developed co-operatively by NASA and the U.S. military and launched to the Moon and beyond in 1994, could serve the purpose.
That kind of research need not wait until a threatening asteroid is actually discovered. In fact, the Galileo spacecraft was aimed to pass near two asteroids on its way to Jupiter. And there are other proposals, waiting for political approval and funding, to send Clementine-class instrument packages to other asteroids. These missions can be at least partly justified as integral to the defence of the planet.
Even more research is required into smaller objects that could hit the Earth. The NASA report emphasizes the importance of NEAs 10 kilometres or more across, because those could cause truly global catastrophes. However, there are many more objects floating around out there which are not so large but would nevertheless cause serious regional damage. For each and every one of the thousands of NEAs one kilometre across, there are perhaps another 100,000 space rocks down to 10 metres in size — as big as a house. Despite their smaller size, they should not be overlooked as potential hazards.
Because there are so many of them, they collide with earth more frequently. On October 9, 1992, a meteor flashed across the skies of the eastern U.S. and a meteorite the size of a football plowed into the rear end of a parked car near New York City. Even more recently, on the evening of June 14, 1994, another meteor zoomed over southern Quebec, and dropped dozens of small rocks on farmers' fields near Sorel. Peter Brown, a graduate student at UWO who specializes in meteors and meteorites, went to Quebec to help search for the pieces, one of which was recently purchased by UWO. Brown says he doesn't know of any incident in which a person was killed by a meteorite, but the threat is always there. To a single individual, being killed by a small meteorite is no less serious than being killed by a large one.
A meteor doesn't have to kill people to cause trouble, however. About once a year, a meteor the size of your kitchen explodes somewhere over the earth with as much energy as the atomic bombs that were dropped on Hiroshima and Nagasaki. There is, of course, no bomb aboard a meteor; the explosion, called an airburst, occurs solely because the incoming rock is moving so fast that it heats up quickly due to friction with the air. Brown says that military warning satellites have picked up such airbursts on numerous occasions, and have mistaken them for bomb explosions. On February 1, 1994, U.S. Defence Department officials woke up President Bill Clinton because of an alarm that, as it turned out, was set off by an airburst over the Pacific Ocean.
In fact, there's no end to it. Very small objects, the size of pebbles or even just grains of sand, are so numerous that they can be seen burning up in the atmosphere on any clear night — they'recalled "shooting stars." These tiny meteors can't reach the surface of the Earth, but they're going so fast that they pack quite a punch. If they hit a commercial satellite, it can get expensive. On August 11, 1993, during the Perseid meteor shower that thousands of Londoners witnessed from county roads around the city, an $850 million experimental European communications satellite named Olympus suddenly stopped functioning. Brown says project engineers at the European Space Agency have concluded that a small meteoroid probably knocked out the satellite, and he's sure that is what happened. During the same meteor shower, the U.S. magazine Aviation Week and Space Technology reports Russian cosmonauts aboard the Mir space station heard rapid-fire pings on the outside of their hull. If one of the meteoroids had punctured the station, the cosmonauts would surely have died.
There is one final aspect of the threat from space: long-period comets. While most asteroids have nearly circular orbits that have mostly not changed in billions of years, long-period comets are knocked down toward the centre of the solar system from well beyond the orbit of any planet. There is no predicting when one might appear, and there is no hope of making a complete survey from Earth. Many are comparable in size to the 10 kilometre asteroids, so long-period comets are a serious menace. David H. Levy, the Canadian amateur astronomer who has discovered 21 comets, including Shoemaker-Levy 9, points out that there are far fewer large asteroids than small ones. But big comets are not much rarer than small comets and "In the past two centuries, the Earth has been grazed by large comets coming in from distance space," says Levy, who lectured at Fanshawe College [1994-11-21]. In fact, Levy notes, comets passed closer than 6 million kilometres from Earth several times, most recently in 1983.
Instead of trying to catalogue the large comets, a system of telescopes like Spaceguard would have to be maintained indefinitely. A constant vigil could provide a few years warning — time enough to execute a defensive plan, if mankind is prepared.
When Comet Shoemaker-Levy 9 hit Jupiter in July, 1994, it was the first time that human beings had ever witnessed a major impact of bodies in our solar system. In dozens of media interviews, Levy was asked if something similar could happen to the Earth, and the answer is yes. The event sparked discussion in the U.S. Congress, and a resolution was passed creating the Near Earth Object Survey Committee to pursue further the idea of hunting all large asteroids. Shoemaker chairs the new committee, and he says a report will be ready in late January. Many commentators expect the report will recommend that Spaceguard be built at once.
It must be remembered that, serious as the consequences of a cosmic collision would be, the odds are in our favour. It may be many millions of years yet before another major comet or asteroid hits the earth, and there's certainly no need to panic. But another impact will definitely occur sometime, and it is worth knowing more about these potentially-deadly objects.