Re: Asteroid, invisible

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Tue, 19 Mar 2002, Gary Turner wrote:

> > When it passed
> > closest to the Earth - just 1.5 times the distance to the Moon - it was
> > too close to the Sun to be visible. Asteroids approaching from this
> > blind spot cannot be seen by astronomers.
>
> That sounds like a long way - its 375 000 miles.
>
> However, think about that in terms of time.  The Earth moves at a speed of
> about 70 000 miles per hour, so it missed by a little over 5 hours.  Floats
> around for 3 or 4 billion years and misses by 5 hours.  Thats like driving
> a half hour to a meeting and being late by ~10 nanoseconds.
>
> [Of course, this is assuming the very unlikely event that it passed through
> the orbital plane at closest approach.  It probably passed through the
> orbital plane elsewhere, but its still a useful analogy when someone says -
>  "but that's so far away"]

Here's how I'd think about it: This asteroid would have hit Earth
had Earth presented to it a cross section that was a little more
than 2000 times its physical cross section. Thus, we should expect
something like 1 out of every 2000 such asteroids to hit Earth.

This number would be reduced a little bit if you take into
account the gravitational attraction of Earth, but it would take a
fairly slow moving asteroid to have its trajectory altered enough
to significantly increase the collision cross section.  I
calculate that an asteroid with a relative speed of about 1/3 the
orbital speed of Earth would have its collision cross section
doubled as a result of gravitational attraction.  So let's say 1
out of every 1000 such asteroids would hit Earth.

So now the question is, how often does an asteroid this size pass
this close to Earth.  If this were a Tunguska-sized object, we
shouldn't be too surprised to find that many of them pass this
close each year since that event was just 100 years ago.  Judging,
however, by the fanfare this event caused, I would be surprised if
that were the case.

John Mallinckrodt      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm