Australian Sky & Telescope - May 2018

50 AUSTRALIAN SKY & TELESCOPE May | June 2018

COMETS by David Seargent

Comet spotting in Sagittarius

A good comet for the winter months, and another one on the way.

T

he best prospect for amateurs

with small telescopes during

May and June will be C/2016 M1

(PANSTARRS). Discovered on June 22,

2016, with the Pan-STARRS 1 telescope

at Haleakala in Hawaii, this object was

estimated at magnitude 19.7 at the time,

but subsequent observations revealed

that it was still well in advance of

perihelion, which would not occur until

August 10, 2018 at a distance of 2.21

astronomical units (a.u.) from the Sun.

C/2016 M1 is relatively bright

intrinsically and is also dynamically

evolved, implying that its relative

brightness indicates a fairly large and/

or active nucleus rather than a surface

accumulation of very volatile substances

likely to boil away as it nears perihelion.

Fortunately for southern observers,

C/2016 M1 is also travelling southward

through the skies. Progressing from

Aquila into Sagittarius during the

opening week of May, the comet

will traverse the latter constellation

throughout the rest of May and the first

half of June, before crossing Corona

AustralisandenteringArabytheendof

that month. On June 24, it will make its

closestapproachtoEarthatadistance

of 1.29 a.u.

Visual magnitude estimates by

northernobserversearlyintheyear

indicate C/2016 M1 to be brightening

steadily(evenifnotespeciallyrapidly)

and, if this early trend persists, it

should increase in lustre from about

magnitude 10.5 to near 9 through the

MayandJuneperiod.Itislikelytostay

around this maximum magnitude for an

extendedperiod,allthewhileremaining

wellplacedforsouthernobservers.On

presentindications,itislikelytostillbe

around 10th magnitude at the end of

Augustandbrighterthan11thuntilthe

closeofOctober.

Keen-eyed observers with a clear

easternhorizonmayalsoberewarded

with a glimpse of another dynamically

evolved, long-period comet during the

closing days of June. Designated C/2017

T3 (ATLAS), this object was discovered

by the Asteroid Terrestrial-impact Last

Alert System (ATLAS; see fallingstar.

com) of the University of Hawaii

on October 14, 2017. At the time of

discovery, the comet was estimated at

magnitude 18.3. Perihelion will occur

on July 19 this year at a distance of 0.82

a.u. from the Sun.

The orbit of this comet passes quite

close to Earth. Unfortunately, the comet

itself does not! Indeed, C/2017 T3 will

not be at all well placed for observing,

but it may be possible to find it low in

the dawn during the last few days of

June, when it will be located in Orion

and probably no brighter than about

magnitude 10. Prospects do, however,

look a little better following perihelion,

but I’ll have more to say about that in

our next issue.

■ David Seargent is a long-time comet

observer and author of several books on

the subject, including the recent Weird

Comets and Asteroids and Visually

Observing Comets.

S C/2016 M1 (PANSTARRS) showed as a condensed fuzzball in July,

2017, when Michael Jager took this image. It’s expected to reach about

magnitude 9 to 10.5 during May as it moves through Sagittarius.

S Comet C/2017 T3 (ATLAS) appeared as a tiny dot when this image,

also by Michael Jager, was taken on November 14, 2017.