Index Home About Blog
From: (Henry Spencer)
Subject: Re: A New Moon
Date: Sat, 11 Nov 2000 17:32:02 GMT

In article <8uh75i$5m8$>,  <> wrote:
>So: can Earth capture an asteroid (with no human help)
>as a new moon rather than just kicking it away or
>suffering an impact?

It is marginally possible.

A classical two-body system, just the Earth and the asteroid, can't do a
capture.  It's easy to get a capture if you arrange for the asteroid to
skim through the upper atmosphere, or do a close encounter with the
Moon... but the resulting orbit would lead to more encounters with the
atmosphere or the Moon, and it wouldn't last long -- months in the former
case, perhaps centuries in the latter.

It's just possible to arrange a capture by exploiting the Sun's gravity as
a perturbing force.  The result is a highly elliptical orbit, and the
comments above about long-term instability do apply... but the time scale
is potentially much longer, and tidal evolution conceivably could
stabilize such an orbit.  However, such a capture requires *exactly* the
right initial conditions, and is extremely unlikely.

>It seems to have worked for Mars...

Actually, just how Mars captured its moons is still most unclear.  They
undoubtedly are captured asteroids, but how they ended up in circular
equatorial orbits -- as opposed to inclined elliptical ones -- is a major
mystery.  It's hard to clean up the orbits that drastically, and it's also
hard to reconcile the details of the timing with current theories of the
origin of the asteroid belt.
When failure is not an option, success  |  Henry Spencer
can get expensive.   -- Peter Stibrany  |      (aka

From: (Henry Spencer)
Subject: Re: A New Moon
Date: Mon, 20 Nov 2000 21:08:35 GMT

In article <>,
Gordon D. Pusch <> wrote:
>>> On the other hand Mars has somehow managed to collect 2 asteroids...
>> Provided you are willing to accept a low probability of such an event, this
>> is not in the least difficult...
>An asteroid on such an orbit as you describe does not become ``captured.''...
>In either case, absent additional perturbations caused by a third body,
>``trapping'' an asteroid in the simple fashion you describe is not possible.

However, the Sun is sufficient to serve as the perturbing third body.

For little planets close in, like Earth and Mars, the region where the
planet dominates but the Sun's effect remains significant -- and captures
are thus possible -- is quite small, and the trajectory of the incoming
asteroid must be *exactly* right... making this rather implausible as the
route by which Mars acquired its moons.

But Jupiter does this sort of thing all the time; the transition region
around it is quite broad, and stuff wanders by on suitable paths fairly
often.  Several of Jupiter's retrograde outer moons are thought to be
captured asteroids, and it captured comet SL9 several years before the
When failure is not an option, success  |  Henry Spencer
can get expensive.   -- Peter Stibrany  |      (aka

Index Home About Blog