These processes are direct collisional ejection from the asteroid belt and gravitational acceleration by dynamic resonances with the planets. ![]() Only two processes are known that can put meteoroidal fragments into Earth-crossing orbits on the short timescales indicated by their cosmic-ray exposure ages. The longer exposure ages of iron meteorites suggest that their greater strength allows them to survive longer in space. For most meteorite types, the time it takes for approximately half of a population to be eliminated by collisions is about 5–10 million years. To some extent the ranges of exposure ages reflect the time it takes for meteoroid orbits to become Earth-crossing, but for the most part they are determined by collisional lifetimes, the characteristic time a meteoroid can exist before suffering a catastrophic collision. Iron meteorites have a much broader range of exposure ages some are between one and two billion years old. Achondrites, another stony type, have ages that cluster between 20 and 30 million years. Most ordinary chondrites have exposure ages of less than 50 million years and most carbonaceous chondrites less than 20 million years. For chondritic meteorites, there are significantly fewer older ones than younger ones. The length of time that meteoroids spent in space as small meteoroids (a few metres across or less) can be estimated from the effect of their exposure to high-energy cosmic rays in the space environment ( see meteorite: Cosmic-ray exposure ages of meteorites). The grinding extends down through yet smaller meteoroidal bodies to fine dust. The present-day smaller asteroids consist of debris formed by the fragmentation of larger asteroids that is caused by this natural grinding process. Because of this, few asteroids larger than about 75 km in diameter have survived collisional destruction over the entire history of the solar system. These bodies have orbital eccentricities and inclinations great enough that they collide with one another at velocities averaging about 5 km per second. Hundreds of thousands of asteroids have been identified orbiting between Mars and Jupiter, although there may be more than a million such objects greater than 1 km across and many more smaller ones. SpaceNext50 Britannica presents SpaceNext50, From the race to the Moon to space stewardship, we explore a wide range of subjects that feed our curiosity about space!.Learn about the major environmental problems facing our planet and what can be done about them! Saving Earth Britannica Presents Earth’s To-Do List for the 21st Century.Britannica Beyond We’ve created a new place where questions are at the center of learning.100 Women Britannica celebrates the centennial of the Nineteenth Amendment, highlighting suffragists and history-making politicians.COVID-19 Portal While this global health crisis continues to evolve, it can be useful to look to past pandemics to better understand how to respond today.Student Portal Britannica is the ultimate student resource for key school subjects like history, government, literature, and more. ![]() Demystified Videos In Demystified, Britannica has all the answers to your burning questions.This Time in History In these videos, find out what happened this month (or any month!) in history.#WTFact Videos In #WTFact Britannica shares some of the most bizarre facts we can find. ![]()
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