In article , Jake
McGuire wrote:


With close flybys in 3-body systems you can either eject or capture
something, but that's not the situation you're describing.

If Cynthia originally had a sister, a double asteroid like Hermes was
just found to be, then you have a three-body system that might allow
capture. It would take the same hand-of-god that placed Luna at just
the right size and distance to give us nice solar eclipses, but it's
possible.

Especially with an aerobrake to bleed off some energy, then the second
component circularizes Cynthia's orbit before being flung into the
utterdark. (A single asteroid aerobraking gives an orbit that passes
inside the atmosphere on subsequent passes, which quickly leads to
lithocapture and a nice iridium layer for the next intelligent species
to find.)

Of course, the protagonist realizes this after single-handedly
recapitulating the works of Galileo, Newton, Halley etc. from our time
line to develop orbital mechanics, Percival Lowell to find the cast off
sister, Shoemaker to discover that it will hit Earth in 3 years,
Goddard, Tsiolkovskii and Korolev to build a rocket, Oppenheimer and
the gang for the payload, and Bruce Willis to get the box office.


Bogen:
Luna will give off more light in total because it's larger but Cynthia
is much closer so each solid angle measure (steradian?) should be
brighter. I think Cynthia will lokk like a brighter, fast moving
Venus.

No, brightness per steradian depends on illumination (how far from the
Sun it is--the same as the Moon is, plus a bit of Earthshine) and how
reflective it is, but not on how far away it is. (The inverse square
law, an approximation in this case, comes entirely from the solid angle
shrinking with distance.) That's why a tree nearby doesn't burn your
eyes out while a tree-covered distant mountain is other than black.

--
David M. Palmer (formerly @clark.net, @ematic.com)