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This Sunday at 3:54 p.m. Eastern Time, Canadians under clear skies will be able to watch a thin sliver of the moon as it heads toward the western horizon, setting at just after 6 p.m.
Did I say THE moon? I meant A moon. Because the afternoon of Sept. 29 is also when Earth will briefly pick up a second moon, much smaller than our well known satellite; a so-called mini-moon or moonlet.
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The itsy bitsy companion, with the unprepossessing name of asteroid 2024 PT5, won’t be visible to the naked eye. That’s because it’s only about 10 metres in size, roughly as big as a house, compared to the moon’s 3,500-kilometre diameter, which makes it the size of — well, the moon. It will also be almost 10 times further from the Earth than our regular moon, which also means there’s no chance of it hitting us.
But it will be a satellite of the Earth, or at least until Nov. 25, when it will slip out of our planet’s gravitational grasp and wander back into the small asteroid belt whence it came. In other words, by the time big moon goes through two cycles, little moon will have left.
“The object that is going to pay us a visit belongs to the Arjuna asteroid belt, a secondary asteroid belt made of space rocks that follow orbits very similar to that of Earth,” astronomer Carlos de la Fuente Marcos told Space.com. “Objects in the Arjuna asteroid belt are part of the near-Earth object population of asteroids and comets.”
As its name suggests, 2024 PT5 was discovered just this year by the Asteroid Terrestrial-impact Last Alert System (ATLAS), a robotic survey mission funded by NASA and tasked with detecting small near-Earth objects before they collide with our planet — or, in this case, go into brief orbit around it.
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It’s not the first time Earth has picked up another moon. Scientists have documented two prior “short captures” events, which last for weeks or months and are estimated to occur several times per decade. There are also two prior “long-capture” events that can last years rather than weeks, and in which the mini-moon makes one or more complete orbits around the Earth. According to Marcos, 2024 PT5 won’t quite make a full orbit in its brief time as a moon.
One of our previous mini-moons was 2020 CD3, discovered in 2020 already in orbit, and believed to have been sneakily circling the Earth since about 2016. But the shy, metre-wide rock left almost as soon as it was discovered, and won’t even be back in the neighbourhood for another 20 years.
Before that the most recent moonlet known to have been captured even into temporary orbit by Earth’s gravity was 2006 RH120, a similar-sized asteroid that was in orbit for a few months ending in June 2007.
Last year, scientists discovered 2023 FW13, a 20-metre-wide so-called quasi-satellite which circles the sun in sync with the Earth, but in an eccentric path that takes it halfway to neighbouring Mars and Venus while it executes a long, lazy orbit around our planet.
Scientists calculated the path of the space rock and hypothesized that it’s been circling the Earth for at least 2,100 years and will keep it up for another 1,700 years or so until it wanders off into deep space again. “It seems to be the longest quasi-satellite of Earth known to date,” French astronomer Adrien Coffinet told Sky & Telescope magazine after running the numbers on the moonlet.
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