Near-Earth asteroid 1995 HM: a highly-elongated monolith rotating under tension?

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.