We stopped printing the annual ICQ Comet Handbook in 2022.

[last updated 2023 November 7]

INTERNATIONAL COMET QUARTERLY             H2               December 2021

The International Comet Quarterly (ICQ) is a journal devoted to news and observation of comets. Regular issues are published four times per year (January, April, July, and October) by the Cometary Science Center at the Department of Earth and Planetary Sciences, Harvard University, with an annual Comet Handbook of ephemerides published as a special fifth issue.

The Editor may also be contacted via the Internet (ICQCSC@EPS.HARVARD.EDU); see our web site at http://www.icq.eps.harvard.edu/index.html.

ICQ COMET HANDBOOK EDITORIAL STAFF::
Daniel W. E. Green..................Editor 
Syuichi Nakano......................Ephemeris Editor

This special issue is Vol. 42, No. 1a, of the ICQ Copyright 2021, International Comet Quarterly [ISSN 0736-6922]

This is the thirty-sixth edition of the Comet Handbook, a collection of up-to-date (as of January 2022 orbital elements and ephemerides to aid observers in locating comets for most of 2022 and the first part of 2023. Syuichi Nakano compiled the ephemerides in this Handbook, with some consultation with Daniel W. E. Green. The content of this Handbook is similar to that of previous ICQ Comet Handbooks, and more complete introductory information, especially concerning such data as magnitude parameters, may be found in the "Introduction" to the 1990 ICQ Comet Handbook and in the 1997 ICQ Guide to Observing Comets. Up-to-date ephemerides for other new comets will be found in the IAU Circulars and the Minor Planet Center's publications (and via the ICQ/CBAT/MPC website). Note that the orbital elements for some comets will be improved (sometimes considerably) during the year of coverage here, and observers should consult the MPC website for revised elements. Some comets, for example, are considered lost, and their predicted positions are likely to be considerably different from the actual locations (and any associated brightness prediction is nothing more than a guess); generally one or two comets per year appear to split or even apparently fall apart and fade or disappear unexpectedly.

Pages H3-H15 of this issue contain the orbital elements for the 390 comets included in this Handbook, listed in order of T (the time of perihelion). As noted in the "Introduction" to recent Handbooks, we have been including information for both B1950.0 and J2000.0 equinoxes, noting that it seems appropriate to have a transition period of several years in which ephemerides for both equinoxes are provided. Note that, as the 2021 Comet Handbook has ephemerides through 2022 Mar. 7, one should also consult that Handbook for comets visible for a few months around early 2022 that may not have ephemerides in this 2022 Comet Handbook (due to poor placement or rapid fading shortly after this time period).

Orbital Elements. The top-half of each of pages H3-H15 contains orbital elements for the comets with the columns containing the following information: (1) No. is the orbit number; (2) T/ TT is the time of perihelion in 'Terrestrial Dynamical Time' (last two digits of year; month; date), which in 1994 was about 60 sec ahead of Universal Time; (3) q (AU) is the perihelion distance; (4) e is the orbital eccentricity; (5) P is the orbital period in years; (6) Peri. is the argument of perihelion; (7) Node is the longitude of the ascending node; (8) Inc. is the orbital inclination (these three angles are for equinox J2000.0); (9) A1 and A2 (if given) are the nongravitational parameters instituted by B. G. Marsden [see the 2000 Comet Handbook]; (10) Epoch is provided with year, month, and date (usually the 40-day Julian Date nearest perihelion passage); (11) Obs. is the number of astrometric observations used in the orbital computations (an asterisk indicates that non-gravitational-force parameters were solved for); (12) (Delta)T is the correction to T in days for a recent periodic-comet recovery; (13) Arc is the arc of time spanned by the observations used in the calculation; and (14) Per. indicates the planets whose perturbations were allowed for (ME-PL = Mercury-Pluto; MEmPL = ME-PL plus 5 minor planets; MEcNE = Mercury to Neptune including minor planet Ceres; MEvNE = MEcNE + minor planets Vesta and Pallas). [Note that elements for each comet are also listed at the top of each ephemeris page, but for an epoch 2022 xxx. yy.0 TT to aid observers (instead of the epoch closest to the time of perihelion, which is sometimes far from the ephemeris dates).]

The bottom-half of each of pages H3-H15 contains the following supplemental information, by column: (1) comet number, to identify easily with the orbital elements on the top-half of the page; (2) Comet Designation (and name), where "P/" indicates "periodic comet" (one with orbital period < 30 years), "C/" indicates "long-period" or "intermediate-period" comet (P > 30 yr, a recent change), "A/" indicates an object reported initially as asteroidal in appearance but which has a comet-like orbit and may be cometary, and "D/" indicates a comet with a very-uncertain prediction (usually short-period comets not observed in many years); (3) Calculator of the orbital elements; (4) References, or sources, of the (published) orbital elements [NK represents Nakano's personal circulars; CBET = Central Bureau Electronic Telegrams; HICQ = earlier annual editions of the ICQ Comet Handbook], including year in which calculation or publication was made; (5) m.e. is the mean error of the observations with respect to the elements, in arcsec; (6-7) H1, K1 are the comet's estimated total magnitude (m_1) parameters, to satisfy the equation m_1 = H_o + 5 log Delta + 2.5n log r, where H_o = H1 and 2.5n = K1 (note that, when both pre-T and post-T parameters are used for the ephemerides, only pre-T values are given in the tables on pages H3-H15, while both sets of parameters are then given on the relevant ephemeris pages); (8-9) H2, K2 are the comet's estimated so-called 'nuclear magnitude' (m_2) parameters, replacing m_1 with m_2, H_o with H2, and 2.5n with K2 in the above equation [but the concept of "nuclear magnitude" is being phased out and will only occasionally be used in this annual Handbook; see comments in the 2003 Comet Handbook (last paragraph) regarding this topic]; (10) the orbital semi-major axis, a (or 1/a in the case of long-period comets, in which case are given respectively the osculating, 'original', and 'future' values of 1/a, which represent the situation during, before, and after the comet's current passage through the inner solar system); (11) date of the last known (usually astrometric) observation at press time, with observatory code given according to the 3-digit key used in the MPCs; and (12) Remarks contain further information, such as values of 1/a at the given epoch, estimated uncertainty in orbital period for comets with more uncertain predictions, notes regarding special magnitude equations, or sources of these or other (similar) orbital elements. Ephemerides (pp. H16-end). The comets chosen to appear in this 2021 edition are generally those that are estimated as having a reasonable chance of being recovered or observed sometime in 2021. While no exact formula was followed, comets with predicted m_1 > 20-21 throughout this span generally were not included in this Handbook. Also, we include here some (but not all) of the 'traditional' "annual comets", which are those regarded as being "easily" observable essentially around their entire orbit: 2P, 10P, 29P, 49P, 65P, and 74P (although more objects do fall into this category for non-"routine" observations with very large telescopes). Comet 1P/Halley is included because of general interest in this comet. While some of the other included comets will not be observed (because they are too faint), most of the comets listed herein should be observable with telescopes of 1- or 2-meter aperture with CCDs. Observers are encouraged to monitor the faint comets for possible outbursts in brightness, and astrometrists are encouraged to report photometric results (with explanatory details) to the ICQ Editor for archiving.

The top line of each comet ephemeris lists: (1) the comet's designation and name, (2) individual who computed the orbital elements, (3) with reference, and occasionally (4) the last known astrometric observation (with associated MPC observatory code given parenthetically). The next three lines contain the J2000.0 orbital elements (with headers) for epoch 2021 July 5.0 TT (those corresponding to the J2000.0 elements for "perihelion epochs" on pages H3-H16); added here is the column titled Tail(AU), which indicates a chosen "standard" tail length in AU for the comet, which is used in computing an estimated apparent tail length in the ephemeris below.

The magnitude parameters (discussed further below) are placed into formulae on the line below the elements for each ephemeris. The actual ephemerides have columns that contain the following information, computed with allowance for perturbations (i.e., epoch of ephemeris date): (1) Date for 0h TT, (2-5) right ascension (R.A.) and declination ((Delta)) for both equinoxes B1950.0 and J2000.0, (6-7) geocentric ((Delta)) and heliocentric (r) distances of the comet, in AU.

Columns 8-9 contain as a heading one of two possible titles ("Daily Motion" and "Variation for T=+1 day"), depending on the individual comet. The "Daily Motion" (in minutes of R.A. and arcmin of Decl.) is given for all long-period comets and all "recently recovered" short-period ones. In ephemerides for yet-to-be-recovered short-period comets, where this column is titled "Variation for T=+1 day", the so-called "line of variation" is given; this is for locating the comet when it hasn't been seen in years (because the most uncertain element is often T). Thus, if one adds one day to T, then one must add the value given in Column 8 to R.A., and add the value listed in Column 9 to (Delta), in order to find the comet; the orbits of most short-period comets are known well enough such that (Delta)T is < 1 day, so the correction to the ephemeris will normally be quite small.

Column 10 contains the predicted magnitude (usually total, m_1; described above and below). Column 11 (Mot./PA) gives the comet's daily rate of motion in arcmin and its direction (position angles are measured with north = 0 degrees, west = 270 degrees). Column 12, Elong., lists the comet's apparent elongation (in degrees) from the sun, and column 13 (P) lists the phase angle of the comet. Column 14 (PA/Tail) lists the computed anti-solar direction for a possible tail, and the estimated tail length (based on the "standard" parameter described above). Columns 15 (Moon) and 16 (Age) list the elongation of the comet from the moon and the percent-illumination of the moon (1.0 = full moon, 0.0 = new moon), respectively.

Observers using these ephemerides should be aware that the predicted brightness could be off by several magnitudes in either direction. Our list of magnitude parameters is updated each year based on brightness data contributed by observers to the ICQ and to the Minor Planet Center (this procedure is described in a paper by Green in the July 1996 ICQ; see also the ICQ Guide to Observing Comets), and the included magnitude parameters generally produce results within 1-2 mag of reality for most comets. We take m_1 as the total, integrated, visual magnitude of a comet's coma that is visible in the smallest instrument needed to easily observe the comet (which may be generally 1 or 2 mag brighter than CCD magnitudes of comets in the range 12 < m_1 < 16). Note that those numerous comets that have been observed to split (i.e., multiple nuclei) have extremely unpredictable light curves; returning short-period comets such as 51P, 57P, 69P, 73P, and 141P that have split have shown great variations in brightness from one return to the next, and it can be expected that such split comets will be difficult to predict at future returns to perihelion, in terms of brightness. Other periodic comets have not been seen at recent returns to perihelion, their brightness being absolutely uncertain, and even their predicted positions must be highly suspect (18D being a good example). We generally do not give H_o to more precision than half a magnitude (or 2.5n to more precision than 0.5), as the magnitude formula in this form is usually not accurate to such levels over a large range in r. Observers are encouraged to try observing comets whose ephemeris magnitudes are a couple of mag fainter than the limiting magnitude of the available telescope, but use utmost caution to make unambiguous identification.