Can Earth capture a Second Moon?
Dear Dr. Stern,
If a large asteroid came close enough to Earth's magnetosphere, at the
precise moment the Sun had a massive solar eruption aimed at Earth, would it
be possible for the asteroid to get caught in Earth's gravitational force
and become a secondary moon?
The answer is no, for several reasons, and I hope you did not plan to write a science fiction story around this. The momentum of an asteroid is much too big, and it is concentrated in a small object. The momentum of the matter ejected in a solar eruption is probably smaller, and in any case, it is spread out over a huge volume. To act on the asteroid it would have to hit its body. Magnetic forces are again much too weak and asteroids we know about are probably not too magnetic.
However, it is theoretically possible that an asteroid could be injected into a trapped orbit by the gravity of the Moon (and also by the combination of gravities Earth-Sun). In section #35 of "From Stargazers to Starships" you can read about "gravity assist" (or "swingby") maneuvers of spacecraft encountering a moving planet like Jupiter, or our own Moon. If the two come from opposite directions (like ping-pong ball and paddle in the example there), the spacecraft gains energy; if it overtakes (like the water jet in a Pelton turbine, section #35a) is loses energy. It is possible that an asteroid overtaking the Moon loses energy and ends up trapped around the Earth. Something like that apparently happened to Comet Shoemaker Levy, whose fragment crashed onto Jupiter 8 years ago, in July 1994. The capture itself was never observed, the comet was only discovered afterwards.
Of course, an asteroid coming close enough to Earth to become captured would be a definite collision hazard. Not likely, but not impossible, either.
Thank you for your reply. However, I wonder why you would hope that I am not writing a science fiction book or screen play about the effects on humans, ... planet Earth, tides, real estate, animals, weather... after our planet does in fact acquire a secondary moon. The last bit of polishing to the story is to present a possible and/or feasible way in which we do acquire a second moon, which occurs at the onset of the story.
I have done research, (perhaps not enough) and have found no other novels or works of literature in which Earth gets or has two moons. Do you know of any? if so, please inform me.
My comment about a science fiction story was meant as a joke, and I did not realize it scored a hit. A story is OK, but the science better be plausible, or at least hard to disprove. A solar eruption if too far out.
You wondered about effects on humans. If I wrote such a story, the main emotion would be sheer terror, since an asteroid approaching us so close could well hit Earth. Even if it went into an orbit around Earth, that orbit would most likely be very eccentric (like that of Shoemaker Levy) and chances would be high of it hitting later (again, like Shoemaker Levy), after the orbit had undergone perturbations. The second moon would be a clear and present danger and humanity's main concern would be probably getting rid of it.
Have you read "The Star" by H.G. Wells? Its premise is somewhat similar. See