Perihelion Studies of Comet ISON

Remember Comet C/2011 W3 (Lovejoy), that spectacular Kreutz sungrazer of a couple of years ago? We only had very brief warnings that it was on its way -- about three weeks, to be precise. Nonetheless, in that time we were able to raise some alarm bells with the operations teams for the SOHO , STEREO, SDO , and Hinode satellites, and at short notice we put together a plan to get as much data as possible with instruments really not designed for comet science. Despite several unknowns, including uncertainty in the orbit of the comet, it could not have gone much better, and with the additional help of the PROBA2 satellite, we ended up getting spectacular data from 18 instruments on 6 different satellites.

Perihelion, or the days surrounding it, when Comet ISON is at its closest point to the Sun, is a very difficult time to observe Sungrazing comets. On Earth, we rely on dark skies to enable us to point our telescopes out into space and view astronomical objects, but if our target of interest is right next to the Sun then clearly nighttime viewing is not an option. Furthermore, it is during the days and hours surrounding perihelion that a Sungrazer is experiencing the greatest intensity of gravitational stress and solar radiation. In short, if a comet is going to do something wild (outburst, fragment, vaporize, etc) then this window is when it's most likely to occur, and is when we are most interested in watching it. This is where Sun-watching spacecraft come in to play, as they are designed specifically to look at the Sun and the region of space surrounding it. As we just said, we took great advantage of these unique observatories when Comet Lovejoy blazed by the Sun in 2011 and were able to return some unprecedented result. Our strategy for space-based observations of Comet ISON will be much the same, but this time with much more planning, and potentially aided by a fleet of NASA spacecraft including MESSENGER (at Mercury), Dawn (asteroid mission), Venus Express, and theoretically any of the currently active Mars missions. We're not in a position to speak for many of those missions, but team member Karl Battams can speak to what we plan do with the SOHO and STEREO spacecraft, and can also speculate on what the SDO team might choose to do. Here's what he says on the subject...

Observing ISON with Sun-watching spacecraft

A composite EUV image from STEREO-A

On its way into the solar system, the first view we will get in some of our instruments will be in the , SECCHI HI-2 camera on the STEREO-A spacecraft. It should appear in the field of view in early October 2013, and we will be able to monitor it continuously throughout its passage past the Sun. I think our only “blackout” will be shortly after perihelion as it passes behind the Sun, as seen from that particular spacecraft. So it all sounds very promising for STEREO, but there is a snag. Both of the spacecraft are very far from Earth, and will be even farther by the time ISON gets here (no, we can’t move them!). This means that will we have only a limited amount of data that we can record because the distance means we need a larger and more powerful receiver on Earth, and to use those really big Deep Space Network receivers costs an awful lot of money! But we have some clever folks here in our operations team, and we will certainly do all we can to take as much data as possible.

The STEREO-B spacecraft looks in a slightly different direction, and thus we will not be able to get such continuous coverage until ISON gets very close to the Sun, where it will appear in the COR-2 coronagraph images about a day before perihelion. (If I ask the STEREO project management nicely, I might be able to convince them to roll the spacecraft for me once or twice so that we get to see ISON a little sooner... but I haven’t asked them yet so don’t say anything!) Once it reaches COR-2, will subsequently get to follow it through the inner COR-1 coronagraph, and should be well placed for it to cross our Extreme-Ultraviolet (EUV) images of the Sun. Whether it will be visible in EUV -- like Comet Lovejoy was in the SDO images -- is uncertain, simply because ISON will be about twice as far from the Sun as Lovejoy was.

Comet Lovejoy visible in SDO.Credit:NASA/SDO

The NASA SDO satellite, which captured spectacular EUV images of comet Lovejoy, does have potential to view ISON, but the details will depend on a couple of factors, with ISON’s brightness and size being foremost among them. Comet ISON will pass about 2.5-times further from the Sun than Comet Lovejoy did, and even though it will pass through parts of the field of view of the SDO instruments, it may be too far away from the Sun to show any signature in the extreme-ultraviolet.

That said, ISON could potentially be a kilometer-sized comet (Lovejoy was less than half a kilometer), and a few hundred times brighter than Lovejoy, so I will fact-check this as we get nearer to the time. I know the SDO team are monitoring ISON very closely and if there’s any chance of being able to detect it, they will do all they can, I’m sure. For Lovejoy, they actually pointed the satellite somewhat away from the Sun -- a very risky venture for a satellite like SDO, and something you only do if you’re really confident the payoff is in the data... which it was of course! If SDO is to even attempt to image Comet ISON then it will have to repeat this maneuver to capture it in their field of view. My understanding is that they are willing to do this so long as the odds of being able to detect the comet are indeed good.

Finally, but certainly not least of all, we have our favorite old workhorse, the ESA/NASA SOHO satellite. As you perhaps know, SOHO is currently in its 18th year of a 2-year mission... Yes, you read that right -- it’s already 16-years beyond its nominal mission, and while it is still working well at the moment, it’s certainly getting up there in years. So I’m delighted that, after rewriting the record books and discovering nearly 2,500 previously unknown sungrazing comets, SOHO will get to sit back and watch (potentially!) one the brightest Sungrazers in history!

Comet ISON's trajectory through SOHO/LASCO C3.
So, what will SOHO see? Well, it will only get to view ISON for a few days either side of perihelion, with ISON entering the SOHO/LASCO C3 camera in the lower right-hand side, zoom towards the Sun, pass through the inner C2 camera, and exit out of the top-left hand side of the images a couple of days after perihelion.

I have already verified with the SOHO operations team (which these days comprises exactly one awesome guy: Kevin) that we should have no data connection issues with SOHO around that time, the concern there being SOHO’s periodic “keyholes”, where we have to make do with less data for a couple of weeks a few times per year. So that is good news. The less good news is that if -- and I do mean IF -- Comet ISON is ridiculously bright (more than ~mag -5), then the comet will be a lot brighter than our camera can comfortably handle. We’ll still take images, of course, but ISON will appear very saturated and overexposed. I think the shortest exposure time we can do is one-second, and that won’t be enough to take care of a magnitude -5 or brighter object. But we’ll still get to see it, of course; we won’t stop taking images because of saturation, and we might still have one or two tricks up our sleeve that we can employ to get some pretty pics of the comet. The saturation will also be a problem for our SECCHI imagers on the STEREO satellites, but I think the newer technology there might allow us a tad more leeway in exposure times.
So that's the basic outline of our perihelion plans. We can't get much more detailed than that right now as we don't know what the comet will do. Once we have a handle on its brightness then we can start planning exact exposure times and such, and really get a handle on what we're going to do with those spacecraft.

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