"Hello,

here's something concerning the "NEO flux" that I do not understand (yet).

Suppose the impact frequency of tunguska-sized (H= 23...25mag) objects is 1 per 300 years.

The earth's radius is about 1/60 of the moon's distance, so the target area of a "moon crosser" would be 3600 times larger than that of an impactor, and now divide 300 years by 3600 - looks like there should be 1 of these objects passing by the earth at a distance closer than the moon each *month*. While tunguska-sized objects are found quite frequently by the NEO surveys, none has ever been been recorded closer than the moon (0.00256 au, see http://cfa-www.harvard.edu/iau/lists/Closest.html , the biggest "moon
crossing" object listed there is 2001 BA16, H= 25.8)

So what's wrong with these statistics?

Regards,
Christian

Christian Glowinski
Bremen, Germany
glowi@gmx.net

============
(11) ... AND AN ANSWER

>From Duncan Steel <D.I.Steel@salford.ac.uk>

Dear Benny,

With regard to the question from Christian Glowinski:

(a) Nothing is wrong with the statistics (except that I believe the
"Tunguska rate" is likely closer to once per century rather than
once per 300 years - but that is a triviality).

(b) Another triviality that upsets the sums very slightly is the fact
that objects striking the Earth have been subject to gravitational
focussing. This has the effect of producing a collision cross-section
that is larger than the Earth's geometrical cross-section. The
enhancement factor is speed-dependent, but it is typically around
two or less (it is only much greater for very low-speed arrivals). So
again it's a small factor.

(c) The real answer to the basis of Christian's question is that indeed
there are many small (Tunguska-type) asteroids passing "close" by the
Earth (that is, passing through cis-lunar space), but the majority of
them are not detected.

This is not news: many of us in the game have pointed this out in books,
magazines, and radio and TV interviews over the past decade or more. For example, in "Rogue Asteroids and Doomsday Comets" (Wiley, 1995), referring to the few *detected* near-missers, I wrote that "These comprise only a small fraction of the total that passes closer than the Moon, however, with more than 99% undoubtedly escaping unseen."

Nor is this surprising. Even now the sky is being searched typically only
once per month (to various limiting magnitudes): perhaps several times for parts of the northern sky, but the southern sky not at all. On the other hand, it takes a typical asteroid that is passing through cis-lunar space less than ten hours to do (i.e. transit time through the centre of a sphere with radius 384,000 km), and that is when it would be brightest. It would also be moving at its highest angular speed, making the probability of detection lower than if the angular speed were moderate (i.e. during an exposure its reflected light is "smeared out"). So what do you expect?

Bottom line: most small NEAs pass by the Earth unobserved.

Regards,

Duncan Steel"
From CCNET:
http://abob.libs.uga.edu/bobk/ccc/cc022102.html