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Most asteroids orbit the Sun in the main belt between Mars and Jupiter. Most of the asteroids are the raw material leftover from the formation of the Sun, clumps of dust, gas and ice that never coalesced into a planet because of the gravitational influence of Jupiter. The planets were assembled in the waste material from the birth of the Sun. While we know of only eight planets orbiting the Sun, there were between 10 million and one billion planetesimals in the infancy of the Solar System, seeds of worlds that never germinated.
Illustration of the asteroid belt. (Image Credit: NASA).
Planetesimals are seeds of worlds while protoplanets are larger, embryonic worlds with differentiated interiors. Most of these were battered to bits by violent collisions, with their fragments also making up populations of asteroids. The gravitational influence of the planets caused these fragments to shift their orbits. There are swarms of asteroids occupying all the regions of the Solar System, including in the neighbourhood of Earth’s orbit around the Sun. These are called Near Earth Objects (NEOs), and some of them pose an impact threat to the Earth, called Potentially Hazardous Asteroids (PHAs).
An asteroid is a rocky, airless body that orbits the Sun, typically smaller than a planet. These are remnants from the infancy of the Solar System, dating back to about 4.6 billion years ago. They are an admixture of mostly rock and metal. The asteroids are classified by their compositions as stony, metallic or carbonaceous. The composition differs depending on where in the Solar System they were formed, with the asteroids at greater distances from the Sun containing more ice.
The bodies in the outer Solar System have more icy compositions and are known as comets. Straddling the line between asteroids and comets are strange objects known as centaurs, that resemble an asteroid in appearance, but behave like comets, sprouting tails when approaching close to the Sun. Asteroids have a tenuous gravity, and do not have atmospheres, so they are not habitable. Most asteroids also continuously tumble through space, their spins giving them a bulge and a top-like appearance.
There is not a single asteroid on a confirmed collision course with the Earth in the immediate future. No large impact is expected for at least 1,000 years into the future. Scientists are continuously tracking the NEOs that approach within 1.3 times the distance between the Earth and the Sun, to keep tabs on any potential threats. Telescopes and automated systems continuously monitor these objects.
After their initial discoveries, the orbits of asteroids are not well understood, and can cause scares as they appear to be potential impactors. The asteroid Apophis discovered in 2004 is a great example of this. It was considered a potential impactor at the time of discovery, but follow-up observations indicated that it would safely hurtle past the Earth in 2029. A more recent discovery is that of 2024 YR4. At the time of initial discovery, the chances of striking the Earth was pegged at over three per cent, which reduced to 0.004 per cent after follow-up observations. The short and simple answer is that no, there is no large asteroid coming for the Earth, at least not that we know of.
Asteroids vary in size from tiny pebbles to massive mountains, their sizes ranging from less than one metre across to 1,000 kilometres wide. The smallest are called meteoroids, and are less than a meter across. These fragments are dust to boulder sizes. The smallest asteroids range in size from one to 10 metres across, and are hard to detect till they almost strike the planet. Medium-sized asteroids, measuring between 10 and 100 metres across can cause spectacular airbursts and local damage to fragile structures. The Chelyabisnk meteor, at 20 metres wide is an example of such an asteroid.
A comparison of sizes of selected asteroids. (Image Credit: NASA).
Large asteroids measuring between 100 and one kilometre across are rarer, but tracked closely. The biggest asteroid in the Solar System is a protoplanet named Ceres. As the asteroids grow larger than 400 kilometres across, they start assuming a spherical shape under the influence of gravity. Ceres blurs the line between asteroids and dwarf planets. Most asteroids are less than one kilometre across, with the numbers dropping as the size increases. There are only about 1,200 asteroids in the main belt larger than 30 kilometres across.
