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Planetary News: Asteroids and Comets (2005)

Astronomers Revise Torino Scale Asteroid Advisory System

by A. J. S. Rayl
20 April 2005

Astronomers have revised the Torino scale, the color-coded advisory system to assess the threat of asteroids and other near-Earth objects (NEOs) to make it easier for the public to understand.

"The purpose of the Torino Scale is to allow scientists to convey concisely how much real danger, or lack thereof, we face from newly discovered asteroids and comets, especially those predicted to make very close passes by our planet in the 21st century," Richard Binzel, the planetary sciences professor at the Massachusetts Institute of Technology (MIT) who created of the scale told The Planetary Society in a recent interview. "The new, revised version basically provides an easier-to-understand gauge that better communicates the risks with the public and to help assuage concerns about doomsday collisions."

Based on the probability of impact as determined from observations and orbit calculations, as well as the energy of the impact deduced from the object's estimated size, density, and velocity, the Torino scale enables NEO researchers to place objects within a potential threat range from zero -- where there is virtually no chance of collision, to 10 -- where global catastrophe is certain. It was first adopted in 1999 by a working group of the International Astronomical Union (IAU) at a meeting co-sponsored by The Planetary Society in Torino, Italy.

In the days and months following its adoption, critics complained that first Torino scale was actually scaring people -- "exactly the opposite of what was intended," said Binzel. Although he had always considered the scale a "work in progress" that would evolve with time, those criticisms served to hasten the revisions. During the past year or so dozens of astronomers around the world offered input and took part in the process, and the new, improved Torino scale was published late last year in a chapter of Mitigation of Hazardous Comets and Asteroids (Cambridge University Press).

For the most part, Binzel said, it was the wordage that was worrying people. "It was the semantics, and, for the astronomers it was really trying to figure out the audience," he reflected. "In the original development, this was as much for the audience of astronomers as it was for the public, even though we always meant to talk to the public. So the first version was our first attempt to speak to the public. As we've had more and more discoveries, we've learned how well this works and how well it doesn't work."

In Category 1 -- the green zone -- for example, the events so labeled were defined as "events meriting careful monitoring" a phrase that turned out to mean distinctly different things to the public than it did to astronomers. "For astronomers, it meant that this was an object that has some small but non-zero chance of hitting the Earth and astronomers should track it -- meaning we shouldn't ignore it," explained Binzel. "For John Q. Public the 'careful monitoring' meant 'uh-oh, astronomers are worried, scientists are on the case and monitoring this thing to find out if we're going to die. John Q. Public really didn't understand what we really meant by Category 1 before, but now we characterize Category 1 as being 'a routine discovery in which a path near the Earth is predicted that poses no unusual level of danger,' which is much clearer."

Throughout the revision process, the astronomical community honed its skill for communicating asteroid risks with the public. "We now have a better idea of what questions the public wants to know, the first of which is -- 'Should I be concerned?'" said Binzel. "The original scale did not make the distinction as to when an object is of interest to astronomers and when it is that an object should be of interest to the public. What we're saying now is a little more understandable."

The general process of classifying NEO hazards is roughly analogous to hurricane forecasting, where predictions of a storm's path are updated as more and more tracking data are collected. "It's the same thing with astronomers," said Binzel, "and equally important in the revision of the Torino scale is the emphasis on how continued tracking of an object is almost always likely to reduce the hazard level to 0, once sufficient data are obtained."

"The revisions in the Torino Scale should go a long way toward assuring the public that while we cannot always immediately rule out Earth impacts for recently discovered near-Earth objects, additional observations will almost certainly allow us to do so," said Donald K. Yeomans, of JPL, manager of NASA's Near Earth Object Program Office.

Binzel first began thinking about developing an asteroid alert system back in the early 1990s. By 1992, he was ready to talk about it out loud and his first audience turned out to be Carl Sagan and Louis Friedman, co-founders of The Planetary Society. They were all having dinner together following an event at the Museum of Science in Boston.

"The table's company had broken into several small conversations, and I remember explaining to Carl that I was developing a numerical rating system to help the public comprehend relative threats from asteroid and comet impacts," recalled Binzel. Sagan responded immediately, enlisting Society support. Later, after "a bit of arm twisting," Friedman succeeded in persuading Binzel to write up some preliminary ideas for The Planetary Report's March/April1995 issue. At the same time, he prepared a formal presentation for a United Nations Conference on Near-Earth Objects (NEOs), which was, as it turned out, cosponsored by The Planetary Society. Despite the discussions it spawned in the early and mid-1990s, Binzel's concept of a numerical ratings system did not take root then.

Binzel carried on, pondering the scale with colleagues and members of the science media. Finally, in 1999 he presented a reformulation of his original ideas at the international workshop on impact hazards in Torino, and there his system found life -- and its name.

While the revised Torino scale offers significantly more clarity, the basic numbering system and color-coded delineations remain the same. "For a newly discovered NEO, the revised scale still ranks the impact hazard from 0 to 10, and the calculations that determine the hazard level are still exactly the same," Binzel confirmed. "The difference is that the wording for each category now better describes the attention or response merited for each."

In the original scale NEOs that ranked 2 to 4 -- in the yellow zone -- were described as "meriting concern." The revised scale describes objects with those rankings as "meriting attention by astronomers, and not necessarily the public," informed Binzel. Now for objects at Level 4, "we suggest attention by the public and public officials is merited if the encounter is less than a decade away. In other words, as long as we have more than 10 years, a Level 4 means, that means there is a few percent chance of hitting the Earth. For example, say it's 2% or a 1-in-50 chance of hitting the Earth. That means it's 50-to-1 odds that when we get enough data it will go away," explained Binzel.

"We're trying from an astronomer point of view to give some indication as where we think public concern is merited, or even concern by any government official is actually merited," he expounded. "These are only suggestions, and everyone's going to have their own threshold. But the Torino scale now gives some suggestion as to where that threshold should probably be."

From the outset, a school of opposing thought emerged from the astronomical community's debates about the Torino scale, which held, in essence, that astronomers should just keep their mouths shut. "We talked a lot about whether we should keep potential NEO collisions secret or be completely open with what we know when we know it," Binzel recalled. "Say we discover an object and based on a preliminary analysis of its orbit we can see it's going to come close to the Earth in 20 to 30 years from now. Astronomers have to ask -- 'Do we say something or wait and get more data?' It is a conundrum."

In fact, during the summer of 2002, a news reporter picked up calculations at NASA/JPL NEO pages that asteroid 2002 NT7 was on a collision course with Earth, to hit February 1, 2019, and disseminated it pretty far and wide before astronomers could put the skids on the story. In fact, just a few additional observations thrust the asteroid into "the typical, inexorable decline that we normally see in these cases," as Steve Chesley, senior engineer at NASA's NEO program office at JPL, put it. Within days, the calculated probability of an impact was eliminated entirely.

The IAU working group, for which Binzel serves as secretary, concluded that to best serve the public that funds them they should make available whatever data they have when they know it, given they have completed at least a few days of checking calculations and acquiring a little more data.

In a sense, astronomers are between a rock and a hard place, darned if they do and darned if they don't. "If we discover an object and find there's 1 in 50 chance of it hitting the Earth and then we get more data and find it's going to miss, many reporters will write that we made a mistake," Binzel pointed out. "It's not a mistake at all, but simply a scientific method at work. So astronomers face the risk of the media reporting that we've made a mistake and we don't know what we're doing, when in fact we're just trying to convey the best information we know when we know it, just as meteorologists do with hurricane forecasting.

Ultimately the consensus -- "which doesn't mean everyone agrees," Binzel underscored -- held. The astronomical community should be open with data, even though it leaves the scientists vulnerable to criticism.

"If we have the responsibility of making data known effectively in real time, then we also have the responsibility of having good tools for explaining what we find," Binzel said. "That's the goal of the Torino scale."

Since the IAU's adoption of the Torino scale, at least two other tools have been put forth:

- The Palermo Technical Impact Hazard Scale, introduced in 2001, compares the probability of the impact of an object, taking into account time duration between the present and potential impact date, with the chance of a random object of the same size colliding with Earth in that same time interval.

- The Purgatorio Ratio, devised and put forth in 2003 by Brian Marsden of the Minor Planet Center, the grand central clearinghouse for asteroid data, focuses on the ratio of the known path of a newly discovered object to the amount of time before any predicted possible impact would occur.

"Both the Palermo scale and the Purgatorio Ratio give measures of the quality of the available information, but you still need a scientist to interpret what the number means," said Binzel. "With the Torino scale, the interpretation is all done ahead of time, so you can quickly determine what it means. If you said it's a 1 on the Torino scale, you could go look at your decoder ring -- think of the Torino scale as a decoder ring -- and you would discover that it's 'a routine discovery in which a pass near the Earth is predicted that poses no unusual level of danger.' 'Oh, okay, fine.' It's not news. The Torino scale is really a streamlined approach -- almost a sound-byte approach -- to help science reporters and the public to understand what category a given object falls into and whether they need to be concerned."

The Palermo Technical Impact Hazard Scale and the Purgatorio Ratio have been used less widely or frequently than the Torino Scale, which has benefited from promotion by asteroid experts at NASA, which provides most of the funding for asteroid search programs. Nevertheless, Binzel views them all as viable tools, complementary and not necessarily competitive. "This is a very multi-dimensional problem, both in terms of the science and in terms of public communication, and different tools have their different utilities," he said. "My view is that any tool that helps understanding is a good tool."

The Torino scale really is directed at the public, and it's goal in effect is to become the Richter scale of the asteroid world. "If someone said, 'Oh there was magnitude 2 earthquake today in California,' you would go, 'So what?' If they said it was a 6, that would elicit a completely different reaction," Binzel noted. "The Torino Scale is similar in that sense. If you're dealing with a NEO that is a 1 or 2, there should be an immediate reaction that it is of no concern to you or your well-being. When you get up to 8 on the Torino scale you have finally reached certainty that an object coming by the Earth is going to hit and cause local effects," much like you know an 8 on the Richter scale will raise eyebrows and level buildings. Objects that fall in the 8, 9, or 10 level on the scale are certain to hit Earth. A 9 on the Torino scale denotes large regional effects, and 10 signifies ultimate catastrophe. "But a 1 on the Torino scale is routine."

Now it's just a matter of the scale becoming more well-known and understood. "I've always believed and understood there is a long learning curve in the public at large having some understanding of what the numbers means," Binzel acknowledged. He is confident recognition of the scale's numeric system will grow in time.

When one looks at the brightly color-coded Torino scale, it is hard not to notice its similarity to the Homeland Security Terrorist alert system. "I read somewhere -- and I don't know how to document this -- that when they were working on the Homeland Security scale, they used the Torino scale as a model," Binzel said.

Whether or not that's true, what is true is the Homeland Security Terrorist alert system was slammed as worthless. "Having invented a scale of my own, I watched and listened to the criticisms made of the Homeland Security scale with a great deal of interest," Binzel said. Among the key complaints of the Homeland Security scale is that it does not tell the public that they should do or at what level they should actually be concerned. "In the revisions to the Torino scale, I tried to pay attention to that and other criticisms and responded to those criticisms as if they were criticisms of the Torino scale."

So, does the Torino scale work more effectively than Homeland Security's Terrorist alert system?

Consider the near Earth object classified highest to date on the Torino scale -- asteroid 2004 MN4, which was categorized at a Level 4 last December. "It was said then to have about a 2 percent chance of hitting Earth in 2029 or in about 25 years," reminded Binzel. "After extended tracking of the asteroid's orbit, however, that asteroid has now been reclassified to a level 1, effectively no chance of collision, the outcome correctly emphasized by Level 4 as being most likely. I leave it to individuals to make that decision as to whether the revised Torino scale did a good job there."

While the chance of something hitting the Earth and having a major impact is "very unlikely," according to Binzel, it is still possible. "The only way to be certain of no asteroid impacts in the forecast is to keep looking."

Astronomers are spotting and tracking more and more NEOs with telescopes around the world. NASA funded efforts include projects like the Lincoln Near Earth Asteroid Research project at MIT, the Lowell Observatory Near-Earth Object Search (LONEOS), and the Near-Earth Asteroid Tracking (NEAT) program developed by the Jet Propulsion Laboratory. As new NEOs are discovered, each needs to be classified.

"There's no increase in the number of asteroids out there or how frequently they encounter our planet," Binzel said. "These objects have always been there. The cratering record of the Moon and even of the Earth tells us they have always been there. What's changed is our awareness of them. We have knowledge now that they're there and knowledge of when they might be passing closely by, and once astronomers have that knowledge, our challenge is in how to communicate that knowledge."

The goal now, said Binzel, is to get the Torino scale in peoples' hands, or file drawers, or at their fingertips, so every time a new asteroid is discovered, we can say: 'It's a 1 or 2 or whatever on the Torino scale' -- and people and general assignment science reporters can look that up and know what that means, and know that it's nothing unusual. We want to make sure this is available as a tool for people to interpret new discoveries."