In September 2024 Earth acquired an additional minimoon for a brief period of two months. To a common reader this sounds curious and puzzling but fascinating because we see a single moon every night.
Space scientists define mini moons as asteroids that are gravitationally captured by Earth for a temporary period of timeand are not visible to our naked eye. Discovered a month earlier, this mini moon is named as Asteroid 2024 PT5 that is of 10 meters diameter, in size. This is the fifth detected mini-moon captured by planet Earth.
Our solar system is filled with millions of asteroids that are made up of stony materials like silicate and nickel-iron found in the crust and mantle of rocky planets. These are bodies that are leftovers from the formation of our solar system and orbits the sun like other planets.
A majority of these asteroids varying in size from about 500 km in diameter to less than 10 meters across are found in the ‘asteroid belt’ in an orbit between Mars and Jupiter orbiting the Sun. Occasionally, some of the asteroids leave their orbits and head towards Earth due to several factors.
A few of them are, i) Gravitational perturbations and resonance under the influence of Jupiter that alters the velocity and trajectory leading to paths that intersects Earth’s orbit, ii) Collision of two or more asteroids ejecting the fragments towards Earth’s orbit, iii) Yarkovsky effect, a phenomenon that drift the path of a rotating asteroid towards Earth due to uneven heating of its surface by Sun. As the asteroid rotates, it absorbs solar energy and re-emits it as heat that acts as mini-thruster over a period of time altering the orbit, and iv) Other forces like radiation pressure or the outgassing of materials from an asteroid can also alter the trajectory.
All these phenomena push these asteroids towards Earth to become “Near-Earth Objects” (NEOs) that can be called quasi-satellites as they tend to share or cross Earth’s orbit while orbiting the Sun.
That is why, sometimes we hear that an asteroid is approaching Earth and will pass very close by.A few of the well-established groups of NEOs with their own orbits around the Sunare Arjuna, Apollo, Amor, Aten and Atira. The 2024 mini moon belongs to Arjuna asteroid group captured by the Earth’s gravity and will be a companion of our regular moon for a brief period of two months. It will swing around the Earth in a horseshoe-shaped path before continuing on its course around the Sun.
Though, not the same, the mechanism is similar to what Indian scientists adopted for Moon or Mars missions where they made a rocket spin around the earth, gain momentum and then shoots off to their respective destinations. For two months from 29 September to 25 November, Asteroid 2024 PT5 will be alongside the Earth like a mini moon and then disappear. Some rough calculations show that it will be back to do the same again in 2055.
–Dr R K Chadha
