(Contrary to scenes from the movie Star Wars, there now are very large distances between each of the asteroids, but they do get hit by other orbiting objects, or are gradually moved into transfer orbits from which they can be perturbed into Earth-crossing trajectories).
Scientists say they have developed a means of accurately dating Earth's oldest and densest polar ice by analyzing the composition of krypton gas trapped within ancient air bubbles.In a study published Monday in the journal PNAS, researchers used the new dating method, called Atom Trap Trace Analysis, or ATTA, to study ice recovered from the Mc Murdo Dry Valleys in Antarctica."The oldest ice found in drilled cores is around 800,000 years old, and with this new technique we think we can look in other regions and successfully date polar ice back as far as 1.5 million years," lead study author Christo Buizert, an Oregon State University paleoclimatologist, said in a statement."That is very exciting because a lot of interesting things happened with the Earth's climate prior to 80,000 years ago that we currently cannot study in the ice core record," Buizert said.Meteorites are objects composed of rock and sometimes metal which are derived from various solar system bodies, and after traveling through space land on the surfaces of other solar system bodies (notably Earth, but also the Moon and Mars, and no doubt other worlds).Most meteorites are not observed to fall, but instead are found long afterwards, and then must be subjected to a variety of forensic tests to ascertain their provenance. Most meteorites are reasonably inferred to come from relatively small bodies (up to several hundred kilometers in diameter, but mostly much smaller) located within the Main Asteroid Belt, which is a collection of more than 100,000 objects larger than 1 km constrained by celestial mechanics to orbit the Sun between Mars and Jupiter.Think of air bubbles frozen in an ice cube--the air is prevented from escaping by the solid ice, but will escape when the cube melts because the energy of the system is lower without the bubbles.
(Further separation could occur during solidification, due to different crystallizations of the parent and daughter isotopes.) This means that the information about how much decay took place prior to melting is lost.
Other meteorites lacking chondrules are called achondrites, and a subset of those are commonly referred to as planetary meteorites.
This latter group comprises meteorites that derive from current or former bodies of sufficient size and with enough heat that they became internally layered (or differentiated), with a dense core and an overlying lithosphere (composed of a mantle and sometimes a crust as well).
The smallest meteorites, called micrometeorites, range in size from a few hundred micrometres (μm) to as small as about 10 μm and come from the population of tiny particles that fill interplanetary space ( The effect of the final impact with the ground of meteoroids about a kilogram or less in mass is usually an anticlimax.
The fall can go unnoticed even by those near the impact site, the impact being signaled only by a whistling sound and a thud.
There are other known asteroids that do not orbit within the Main Belt (such as various near Earth objects or NEOs), and some of these also could be sources of meteorites.