So Sky Safari has shown that comet 289P/Blainpain has had a sudden increase in brightness, and is currently sitting at mag -1.4!!! Last week it was sitting at +15. It is due to reach Perihelion on 20th December (2 days time) so is this due to some outburst? Or is there a glitch in Sky Safari? Looking at THIS site, it states:

When the perihelion distance is larger than 1.0 A.U., it becomes very bright if it passes the perihelion on a special day in early December. However, only if the perihelion passage shifts by a several days.

But the Perihelion distance is 0.089AU for this year according to Google

This comet could be prone to outbursts as it was recently rediscovered in July 2013 due to a sudden brightness increase from an outburst (although nowhere near this much!)

However, currently TheSkyLive is showing that it is currently at mag +18.

I have had a quick search to try and verify this, unfortunately, I live in England and our skies are very cloudy at the moment so I can't go out and see for myself. Does anyone have any information as to whether this is a Sky Safari glitch or if not, what has happened to cause such a huge brightness increase?

  • $\begingroup$ Sky Safari is a program/app, correct? The probability of a program calculating the brightness of a comet accurately is not very high :-). It is a glitch or inaccurate calculation in my opinion. Also, you are confusing perihelion (closest distance to the Sun) with the distance to the Earth. The 0.089 AU listed by Google is the Earth's distance. $\endgroup$ – JohnHoltz Dec 18 '19 at 14:29
  • $\begingroup$ @JohnHoltz yes, SkySafari is a program/app and at roughly £40 for the pro version, it is usually very accurate. As for the perihelion distance, I probably should have clicked one of the pages and read it before putting the numbers in the question! $\endgroup$ – MCG Dec 18 '19 at 14:38

When I add 289P/Blanpain in the solar system editor of Stellarium, it too shows the the comet blazing away at apparent magnitude -1.4. The problem must be upstream. Planetarium software typically gets comet and asteroid data from the Minor Planet Center, whose own ephemeris service gives a similar result under m1:

Date       UT      R.A. (J2000) Decl.    Delta     r     El.    Ph.   m1     Sky Motion
            h m s                                                            "/min    P.A.
2019 11 30 000000 22 29 37.8 -16 25 42   0.280   1.006   85.9  78.0  -0.2    1.25    023.6
2019 12 15 000000 22 46 29.4 -06 51 49   0.201   0.963   78.0  90.2  -1.1    2.39    021.8
2019 12 30 000000 23 17 53.0 +14 42 59   0.123   0.968   79.2  93.7  -2.2    6.04    019.2

However, MPEC 2019-W179 from the same organization lists some actual observations from late November and gives this ephemeris:

Date    TT    R. A. (2000) Decl.     Delta      r    Elong.  Phase   m1    m2
2019 11 30    22 29 37.7 -16 25 45   0.2802  1.0058    85.9    78.0  17.3
2019 12 15    22 46 29.3 -06 51 54   0.2013  0.9628    78.0    90.2  16.4
2019 12 30    23 17 52.7 +14 42 50   0.1232  0.9678    79.2    93.7  15.3

MPC has collected other observations since then. The JPL Small Body Database record, based on the same observations, estimates a total absolute magnitude M1 = 20.8±1. HORIZONS gives this ephemeris:

Date__(UT)__HR:MN     R.A._(ICRF/J2000.0)_DEC  T-mag  N-mag
2019-Nov-30 00:00     22 29 37.44 -16 25 42.6  18.05   n.a.
2019-Dec-15 00:00     22 46 28.88 -06 51 47.8  17.25   n.a.
2019-Dec-30 00:00     23 17 52.05 +14 43 16.0  16.19   n.a.

If I edit my .stellarium/data/ssystem_minor.ini

absolute_magnitude = 2.5

and replace the faulty 2.5 with 20.8, Stellarium predicts a more realistic magnitude 16.9 for the comet today. The MPEC ephemeris is consistent with M1 ≈ 20.0.

The MPC data file CometEls.txt lists a few other comets at absolute magnitude 2.5. JPL estimates for those comets range from 4.8±0.8 to 10.7±1.

  • 1
    $\begingroup$ Thanks for the detailed answer. I was very skeptical about this, but wasn't too sure where to look for the Data, especially as I had asked a few people, all of which had disagreeing data. Some others using Sky Safari had it as mag 18, and some Stellarium users had mag 18, some had -1.1, and some -1.4. It was very strange, but thanks for the detail in this answer. Very helpful $\endgroup$ – MCG Dec 19 '19 at 8:18

MPC has always used magnitude parameters from previous apparitions in these files. But in this case, there hardly are any. In more detail:

This small Jupiter-family comet was discovered independently by Jean-Jacques Blanpain and J.L. Pons in 1819, but then lost until it was eventually recovered in 2003, appearing asteroidal. A faint coma was observed in 2005, further confirming that the object was indeed the lost comet. Then in 2013, a very large outburst was observed by Pan-STARRS, when the comet was at no less than 3.9 AU from the sun. Unfortunately, because this was the first time since 1819 that the object appeared truly cometary, magnitude parameters were published that were based solely on this outburst, causing many planetarium programs to severely overestimate the comet’s brightness...

Now for the good news: I'm maintaining a list with up-to-date magnitude parameters based on cobs.si observations and some total-magnitude MPC observations on my website:

See http://astro.vanbuitenen.nl/cometels

  • $\begingroup$ Outstanding! Added your MPC-format list to my Stellarium bookmarks. $\endgroup$ – Mike G Dec 20 '19 at 17:00
  • $\begingroup$ I just had a look at your website... Is the data for C/2019 Y4 correct?! -26 is super bright! Surely this would be quite big news if this were the case? theskylive has it reaching mag +12 at brightest $\endgroup$ – MCG Feb 25 '20 at 10:22

Most planetarium software get their comet elements from the Minor Planet Centre. It looks like there is a glitch in the data for 289/p. It did suddenly brighten by a couple of magnitudes in October (I think) from 20 to about 18. None of the forums I look at have reported any spectacular brightening more recently.


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