[meteorite-list] Dead-comet reflectivity

From: Rob Matson <mojave_meteorites_at_meteoritecentral.com>
Date: Wed, 14 Jan 2009 00:41:15 -0800
Message-ID: <GOEDJOCBMMEHLEFDHGMMIEKKDKAA.mojave_meteorites_at_cox.net>

E.P. asked about dead-comet albedos:

> But what is the reflectivity of carbonaceous chondrite, the darkest
> of the dark? I'm sure some list members can give us the exact numbers.

The very darkest "asteroids" in comet-like orbits (Tisserand value < 3)
have geometric albedos as low as 3%, though 5% is more common, and
anything less than 7.5% is considered "comet-like". The average NEO
albedo is around 10%, so a little more than three times brighter than
the darkest dead comet core. In terms of detection distance, this
means that spotting the darkest comet at the distance of the Moon is
equivalent to spotting a NEO of the same size at 1.83 times the Moon's
distance (1.83 x 384,000 km = ~703,000 km).

2008 TC3 (the Sudan impactor) was discovered at a distance of 487,000 km
(1.27 lunar distances) at magnitude +18.8 (not especially dim). If it
had an albedo of 10%, then its size was around 3.2 meters. If instead
it was a dead comet with a 3% albedo, then its size was around 5.8 meters.

So let's assume it ~was~ a dead comet core, but instead of 5.8 meters
let's make it a 50-meter object. In this case, it would have reached that
same magnitude +18.8 at a distance of 1.43 million km (3.7 lunar distances),
providing nearly three times the warning time of 2008 TC3. The point I'm
trying to illustrate is that spotting a 50-meter extinct comet at two or
three lunar distances is not a problem for current ground-based survey
instruments (provided the solar elongation is greater than, say, 70
degrees).
Yes, a Moon-based instrument could detect objects at lower solar
elongations,
but the poor phase angle unfortunately means that you would still have very
little warning time -- less than 24 hours. I doubt that would be sufficient
to evacuate a medium-sized city.

--Rob
Received on Wed 14 Jan 2009 03:41:15 AM PST