Bengaluru: From the end of September to the end of November, the Earth will experience a rare celestial treat—a second ‘moon’ will be visible in the night sky. Along with the planet’s only natural satellite that humans have walked on, a smaller rock will orbit the Earth as a temporary companion to the Moon during its journey around the Sun.
Just a day ago, on the intervening night (IST) of 29 and 30 September, asteroid 2024 PT5, a near-Earth object (NEO), entered the planet’s gravitational sphere of influence, becoming an official ‘mini moon’ to the Earth.
Space rocks of various sizes that orbit the Sun and come close enough to the Earth, warranting monitoring, are called NEOs. They are studied and observed closely, and the risk they pose to the Earth is evaluated.
Asteroid 2024 PT5 was identified by NASA’s Asteroid Terrestrial-impact Last Alert System (ATLAS, South Africa)—a sky scanning project meant to detect NEOs a few days before they reach the Earth—in August this year.
The findings were published this month by ATLAS researchers in the non-peer reviewed journal Research Notes of the AAS (American Astronomical Society).
Asteroid 2024 PT5 flies at a speed of nearly 3,540 km per hour, slow enough to be temporarily trapped by the Earth’s gravity.
Such rocks, which get captured temporarily around or near the Earth before exiting the planet’s gravitational sphere of influence, are called mini moons.
Travelling in a horseshoe-shaped orbital path, asteroid 2024 PT5 will not even complete one revolution around the Earth before leaving the planet’s neighbourhood.
The bus-sized asteroid entered Earth orbit Sunday at 7.54 pm UTC (Monday at 1.24 am IST) and will exit the planet’s gravity on 25 November at 3.13 pm UTC (8.43 pm IST). It will be Earth’s mini Moon for about 57 days.
However, this is not a shiny dot that can be seen in the sky with the naked eye, powerful binoculars or even amateur telescopes. The rock, which is merely 10 metres wide, will require powerful, professional astronomical telescopes in observatories to be seen. Images are likely to be released by space agencies like NASA over the coming days.
The asteroid is not the first to either be captured by the Earth or be pulled in by the planet’s gravity before escaping. Earth has had at least three temporary rocks that acted as a companion moon on different instances, before escaping the planet’s gravitational sphere of influence.
While these temporary rocks were observed only during the past couple of decades, the Earth might have had more companion moons in ancient times, when we did not have the technology to spot them.
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Does the latest mini moon pose a risk to Earth?
So far, no major asteroid has posed a significant risk directly to the Earth. However, the possibility of such risks cannot be completely ruled out for the future. Hence, astronomers are already conducting planetary defence tests against space rocks.
NASA’s Double Asteroid Redirection Test (DART) mission is a first-of-its-kind mission that impacted the moon of a distant asteroid in September 2022, changing its orbit.
In future, for space rocks that pose risk to the Earth, governments can carry out asteroid impact avoidance missions and potentially launch an impactor towards the asteroid to deviate it off its original orbital path slightly, far enough for it to completely avoid the Earth.
The riskiest of rocks that can come close to the Earth are classified as potentially hazardous objects. Several are known to us, and over 99 percent of these are believed to be completely risk-free for a century.
Some are influenced by the Earth’s gravity, and experience changes in their orbit. Asteroid 2024 PT5, will not pose a risk to Earth because it will travel in a horseshoe-shaped orbit before flying away.
Various types of companion moons Earth has had
The Earth was previously host to two mini-moons that orbited the planet and subsequently left its gravitational sphere of influence. Asteroid 2006 RH120 was gravitationally bound to the Earth from July 2006 to July 2007, while asteroid 2020 CD3 orbited the planet from around 2016-17 to May 2020.
These two mini moons were also technically classified as temporary satellites. When a space rock fully enters the Earth’s gravitational sphere of influence and actually orbits the planet like our Moon does, it becomes a temporary satellite.
As the name suggests, eventually, it leaves the orbit or collides with the planet. Earth had another temporary satellite: asteroid 2022 NX1. It was in Earth’s orbit for 22 days in July 2022, and prior to that, it was a mini moon to the Earth in 1981.
Temporary satellites and mini moons are different from another class of rocks called quasi-satellites. These are also considered a kind of moon to the Earth.
Quasi-satellites orbit the Sun in resonance with the Earth’s orbit, often along or around the same orbit the planet takes, and some are visible at a distance from the Earth like the Moon is. The Earth has seven known quasi-satellites.
Asteroid 469219 Kamoʻoalewa, discovered in 2016, is the Earth’s most stable quasi-satellite. As it goes around the Sun, the quasi-satellite will also appear to orbit the Earth to an observer on the planet.
However, it is too distant to be under the Earth’s true gravitational influence and hence, it is technically not a satellite of the Earth like the Moon is.
The asteroid orbits the Sun at a distance 38 to 100 times that between the Moon and the Earth.
Trojans are the last class of space rocks in Earth’s orbit. These are rocks present in the same orbit as that of a planet, and either lead or follow (or both) the planet in its orbit. Mars, Jupiter and Uranus are other planets in the solar system with trojan asteroids.
Trojans go around the Sun along with the Earth, and are located on the planet’s orbit itself, like embellishments on either side of the planet.
While larger planets like Jupiter have hundreds of trojans, the Earth only has two leading trojans: the 300-metre wide 2010 TK7 asteroid, and the 1.2-kilometre wide 2020 XL5 asteroid.
(Edited by Radifah Kabir)
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