Jonathan Silverlight wrote:

Isn't the point that the transverse velocity is unknown?

Yes, it is, but unfortunately, that isn't what many of the quotations from
press releases said. More than a few used the words "will collide", instead
of "may collide". Again, the idea of a possible "head-on" collision gets the
most attention (and maybe the most bucks depending on the researchers). The
most likely scenario is that M31 and our Milky Way galaxy will pass each
other, perhaps becoming slightly distorted due to tidal interactions, as M51
and its large companion galaxy NGC 5195 are doing. It is just too soon to tell.

How big would it have to be before we can be certain that M31 will miss us

Oh, I imagine 300 to 800 years would probably be enough, although with
extended baseline radio interferometry (and suitable reference radio "noisy"
quasars along or close to a visual line close to that of M31's nucleus), this
number range could easily be cut in half.
Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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