About NEOs

a radar image of 2-mile (3.5-km) wide 1999 JM8. picture courtesy Lance Benner, JPL

Near-Earth Objects (NEOs), technically speaking, are asteroids and comets approaching Sun at 1.3 AU (121 million miles -195 million kilometers) or or less. For the general public and the media the term however applies to any space rock able to make a grazing passage relatively to Earth. Worrying about such object is relatively new as until lately no systematic search had been launched about their threatening probabilities. Asteroids, in the solar system, are rocky -or icy- bodies mostly grouped into the Asteroid Belt located between the orbits of Mars and Jupiter, with stable orbits. Comets are icy and dust objects too with more elongated orbits into the solar system

Impact Odds

The April 13th, 2029, 99942 Apophis, which was considered a threat, grazing under the geosynchronous orbit, has been retrograted to no threat as it's remaining such for April 13th, 2036 due to that Earth will modify its orbit during the fist graze, placing it into an orbit on which it might be captured into a keyhole by Earth. More will be known by 2011 or 2013, when the decision will be eventually made to send a mission to modify the orbit of the asteroid. In any case, April 13th is the anniversary date of the webmaster of this site :-) The asteroid is 690 and 1080 feet (210 and 330 meters) wide, and, if striking, would in the Atlantic Ocean triggering large tsunamis and expandind ashes and dust in the atmosphere for an undetermined period of time. A good new is maybe that the asteroid might collide with another one

Most of the NEOs larger than 1 km in width are in the process of being catalogued at the horizon 2008, as such bodies could yield civilization or regional-sized damages with an impact frequency of one each half a million years (or one under each 100,000 years). On the 4,300 in the catalog, 700 have been found until now, of them 120 having the potential to hit Earth -albeit statistics are showing that one will do. What will be done after 2008 about objects of less of 1km in width is that a new survey aiming at a cataloging 90 per cent of such NEOs, larger than 140 meters, has been set up, with the survey targeting 2020 for an end. 40,000 such objects might be spotted. Long-period comets, as they represent 1 per cent of the threat only, would not be surveyed as they represent only 1 per cent of the risk, as short-period comets and the remaining large NEOs the present survey did not find would be looked for. This second survey would take between 7 and 20 years, as it would further cope with 60-90 per cent for objects between 140 and 50 meters (which is the limit for an object to reach Earth; objects between 50 and 100 meter wide would be about half a million, with an impact frequency of one every 1,000 to 3,000 years (a few hundred to a thousand years for 100 m-wide ones). These rocks are the size of the Tunguska object (an asteroid which crashed into Siberia in 1908) and cause local damages and tsunamis). A last point is the lack of any program for the Southern hemisphere. Some lesser asteroids travel in swarms or streams, as 1,300 ft (400 m) wide asteroids pass within 2 lunar distances every 5 years. There is, at last, an estimated total of 1 million of asteroids, generally. Everything under 160 ft (50 m) explode in the Earth's upper atmosphere

The current survey program aims at detecting, tracking, and cataloguing and characterizing the NEOs equal or larger than 140 meters in diameter, with a perihelion distance of less than 1.3 A.U. and this aim be achieved by 90 percent within 15 years from now. A NASA report, by March 2007, is setting as new goals to detect, track, etc, by the end of 2020, 90 percent of all the NEOs larger than 140 meters, with their orbits passing within 0.05 A.U. of the Earth's orbit (such objects called PHOs -Potentially Hazardous Objects), hence letting down the survey, practically, of 'all' the NEOs. New survey techniques -mostly an infrared telescope in an orbit at the distance of Venus are also considered
As far as the diversion methods are concerned, this report, a nuclear explosion triggered near the asteroid are considered 10 to 100 times more effective than any other method as the impacting of a spacecraft unto the body is considered the most mature scenario. 'Slow push' methods, for now, are the most expensive and the less technically-ready

for a more detailed view of the threats and their frequency, see a table and diagram of NEOs frequencies

Solutions

The latest in matter of deflecting an asteroid lies into sending a satellite which will impact the body -from front or behind- just slowing -or accelerating-it enough for, on the long term, modify the trajectory enough; should that be seen not enough, a second mission would be sent. The first question is that such a slight change in the orbit is useable for objects which may hit Earth provided they pass through a 'keyhole' only (such bodies may orbit several times with no threat before they are played with by the gravities in the solar system). NEOs bound for a direct hit would need to be deflected by thousands of miles. Asteroids, further with satellites or with compagnon asteroids are discovered, as this might complicate the case. The matter is complicating too due to that it's becoming political with some wanting that the United Nations manage the question

Other methods related to deflecting an asteroid, are of the like: electromagnetic device ejecting dust, parabolic mirror orbiting around the object, heating the surface and creating a cloud of vaporised material, painting or covering the asteroid with dust to modify its albedo (or to scratch the surface with explosive to expose a possible underlying material with a different albedo). One could too attach a little rocket engine to the asteroid itself (in this case, some trouble would come from the porous nature of the asteroid). It has been too evoked to wrap the asteroid into a solar sail (in the same purpose than paint or dusting) -which would have been used by the satellite sent for the mission. The nuclear option would lead to none as the asteroid would become a clump of fragments, or that it could absorb the shock wave. Asteroids, further, are thought to be agglomerates of rocks, hence porous. A nuclear mission however might remain the sole option as long as some bodies might still not be detected early in time and would need a swift solution, or too for an object with a delay enough for some measure but that a preliminary mission would have found improper for any other measure

for the official taxinomy of the various NEOs and the detailed date of NEOs' orbits, the definite site is the one of NASA's Near Earth Object Program

The sketches above are showing the various types of NEOs according to their orbits. click to a larger picture (courtesy NASA)

The Tunguska Impact Detailed
Near the Podkamennaya Tunguska River, on June 30th, 1908, the people at the trading post in the aera got to be the first-hand viewers of an asteroid impact at Earth. Eye-witnesses testify that around 07:17 a.m. local Siberia time, the sky, North, was split in two, with, high above the taiga forest, the whole northern part of the sky covered with fire! A bang, and a mighty crash followed, with noise of stones falling from the sky, or guns firing as the Earth trembled. Eight hundred square miles (2000 square kilometers; 20 by 40 miles) of remote forest got been ripped asunder, having eighty million trees lying on their sides in a radial pattern as the explosion just killed hundreds of reindeer, the livelihood of local herders. Some people at the trade post were pushed away by some yards...
It's not before 1927 that a Soviet scientific expedition could really reach the impact zone. The inhabitants there believed the blast had been a cursing visitation by their god Ogdy. It is now estimated that the asteroid entered the Earth's atmosphere traveling at a speed of about 33,500 miles per hour (54,000 km/h). During its quick plunge, the 220-million-pound (110,000-ton) space rock heated the air surrounding it to 44,500° F (24,500° C). At a height of about 28,000 feet (8.5 km), pressure and heat eventually had the asteroid fragmenting and annihilating itself, producing a fireball and releasing an energy equivalent to about 185 Hiroshima bombs. That's why there were no crater produced at the impact. That created a shockwave instead, as the fast moving one, near the impact just debranched the trees, leaving them standing and with all the forest around just blown up
The seismic signatures of the events, at the time, were recorded up to in England as dense clouds formed over Siberia at high altitudes, reflecting sunlight from beyond the horizon. Glowing night skies were reported from as far as from Asia, where people could read newspapers outdoor until midnight