News: we have spotted a new “Mini-Moon” (Temporarily-Captured Orbiters, or more specifically a Temporarily-Captured Flyby):
iopscience.iop.org/article/10.3847/2515-5172/ad781f
de la Fuente Marcos, C. de la Fuente Marcos, R. A Two-month Mini-moon: 2024 PT5 Captured by Earth from September to November, Res. Notes AAS, vol. 8 p. 224
These TCOs are passing asteroids (i. e., heliocentric orbits) that happen to drift slowly past Earth. If the orbital crossing is close enough, slow enough, in a favorable vector, etc., the asteroid may be captured by Earth’s gravity. However, since “favorable” and “slow” are relative terms with lots of grey area, the body is (generally) only weakly captured. Its Earth orbit is high and slow, and destabilized by the residual pull of the Sun, of other planets, effects of the Earth’s moon, etc. Generally, the body orbits for a few laps around the Earth, before breaking loose again, and returning to heliocentric space. Less often, the body is destabilized into Earth or lunar impact. And another grey area is a capture so weak, the body never even completes a full orbit of Earth. This case is called TCF, or Temporarily-Captured Flyby.
2024 PT5 is one of these bodies. Discovered in August, the follow-up observations and trajectory tracking indicated, as the de la Fuente Marcos et al. group report, that the body will linger in Earth’s gravity well for months, before breaking out again in November. No, it’s no threat; this is a weak, distant capture and trajectory. And even if PT5 did go into an Earth-impact trajectory, it’s dim (and therefore, tiny). The body is too small to cause injury, assuming pieces survive through Earth’s atmosphere.
We first noticed TCOs/TCFs with 2006 RH120, spotted in- you guessed it- 2006. It is now back in Solar orbit. Then came 2020 CD3, which also spent a few months in our immediate neighborhood. Two instances are not coincidences. There’s also 2022 NX 1, but that was a TCF, not a full TCO. Now we have four, which we can start calling a “population” (hesitantly). And of course, phenomenon are happening that we aren’t catching- yet. The number of NEOs is high, and it’s just a matter of time before one meets the close-slow-favorable criteria.
TCOs are important as dynamical examples, and as flight targets. A disadvantage is that, due to the random incoming vectors, their Earth orbits are inclined and eccentric. But, due to their low (!) gravities, rendezvous and return would take negligible fuel. Even a flyby mission, if timed at a point around Earth’s equator, would be a low-energy, low-propellant, and therefore low-cost flyby mission. The trouble, of course, is the short warning time. The warning we got on all three objects is too short for a “from-scratch” mission- and that’s for a TCO, not a TCF. If a spacecraft were pre-built and pre-stored, there would be a chance. Or, we could task this to a NewSpace company (such as AstroForge) that can move fast. When the Vera Rubin Observatory is running next year, we should be seeing these bodies at about once per year, and with more warning, too.
The universe is more interesting than we can imagine. And Earth’s little corner of the universe is, also, more interesting than the average person is imagining. Too bad our preparations are not imaginative, much less fast.