2024 PT5

2024 PT5 is the minimoon of autumn 2024, a rock about 10 m across that spent two months in Earth's company and is probably a piece of the Moon.^[1][2] It is also the one the experts disagree about. The astronomers who predicted its visit call it a mini-moon, bound to Earth from 29 September to 25 November 2024 without completing a revolution; NASA's planetary defense office says Earth's gravity never captured it and calls it not quite a mini-moon.^[3][1] This page gives both. The guide Minimoons explains the definition behind the disagreement.

Found in August, captured in September

The NASA-funded ATLAS telescope at Sutherland in South Africa first observed it on 7 August 2024.^[1] The day after, it made its closest pass, 0.0038 au from Earth, about 570,000 km, one and a half times the Moon's distance.^[4] The Minor Planet Center designated it 2024 PT5 on 14 August with an absolute magnitude of 27.5.^[5]

Within weeks, Carlos and Raúl de la Fuente Marcos of the Complutense University of Madrid predicted that it would become a mini-moon at 20:02 UTC on 29 September 2024 and return to an orbit around the Sun at 10:33 UTC on 25 November, a stay of 56.6 days.^[3] Because it would not complete one revolution around Earth while bound, they classed it as a temporarily captured flyby, not an orbiter, and placed it among the Arjunas, small asteroids on horseshoe paths that almost match Earth's orbit, like 2022 NX1 and 1991 VG.^[3]

NASA saw the same motion differently. Its planetary defense blog of 2 October 2024 says the asteroid's orbit is so similar to Earth's that it stays a distant companion for a few months, roughly nine times farther away than the Moon, and that Earth's gravity will never capture it, so it is not a true satellite and not quite a mini-moon.^[1] NASA's animation of the encounter shows it staying near our planet from late 2024 without being captured.^[6] The two descriptions agree on the path and differ on the word: the dynamical definition counts an object as captured while its energy relative to Earth is negative, however far out it is, and does not require a full orbit.^[3]

A piece of the Moon

On 16 August 2024, a team led by Teddy Kareta of Lowell Observatory measured its reflected light with the Lowell Discovery Telescope and NASA's Infrared Telescope Facility on Maunakea.^[7] The spectrum does not match any known asteroid type: it is rich in pyroxene where asteroids of similar colour are rich in olivine, and it matches lunar samples instead.^[7] The push of sunlight on it is far too small for a hollow artificial object such as a rocket stage, so it is a dense natural rock, about 8 to 12 m across.^[7]

NASA's account of the study, published on 22 January 2025, is that 2024 PT5 probably broke off the Moon's surface when a large impact threw it into space a few thousand years ago; the lack of space weathering suggests it has not been in space for long.^[2] That doubles the known count of asteroids thought to come from the Moon, after Kamoʻoalewa, the quasi-moon described in the guide.^[2]

Where it is now

It came back past Earth on 9 January 2025 at 0.012 au, about five times the Moon's distance, and NASA's Goldstone Solar System Radar tracked it before it moved off along its orbit around the Sun.^[4][1] That orbit has a semi-major axis of 1.02 au and a period of 377 days; JPL's next listed return is in November 2055 at 0.036 au, about 14 lunar distances.^[4] It poses no hazard to Earth.^[1]