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Earth is going to have its very own mini-moon from September 29 until November 25. The regular moon’s new, temporary friend is 2024 PT\u2085, an asteroid captured from the Arjuna asteroid group (called the “Arjunas”).<\/p>\n
Our new mini-moon is approximately 10 meters in diameter and will be captured by Earth’s gravity for 57 days. It’s small and faint, so it won’t be visible by the eye or with small telescopes, but will be visible to larger telescopes.<\/p>\n
NASA defines “moons” as “naturally formed bodies that orbit planets.” Intuitively, we think of moons as big rocks that stick around for a while, like our very own moon.<\/p>\n
Astronomers have defined mini-moons as asteroids or comets that are gravitationally captured by a planet for a temporary amount of time. Small mini-moons are very common, but ones big enough to spot are more rare.<\/p>\n
2024 PT\u2085 is only the fifth ever detected mini-moon captured by Earth.<\/p>\n
A mini-moon is a “temporarily captured orbiter” if it completes at least one full orbit of the Earth before returning to its usual orbit around the sun.<\/p>\n
If a mini-moon is captured by Earth’s gravity but doesn’t make it around for a full orbit, it’s a “temporarily captured flyby” instead.<\/p>\n
Two of Earth’s mini-moons were “temporarily captured orbiters,” while the other three (including 2024 PT\u2085) fall into the “temporarily captured flybys” category.<\/p>\n
Mini-moons are asteroids from the large population of near-Earth objects (or NEOs) that are temporarily grabbed from their orbit around the sun.<\/p>\n
Near-Earth objects are defined as natural physical objects floating in space, such as asteroids (space rocks) or comets (dirty space snowballs made of rock and ice), that are orbiting the sun and approach to within 1.3 times Earth’s distance from the sun at some point in their orbit.<\/p>\n
Because these objects are around the same distance from the sun as the Earth, they can sometimes be captured by Earth’s gravity.<\/p>\n
2024 PT\u2085 and and a previous mini-moon called 2022 NX\u2081 were both captured from a group of asteroids orbiting the sun at a similar distance from Earth, called the Arjunas. Arjuna is one of the main characters of the Hindu epic, Mah\u0101bh\u0101rata.<\/p>\n
The first known mini-moon was called 1991 VG. It arrived in late 1991 and left in early 1992, and like 2024 PT\u2085 it was around 10 meters in diameter.<\/p>\n
In 2002, amateur astronomer Bill Yeung found what he thought was a second mini-moon: J002E3.<\/p>\n
However, on closer inspection, the object displayed a spectrum of light suggesting it was coated in white paint containing titanium oxide. Then, a study of how the object’s brightness changed over time found its shape resembled something like the upper stage of a rocket.<\/p>\n
Astronomers now believe J002E3 is the third stage of the Apollo Saturn V rocket (S-IVB) instead of a natural mini-moon.<\/p>\n
The plot twist of J002E3 made astronomers a bit more cautious. When another mini-moon (2006 RH\u2081\u2082\u2080) was spotted on September 14, 2006, it was first classified as artificial. However, after more observations it turned out to be a natural mini-moon about 2\u20137 meters in diameter. 2006 RH\u2081\u2082\u2080 stuck around from July 2006 until July 2007.<\/p>\n
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