[I tried posting this earlier but didn't see it appear; trying again
via Google groups.]

In article ,
Keith Thompson writes:
I'm curious about one thing. We can measure the approach velocity
accurately via the Doppler shift, but how accurately can we estimate
the lateral velocity? I presume the lateral motion is too small to
be
measured directly.

Certainly not _easily_ measurable. If the lateral motion is 150 km/s
(similar to radial motion) at a distance of 750 kpc, it is about 40
micro-arcsec per year. This might be measurable with a serious VLBI
program over several years if bright radio sources could be found in
M31 and if their individual motions could be disentangled from the
motion of the galaxy center of mass.

[Since my original posting attempt, I've found out that SIM = Space
Interferometry Mission should be able to measure proper motions with
the needed precision.]

Could the motion vectors be such that it will be a
near miss rather than a collision?

Yes. In fact, that's probably the way to bet. Over enough time,
dynamical friction will cause the M31/Milky Way orbit to decay, and
there will eventually be a collision and merger, but there's no
reason to suppose it will occur during the next periapsis.

--

Steve Willner Phone 617-495-7123

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