Massive space rock impact could have 'instantly' created the moon

Quite a detailed simulation report in the reference link, Immediate Origin of the Moon as a Post-impact Satellite - IOPscience

I digested an earlier version of this simulation report, Giant impact could have formed the Moon more rapidly, scientists reveal in new simulations,

ref paper - Immediate origin of the Moon as a post-impact satellite,, 2-page PDF report.

My observation. As the model simulations show, a Moon forming in 5 hours after giant impact could take place, about 0.69 lunar masses and begin orbit at 7.1 earth radii for periapsis in a nearly circular orbit about the proto-earth. It could form near the Roche limit about 2.9 earth radii and survive but a bit farther out seems better :) More specific details like lunar diameter at 7.1 earth radii I did not see or the original proto-earth mass and spin rate before giant impact with Theia. There are 9 references to proto-earth and 7 references to Theia. 5 references to iron and the model with more than 400 simulations seeks to explain the iron content difference between the Moon and Earth too. Using my spreadsheet with lunar mass 0.69 (69% of present Moon mass), proto-earth mass 0.9 (90% present earth mass), e = 0, I calculate an orbital period for the proto-Moon formed at 7.1 earth radii for semi-major axis ~ 28 hours. The angular size could be some 4-degrees across in the sky.

My note, the reference paper link used by article is much more detailed than the 2-page PDF link I read earlier. Immediate Origin of the Moon as a Post-impact Satellite,, 04-Oct-2022.

The proto-earth could have original LOD 5.2 hour or 3.0 hour. Different scenarios are run using different masses and prograde or retrograde spins for the proto-earth and Theia with different impact angles, etc. Some greatly different outcomes arise in the 400 or more simulations run using 10^7-10^8 particles vs. the older simulations using 10^6 particles. It is good to keep refining the giant impact model for the origin of the Moon because it seems more and more constraints show up now in the simulations to make this scenario viable to explain the origin of the Moon we see in the sky. Other recent reports demonstrate how difficult the angular momentum problem of the Earth-Moon system is when extrapolated over 4.5 Gyr period.

The Resonant Tidal Evolution of the Earth-Moon Distance,

The present Earth-Moon system and tidal dissipation rate does not work for the pair past 1.6 Gyr. Plenty of juggling is needed to show that the Earth-Moon as a system formed some 4.5 Gyr ago and remained together.
Now we have new reports of an exomoon that challenges moon formation scenarios :)

An Exomoon Survey of 70 Cool Giants and the New Candidate Kepler-1708 b-i - NASA/ADS (

"...One object is found to pass all of detection tests, as well as a battery of subsequent vetting efforts, for which the host planet is dubbed Kepler-1708b after its successful validation. With a ~2 year period around a 15.7th magnitude Solar-type star, the approximately Jupiter-sized planet undergoes just two transits and has a nearly-circular orbit at 1.6 AU. Light curve fits recover a 4.8-sigma moon-like transit signal intermixed with that of the planet, with the moon-model favoured at a Bayes factor of 11.9. Injection-recovery tests into the target's light curve reveal the false-positive probability of the signal to be 1%. This 2.6 Earth-radius exomoon candidate appears to be on an approximately coplanar orbit, 12 planetary radii from its host, and provides a challenge to traditional moon formation models."

The site lists now, The Extrasolar Planet Encyclopaedia — Exomoon Kepler-1708 b-i (

That is a large exomoon it seems :)


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