Siding Spring is now just 2 days away from buzzing Mars! It is now clear that the drop in brightness we saw beginning a couple of weeks ago has continued, but fortunately it hasn't fallen off the proverbial cliff.
It appears that Siding Spring's rate of brightening as it approaches the Sun has simply changed in recent weeks compared to what it was doing earlier in the year. As Matthew discussed recently this isn't particularly unusual in comets like Siding Spring that are newly arriving from the Oort Cloud.
While everyone is focusing on the October 19 close approach to Mars, it's worth keeping in mind that October 25th is the comet's closest point to the Sun in its orbit, so that's the time it'll be experiencing the most solar radiation it has seen in over 4-billion years...
As far as we know, the sharp drop in brightness won't affect any plans for the Mars fleet and planned observations will continue as scheduled. While it is perhaps disappointing from the standpoint of hoping for cool pictures of the comet from Mars, it may actually make it easier for MRO's HiRISE instrument to image the nucleus since there will be less coma in the way than we previously expected. Hurray! (We're optimists. Most astronomers tend to be.)
We'll be posting images and news from observatories on Earth and at Mars as soon as they are publicly available. We don't know what that timescale will be, particularly with respect to the Mars teams.
We've mentioned this in a couple of our blogs, but it's worth reiterating that this situation is not the same as with Comet ISON last year. With that comet, we were lucky to be observing it with the ESA/NASA SOHO satellite, which is close to Earth and sends down data in realtime. The Mars craft don't do that - it's just not part of their design. So we will have to be patient and wait for the teams to downlink and process their data.
This will probably be our last light-curve update before close approach but, as always, keep checking back on this site for new blogs, keep an eye on Karl's @SungrazerComets Twitter feed for breaking news, and feel free to Contact Us with questions.
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
Jul 25, 2014
Sep 16, 2014
Sep 26, 2014