Asteroid 'murder mystery' solved

Near-Earth objects (NEOs)- asteroids and comets that may pose a hazard to life on Earth - are destroyed very far from the Sun rather than ending their existence in a dramatic plunge into the solar body, a new study suggests.

An asteroid is classified as an NEO when its smallest distance from the Sun during an orbit is less than 1.3 times the average Earth-Sun distance.

The vast majority of NEOs originate in the doughnut-shaped main asteroid belt between the orbits of Mars and Jupiter.

An international team of researchers from Finland, France, the US and the Czech Republic used the properties of almost 9,000 NEOs detected in about 100,000 images acquired over about eight years by the Catalina Sky Survey (CSS) in Arizona to construct a new population model.

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 10 times 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, a research scientist at the University of Helsinki hypothesised that their model would better match the observations if NEOs are destroyed close 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 about 10 solar diameters of the Sun.

"The discovery that asteroids must be breaking up when they approach too close to the Sun was surprising and that's why we spent so much time verifying our calculations," said Dr Robert Jedicke from the University of Hawai'i Institute for Astronomy.

The team's discovery helps to explain several other discrepancies between observations and predictions of the distribution of small objects in our solar system.

Meteors, commonly known as 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.

The 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.