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Re: (meteorobs) OT but interesting-Elongated Asteroids Believed To Be Weaker
>..elongated asteroids are never seen to be rotating faster than once
every four hours. In contrast, more spherical asteroids can rotate as
fast as once every 2.3 hours.
It will be interesting to see what the author does with the following
paper
"Near-Earth asteroid 1995 HM: a highly-elongated monolith rotating under
tension?"
D.I. Steel, R.H. McNaught, G.J.Garradd, D.J.Asher and A.D.Taylor
Planet. Space Sci. 45, 9 pp 1091-1098, 1997
Abstract
Photometry of near-Earth asteroid 1995 HM obtained in 1995 June is
reported. This object has dimensions of at most a few hundred metres.
The 85 observations result in a lightcurve with an amplitude of at least
two magnitudes, indicating a highly elongated shape. The full drop in
brightness occurred within 15 min, and the cyclicity of the lightcurve
indicates that the rotation period of 1995 HM is only about 97 min, the
briefest ever determined for an asteroid. If 1995 HM is indeed spinning
this quickly then it must be a monolithic body rotating under tension,
because the density required for an object with zero tensile strength to
maintain integrity through self-gravitation at such a rotation rate is
unreasonably high (~4 g/cm^3 for a spherical profile, and above 8 g/cm^3
for the elongation indicated by the observed lightcurve amplitude). The
search for an asteroid with such a spin rate has been a long-term quest
(Harris (1996) Lunar Planet. Sci. XXVII, 493-494). Some difficulties in
fitting a single lightcurve over the full 15 day time-span of the
observations may be indicative of this asteroid being in a tumbling state,
rather than undergoing simple principal axis rotation.
(c) 1997 Elsevier Science Ltd
If indeed this study has been ignored or rejected, it would presumably be
on the grounds of the uncalibrated photometry which limits the conclusion
on the amplitude. However, of the 100 or so NEO I have observed, this one
was the only one with an obvious and dramatic light variation over a few
minutes, which made us continue observations to derive the lightcurve. On
individual nights, the period of around 97 min is very well defined and
this period can be reasonable represented over 15 days. 1995 HM was
varying between mag 18 to below 20.
It is my understanding that a well determined amplitude and period for
another asteroid unequivocally shows it to be a monolith.
Sorry, off topic, but I couldn't let it pass.
Cheers, Rob
Robert H. McNaught
rmn@aaocbn.aaodot gov.au
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