he Shroud of Turin--thought by some to be a divine artifact--draws its
name from Torino, the west Italian city where the shroud has resided for the last
526 years. As of June 1999, the city has had another potential act of God
to boast about: Catastrophic Asteroid Impact!
Following comet Shoemaker-Levy's spectacular 1994 run-in with the
planet Jupiter, a rash of movies, TV shows and news reports have educated
Earthlings about the perils of falling rock. Or ice, as the case may be. And
indeed, the Earth is bombarded daily by some 400 tons of material, mainly
dust, pebbles and snow from interplanetary space that drift across our
orbit at just the right (or wrong) time.
Usually these "meteorites" burn up in the atmosphere, but a few of them
reach the ground, where, being hot and heavy and bullet-fast, they
occasionally destroy property. Death or injury by meteor is extremely
rare--only about one out of every 100 million deaths occurs that way--but if a
really large rock or snowball struck the Earth at an interplanetary velocity
of 7-50 kilometers per second, we'd all have reason to fear. In fact, there
are close calls of this sort all the time, such as KA2, the locomotive-sized
asteroid that passed between the Earth and moon in 1993, and the more recent
1997XF11, which initially appeared to be on an encounter course with Earth
in the year 2028, until better astronomical data showed that it wasn't.
And there are not-so-close calls, too, like the K-T impact, which is thought to have
killed off the dinosaurs 65 million years ago, and the Barringer impact that
left a mile-wide crater in Arizona around 50,000 B.C., and the Tunguska
event of 1908--this very century!--in which a suspected cometary impact
flattened Siberian trees over an area of 22,000 square kilometers. Actually,
it wasn't even a real impact: the comet vaporized in the atmosphere,
releasing its energy in the form of a fiery shockwave. It's just our good
fortune that the release took place in one of the remotest wilderness areas
on Earth, 92 kilometers from the nearest village (where roofs and windows
were nonetheless blown out).
Speed kills
Comets, as the Siberians learned, are generally more dangerous than
asteroids, because they move a lot faster. Kinetic energy is measured by
E=(1/2)mV2, meaning the energy (i.e, heat and shock) released by an impact
increases linearly with the object's mass, but exponentially with its
velocity. In other words, twice the mass means twice the damage, but twice
the velocity means four times the damage. In astronomy, as in traffic, speed
kills.
To be a threat to humans, a celestial object must first, obviously, have an
orbit that intersects Earth's. Next, it needs either (a) enough mass to
reach the ground intact and cause damage on impact, (b) enough kinetic
energy to release a destructive burst of heat (i.e., an explosion) into the
atmosphere, or (c) both of the above.
According to our best current estimates, there are at least 4,000 asteroids that fit this
description, plus an unknown number of comets lurking far beyond the orbit
of Neptune, in tall, skinny orbits that bring them sunward on a schedule
measured in decades or centuries. Now, just because these orbits cross ours
doesn't mean a collision is inevitable--by definition intersecting
ellipses touch at only two points, and both the impactor and impactee would
have to occupy one of those points at the same time. Synchronization is
critical. But with that much junk flying around, major impacts do happen.
What does "major" mean?
But both the press and the public have rightly asked: What does "major"
mean? When a new object is spotted that might or might not threaten Earth,
how do we measure that threat in an easily understandable way? MIT
astronomer Richard Binzel proposed a solution at the International
Astronomical Union (IAU) in Torino this summer, where it was immediately
adopted as the "Torino Scale."
Like the Richter scale for measuring the intensity of earthquakes, Torino is
partly a measure of energy release. It includes an additional factor,
though, crucial for assessing the immediacy of concern: probability of
near-term impact. As orbits drift over millions of years, any Earth-crossing
object could eventually sidle into collision position; as with weather,
chaos theory prevents us from predicting these motions beyond a certain time
horizon. What we want to know is, over the span of the decades when our
calculations are accurate, what might and might not be dangerous?
The scale's highest rating, 10, indicates an object with a near 100 percent chance
of causing a global climatic catastrophe, such as a mile-wide asteroid on a
certain collision course. Such Torino 10 impacts, which occur every 100,000
to 1 million years, are a major driver of Earthly evolution (just ask the
dinosaurs), but are unlikely to threaten our current civilization.
Regional devastation
Next down are the "red" ratings of 9 and 8, indicating certain impacts of
varying severity. A 9 points to regional devastation of the Tunguska or
Barringer variety, in which whole nations might easily be destroyed. This
happens on Earth every 1,000-100,000 years. An 8 indicates a more localized
destruction of the sort that occurs every 50-1,000 years, i.e., well within
the time span of human society. It's probably here that we should find the
greatest levels of international concern, as part of an overall disaster
planning effort that encompasses major earthquakes, volcanos and other
assorted "big ones."
In the "orange" zone, Torino ratings of 7, 6 and 5 describe close
encounters with objects that have a sigificant chance of striking our
planet. Again, the high number points to a global problem, the low to a
local one. As such an encounter approached, astronomers' observations would
presumably upgrade or downgrade the threat as orbit predictions improved.
The "yellow" zone, from 4-2, points to objects with a 1 percent or greater chance of
collision within the next few decades, and the "green" rating of 1 indicates
a nonzero but measurable chance of collision. The scale's lowest rating, a
"zero" or "white," is applied to objects with either a negligible chance of
hitting the Earth, or a kinetic energy too low to wreak significant havoc.
Even with a 100% chance of collision, objects momentous enough to destroy a
car or even a house are in the same everyday category as hail and
avalanches--usually harmless, though occasionally fatal to an unlucky few.
The other class of Torino 0 objects are large, distant bodies whose orbits
won't carry them close to Earth anytime soon. These, God love 'em, will
spare 100 percent of us for as long as we care to predict. Interestingly, of the
hundreds of Earth-crossing asteroids and comets astronomers have detected so
far, all fall into this latter category. And with any luck, it'll stay that
way for a good long while.
Wil McCarthy is a rocket guidance engineer, robot designer, science fiction
author and occasional aquanaut. He has contributed to three interplanetary
spacecraft, five communication and weather satellites, a line of
landmine-clearing robots, and some other "really cool stuff" he can't tell
us about. His short fiction has graced the pages of Analog,
Asimov's, SF Age and other major markets, and his
novel-length works include Aggressor Six, the New York Times Notable
Bloom, and upcoming The Collapsium.
