Over the past thirteen days I’ve programmed for myself a way to use the current orbital elements for comets.
First, be warned: none of the comets I can at the moment talk about are of the rare kind that the general public can see: they are from about 5 to 700 or more times too dim for the naked eye. They need a good telescope, skill, and a clear un-light-polluted sky.
But it’s interesting to be able to show what they are doing, and the longer-term purpose is that I’ll be able to repeat the process at any time in the coming year. The Astronomical Calendar 2016 comet pages had to be completed in August 2015. New comets (mostly dim, a few not) keep being discovered, and I hope that this process can be my butterfly-net for catching them.
By “the current orbital elements,” I mean those that can be found in what I’ll call the Minor Planet Center table. You can see it for yourself at:
www.minorplanetcenter.net/iau/MPCORB/CometEls.txt
It contains at present 894 long lines. In case you really want to know, I’ll explain that the columns represent: a code for the scientists; perihelion date; perihelion distance; eccentricity; argument of perihelion; longitude of ascending node; inclination; epoch of the elements; absolute magnitude; slope factor; designation; name; and the source (such as an issue of the Minor Planet Circulars).
I won’t bore you by explaining just why it took me thirteen days of careful thought to weave a way of using these elements into my older way, which uses a catalogue structured differently. That catalogue was derived from older ones formerly issued by the same authorities in Cambridge, Massachusetts, and to it I had to add, manually, more recent elements and new comets. A lot of the intricacy had to do with my choice, in early years of program-building, to use my own short codes, up to only four character, to denote moving bodies, such as “EART,” “MOON,” “JUP,” “A1,” “A433” (asteroids), “1P,” “15V2” (comets). It might be another lifetime’s work to go back and unpick that sort of thing from a vast mass of interdependent programming.
My newly written small program (called so far, for want of a better name, COMNOW), when told to look for comets in a certain time span and brighter than magnitude 13, spits out this ephemeris, or table of positions and other facts by date. (I have to show it as a picture, because I haven’t yet found how to change to a fixed-width font.)
Tthe columns are the comet designation and name; date; right ascension (in degrees); declination; distance from Earth and Sun (in AU, astronomical units, that is, units of the Earth-Sun distance); elongation from the Sun (negative being to the west); and magnitude (smaller being brighter, and 5 or 6 the naked-eye limit).
I checked my ephemeris by using this moderately complex tool provided by the Minor Planet Center: www.minorplanetcenter.net/iau/MPEph/MPEph.html
(Note, if you really do this, that they allow right ascension expressed in hours or decimal hours, but not in degrees.)
Are these really all and only the comets brighter than 13 at these dates?
Periodic comets 43P Wolf-Harrington and 45P Honda-Mrkos-Pajdušáková are fully featured in Astronomical Calendar 2016. In the “Megnitude” graph there, you can see that they were indeed the only comets expected to climb (HMP very steeply) to moderate brightness in the last two months of the year.
C/2015 V2 Johnson was discovered 2015 Nov. 3, too late to get into Astronomical Calendar 2016.
SOHO is the Solar and Heliospheric Observatory, which, hanging in space, uses a coronagraph to mask out the solar disk, and makes incidental discoveries of things skimming or falling into the Sun. Notice 323P’s elongation of only 7 degrees: it evidently will reach a brief peak of brightness as it whips around the Sun.
I used to get from Alan Hale his suggestions of comets that he expected to become observable by amateurs in the coming year. Alan is a comet expert, which I am not. His hundreds of hours of comet-observing and comet-hunting were rewarded by his discovery of C/1995 O1, better known as Hale-Bopp, one of the greatest, perhaps the greatest, of the twentieth century. (Notice that it is the first line in that Minor Planet Center table, which must mean that it is still a candidate for detection despite having departed nearly 20 years ago – longer than any other non-periodic comet.)
Alan would send me a list of nine to fifteen, generally those predicted to reach above a magnitude of, say, 12, while not too close to the Sun. But comets, whose brightness depends on their erratic “activity” as sunlight causes matter to diffuse from them, often fail to follow expectations; and Alan’s experience, his gut feelings, or “seat of the pants” estimates, were often a better guide than the formulae. And then, as I was half through working on the comet section, he would have to tell me that there was a newcomer that should perhaps be included.
I have asked Alan what he thinks of my table of predictions, and he sent me extensive notes. Briefly:
C/2015 V2 Johnson was at 12.8 when Alan observed it on Nov. 11, so it’s about on track, though earlier it had been expected to brighten more.
43P Wolf-Harrington is running a bit dimmer than expected; 13.2 when Alan saw it a few days ago.
45P Honda-Mrkos-Pajdušáková was recovered only a week or so ago, very faint, and will probably brighten steeply as it has done in the past.
On 323P SOHO I quote Alan: it “is one of the SOHO-discovered periodic ‘comets’ (we’re not quite sure just what they are, as far as their physical nature is concerned) that has never been seen from the ground… It might reach 8th magnitude or a bit brighter when it passes through perihelion a couple of days later, but it will only be visible with the coronagraphs aboard SOHO. It will be extremely faint, and probably too faint to be detectable, when its elongation becomes high enough to access from the ground.”
And Alan mentions other comets that my net has not caught (later I may find out why – it may have to do with how frequently the Minor Planet Center table is refreshed):
144P Kushida, at present the brightest in the sky, though fading; Alan saw it at magnitude 12 a few mornings ago.
237P LINEAR, unexpectedly bright, about 12.5 a couple of weeks ago, but low in the dusk and getting more distant.
C/2011 KP36, near magnitude 13 for the past couple of months but also departing.
29P Schwassmann-Wachmann, the great but distant comet famous for “outbursts”: it had one to slightly brighter than 13.0 three weeks ago, but is fading and getting lower in the dusk.
Two more, C/2015 ER61 and the recently-discovered C/2016 U1, just may become detectable in the morning sky by the end of the year.
2P Encke, the comet most often seen, because of its short orbit, may appear on the stage by the end of December. So we’ll catch it later.
For now, let’s picture just the comet discovered a year ago by Jess Johnson with one of the telescopes used by the Catalina Sky Survey. It was then down at magnitude 17. It is falling in from the outer reaches of the solar system, on an enormous near-parabolic orbit. At present it is still 3 AU out and far north in our morning sky. Next June 12 it will speed at about 33 kilometers per second through perihelion, around which time it will be a bit over 1.6 AU from the Sun but only half that distance from Earth, well out in the evening sky in the Virgo-Libra area, and may reach magnitude 7 or 6. But it will find itself deflected into a hyperbolic orbit – that is, it will leave the solar system for ever.
Path of the comet in November and December.
Space trajectory of the comet in 2016 and 2017. Lines on the ecliptic plane are at intervals of 1 AU. Stalks at monthly intervals connect the comet’s position to the ecliptic plane. The thick line is the vernal equinox direction. The large circle is the orbit of Jupiter.