Page 100 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

COMETS
TOP 5
COMETS

1 MCNAUGHT
Closest approach COMETS ARE THE LEAST PREDICTABLE CELESTIAL
to the Sun: WANDERERS, BUT THEY’RE A HIGHLIGHT
25.6 million km
Orbital period: WHENEVER THEY ENTER THE SOLAR SYSTEM
Millions of years
First spotted: Aug 2006
Best visibility: Jan 2007
We live in an age of high-tech telescopes and grace our skies since Ikeya-Seki in 1965. But until
computer modelling, so there’s something curiously McNaught actually whizzed by our planet, we
reassuring about the way comets continue to do knew nothing for sure. It’s diffi cult to predict
2 IKEYA-SEKI their own thing, regardless of what any experts say much about a particular comet because we’re
Closest approach that they should be doing. It’s this unpredictability often missing key information about it. How big
to the Sun: that makes any cometary apparition a unique event. is it? How old is it? Has it visited the Sun lots of
1.2 million km Will the comet be bright? How long will its tail times before? What is it made of?
Orbital period:
1,056 and 877 be? The only thing you need to do to fi nd out the
years (due to break up) answer is to take a look into the night sky, so long FLYING HIGH
First spotted: Sep 1965 as you know when to do it. Think of comets as big, dark, dirty snowballs.
Best visibility: Oct 1965 Thankfully, with the help of the internet, you can These icy balls get warmed by the Sun if they fl y
keep right up to date with the latest discoveries, into the inner Solar System. Its heat melts and
ESO; ROGER LYNDS/NAOAO/AURA/NSF; NASA KENNEDY SPACE CENTER (NASA-KSC); PETER STÄTTMAYER & ESO; M NEWBERRY & J MCGAHA;
as well as predicted viewing opportunities and evaporates their ice, releasing gas and dust. This
magnitudes. This is exactly what many of us did surrounds the nucleus of the snowball, making
3 HALE-BOPP
Closest approach during December 2006 and January 2007 with the it invisible, so we cannot actually measure the
to the Sun: approach of Comet McNaught. All eyes were ready comet’s true size. We’re not even totally sure where
136.5 million km for what looked like a good naked-eye apparition. comets come from, although there is one leading
Orbital period: When it arrived it was much more, becoming a theory about their origins being in the Oort Cloud
2,537 years real evening showpiece and the brightest comet to and Kuiper Belt. It can take millions of years for
First spotted: Jul 1995
Best visibility: Apr 1996

ROBERT MCNAUGHT/SCIENCE PHOTO LIBRARY; GIOTTO (ESA) HMC MPAE; PAUL WOOTTON
WHERE DO THEY COME FROM?
4 WEST
Closest approach Comets are believed to be
to the Sun: the leftover material from a Sun
30 million km nebula that collapsed to form
Orbital period: the Sun and the Solar System. Planetary
560,000 years Forming a great halo around region
First spotted: Aug 1975 the Sun known as the Oort
Best visibility: Mar 1976 Cloud, at a distance of
around one lightyear, these Kuiper
frozen bits are thought to Belt
generally stay out of the 1.5 lightyears
AU
way in ‘hibernation’.
5 HYAKUTAKE
Closest approach However, passing stars 0 1 10 10 2 10 3 10 4 10 5
to the Sun: can nudge one, two or maybe
34.5 million km even thousands of comets our Oort
Orbital period: way. It is possible that some Cloud
72,000 years mass extinctions on Earth were
First spotted: Jan 1996 caused by such events. Short-
Best visibility: Mar 1996 period comets, on the other Long-period
hand are thought to originate comet
in the Kuiper Belt.
100 skyatnightmagazine.com 2012