Five Years and Counting
In 1998, NASA formally embraced the goal of finding and cataloging, by 2008, 90% of all near-Earth objects (NEOs) with diameters of 1 km or larger that could represent a collision risk to Earth. The survey project is ongoing and is called Spaceguard. The 1 km diameter metric was chosen after considerable study indicated that an impact of an object smaller than 1 km could cause significant local or regional damage but is unlikely to cause a worldwide catastrophe. Any one kilometer-diameter object will punch right through the atmosphere regardless of its velocity or composition. But what to do about the much larger volume of smaller objects that pass near earth or potentially collide with the planet?
The total population of near-earth asteroids (NEAs) larger than 1 kilometer is about 1,100. In the last five years, telescopic search programs have found about 640 NEAs, so more than half of the estimate has been found already.
|Impact of the Tunguska explosion is visible even after 90 years.
Credit: Galena HS
Late this summer, NASA's Near-Earth Object Science Definition Team issued a report entitled "Study to Determine the Feasibility of Extending the Search for Near-Earth Objects to Smaller Limiting Diameters". The conclusions of their report are summarized to address the large volume of material that rains down on Earth in the form of dust and micro-particles, and whether such small objects are scientifically valuable or worth detecting for practical reasons.
To give a measure of the problem's significance, the team estimated the probability of these small impactors, as "a population of about 1100 near-Earth objects larger than 1 km, leading to an impact frequency of about one in half a million years. To the lower limit of an object's atmospheric penetration (between 50 and 100 m diameter), we estimate about half a million NEOs, with an impact frequency of about one in a thousand years."
To introduce the study's goal, the science team writes: "In recent years, there has been an increasing appreciation for the hazards posed by near-Earth objects (NEOs), those asteroids and periodic comets (both active and inactive) whose motions can bring them into the Earthís neighborhood. In August of 2002, NASA chartered a Science Definition Team to study the feasibility of extending the search for near-Earth objects to smaller limiting diameters. The formation of the team was motivated by the good progress being made toward achieving the so-called Spaceguard goal of discovering 90% of all near-Earth objects (NEOs) with diameters greater than 1 km by the end of 2008. This raised the question of what, if anything, should be done with respect to the much more numerous smaller, but still potentially dangerous, objects."
For objects this small, a peak in impact damage is local, and arises for objects in the range of 50-200 meters across (about the size of two football fields). The authors' estimate "damage from smaller land impacts below the threshold for global climatic effects is peaked at sizes on the scale of the Tunguska air blast event of 1908 (50-100 m diameter). For the local damage due to ocean impacts (and the associated tsunami), the damage reaches a maximum for impacts from objects at about 200 m in diameter; smaller ones do not reach the surface at cosmic speed and energy."
The team was tasked with providing recommendations to NASA, as well as the answers to the following 7 specific questions:
- 1. What are the smallest objects for which the search should be optimized?
- 2. Should comets be included in any way in the survey?
- 3. What is technically possible?
- 4. How would the expanded search be done?
- 5. What would it cost?
- 6. How long would the search take?
- 7. Is there a transition size above which one catalogs all the objects, and below which the design is simply to provide warning?
|"I was sitting in the porch of the house at the trading station of Vanovara at breakfast time and looking towards the north... suddenly the sky was split in two, and high above the forest the whole northern part of the sky appeared to be covered with fire." - Farmer Sergei Semenov of the Tunguska event, 1908
Since the initiative called Spaceguard will catalog larger objects by 2008, one must weigh the benefits against costs for extending the reach of this already ambitious survey effort. The report concludes that what falls through the cracks of the large-object survey has an associated risk of local damage: "The nominal yearly average remaining, or residual, risk in 2008 associated with potentially hazardous objects (PHO) impact is estimated by the Team to be approximately 300 casualties worldwide, plus the attendant property damage and destruction. About 17% of the risk is attributed to regional damage from smaller land impacts, 53% to water impacts and the ensuing tsunamis, and 30% to the risk of global climatic disruption caused by large impacts, i.e. the risk that is expected to remain after the completion of the current Spaceguard effort in 2008."
"According to the cost/benefit assessment done for this report, the benefits associated with eliminating these risks justify substantial investment in potentially hazardous objects (PHO) search systems."
However one of the challenges of designing a true Spaceguard system for small objects is not just the difficulty of detecting them, but what to do once a candidate is found. Since the effects are mainly local, advanced warning systems might function similar to current weather monitoring efforts, such as hurricane tracking. In that case, migration or temporary stopgaps have succeeded to minimize overall damages.
The Team recommends that the search system be constructed to produce a catalog that is 90% complete for potentially hazardous objects (PHOs) larger than 140 meters. They answer a number of questions that are frequently raised and estimate the quantitative solutions.
Should comets be included in any way in the survey?
The Team's analysis indicates that the frequency with which long-period comets (of any size) closely approach the Earth is roughly one-hundredth the frequency with which asteroids closely approach the Earth and that the fraction of the total risk represented by comets is approximately 1%. The relatively small risk fraction, combined with the difficulty of generating a catalog of comets, leads the Team to the conclusion that, at least for the next generation of NEO surveys, the limited resources available for near-Earth object searches would be better spent on finding and cataloging Earth-threatening near-Earth asteroids and short-period comets. A NEO search system would naturally provide an advance warning of at least months for most threatening long-period comets.
What is technically possible?
Current technology offers asteroid detection and cataloging capabilities several orders of magnitude better than the presently operating systems. NEO search performance is generally not driven by technology, but rather resources. This report outlines a variety of search system examples, spanning a factor of about 100 in search discovery rate, all of which are possible using current technology. Some of these systems, when operated over a period of 7-20 years, would generate a catalog that is 90% complete for NEOs larger than 140 meters.
How would the expanded search be done?
From a cost/benefit point-of-view, there are a number of attractive options for executing an expanded search that would vastly reduce the risk posed by potentially hazardous object impacts. The Team identified a series of specific ground-based, space-based and mixed ground- and space-based systems that could accomplish the next generation search. The choice of specific systems will depend on the time allowed for the search and the resources available.
What would it cost?
For a search period no longer than 20 years, the Team identified several systems that would eliminate, at varying rates, 90% of the risk for sub-kilometer NEOs, with costs ranging between $236 million and $397 million. All of these systems have risk reduction benefits which greatly exceed the costs of system acquisition and operation.
How long would the search take?
A period of 7-20 years is sufficient to generate a catalog 90% complete to 140-meter diameter, which will eliminate 90% of the risk for sub-kilometer NEOs.
Local History in Human Terms
Social anthropologist Benny Peiser, of Liverpool John Moores University in the UK, has written extensively about the influence of NEO impacts on human and societal evolution. He described the effects in human terms from the most famous, locally devastating impact of the 1908 Siberian strike as: "The atmospheric impact over the Tunguska region in Siberia was witnessed by many thousands of people and felt over an area of more than 1,000 miles in radius. Many native Tungus hunters were fairly close to ground zero, and some of them witnessed the large-scale slaughter of their deer herds as a result of the blast. Apparently, one or two hunters were killed by the explosion. Reminiscent of religious leaders who blame natural disasters on a vengeful deity, Tungus shamans told their people that disobedience had brought divine calamity upon themselves."
|While three-quarters of the overall NEO risk is due to large asteroids, the most likely impact to occur in the foreseeable future will be caused by a small asteroid.
"Further away from the epicenter, the disaster also was witnessed with trepidation. Just days after the impact, many Russian newspapers reported the huge explosion. A newspaper from Irkutsk, for example, described how peasants in the village of Nizhne-Karelinsk (200 miles from ground zero) 'saw a body shining very brightly with a bluish white light. When the shining body approached the ground it seemed to be pulverized, and in its place a huge cloud of black smoke was formed and a loud crash -- not like thunder, but as if from the fall of large stones or from gunfire -- was heard. All the buildings shook and at the same time, a forked tongue of flame broke through the cloud'. Everyone thought that the end of the world was approaching."
"Despite many similar reports and eyewitness accounts, newspapers and scientists discarded the whole incident, claiming that such bizarre stories were unsound and unreliable. As a result of this general disbelief, it took some 20 years before the first scientific excursion reached ground zero to investigate the causes of the catastrophe."
"And what about today? It is unlikely that we will encounter another Tunguska event in the near future. Impacts in the 10-megaton range probably happen only once every 500 to 1,000 years. But for argument's sake, let us contemplate how the public might respond if another Tunguska-type impact happened tomorrow. "
"For a start, the reaction of the public fundamentally will depend on the location, extent, and destruction of the impact. In all likelihood, another Tunguska event would occur over an unpopulated or scarcely inhabited region of the world. However, in the unlikely event of fatalities, the global uproar could be substantial. In such a case, 9/11 would look like an insignificant security failure. The blame game would be brutal, and I would certainly not like to be in the shoes of those who had advised the government that small impacts were negligible."