Some thoughts and speculations on Holmes

David Kingsley

1) The change in earth comet distance over the last few days is such a small fraction of the total earth comet distance that I think it makes a negligible contribution to growth in apparent size (change in about 0.02 astronomical units compared to earth comet distance of 1.64 to 1.62 astronomical units from late October to early November.)

2) By contrast, the apparent expansion rate of 1.1 arseconds a day for Holmes, ( on the order of 1000 miles per hour or 0.45 km sec in my previous email), is on par with gas and dust ejection rates that have also been seen in other comets. I therefore think a large gas/ dust outburst probably explains almost all of the rapid growth in size of Holmes from night to night

3) The bright annular appearing coma disc was surprisingly symmetric for several days, with the comet nucleus looking centered in the disc from Wed to Sunday. After several consecutive days of nearly uniform looking expansion, I was surprised to see on Tuesday night that the bright annular disk was beginning to look asymmetric by day 6. The asymmetry appeared in two forms. First, beginning Tuesday night, the N to E edge of the annuar disk began looking much more sharply defined than the S to W edge. (Taking due north as 0, due east as 90, due south as 180, an due west as 270 degrees, I would estimate that that the annular rim was pretty sharp appearing from about the 270 degree to 120 degree position (Western edge, through North and down Eastern side). Conversely, the southern edge from 120 around to 270 degrees began to look soft, or slightly extended, almost like it was being eroded or worn away. At the same time, on Tuesday night the stellar part of the nucleus appeared slightly closer to the N rim than the S rim of the coma, the first night it did not look centered to me with the microguide eyepiece.

4) Last night, on Halloween, the difference in edges was even more apparent, with the sharp portion f the annular disk now extending only from about position 300 to 120 (NW to a bit south of East). The eroded appearance of the S and SW edge had widened somewhat, creeping up somewhat on the western edge of the disk while still extending all the way around the south and southwest (120 to 300 degrees). The near stellar nucleus also looks even less centered than normal, again shifted away from the southern side to the northern side.

What could account for several days of symmetric growth, then a shift around day 6 to an eroded asymmetric appearance (with the erosion occurring on the on the southern , southwest and western side).????

Comet tails grow from interaction with the solar wind. I therefore went to space weather sites and looked up the magntidue and intensity of solar wind since the beginning of the Holmes outburst. As measured from the ACE satellite, there was a burst of proton density about 10 fold above background rates that passed the earth on Oct 25 about 10:00 Universal time, followed by a secondary peak of above background solar wind density in the next 12 hours. The big spike was moving outward from the sun at about 450 km/sec and the secondary peak was moving about 600 km/sec. Since Holmes is currently about 240 million kilometers from earth, these recent peaks in solar wind density, if broad enough, would begin interacting with the comet approximately five to six days later after passing earth (October 30 to October 31 universal time, or October 29 and October 30 for our typical viewing time from California).

As the NASA orbit tool shows, the comet is located to the North of us above the plane of the solar system, and we are not yet at direct opposition in orbits in the sky. Therefore, the solar wind should appear to hit the comet preferentially on the south and western side of the circular coma as viewed from earth. Imagine a badminton birdie with a nascent tail pointed outward from the sun at the position of Comet Homes in the the Nasa orbit visualizer. We are looking at the birdie nearly head on but not quite. As an antisolar badminton tail starts to develop from the head/coma, our view of the process is below and slightly to one side of the birdie. We might thus expect to see the very first morphological signs of solar wind interaction and tail development as an eroded look to the previously symmetric head on view of the coma. And that erosion/solar wind interaction should appear most apparent to us on the southern side (given the comet's location above the plane of the solar system), biased somewhat to the apparent wester edge (because the earth has still not reached true opposition with the comet).

I have no idea of this is actually right. But it has been fun measuring actual rates of expansion, and then thinking about ejection velocities, solar wind rates, distances, tail development, and the geometry with which we see all this in the sky. Holmes has been great eye candy AND great mind candy, my favorite kind of object to watch, sketch, measure, and ponder. I have learned more from wondering about this comet, than from other comet I have ever observed. And like you said at the end of your own thoughtful email, I can't wait to see what happens next.

--David Kingsley