Whenever astronomers detect something new moving through our region of space, like an interstellar object or an unusual asteroid, somebody somewhere claims it could be an alien interstellar space probe. It's like one of those laws about human behaviour – Godwin's Law for example – that should probably have its own name.
This applies to the detection of 1991 VG, an asteroid with an Earth-like orbit discovered by the Spacewatch Project in 1991. Astronomers now know that it's just an asteroid, and they've also found others like it.
Together, they're called the Arjuna asteroids and they're Near-Earth Objects (NEO). There's more than 100 of them and they constitute the so-called Arjuna secondary asteroid belt.
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Now the Arjuna group has one more member: Arjuna 2025 PN7. It's discovery is presented in a new research note in Research Notes of the American Astronomical Society titled "Meet Arjuna 2025 PN7, the Newest Quasi-satellite of Earth." The authors are Carlos de la Fuente Marcos and Raúl de la Fuente Marcos, both from the Universidad Complutense de Madrid, Ciudad Universitaria.
When 1991 VG was discovered, it followed a path very similar to Earth's and remained close to it. That's what led to the 'alien space probe' thinking. But it was really just the first member of a newly discovered class of objects. That only became apparent when more were discovered, and now the natural explanation has eliminated the speculation.
The Arjuna asteroids are known as quasi-satellites. That's because while they move in resonance with Earth, they're not gravitationally bound. Sometimes one of the Arjuna asteroids is captured by the Earth for a brief period of time and becomes a mini-Moon.
"The Arjunas with the most Earth-like orbits can experience temporary captures as mini-moons of our planet," the authors write. That happened with the Arjuna asteroid 2024 PT5, which was captured by Earth and posed a small threat of impact for a while.
"Quasi-satellites are in a resonant orbit but are not gravitationally bound to Earth, allowing for more sustained, though unbound, proximity; while mini-moons are characterized by temporary gravitational captures by Earth, meaning they are gravitationally bound, albeit for a limited time," the authors explain in their research note.
Other current quasi-satellites include 164207 Cardea (2004 GU9), 469219 Kamo'oalewa (2016 HO3), 277810 (2006 FV35), 2013 LX28, 2014 OL339, and 2023 FW13. 2025 PN7 is now the newest member of that group.
2025 PN7 is also an Apollo asteroid, a larger group of asteroids of which the Arjuna's are a small subclass. A critical distinction between them is that Arjunas don't have Earth-crossing orbits while the Apollos do.
The researchers say that 2025 PN7 won't be a quasi-satellite for very long. "The available data indicate that 2025 PN7 is a relatively short-lived quasi-satellite of Earth," they write.
It'll only be a quasi-satellite for about 128 years. Eventually, subtle changes in orbital elements will mean it leaves its resonance with Earth and will like follow a Trojan orbit or horseshoe orbit.