Comet PanSTARRS (C/2015 ER61) in Outburst and Binocular Bright

Dawn comet PanSTARRS (C/2015 ER61) just underwent a bright outburst and is now an easy binocular object. Take a look before the Moon returns!

Comet PanSTARRS (C/2015 ER61) José J. Chambó

Look at the difference in appearance of comet PanSTARRS (C/2015 ER61) pre-outburst (left) on April 1st and in outburst on April 4th.
José J. Chambó

2017 may well go down as the year of the binocular comet. Three have been easy catches, and it's only the start of April: 45P/Honda-Mrkos-Pajdusakova, 41P/Tuttle-Giacobini-Kresak and Lovejoy (C/2017 E4). Now there's a fourth. Overnight, PanSTARRS (C/2015 ER61) joined the club.

Discovered two years ago on March 15th by the Pan-STARRS 1 telescope on the summit of Haleakalā, it was a faint 21st-magnitude midge. But how it's bloomed! By late March and the start of April, the comet had brightened to around magnitude +8.5 while puttering across Sagittarius and Capricornus low in the southern sky before dawn.

Beautiful Sight in Aquarius

This April 5th photo catches the comet in the full glory of its outburst.
Gerald Rhemann

Then it happened. On April 4th, comet observer Juan José González Suárez reported a possible outburst to magnitude +7.4. This was confirmed, both visually and photographically, by several observers including myself early this morning. It's now as bright as magnitude +6.5, a leap of two magnitudes practically overnight! Although the specific cause of the outburst isn't known, it's likely that some sort of outgassing or disruption on the comet's surface exposed fresh ice to sunlight, initiating a new wave of vaporization.

Even from northern Minnesota, where the PanSTARRS only managed to climb to 7° at dawn's approach, I caught it in my trusty 10×50 binoculars crossing the border from Capricornus into western Aquarius.

Comet on Track

Comet PanSTARRS (C/2015 ER61) tracks close to the ecliptic in Aquarius in the coming weeks. The map shows daily ticks (5 a.m. CDT) and is marked at 3-day intervals. Stars are plotted to magnitude +7.5. The view faces east and shows the sky at the start of dawn from mid-northern latitudes (40° N, specifically). Although the comet appears to be moving downward and out of view, it's only because the map date is April 6th. In reality, the downward movement will be compensated by the westward, rising movement of the stars with the comet maintaining a roughly constant altitude of ~10° at dawn.
Map created using Chris Marriott's SkyMap

As twilight swelled, the comet rose higher and became unmistakable. Due to its low altitude, I wouldn't say it was bright, just a soft, round fuzzball easy to pick out from the barren star field. Through a 10-inch telescope at 60×, I estimated the moderately condensed coma at 8′ in diameter with a degree of condensation (DC) of 5. A faint, broad tail extended about ½° to the west-southwest. A splendid sight!

Don't Forget Lovejoy!

Comet E4 Lovejoy with its bright, narrow gas tail on April 3. The comet shines at magnitude +6.5 and is still brightening.
Gerald Rhemann

Compared to comet Lovejoy (C/2017 E4) located further north and east in Pegasus, PanSTARRS has a big coma — nearly twice as large but neither as dense nor as bright. Lovejoy was much more compressed (DC=7) and distinctly blue-green with a short, faint tail about 15′ poking out to the west as seen through the 10-inch. With binoculars I compared Lovejoy to the nearby globular cluster M15; to my eyes they appeared nearly identical in size and brightness, which would put the comet at about magnitude +6.5.

These are wonderful times for comet watchers even if you have to get up at 4:30 in the morning to see these ever-alluring puffballs. Speaking of which, there are few moonless left for pre-dawn comet catching. The waxing gibbous returns on April 7th or 8th and will do its best to thwart the view. But given that both Lovejoy and PanSTARRS are reasonably bright and compact, we might still get good looks while waiting for the Moon to slim down.

Comet PanSTARRS passes closest to Earth on April 19th at a distance of 109.5 million miles. It's expected to continue to brighten on its way to a May 10th perihelion. How the outburst may affect its light curve is anyone's guess!

19 thoughts on “Comet PanSTARRS (C/2015 ER61) in Outburst and Binocular Bright

  1. Joe StieberJoe Stieber

    I looked this morning and easily saw C/2015 ER61 (PANSTARRS) with 16×70 binoculars from my front yard, which is in the light-polluted NJ suburbs of Philadelphia (I’m 8 miles from center city) about 5 minutes before the start of nautical twilight (which was at 5:37 am EDT). I thought it was a little brighter than nearby C/2017 E4 (Lovejoy).

    1. Bob KingBob King Post author

      Amazed you could do so well from such an urban area. Thanks for sharing your observation. I hope other city dwellers read it and take hope!

  2. Graham-Wolf

    Hello Bob.
    Survived tropical cyclone DEBBIE yesterday (only got the tail end of it) but northern NZ is quite trashed. Bucketed down here at 46 South all day and most of the night. Skies cleared just an hour before dawn onset at 18:00UT, so had a quick peek at Lovejoy and Panstarrs… the two comets of the moment. Skies overcast by ~19:40 UT. My latest results are:-

    C/2017E4 LOVEJOY
    Apr 04.742 UT Mv 6.2 6 arcmin coma DC7 10x50B
    04.746 UT Mv 6.4 6.8 arcmin coma DC6 12cm f4 L at 50x (96 arcmin FOV)
    06.750 UT Mv 6.4 9 arcmin coma DC6 10x50B
    AAVSO-VSP Comparisons (15df MLim 9 centred on Enif).

    C/2015ER61 PANSTARRS
    Apr 04.719 UT Mv 7.2 4.8 arcmin coma DC6 20 arcmin tail in PA 260 12cm f4 L at 50x
    04.723 UT Mv 7.0 5 arcmin coma DC6 10x50B
    06.736 UT Mv 6.2 8.2 arcmin coma DC6 12cm f4 L at 50x
    06.740 UT Mv 6.0 9 arcmin coma DC7 10x50B

    Chris Wyatt at NSW has posted his latest data for April 5th, this morning on the Yahoogroups/CometObs website. So… he HAS survived Cyclone Debbie!
    Keep up the great work with those AstroBlogs and “Heads-Up” articles on Sky and Telescope, Bob.
    They always make for a cracking good read. One day, I must stand on my head late at night, so that I can actually see the Equatorial Constellations , in exactly the same orientation as you!!

    For p1/Halley in the mid 1980s, I did exactly that for Steve Edberg…

    Regards from 46 South, NZ. Graham W. Wolf.

  3. Tom-Reiland

    It was finally clear enough at Moonset and just before twilight for me to observe Comets Lovejoy and PANSTARRS from my light polluted yard using my 5″ Refractor at 31X and 10 X 50 binoculars. Lovejoy is small and compact like a Planetary Nebula. I estimate it between 6.4 and 6.7 magnitude. PANSTARRS is larger with a diffuse coma and a gradually brighter nucleus. I put it at 6.3 magnitude. Estimates were difficult under the conditions in my neighborhood. I could not see the latter with my binocs because of the lights and twilight encroaching. Lovejoy appears almost starlike in my binocs. A dim satellite almost passed over Lovejoy while I was viewing it through my 5″ scope. These are comets #179 and 180 for me. Glad that I had a chance to see them before the Moon gets near to and just passed Full and twilights are earlier and longer.

      1. Tom-Reiland

        I just realized that I’ve viewed 6 comets in 50 nights this year: Encke, H-M-P, T-G-K, Johnson and these two. I often think about all of the observing and great comets I missed after grade school in the mid 1950s until I started again in 1972. Kohoutek was my first in1973 and West the first great comet that I saw. I missed Arend-Roland, Mrkos, Ikeya-Seki and Bennett just to name a few.

  4. Graham-Wolf

    Hi Bob

    Just before dawn onset today, had a quick last peek at Panstarrs and Encke.
    The former is definitely in post-outburst decline, but sporting a near 1/2 degree diffuse coma! The latter starting to fade off a little, after holding steady all week. Latest data:- both with 12cm f4 Meade GOTO at 50x (96 arcmin FOV), for your readers.

    C/2015ER61 PANSTARRS
    Apr 09.742 UT Mv 6.6 26.2 arcmin DC3
    Apr 09.746 UT Mv 10.0 3.5 arcmin coma DC5
    The Full Moon was below the local western horizon, for both observations, here at 46 South.
    Now… say “hello” to Lunar Pollution for the next fortnight!

    Congrats to Tom-Reiland on his latest comet achievements. Well done:- Tom!
    It was Comet Bennett in 1970 which inspired me to start up Sidewalk Astronomy here in NZ. I set up a small refactor in Dunedin’s famous “Octagon” and got hundreds of viewers over ~ 2 week period. I also saw Ikeya-Seki in 1965, but not sure about Mrkos in 1957. Was only born in the mid-50’s.
    P/1 Halley was my 120th comet. Alan Hale at Cloudcroft, NM, must be getting very close to 600… C’mon, Alan, just a few more!!!

    Regards from 46 South, NZ. Graham W. Wolf

  5. Graham-Wolf

    Hi again, Bob.

    Got up yesterday morning at ~ 4am NZST to see the Full Moon rather low in the West and about to set.
    As it touched the local horizon ~ 5:30am, I stayed up and got out my trusty 12cm f4 Meade GOTO scope, ready for action! Waited ’til just on 6am (18h UT) and had a go at both Panstarrs and Encke.
    Glad I did, as we now have three days of Cyclone Cook starting last night… bucketing down all day and through to end of week. Won’t even notice the lunar pollution lurking out there… haha!

    C/2015ER61 PANSTARRS
    Apr 10.750 UT Mv 6.4 11.6 arcmin coma DC4 50x (96 arcmin FOV)

    Apr 10.754 UT Mv 10.0 3.2 arcmin coma DC5 50x (96 arcmin coma)

    Regards and best wishes from 46 South, NZ. Graham W. Wolf

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