The actual mechanism causing asteroids to disrupt is still unknown but some obvious scenarios such as tidal forces caused by the Sun and direct sublimation of silicates have been ruled out. One of the remaining scenarios is that volatiles inside the asteroid sublimate at moderate temperatures and create enough pressure to blow up the body. A similar process on a smaller scale called spalling can also break up surface rocks. Credit: Lauri Voutilainen
For 2 decades it was thought that most near-Earth objects (NEOs)—asteroids and comets that may pose a hazard to life on Earth—end their existence in a dramatic final plunge into the Sun. A new study finds instead that most of those objects are destroyed much farther from the Sun than previously thought. This surprising new discovery explains several puzzling observations that have been reported in recent years.
An international team constructed a state-of-the-art model of the NEO population needed for planning future asteroid surveys and spacecraft missions. It describes NEOs’ orbit distribution and estimates the number of NEOs of different sizes. The vast majority of NEOs originate in the doughnut-shaped main asteroid belt between the orbits of Mars and Jupiter. The orbit of a main-belt asteroid slowly changes as it is pushed by the uneven release of excess solar heat from the asteroid’s surface. The asteroid’s orbit eventually interacts with the orbital motions of Jupiter and Saturn changing the trajectory to bring the asteroid close to the Earth. An asteroid is classified as an NEO when its smallest distance from the Sun during an orbit is <1.3X average Earth-Sun distance.
The team used the properties of almost 9,000 NEOs detected in about 100,000 images acquired over ~8 years by the Catalina Sky Survey (CSS) near Tucson, Arizona, to construct the new population model. One of the most challenging problems facing the team was computing which asteroids they could actually detect. An asteroid appears as a moving point of light against a background of fixed stars but detecting it on an image depends on 2 factors – how bright it is and how fast it seems to be moving. If the telescope isn’t looking in the right location at the right time when an asteroid is bright enough and slow enough to be detected, we simply may never find that asteroid.
The team produced the best-ever model of the NEO population by combining information about CSS’s selection effects with the CSS data and theoretical models of the orbit distributions of NEOs that originate in different parts of the main asteroid belt.
But they noticed that their model had a problem – it predicted that there should be almost 10X more objects on orbits that approach the Sun to within 10 solar diameters. The team then spent a year verifying their calculations before they came to the conclusion that the problem was not in their analysis but in their assumptions of how the Solar System works.
Dr. Mikael Granvik hypothesized their model would better match the observations if NEOs are destroyed near to the Sun but long before an actual collision. The team tested this idea and found an excellent agreement between the model and the observed population of NEOs when they eliminated asteroids that spend too much time within ~10 solar diameters of the Sun.
The discovery explains other discrepancies between observations and predictions of the distribution of small objects in our Solar System. Meteors, aka shooting stars, are tiny bits of dust and rock that are dislodged from the surfaces of asteroids and comets that then end their lives burning up as they enter our atmosphere. Meteors often travel in “streams” that follow the path of their parent object, but astronomers have been unable to match most of the meteor streams on orbits closely approaching the Sun with known parent objects. This study suggests that the parent objects were completely destroyed when they came too close to the Sun – leaving behind streams of meteors but no parent NEOs. They also found that darker asteroids are destroyed farther from the Sun than brighter ones, explaining an earlier discovery that NEOs that approach closer to the Sun are brighter than those that keep their distance from the Sun. The fact that dark objects are more easily destroyed implies that dark and bright asteroids have a different internal composition and structure.
“Perhaps the most intriguing outcome of this study is that it is now possible to test models of asteroid interiors simply by keeping track of their orbits and sizes. This is truly remarkable and was completely unexpected when we first started constructing the new NEO model.”
http://www.eurekalert.org/pub_releases/2016-02/uoh-pao021616.php
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