Theory About NEOs

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

A NEO is a asteroid or a comet whose orbit about the Sun brings it into proximity with Earth. Its closest approach to the Sun (parameter called perihelion) is about 1.3 AU, while the Earth evolves at 1 AU from the Sun. NEOs have been nudged by the gravitational attraction of nearby planets into orbits that allow them to enter the Earth's neighbourhood. There are nearly 15,000 NEOs discovered today by various programs. Near-Earth Objects (NEOs) mostly are comets and asteroids that have been nudged by the gravitational attraction of the giant planets in our solar system into orbits that allow them to enter Earth's neighborhood. 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. NEOs are objects ejected from the asteroid belt from collisions there. Astronomers mostly ignored any collision possibility with some asteroid as they considered that most of asteroids lie in the asteroid belt between the orbits of Mars and Jupiter and that collisions were to rare to consider. First recorded close flyby by a asteroid was first noticed on Oct. 28, 1937 by German astronomer Karl Reinmuth (1892-1979) who accidentally photographed the long trail of a fast moving object, as two nights later, that passed within 460,000 miles of the Earth. The asteroid was named Hermes, after Greek god of boundaries and travelers and even with that, astronomers of the time did not felt that such a close approach represented any exception. That is the reason why no any worry about asteroids surfaced before most recent years. Most NEOs are believed to result from the fact that asteroids in the asteroid belt are nudged into the inner solar system through 'resonances' or areas in the asteroid belt where gravity nudges from Jupiter and Saturn to fling them out of the belt and into the region near Earth. A orbital resonance occurs when the orbital period of Jupiter or Saturn is found a divisor of the one of a asteroid. Each time both align, the asteroid's orbit is slightly shifted and eventually becomes more elliptical. Another domain of study in astronomy in that field now is to undersand how the asteroids in the main belt smashed together and mixed up. As NEAs, or Near-Earth Asteroids come to some point of their orbit close to Earth, PHAs, or Potentially Hazardous Asteroids are the most dangerous of those, with their orbit tending to match that of the Earth and only shifted by a mere 5 million miles (8 million kilometers) as they also are large enough to survive passage through Earth's atmosphere and cause damage on a regional, or greater, scale. Approximately 9,000 asteroids of all sizes are flying past Earth at any given moment. Some NEOs lie on the blurry line between darks asteroids strayed from the main Asteroid Belt and comets. About 120 gigantic asteroid craters can be found on Earth as large falling meteorites correspond to nuclear explosions in terms of the damage they may cause and the blast wave that usually follows. The search for NEOs made appear a kind of a new Asteroid Belt, composed with those NEOs and located on the Earth's orbit as astronomers will have to determine whether that appearance is due to the NEOs' search or to a reality linked to the solar system's formation (the 'Near-Earth object zone,' or the 'main belt' is a area about Earth, that is believed to attract asteroids like a magnet, especially given the fact that cosmic rocks are drawn to Earth by gravitational forces with over 700,000 rocks in the category)

->The latest statement by NASA about NEOs' threat, by late 2016, is that approximately 95 percent of potentially hazardous asteroids and comets larger than 1 kilometer in size that could pose danger to Earth have been found as, additionally, there are no detected impact threats for the next 100 years

->20 percent of the Manhattan Island-sized asteroids -and of the meteorites (those of the L-chondrite type)- existing today are originating back to a collision between two massive bodies in the asteroid belt, between Mars and Jupiter, about 470 millions years ago

->A Supplemental Worry comes from that important NEOs might have a trail of smaller objects due to permanent collisions with meteoroids, for example. Dust also, which finally wanes into the solar wind as such a trail is also source of a magnetic field. Such derivative NEOS might be more easily attracted by planets' gravity, of which the Earth'

Impact Odds

->Any Passage At Earth Increasing the Risks of Further Close Calls for Any NEO?
Some think that the dangerosity of a NEO might be increased during its first close flyby to Earth, with the gravity of our planet interacting on the orbit of it and potentially modifying it in such a way that that NEO might be brought to further close flybys!

->The case of the 99942 Apophis NEO. 99942 Apophis which was considered a threat, grazing under the geosynchronous orbit, has been retrograted to no threat. The NEO however will pass under the geosynchronous orbit, at a low altitude of 29,450 kilometers (18,300 miles) above the Earth's surface, on April 13th, 2029. The question of the passage of the NEO on April 13th, 2036 was also ruled out like a threat by NASA in 2013. The asteroid, with a orbit modified at its first graze might have been captured into a keyhole by Earth. 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. The asteroid might also be bound to collide with another one. In case of a real threat, NASA would have planned to change the NEO's orbit with a Deep Impact-type mission. Russians, as far as they are concerned, through the Makeyev Design Bureau, one office in the domain of launchers, stated by early 2016 that they could tweak their intercontinental ballistic missiles (ICBMs) to target NEOs in the 67-167 ft in size, generally as they plan to first test their missiles unto asteroid Apophis

->Asteroid 2005 YU55. Asteroid 2005 YU55 passed by Earth on November 8th, 2011. At the point of its closest at 3:28 p.m. PST (6:28 p.m. EST/2328 UTC), it was at about201,700 miles (324,600 kilometers), as measured from the center of Earth. Although 2005 YU55 is in an orbit that regularly brings it to the vicinity of Earth (and Venus and Mars), the 2011 encounter with Earth is the closest this space rock has come for at least the last 200 years.The last time a space rock as big came as close to Earth was in 1976, although astronomers did not know about the flyby at the time. The next known approach of an asteroid this large will be in 2028. 2005 YU55 is about 1,300 feet (400 meters) in diameter with a rotation period of about 18 hours. Due the object’s size and whisking by so close to Earth, an extensive campaign of radar, visual and infrared observations have been performed. The dark object however, in term of amateur observation, was not expected to reach more than the 11th magnitude and need a aperture of 6 inches (15cm) or more

Some state that it is 2010 RF12, a 23-foot wide NEO, which has the greatest known probability, at 5 percent, of impacting Earth. On Sep. 5-6, 2095, it will pass by 5,400 miles from the Earth at the -12th magnitude and in case of encounter it would just burst into the upper atmosphere. The most elevated risk is from the 2009 FB asteroid, at 1 to 714, or less than 0.2 percent by 2185

. check a diagram of the NEOs' NEOWISE census by fall 2011 (diagram courtesy NASA/JPL-Caltech modified by site 'Amateur Astronomy')

NEOs' research programs progressively set up. Two minotoring programs nowadays in particular, are run by NASA: the Catalina Sky Survey (CSK) in Arizona, and the Pan-Starrs (Panoramic Survey Telescope & Rapid Response System) in Hawaii. In the summer of 1998, NASA established the Near-Earth Object Observations Program and JPL became the home for the agency's research data and analysis on NEOs, the 'Near-Earth Object Program Office,' to work with the data provided by the International Astronomical Union-sanctioned Minor Planet Center for submission of all observations of asteroids and comets, and to coordinate with observatories operated by academic institutions around the United States, as well as U.S. Air Force space surveillance assets. In 2016, the office was renamed the Center for Near-Earth Object Studies (CNEOS) in conjunction with the establishment of the Planetary Defense Coordination Office (PDCO) at NASA Headquarters in Washington. A CNEOS system called 'Sentry' searches ahead for all potential future Earth impact possibilities over the next hundred years -- for every known NEO. More recently, CNEOS also developed a system called Scout to provide more immediate and automatic trajectory analyses for the most recently discovered objects, even before independent observatories confirm their discovery. NASA's NEO Observations Program eventually is responsible for over 90 percent of near-Earth asteroid and comet discoveries. There are now over 18,000 known NEOs and the discovery rate averages about 40 per week. Although the original Congressional goal from 1998 of detecting and cataloguing at least 90 percent of all NEOs larger than two-thirds of a mile (one kilometer) in size has been exceeded and much progress has been made in asteroid discovery and tracking over the past two decades, the work isn't over. In 2005, Congress established a new, much more ambitious goal for the NEO Observations Program -- to discover 90 percent of the NEOs down to the much smaller size of 450 feet (140 meters), and to do so by the year 2020. The U.N. General Assembly endorsed on Feb. 15, 2013 -- the very day of the Chelyabinsk meteor event -- the establishment of a International Asteroid Warning Network (IAWN) for worldwide collaboration on the detection and tracking of potential impact hazards and a Space Missions Planning Advisory Group (SMPAG) as a forum for the national space agencies to collaborate on plans for preventing any possible asteroid impact. In January 2014, the IAWN steering committee held its first meeting, and SMPAG met for the first time later that year. TOTAS, a amateur software-based survey by the 1m-aperture telescope at ESA's Optical Ground Station at Teide on Tenerife in the Canary Islands is helping to lay the foundation for a future European asteroid survey as part of the full ESA's Space Situational Awareness (SSA) program, which was to be decided in 2012. Such a survey would use multiple 1m telescopes to scan the complete sky every night, a much larger effort than at present, and is expected to discover several NEOs per week. It would use a mix of professional and 'crowd-sourced' astronomers. The only significant asteroid survey in Europe was then the La Sagra Sky Survey, undertaken by amateur astronomers in southern Spain as they joined in 2012 the European Space Agency and a system of automated optical telescopes that can detect asteroids, with the goal of being able spot them at least three weeks before closest approach to Earth. ESA specialists are planning a network of 1 m-diameter telescopes with a combined field of view large enough to image the complete sky in one night. Europe claims it now remains a estimated half a million undiscovered near-Earth objects up to 30-meter (10-foot) across

All telescopic observations of NEOs (professional and amateur) to determine their position and orbit are transmitted to the Minor Planet Center (MPC), which is the official International Astronomical Union clearinghouse for all such observational data. Once an initial orbit is determined, the MPC delivers the observational data for NEOs to JPL, which then computes a higher precision orbit for the NEOs based on the observational data. The orbit data for each NEO can be accessed through JPL's Small-Body Database or the JPL Horizons system

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

NEOs' physical properties are not well known as the study in the infrared by the Spitzer telescope, since 2009, is showing how different the composition of NEOs is. That hints further to varied locations of origin, like the main asteroid belt between Mars and Jupiter or from farther out in the solar system. The NEOs, generally, like the asteroids likely are made from materials resulting from a mixed up soup appeared early in the history of the solar system. The NEOs diversity, in any case, is greater than previously though. The study is showing too that the NEO population is continuously renewing with the perpetual creation of new objects

Solutions

->Most Recent! The U.N. Called to Confront the NEOs' Threat
The most recent trend into confronting the NEO hazard is to consider a NEO like a natural disaster of a large size, and to call for an U.N. action plan to counter asteroids. To press for such action seems too to trigger more catastrophic stances, like warning that, in 2009 alone, six NEOs have a 'very small' probability to hit Earth. Early warnings of such collisions might lead to the ability of deflection operations

The matter of protecting the Earth against a NEO is complicated due to that it's becoming political with some wanting that the United Nations manage the question. Most currently envisioned techniques to protect the Earth from the threat of a NEO are the gravity-tractor method, with a spacecraft sent to anchor the asteroid and exerting a tiny but constant gravitational tug to change its orbit. A spacecraft slamming, from the front or the rear and slowing or accelerating it into is a more agressive method to change that course and when a threat not immediate a nuclear weapon could also be a alternate with the risk that low density of the NEO would turn into a spray of chunks toward Earth. As far as the slamming technique is concerned, should one not be enough, a second mission would be sent. NASA's Dawn mission which fired a impactor at comet Tempel 1 in 2005 partly was a experiment about the fact. 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. 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. By early 2013, a joint US-European AIDA (Asteroid Impact & Deflection Assessment) mission has been deviced. Two small spacecraft should launch to intercept a binary asteroid Didymos, which is projected to travel past Earth in 2022. One 600-pound (300-kg) spacecraft, the Double Asteroid Redirection Test (DART) craft would smash into the smaller of the two asteroids and knock it from its regular orbit. Meanwhile, the Asteroid Impact Monitor (AIM) craft would survey the collision. Such a crash would take place about 6.5 million miles (10.5 million kilometers) from the Earth

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. Most recent ways to deflect a NEO are the so-called 'gravitational tractor', which consist into a planetary satellite placed close to the NEO and gravitationally interacting with it, giving some slight push, or 'parasol missions' which would consist of several large shades placed at distance, between the NEO and the Sun, affecting the Yarkovsky effect. Preventive actions based upon the concept of gravitational tug look like the best appropriate now as most asteroid are considered loose agregates and they may absorb shocks as when more rigid any impact or explosion would just multiplicate the collision risks with numerous fragments created

->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. Some scientists think that those luminescent clouds which were observed after the impact might better have been due to a comet than to an asteroid as a comet alone could have released such a large amount of water in the atmosphere which later turned into the clouds
A most recent, fancyful claim, by a Russian scientist in 2009, is that the Tungunska object indeed was a UFO, the alien pilot of which deliberately crashed his craft into Siberia, sacrificing himself in a collision against a gigantic meteor which was to slam! Quartz slabs with strange markings found there would evidence an alien control panel