"Jim Wilkins" wrote in
:

"Doug Miller" wrote in
message ...
"Jim Wilkins" wrote in
:


Second, there is no such thing as "an object approaching along
the line of Earth's orbital
path": the orbital path is an ellipse, not a straight line. The
only way anything can be
*always* in "the line of Earth's orbital path" is if it is in
the same exact orbit -- and if it's in the
same orbit, it can't be approaching us, because two objects in
the same orbit are
necessarily moving at the same velocity.


Okay, I took a nap and daydreamed of orbits, and realized that
anything nudged loose from the Oort cloud would have to pass
through Earth's orbit at a significant angle.

You're still not quite grasping this, I'm afraid. The point is that *anything*, regardless of
*where* it's coming from, will have to be at a significant angle to the planet's current path at
some seaon of the year. We're not at a fixed point in space, we're moving on an elliptical
orbit that's nearly two hundred million miles wide -- meaning that whatever its angle to us
today, that angle is going to be radically different three or six months from now, unless it's
light-years away (in which case it poses no danger).

The only way to
make it tangent is to assume a high initial velocity as it
enters the Sun's gravity well, so its hyperbolic path grazes our
elliptical one.

OK, let's assume the existence of an object on such a path. For it to pose any danger to us,
we'd have to be at the point of tangency at roughly the same time as the object. Let's further
suppose that to be the case.

A week *before* that, the Earth was ten million miles away from the point of intersection;
three months before, some 120 million miles -- and the object's path then would have not
been even remotely close to being tangent to ours, then. There would have been more than
enough of a difference in position for it to be spotted.

Bottom line: the probability of being hit by something big, without ever seeing it coming, is
effectively zero.