Ordinary Meeting, 2007 November 24


Update on Comet 17P/Holmes

Mr Shanklin explained that the story of Comet 17P/Holmes went back to the earliest days of the BAA: its discoverer, Edwin Holmes, had been among the Association's founder members, and its discovery had come a mere two years after the Association's foundation. According to Holmes' records, on 1892 November 6, at the end of an evening's observing, he had directed his telescope towards M31, whereupon he had noticed an unexpected new object nearby. In time, this had been verified to be a new comet, lying in a short-period 6.9-year orbit. Since it had never been seen previously, and there had been no evidence for any recent change in its orbit, it had seemed certain that it must have outburst prodigiously to have appeared so suddenly. The speaker noted that he was uncertain of the exact location of Holmes' observing site, except that it was somewhere in London; he hoped that further research might throw light on this.

Two months later, in 1893 January, the comet had undergone a second, albeit less dramatic, outburst, attaining a peak brightness of mag 6. But, after fading for a second time, it had returned to a quiescent state, becoming a faint object which had brightened no further than to around mag 16 at each perihelion. It had continued in this way for over a century, until 2007 October 23-24, when it had again shown a very rapid brightening, suddenly becoming easily visible to the naked eye. The rapidity of this outburst had been quite staggering: the comet's magnitude had risen from 17 to 3 – a one million-fold increase in its luminosity – in less than 24 hours. The parallels between the 1892 and 2007 outbursts also seemed striking; the speaker even remarked that the position of the comet in the sky had been almost identical on both occasions, although this was almost certainly a coincidence.

Turning to describe observations of the latest outburst, Mr Shanklin explained that immediately after its appearance, the comet had appeared point-like or stellar; some observers had even mistaken it for a new nova. Within hours, however, it had grown in size and developed a remarkable disk-like morphology which had appeared rather like a planetary nebula. Over time, a diffuse outer halo and an inner condensation had become distinct. Initially no tail had been visible, although the leading edge of the disk had gradually grown to be much more sharply defined than its trailing edge, suggesting that a modest tail was present.

Mr Shanklin explained that the cause of the outburst remained poorly understood. It seemed clear that there had been a sudden and explosive ejection of a cloud of solid dust particles from the surface of the comet's nucleus. These particles were now spreading out in all directions to fill a spherical volume, and the comet's disk-like morphology was thought to result from the reflection of sunlight from them. Its light curve supported this interpretation: its magnitude was declining as an inverse-square law, as would be expected for a steadily expanding cloud of reflecting dust particles. The velocity with which these dust particles had been thrown out from the nucleus could be easily inferred from the observed expansion rate of the coma, and the answer produced was around 500 m/s. Here lay the puzzle: considerable explosive force would be required to eject so much dust at such a speed, and it was unclear what could have triggered such a sudden and violent explosion to take place across the whole surface of a comet.

The speaker closed by reporting that the comet was very well positioned for observation from the UK, and it would remain so for the next few months. It lay close to the zenith at midnight, which was ideal for telescopic observers, though rather uncomfortable for binocular users. If its light curve continued in its present decline, it would remain bright for some weeks to come, sinking below mag 6 in 2008 February. However, given its current rate of expansion, it was likely to become very large and diffuse by this time, and it would probably disappear into the sky background much sooner.

Following the applause for Mr Shanklin's report, the President adjourned the meeting until 2.30pm on December 15 at the present venue.


Dominic Ford




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