The Earth, Moon and Petey? Our New and Transient Companion

On September 29th, our Earth and its moon were joined by an unexpected guest. Its name? 2024 PT5. It isn’t exactly the most memorable or pretty name conceived by humanity thus far. Therefore, for the purpose of this article, I will be referring to this object as Petey, due to its resemblance to the name PT. Now, hopefully, a fun name like that will make our subject matter, a large boulder in space, sound more fun.

Petey is, to my great disappointment, not in a stable orbit. As such, it will only be a part of the Earth-Moon system from the end of September to the end of November. After that period, Petey will leave the influence of Earth’s gravity and continue journeying across the Solar system. Since Petey is not in this world for long, let’s take some time to talk about our new little companion.

Petey, unlike our permanent celestial companion the Moon, is rather small. At just 11m wide, it would likely fit within St Salvator's quad. This is, of course, presuming we could manage to get it there. Due to Petey’s small size, it is not possible to see or take photos of it. However, this does not prevent us from making some educated guesses as to its appearance. Petey is most likely a rocky or metallic asteroid with an oblong potato-like shape. Due to its small size, it lacks the gravitational field necessary to pull itself into a spherical shape like Earth or the Moon. 

If Petey is a rocky or metallic asteroid, it is likely older than the Earth itself and even slightly older than our Sun. While some believe asteroids are random rocks floating aimlessly through space, they’re actually primordial leftovers of planet formation. Since before our Sun ignited, around 4.6 billion years ago, it has sailed calmly through this solar system. Petey was there when Earth had its first sunrise and may even see Earth’s last sunset when the Sun finally goes out in the next billion years. 

Asteroids like Petey are extraordinary treasure troves of knowledge — they act as time capsules to the early solar system, or in rare cases, the early universe. While NASA or other space agencies are unable to gather any samples from Petey, they have been lucky enough to gather samples from other asteroids. The Osiris-Rex mission was launched by NASA in 2016, its goal was simple: rendezvous with the asteroid Bennu and return to Earth with samples. These samples were returned last year and they will answer many questions about the formation of the planets and the early solar system. 

Petey is destined to leave us in November, and while our time together was short, take comfort in knowing Petey was not the first, nor will it be the last, of our temporary moons. It is estimated that Earth will always have at least one additional temporary moon at any given time. At this rate, the Earth has had billions of wanderers come by to say hello, and will have billions more —is just one small part of this ancient pattern. While this does make Petey seem less special in retrospect, remember that we are lucky to have even spotted it.

Image by Elizabeth Lang