LONDON: Meteorites are mostly chips off bigger blocks. Out in the main belt, between Mars and Jupiter, there are billions of asteroids. Inevitably there are collisions between them, and some of the debris eventually reaches us.

A simple picture, but there is a puzzle in the details. Most meteorites appear to be too young, in terms of the time spent on independent orbits after escaping their parent asteroids. Subject to the assumption that the gravitational tugs of the planets are the only forces at play, astro-mathematicians are able to trace how the paths of interplanetary objects wander. Such calculations lead to an estimate that meteorites need about a hundred million years to reach us, much longer than they actually take.

This transit time is known from a meteorite’s space exposure age. This duration is quite different from the period it may have lain on the ground before discovery (between seconds and millennia), or its age from formation as measured using radioactive dating.

Space exposure ages are determined using cosmic rays. Within a much larger asteroid, an eventual meteorite is shielded by an overlying layer of rock. After an inter-asteroid collision, the freed meteoroid is suddenly exposed to the high-energy elementary particles that permeate space.

When these cosmic rays hit the meteoroid, they penetrate a centimetre or so. Characteristic tracks are left in the rock, which may be studied under a microscope. By counting the numbers of tracks it is possible to determine how long it took for the meteorite to travel from its parent asteroid to the Earth’s surface. Typical values are a few million years.

This implies that meteoroid orbits must evolve much faster than purely gravity-based computations would indicate. Something else must be going on. What could it be?

Consider the famous experiment of two cannonballs of different size being dropped from the Leaning Tower of Pisa. Both reach the ground at the same time, despite their differing masses: only gravity matters here.

This is not the case if a feather is substituted, because its large cross-section compared to its mass means that air resistance is substantial. In a vacuum the feather falls at the same rate as the iron balls. Now think again about meteoroids in space. Are there any influences that are size-dependent, causing them to evolve dynamically at a rate faster than pure gravity would allow? There is no air, but is there some other sort of resisting medium affecting their orbits, helping them migrate inwards on a crash course with Earth?

The solar wind, the stream of charged particles moving outwards from the sun, imposes a small force. A greater pressure derives from the photons of sunlight. These two factors are important for tiny interplanetary dust grains, but a meteoroid the size of a basketball is essentially unaffected.

The sunlight absorbed by meteoroids can have other effects. They are heated by this flux, and that energy is then re-emitted as infrared radiation. The emission is not isotropic, though: it is not the same in all directions. —Dawn/The Guardian News Service.