At 12:51 PM 9/29/2011, dfischer@... wrote:
>You've all guessed exactly right - here is was the NEO Observation Program
>Executive at NASA Headquarters just wrote to me...

Well, yes, but you've got the argument a little bit backward. The mean
albedo of NEAs determined by WISE is a bit higher, not lower, than we have
been assuming. Thus, the 1-km diameter equivalent is probably better given
as H = 17.62, rather than 17.75. So, if you make the cut at a lower H
value, you get a smaller number, both discovered and estimated
total. Because of the steep function of completeness with respect to size,
cutting off the bottom 0.13 magnitude where completeness is only around 75%
substantially increases the integral completeness going up to larger
sizes. To give you an idea of how little the difference is, the number
released for N(D>1km) is 981. My most recent estimate, sent to Lindley
Johnson and Don Yeomans back in March, was N(H<17.75) = 989.5; that's less
than a 1% change. If I back out of my size-frequency estimate the H
magnitude where N = 980, I get H = 17.62. Finally, if we were to take H =
17.62 as the equivalent of D = 1 km, we would obtain a mean albedo of 0.158
rather than the currently used 0.14.

There is of course some adjustment for the distribution of albedos about
the mean, but as NASA and the NEOWISE folks indicate, that will take a bit
more sorting out to quantify. Perhaps the biggest issue remaining is that
there is a strong gradient of albedo versus semimajor axis. Thus, NEOs
that have SMA out around 2.5 to 3 AU, that is, go almost out to Jupiter at
aphelion, are much harder to find than those in closer, and are also much
darker, on average. So there is a strong correlation between albedo and
ease of discovery, even for WISE which is more sensitive to darker objects.

cheers,

Alan

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Alan W. Harris
Senior Research Scientist
Space Science Institute
4603 Orange Knoll Ave. Phone: 818-790-8291
La Canada, CA 91011-3364 email: awharris@...
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