On April 22, a space rock, measuring roughly 73 metres, made its closest approach to Earth at a distance of 2.2 million km. Named 2024 HY by astronomers, this ‘aeroplane-sized’ small body (a term used by global space agencies to describe comets, asteroids, objects in the Kuiper Belt and Oort Cloud, small planetary satellites, namely Triton, Pluto, Charon, and interplanetary dust), and several other near-Earth objects are continuously monitored by NASA’s Jet Propulsion Laboratory (JPL).
Similarly, on April 23, 81-metre small body ‘2024 HS’ came at a distance of 7.45 million km, while on April 25, it is estimated that a tiny 6-metre ‘2021 VH2’ will zip past Earth at a distance of 3.6 million km. Each day, there is a multitude of such near-Earth fly-pasts that get recorded – of asteroids, comets and meteoroids of various shapes and sizes – which also call the Solar System home.
DK’s The Practical Astronomer defines the Solar System as the region of space that contains the Sun and all the celestial objects that fall within its gravitational influence. It consists of eight planets, a handful of dwarf planets, and scores of other bodies including asteroids and comets. The Sun has been shining for over 4.57 billion years, approximating the age of the Solar System itself. Beyond the main planets, lie the outer reaches of the Solar System, a dense mysterious world of numerous rocks, ice particles and dust.
“Here sits the Edgeworth-Kuiper Belt, a disc of small, icy orbiting bodies. This merges further out in space with a vast cloud of comets, known as the Oort Cloud, which encompasses the entire Solar System,” the field guide says. While the eight planets, including the massive gas giants, are formidable in their size, it is our star – the Sun – which makes up 99.8% of the entire mass of the Solar System.
Compared to this behemothic figure, the millions of smaller rocks and pieces of cosmic remnants and debris are miniscule, and yet are integral to the study of the Universe. Scientists term most of these objects as ‘Near-Earth Objects’, which are free-moving through the vastness of the Solar System, adhering to clockwork trajectories that may sometimes take even millions of years to complete one orbit around the Sun. And often they come within some degree of Earth’s sphere of influence, for why they are said to be ‘near-Earth’.
NASA’s JPL describes Near-Earth Objects (NEO) as asteroids and comets with orbits that bring them to within 195 million km (1.3 AU) of the Sun (perihelion), which means they can circulate through the Earth’s orbital neighbourhood. “Most NEOs are asteroids that range in size from about 10 feet to nearly 40 km across,” it says. Asteroids relate to the rocky fragments left over from the formation of the Solar System.
Most asteroids orbit the Sun in a belt between planets Mars and Jupiter. It is believed that there are probably millions of asteroids, ranging widely in size from hundreds of kilometres across to less than 1-km wide. They differ from comets, which are relatively small, fragile, irregularly-shaped bodies, left over from the Solar System formation process. “Comets, however, are icy dirtballs that form in the outer Solar System. The icy surface is embedded with dust, grit and particles from space,” explains JPL.
Many comets have elliptical orbits that cut across planetary orbits, taking them very close to the Sun and then catapulting them as far away, as even past Pluto. The most distant comets may take over 30 million years to complete one orbit. Comets with smaller orbital paths can take less than 200 years to orbit the Sun, making them more predictable.
The Haley’s Comet takes 76 years to complete one orbit. Interestingly, when far from the Sun, comets are very cold and icy, but as they approach our star, their surfaces warm up and volatile materials vaporise, releasing gases that carry small dust grains with them, appearing like a bright tail when seen from Earth.