"No news is good news" - that's an old phrase, but it seems somewhat pertinent for C/2013 A1 (Siding Spring). The comet is currently trending very well compared to the predicted JPL curve and, if anything, it's actually over-performing somewhat. That is always a nice thing if you're in the business of watching comets!
This was never going to be a hugely bright object, and we still maintain that it will be a tricky one to observe in October when it is very close to the much brighter Mars. However, it's behaving very nicely and as always we encourage astronomers to attempt observations and share those results with us. We see a report that Dennis Bodewits has obtained another set of Swift observations between July 8-10, 2014. He reports Q(H2O) = 6.1 +/- 0.4 x 10^27/s and notes that "Extrapolation to rh 1.5 AU, assuming Q ~ rh^2, yields a production rate of 1e28/s at Mars.".
What does that mean? It means the comet is doing what comets do, and doing it in a reassuringly predictable manner. And if you're the owner/operator of a billion-dollar fleet of spacecraft orbiting Mars, predictability equals a good night's sleep when a big dust/ice spewing snowball is going to be passing through the neighborhood!
Some notes about the lightcurve plot:
We plot three sources of data. The first, shown as red circles, are the magnitude measurements being reported by the Minor Planet Center. These measurements vary widely from observer to observer based on each person’s technique and instrumentation. Most are only measuring a relatively small region near the comet, yielding a fainter brightness than if they measured the whole coma. So you should expect to see fairly significant scatter within these data points.
The second, plotted as purple squares, are from the Comet Observations Database. The COBS data are generally “total magnitude” estimates that try to encompass all of the light from the comet. Many of these estimates are made by very experienced observers using binoculars or even their naked eye who compare the comet to nearby stars of known brightness. It takes a lot of work to be good at this technique, but as you can see from the relatively small scatter in the points, yields quite reliable estimates.
The third kind, plotted as blue triangles, are from the International Comet Quarterly. This data is recorded in much the same way as that archived by the COBS.
The black line is just one possible model of the comet's brightness behavior. It makes no prediction for what the comet will actually do, but for a comet such as this one, it is likely to end up being a pretty good representation. With comet ISON, for example, we had many unknowns, and with that object entering such a hostile environment so close to the Sun, there were literally no historical precedents we could use to predict its behavior. Comet Siding Spring is a little less of a wild-card in that sense, as it remains at the relative safety of Mars' orbit. That said, comets can and do surprise us...
For Comet ISON, we found it handy to maintain an archive of the "Current Status" page so that we could refer back to previous thoughts, analyses, events, etc. We are doing the same with Siding Spring. So over time, expect to see a growing list of archived snapshots of this page listed here:
Jun 06, 2014