From: Ames Research Center
Posted: Friday, July 26, 2002
Dear Friends and Students of NEOs:
As you probably all know, we are in the midst of an orgy of misinformation and confusion in the press, concerning asteroid 2002 NT7.
There have been "false alarm" stories in the past about threatening asteroids, some originating in poorly informed or misguided statements made by astronomers. The general pattern has been that a warning is issued and a day or two later retracted, reflecting either improved calculations or new data or both. It is entirely normal that a very low probability prediction of impact will go to zero as more information is processed. This is not a "failure" of the system, but rather the normal working of the Spaceguard Survey and supporting dynamical calculations. However, the press sometimes portrays this as a "mistake" by astronomers. Consequently, most of us prefer to see no press coverage of such low-probability predictions. It was in this spirit that no formal announcement was made concerning 2002 NT7, since new observations are accumulating and the whole situation is likely to resolve itself within a few days.
Unfortunately, the media themselves seem to have created the current flood of publicity surrounding NT7. Initial statements from the British press stated that 2002 NT7 was on a collision course with Earth with the impact predicted for February 1, 2019. The only qualification was that the prediction of an impact was still somewhat uncertain. There was no hint of the true situation, in which the probability of impact was at the 1-in-a-million level. As noted by Don Yeomans and others, the position of 2002 NT7 on February 1, 2019, is actually uncertain by many millions of kilometers!
The situation has been made more complex by press references to the Palermo technical scale for classifying an asteroid risk. Astronomers use this Palermo scale in communicating among themselves, but several years ago they agreed to use the simpler Torino risk scale in talking to the press and public. The Torino scale was invented to facilitate such communication. As recently noted by Rick Binzel: The largest rationale for the Torino Scale is that we all have a common lexicon for communicating with the public. If each and every one of us would say: "That object is only zero (or one) on the Torino scale, meaning we are carefully monitoring it, with no cause for public concern." eventually the responsible press and responsible people would learn to categorize these "events" as being the same as the last ones that "just went away."
Below are two items: (1) a statement that I am sending to people who e-mail me expressing their concern that this asteroid will hit in 2019, and (2) a selection of the distorted initial press coverage in the UK taken from Benny Peiser's CCNet.
(1) NOTE TO CONCERNED CITIZENS
This newly discovered asteroid (2002 NT7) is very unlikely to hit the Earth. The current odds are a million to one against hitting. In any case, astronomers are continuing to measure its position, and within a few days we should know for sure what the circumstances are for 2019. Meanwhile, the story has been blown out of proportion. Statements from the press that the asteroid is on a collision course with Earth are simply false.
The probability of impact is so small that this asteroid remains at a risk level on the Torino scale of 0 or 1 - meaning that the chances of impact from another unknown NEA of the same size or larger is similar to the chances of being hit by 2002 NT7. However, the impact probability is not yet zero, and additional observations are needed to ensure that this object will not hit the Earth in 17 years.
Although 2002 NT7 is unlikely to pose any danger, the long-term risk of asteroid collision is real.
The Earth orbits the Sun in a sort of cosmic shooting gallery, subject to impacts from comets and asteroids. It is only fairly recently that we have come to appreciate that these impacts by asteroids and comets (often called Near Earth Objects, or NEOs) pose a significant hazard to life and property. Although the annual probability of the Earth being struck by a large asteroid or comet is extremely small, the consequences of such a collision are so catastrophic that it is prudent to assess the nature of the threat and prepare to deal with it.
Studies have shown that the risk from cosmic impact increases with the size of the projectile. The greatest risk is associated with objects large enough to perturb the Earth's climate on a global scale by injecting large quantities of dust into the stratosphere. Such an event could depress temperatures around the globe, leading to massive loss of food crops and possible breakdown of society. Such global catastrophes are qualitatively different from other more common hazards that we face (excepting nuclear war), because of their potential effect on the entire planet and its population. Various studies have suggested that the minimum mass impacting body to produce such global consequences is several tens of billions of tons, resulting in a groundburst explosion with energy in the vicinity of a million megatons of TNT. The corresponding threshold diameter for NEOs is between 1 and 2 km. Smaller objects (down to tens of meters diameter) can cause severe local damage but pose no global threat.
We don't know when the next NEO impact will take place, but we can calculate the odds. Statistically, the greatest danger is from an NEO with about 1 million megatons energy (roughly 2 km in diameter). On average, one of these collides with the Earth once or twice per million years, producing a global catastrophe that would kill a substantial (but unknown) fraction of the Earth's human population. Reduced to personal terms, this means that you have about one chance in 20,000 of dying as a result of a collision. Such statistics are interesting, but they don't tell you, of course, when the next catastrophic impact will take place-next year or a million years from now.
How much warning will we have? With nearly half of even the larger NEOs remaining undiscovered, the most likely warning today would be zero -- the first indication of a collision would be the flash of light and the shaking of the ground as it hit. In contrast, if the current surveys actually discover a NEO on a collision course, we would expect many decades of warning. Any NEO that is going to hit the Earth will swing near our planet many times before it hits, and it should be discovered by comprehensive sky searches. This is the purpose of the Spaceguard Survey. In almost all cases, we will either have a long lead time or none at all.
Meanwhile, the Spaceguard Survey has already discovered more than half of the near Earth asteroids 1 km or larger, and we are on track to find 90% of them before the end of this decade.
For more information see the NASA websites:
NASA Ames Research Center // end //
NASA Ames Research Center
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