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Torino Scale - Part 2 of 3

Binzel presented his first hazard scale in 1995 at a conference
sponsored by the United Nations. That was a 0-to-5 system derived from
the probability of collision only. But the scheme never caught on with
impact specialists, some of whom openly scoffed at the notion, and
Binzel struggled with the rejection. A later conversation with Carl B.
Pilcher, NASA’s associate administrator for space science, homed in on
what was really needed. "I was expecting a single number to tell me more
than just the probability," Pilcher explained. But how could the chance
of a collision, its consequences (kinetic energy), and forecast interval
(time) be conveyed in a single number?
Then came last year's saga of 1997 XF11, a watershed that showed how
unprepared impact specialists were to deal with such events. "I was
incredibly frustrated that we didn't have a hazard index at our disposal
during the XF11 affair," Binzel says. "The close pass in 2028 would have
plotted as 0, and none of the subsequent returns would have exceeded 1.'

With astronomers smarting from a lack of internal coordination and NASA
managers (who fund many of the search efforts) demanding more
accountability, Binzel felt the time was right to try again. But how to
avoid another thud? The "aha!" moment came earlier this year with the
realization that using color with an expanded 0-to-10 scale could convey
all the essential information. Moreover, he sensed that the obvious
similarity to the Richter scale (for gauging earthquake severity) would
make it more palatable to the news media and the public.
Binzel offered up his new, improved version at the Torino meeting in
early June. The reaction was favorable, though most everyone had a
different opinion of how best to carve up the 10 regions, how to color
them, and how to describe each category. Armed with a vote of
confidence, Binzel returned to Massachusetts and hammered out details
over the next two months with the help of astronomer Clark R. Chapman
(Southwest Research Institute), the author, and science writer David
Chandler (Boston Globe). After a final review by Pilcher and IAU
officials, the Torino scale made its debut on July 22nd.

Voices of Reason

The problem with predicting close-brush encounters with asteroids is
that the most serious threats will likely come from as yet undiscovered
interplanetary foes. Roughly 1,000 asteroids 1 km or larger have orbits
that some day could cause them to slam into Earth, yet fewer than a
third of these have been found. Astronomers hope to locate 90 percent of
such objects within the next decade, but at the present rate of
discovery they will fall short of that goal. Moreover, impact
predictions become less certain when forecast far into the future.
Still, Binzel underscores that all foreseeable encounters with known
near-Earth asteroids plot as 0 on the Torino scale. He notes that
predictions for both 1997 XF11 and 1999 AN10 would have been initially
classified as 1 soon after their discovery, then downgraded to 0 after
further observation. "Imagine if this scale had been in place two years
ago," he muses.
The continued support of NASA and IAU officials will be key to the
Torino scale's longevity. "This is an international problem that
deserves an international solution," observes Paul W. Chodas (Jet
Propulsion Laboratory), a member of NASA’s recently established
Near-Earth Object Program Office. Notably, the IAU has already proposed
that asteroid hunters voluntarily allow their results to be reviewed
independently - whenever a predicted close approach rises above level 0
on the Torino scale. Milani estimates that predictions of Earth-buzzing
asteroids will become more common, roughly one per year, so having a
useful yardstick in place will help everyone distinguish the false
alarms from the real thing. "No near-Earth asteroid has ever exceeded
level 1 on the Torino scale," Binzel notes. "Let's hope it stays that
way!"

Color: pale, light green - Events having no likely consequences
Color: green - Events meriting careful monitoring
Color: ocher - Events meriting concern
Color: orange - Threatening events
Color: brown - Certain collisions

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