Australian Sky & Telescope — January 01, 2018

50 AUSTRALIAN SKY & TELESCOPE January 2018

COMETS by David Seargent

Close encounter with the Sun

Will comet C/2017 T1 (Heinze) survive its perihelion passage?

O

n October 2 last year, Aren

Heinze of the University of

Hawaii Asteroid Terrestrial-

impact Last Alert System (ATLAS),

discovered an 18.2-magnitude comet

sporting a coma approximately 7

arseconds in diameter and a tail about

10 arcseconds long. The comet was

found in images secured with the 0.5-m

f/2 Schmidt telescope at Mauna Loa.

Orbital computations revealed the

new object to be moving in an almost

parabolic orbit toward perihelion on

February 21, 2018 at a rather small

0.58 astronomical units (a.u.) from the

Sun. On its way to perihelion, C/2017

T1 (Heinze) will pass just over 0.2 a.u.

from Earth on January 4.

Although the comet favours

northern latitudes at the time of closest

approach, it should be visible during

late December and during the first few

days of January as it moves northward

from Cancer into Lynx across the New

Year period. Observers in the north of

Australia will be favoured, although

thosefurthersouthwhoareblessed

withaclearnorthernhorizonshould

be able to locate the comet at low

elevation. On present indications, it will

probably be about magnitude 9.5–10 at

the time.

Following perihelion, C/2017 T1

willtheoreticallyemergeintothe

southern morning sky in March.

Normally,aperiheliondistanceof

around0.5a.u.shouldholdpromiseof

afairlyimpressiveobject,eventhough

this comet will then be on the farthest

sideoftheSolarSystem.Nevertheless,

present indications

suggestthatthisis

an intrinsically faint

object that may not

survive its encounter

with the Sun.

Survival instincts

Back in the 1980s,

veteran comet observer

John Bortle found

that comets that are

either dynamically

new (as this one

appears to be) or of

very long period, fail

to survive perihelion

if their absolute

magnitude—thatisto

say, the hypothetical

brightness that they

would have if located

at1a.u.frombothEarthandSun—

isfainterthanavaluedefinedbythe

formula H 0 = 7 + 6q, where H 0 is the

absolute magnitude and q is the comet’s

perihelion distance in a.u.

This formula holds quite well for

very long-period comets, although it is

less well-defined for those of moderate

period and is not applicable at all for

short-period objects. Indeed, the few

instances where very long-period/

dynamically new objects have departed

from the formula’s predictions involved

comets brighter than the ‘survival limit’

disintegrating, rather than fainter ones

surviving! Maybe these comets were

experiencing prolonged outbursts that

made them seem brighter than their

‘true’ value?

Be that as it may, for C/2017 T1,

the ‘Bortle Limit’ (as it has come to be

known) is 10.5. However, according

to the estimates available thus far, the

comet’s absolute brightness seems to

be about two magnitudes fainter than

this! Admittedly, all available estimates

are derived from CCD observations

made for astrometric purposes and

are most probably rather conservative.

Yet, it is questionable whether they are

sufficiently pessimistic for the comet to

be rescued. Visual observations during

December and January, especially

binocular brightness estimates as the

comet passes close to Earth, will be

important in assessing the survival

prospects of this object.

As mentioned in our Nov/Dec issue,

C/2016 R2 (PANSTARRS) should also

be visible through moderate-sized

telescopes during January as it drifts

toward its May 9 perihelion at 2.6

a.u. from the Sun. Recent brightness

estimates suggest that the magnitude

predictions given in that issue may

have been a little optimistic, and that

the comet might be closer to 12 than

10 during January. However it should

be well-condensed in appearance and

rather well placed in Taurus. As January

opens, the comet will be found within

the Hyades open cluster, slowly trekking

northward as the month progresses.

■ DAVID SEARGENT is the discoverer of

comet 1978 XV. His book, Snowballs in the

Furnace, is available from Amazon.com.

S Comet C/2017 T1 should briefly be visible at the beginning of

January, and hopefully again in March. This image was taken by

Michael Jäger on October 15.