BAA Comet Section : Comets discovered in 2017

Updated 2021 November 3

The equations for the light curves of comets that are currently visible use only the raw observations and should give a reasonable prediction for the current brightness. If the comet has not yet been observed or has gone from view a correction for aperture is included, so that telescopic observers should expect the comet to be fainter than given by the equation. The correction is about 0.033 per centimetre. Values for the r parameter given in square brackets [ ] are assumed. The form of the light curve is either the standard m = H0 + 5 log d + K0 log r or the linear brightening m = H0 + 5 log d + L0 abs(t - T + D0) where T is the date of perihelion, t the present and D0 an offset, if L0 is +ve the comet brightens towards perihelion and if D0 is +ve the comet is brightest prior to perihelion.

Observations of new and periodic comets in 2017 are given in ICQ format. 

Full details of recently discovered objects will not appear until they are available on the CBAT web pages. The actual accuracy of preliminary orbits is often (nearly always) much worse than the published accuracy implies.  In part this is because each orbital solution is treated as a mathematical construct and does not take account of observational error.  JPL does publish the errors, whereas the MPECs do not.

In a post on the comets-ml on 2021 April 6, Sam Deen notes:

Unique among the comets listed [see other objects], 2017 GZ8 is actually quite a well observed object with a respectable 3-opposition orbit. This object, despite having a Jupiter MOID of less than 0.05 au, is in a near-4:7 resonance with the planet so is quasi-stable for the time being. This wasn't always the case though, and it was making close approaches to the planet as recently as the 1700s, most notably a 0.154 +/- 0.118 au approach in 1777 that lowered the comet from a ~32 year period (q = 4.3 au) to just ~18-21 years (q = fluctuates between 3.75 to 3.95 au) The comet's fairly low activity can be attributed to its perihelion currently being as high as it's been in several centuries, but that doesn't seem to have stopped it from being active anyway.  The only activity I saw was on 2017/04/01 (r=3.98) with even an observation on 2017/05/05 (r=4.00) showing the object as completely pointlike. Sadly no other available observations are closer to perihelion than 2017/04/01, but PANSTARRS was observing around then so should have been able to see something as well. 2017 April 1: no apparant coma but with a very faint, stubby tail at least 5" long (possibly up to 20") at PA 290. The seeing is about 1.4 arcseconds. 

Early studies of the orbit suggested that most likely it was a dynamically old comet, though there was a 30% chance that it might be an interstellar object. CBET 4966 [2021 May 12] gives it a parabolic orbit, however JPL still designate it as a Hyperbolic Comet using the same observations.  The CBET also notes that pre-discovery observations have been found back to 2013 May.  In late 2020 June it briefly appeared on the PCCP as M7blPbR as a 16th magnitude object, which is in line with a slow rate of brightening.  Some professional observations suggest that the nucleus may be a slow rotator, with a period of around 156 days and two active areas.

It is likely that the future brightness evolution is still not well determined.  The latest magnitude parameters suggest that it might reach 7th magnitude, however it has been brightening more rapidly during 2021 as it approaches the sun.  Recent observations are over 1 magnitude brighter than the mean curve. The comet will be a southern hemisphere object when at its brightest, but could be a binocular object for UK observers prior to perihelion.  It could remain at around 7-8th magnitude for around a year from mid 2022 to mid 2023.

The orbital determinations have a rather complicated history, which are partialy elucidated under the entries for comets 282P and 358P

As suggested in 2017, it was worth monitoring for cometary activity, and it showed such features when recovered in 2018 October.

Further observations of this object are very much desired. Unless there are serious problems with much of the astrometry listed below, strongly hyperbolic orbits are the only viable solutions. Although it is probably not too sensible to compute meaningful original and future barycentric orbits, given the very short arc of observations, the orbit below has e ~ 1.2 for both values. If further observations confirm the unusual nature of this orbit, this object may be the first clear case of an interstellar comet.

K. Meech (Institute of Astronomy, University of Hawaii) reports that in a very deep stacked image, obtained with the VLT, this object appears completely stellar. The prefix for the designation 2017 U1 is therefore being changed to A/, in line with the 1995 IAU Resolution on the system of comet designations.