news.nationalgeographic.com
Killer Asteroids: A Real But Remote Risk?
By John Roach
for National Geographic News
June 19, 2003
It is almost certain that Earth will be hit by an asteroid large enough to exterminate a large percentage of our planet's life, including possibly over a billion people, according to researchers. But as such cataclysmic collisions occur on average only once in a million years or so, are they really worth worrying about?
At some point in the geological future a large chunk of rock and ice will smack into Earth and destroy life as we know it. This is a cold, sober, scientific fact, according to Andrea Milani, a researcher at the University of Pisa in Italy.
"A future impact from, say, a 1-kilometer [0.62 mile]-diameter asteroid is, rather than just probable, almost certain over a time span of a million years," he said.
The heavily impact-cratered front side of the moon, with older impact basins ranging in size from more than thousands to hundreds of kilometers in diameter. Fresh impact crater Tycho, at bottom, measures 85 km (53 miles) in diameter. This is a false-color composite of 15 Galileo images from December 8, 1992. The moon is evidence of constant asteroid impact in the earth's orbit.
Photograph courtesy NASA/JPL
Stunning photography, every month of the year. Click here to get one year of National Geographic magazine and a free gift.
Wolf Reimold, a geoscientist at the University of Witwatersrand in Johannesburg, South Africa, said a 1-kilometer-wide asteroid would produce an impact crater of about 12 miles (20 kilometers) in diameter and wipe out an area the size of the United Kingdom. The human toll would depend on where such an impact occurs.
"Estimates may range from 500,000 to 1.5 billion casualties," he said. "This latter number certainly smells of global nuclear war. Such an event would in all likelihood not wipe out mankind, but it would cause a global economic crisis."
Given the real threat of impact by a so-called near Earth object (NEO) and the consequences for human life, Milani and Reimold are urging the worldwide scientific community, and the agencies that fund their research, to take the field of impact mitigation seriously.
In separate papers appearing in the June 20 issue of the journal Science, Milani and Reimold outline what is known about the impact threat and how impacts have shaped the geologic and life history of Earth.
They agree that the developed world has made great strides over the past few decades in NEO research, but say that more funding is required to raise public awareness of the impact risk and to determine how to thwart an incoming object.
"Governments have the responsibility to deal with a lot of problems afflicting humankind. But these same governments must realize that large asteroid or comet impact has the potential to wipe out all other problems, including mankind," said Reimold.
Impact Science
Impacts of meteorites, asteroids, and comets are frequent events on a geological time scale, said Milani. They have shaped the surface of the Earth and altered the course of life that thrives upon it.
For example, 65 million years ago a 6.2-mile (10 kilometer)-diameter asteroid impact resulted in a 100-million-megaton explosion that excavated a 112-mile (180 kilometer)-wide crater on the Yucatán Peninsula in Mexico and brought the dinosaur era to an end.
Events such as the impact implicated in the dinosaur extinction happen on the order of once every 100 million years. Smaller objects collide with Earth with greater frequency. Asteroids large enough to cause ocean-wide tsunamis, for example, happen once every 63,000 years.
In 1998 NASA accepted the responsibility of compiling a catalog of at least 90 percent of NEOs of 1 kilometer (0.62 miles) in diameter or greater and to assess the probability that any of them will impact Earth. Such events are believed to happen on the order of about once every 1 million years.
To date the NASA initiative, known as Spaceguard, has identified 585 objects of 1 kilometer or greater. Most of them have no chance of impact and those that do have only a very low probability. Scientists estimate there are about 1,000 NEOs, so NASA is more than halfway to accomplishing its goal.
Reimold notes that this initiative and projects such as the British Taskforce on Potentially Hazardous Near Earth Objects and the Intercontinental Scientific Drilling Program into the Chicxulub crater in Mexico have helped scientists understand the risks and consequences of collisions with asteroids and comets.
The developing world, he said, is slower to catch on, but a movement by astronomers and geoscientists in South Africa to establish a National Working Group to assess NEO impact risk and mitigation is gaining traction.
"On the other hand, the general public in developing countries has a host of other problems than the possibility that a large bolide could wipe out mankind," he said. "If your first concern is to have shelter and food, if HIV/Aids and unemployment are your daily worries, you cannot be expected to be wary of meteorite impact."
More Mitigation Funding?
Writing in Science, Milani says that the scientific community should take on the responsibility to investigate all objects that could potentially impact Earth "down to the size compatible with available technology and with the public perception of acceptable risk."
According to Milani, a reasonable goal would be to detect within the next ten to 20 years 90 percent of the NEOs over 1,000 feet (300 meters) in diameter and 97 percent of those greater than 1 kilometer in diameter.
To accomplish this goal, Milani says that understanding and awareness of the impact risk must be raised amongst the public and the agencies that provide the requisite funding to perform the work.
"If [funds] are provided, the scientists would know how to use them efficiently," he said. "If resources dedicated to this task are not provided, the scientists have difficulties in canceling other worthwhile basic research to make resources available for impact risk assessment."
Reimold said that more money ought to also be made available for research into known and potential impact sites. Currently, he said, only a few impact sites older than 300 million years are known, but that many more should be out there.
"Ongoing detailed geological analysis of known impact structures is a must in order to further improve our knowledge of the impact process and its devastating results," he said.
Robert Jedicke, an asteroid expert with the Institute for Astronomy at the University of Hawaii, said that "it would be nice" if asteroid researchers had more money but that current funding for the NEO impact risk assessment programs is sufficiently supported given the available funding for all scientific research.
"There's only so much money to go around," he said. "So if the pot gets split there's less stew for the rest of the astronomical/scientific community."
NEO Deflection
As NEO researchers continue to search the skies for objects that pose an impact risk, they are also beginning discussions on how to deflect an object on a collision course with Earth.
One of the issues being explored is the interior structure of asteroids. If the interior is weak, for example, an attempt to deflect it with a nuclear warhead (an option under consideration) may simply breakup the asteroid into many smaller and uncontrolled pieces.
Milani writes that such investigations are a valid extension of the NASA and European Space Agency NEO programs and make logical sense: "We cannot justify the effort for discovery unless we can safeguard our planet."
Jedicke said that we are not currently prepared to deflect an incoming asteroid, but that there is no reason to be alarmed because there is little chance that an asteroid even as small as 330 feet (100 meters) will hit Earth within the next 100 years.
"They don't build tornado shelters in Germany. Cities don't buy snowplows in Florida. And there's no pressing need to worry about deflection of incoming NEOs at the moment," he said. |
