Sen—Hundreds of thousands of known asteroids are circling the Sun, and at first glance they may appear to be just a disorganised jumble of rocks.

The vast majority of them are found in a swarm that lies between Mars and Jupiter but they seem randomly scattered in that band of debris left over from the formation of the Solar System.

In truth many are related because the numbers have grown in the course of countless collisions that caused larger bodies to break into fragments.

Now a NASA heat-seeking probe called the Wide-field Infrared Survey Explorer (WISE) has managed to match up many of these chunks of cosmic flotsam. As one of its all-sky surveying tasks, WISE took millions of images of the sky to hunt down previously unknown asteroids before it was put into hibernation mode in 2011.

Astronomers have used these infrared pictures gathered in the survey, called NEOWISE, to group together thousands of previously unknown or uncategorised asteroids. They identified 28 new “families” of asteroids, a result which will be invaluable in helping to understand the collisions that created them and so how such families originated.

Spacecraft have visited several asteroids now, giving us detailed close-ups of their surfaces. These have revealed how violent their histories must have been, with countless craters scarring their surfaces, caused by impacts with other asteroids.

The Dawn probe’s recent mission to orbit Vesta, one of the largest asteroids, showed that its southern side had been gouged by two big impacts. However when a colliding body is big enough, is doesn’t simply leave a crater but breaks the asteroid up.

Such catastrophic crashes shatter an asteroid into numerous fragments, as happened with one group of related asteroids known as the Eros family. At first such fragments will continue to orbit the Sun together in a pack, but they become more widely spread over time.

Part of the detective work that astronomers use to tell which rocks come from the same family is to check their physical features such as what minerals they are made of an how reflective their surfaces are. Infrared imaging can find the answers by measuring the heat from the individual objects.

An artist's concept of the WISE space telescope operating in Earth orbit. Credit: NASA/JPL-Caltech

It is important to understand them because the asteroid belt is a major source of the near-Earth objects (NEOs) that have been knocked by collisions into orbits that come close to our own, posing a potential threat.

The NEOWISE team checked out around 120,000 main belt asteroids in their space telescope’s data out of the approximately 600,000 known. They found that about 38,000 of these objects - approximately one third of the observed population - could be assigned to 76 families, 28 of which are new. At the same time, some asteroids that had been thought to belong to a particular family were reclassified.

Lindley Johnson, of NASA’s Near-Earth Object Observation Program based in Washington, said: “NEOWISE has given us the data for a much more detailed look at the evolution of asteroids throughout the solar system. This will help us trace the NEOs back to their sources and understand how some of them have migrated to orbits hazardous to the Earth.”

Amy Mainzer, NEOWISE principal investigator at NASA’s Jet Propulsion Laboratory in California, said: “It’s as if you have shards from a broken vase, and you want to put it back together to find out what happened. Why did the asteroid belt form in the first place and fail to become a planet? We are piecing together our asteroids’ history.”