3/2/2023 0 Comments Meteoroid origin![]() ![]() Since the early days of meteor photography, much progress has been made in mapping visual meteor showers, using low-light video cameras instead. New data is presented on the long duration showers that wander in sun-centered ecliptic coordinates. ![]() Recent work on meteor showers is reviewed. A better understanding of the composition of Near Earth Objects will allow the most efficient deflection techniques to be developed so that they present no hazard to human beings. New missions aimed at returning samples of Near Earth Asteroids to Earth for analysis (Osiris-REx and Hayabusa 2) are very important because they will deliver to our laboratories materials probably non-sampled in meteorite collections. ![]() However, events such as that over Tunguska in 1908, where an air burst caused considerable damage over a large area, indicates that we should not underestimate fragile bodies as potentially hazardous sources. However, the typical strengths of such meteoroids are too low to survive ablation in the upper atmosphere, so that they are unlikely to impact the ground. If meteoroids of cometary origin are included the flux of objects into the Earth’s atmosphere will be increased (Space Sci Rev 84(3/4): 327–471, 1998). From all the available data, a better understanding of the rate at which asteroids impact the Earth can be derived. We will introduce some examples that meter-sized meteoroids following high-inclination, and eccentric orbits are not necessarily fragile, and can trace the existence of hazardous objects: dormant comets or Damocloids being an example. The behaviour of stony bodies interacting with the atmosphere is reasonably well known, but little is known about the either the flux or the behaviour of materials from dormant comets that are often associated with meteoroid streams and small Near Earth Objects. Of course, larger bodies will do more damage, but impact less frequently. Statistically, bodies with a diameter from a few meters up-to about a 100 m can be considered as the most direct source of contemporary hazard. Telescopic surveys provide significant data on Near Earth Asteroids of few tens or hundreds of meters in diameter that can be only detected when these bodies are close to the Earth. Impacts by larger bodies are less frequent and telescopic surveys to find potentially hazardous objects are still crucial to infer the flux of these over long timescales. Ground-based observations of meteors and fireballs increase our data and statistics on meter-sized events entering the Earth’s atmosphere. video, photographic and radar, in general it decreases with decreasing brightness of the observed meteors. This fraction can differ for meteors observed by different techniques, i.e. Using these criteria on a set of ∼78000 sporadic meteoroids 66–67% have comet type orbits. Two parameter criteria have been suggested to remove this bias. Using a single criterion can introduce a serious bias into the results with the fraction of comet orbits understated by up to 29%. Several criteria have been proposed and applied to the present day orbits of sporadic meteors. All that can be done is to classify the orbits as being of comet or asteroid origin. For sporadic meteoroids the situation is more complicated as they can not be associated with a given parent body. In this case, the behaviour of the meteor in the atmosphere will indicate whether the parent body was likely to be an asteroid or a comet. If the parent is active, then it is a comet, but if no activity is found then it could either be an asteroid or a dormant comet. When a meteor shower is observed, a similarity in the orbits should indicate the parent and several test for this are discussed. In this region, orbits evolve rapidly, hence, over time the orbits of stream meteoroids will progressively diverge both from each other and from the orbit of the parent body, so that instead of being observed as a meteor shower, these meteoroids become part of the sporadic background. If the Earth passes through such a stream, the meteoroids will ablate and produce meteors that are as meteor showers. Any dust ejected from them, meteoroid streams will formed a meteoroid stream with orbits that are similar to that of the parent body. Asteroids and Comets that come close to the Earths orbit are called Near Earth Objects (NEOs). ![]()
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