I'm going to talk about comments today
specifically a comment that is on its
way into the inner solar system and may
well treat us to quite a spectacular
sight in a few weeks time
now we have no idea what's going to
happen to this comment but we know a few
things about it that indicate that it
might well put on quite a show
it's called comet Ison and it's from a
long way away it is a kind of comet that
his never ever been in our solar system
before and it never ever will be again
it's a one time visitor and not only
that but it's going to pass very very
close to the Sun it's called a
sungrazing comet and those two facts
together make it quite interesting
comets can orbit the Sun just like the
planets do but they tend to orbit it in
a different way
so I can I can make a little model in
the solar system here got got our Sun
this is absolutely not to scale if most
of the planets orbit in a plane and they
tend to have orbits that are technically
ellipses
those orbits are governed by Kepler's
laws of orbital dynamics which are very
well known have been known for hundreds
of years but comets tend to do something
very different so instead of a slightly
squashed circle they can have orbits
that are very very elliptical sort of
the shape of this whole table and they
don't have to be located in the plane of
the ecliptic the same plane that all of
those planets do they can be tilted up
at quite high angles they can be
oriented in all sorts of different
directions they can have orbits that
take only a few years or a few tens of
years to go around the Sun they kind of
orbits that take tens of thousands of
years and they get flung way out into
the outer solar system and beyond and
then back in again this special kind of
comet is visiting us from the Oort cloud
the Oort cloud is probably the source of
a lot of these very distant orbiting
comments and it's probably we think a
remnant of a leftover bit of when the
solar system formed from a big disk of
debris and material that got puffed up
and sent out got sent out so far that is
actually we think probably billions of
tiny little icy objects in a cloud about
a Lightyear in radius that's a quarter
of the distance away from the next
nearest star that's
way way beyond where the actual planets
orbit those objects probably stay out
there millions and millions of years but
they're very vulnerable to being
perturbed and bumped around a bit so a
passing star or the gravitational field
of the Milky Way might have nudged one
just enough that it it left the Oort
cloud and it is plunged all the way into
the inner solar system but it's not on
an elliptical orbit it's not this close
squashed circle it's on a hyperbolic
orbit it's following a hyperbola that
will take it in towards the Sun whip it
round the Sun in a sort of slingshot
manner this is exactly the kind of orbit
that we send spacecrafts on round
Jupiter to slingshot them out into the
outer solar system and then it's going
to fly back off away we're never going
to see it again well what happened to it
huh well it'll it'll it'll experience
some very exciting and fun things around
the Sun it may or may not make it round
the Sun as we can talk about in a minute
and then it'll just it'll just head back
out become cooler become if it if it
survives it'll it'll just wander off
back into the outer solar system so a
generic name for a comet is known as a
dirty snowball which kind of reflects
its composition it's rock it's ice it's
some organic compounds and some gas all
mixed up together when it's out in the
cold reaches of the outer solar system
it's just a lump it's not varying very
much but as it plunges in towards the
Sun it gets gradually heated and then
interesting things start to happen
material will vaporize off what you
start off as a very solid ish lumpy
nucleus that might be meters or
kilometers across it will gain an
atmosphere called the coma and that will
be puffy and gaseous and and big and
then what we think of with the comment
where they get their name is from the
tails that form and you can actually get
two kinds of tails so if this is the
coma
this is the nucleus plus the outer
atmosphere of our comet this comet is
coming into the Sun and the material is
now being heated and it's starting to
stray
you mount behind it and it's forming
this beautiful tail that is probably
largely made up of dust and that's what
we tend to think of when we think about
beautiful comments streaking across the
sky and in fact the origin of the word
comet refers to something with hair so
these are hairy objects if you like and
this comet this tail this is an
important point it's not it's not
following the direction of motion so
it's not streaming out behind the comet
in the sense that my hair might be
blowing behind me if I'm running it's
it's it can actually follow kind of a
curved path as it gets left behind in
this orbit so you can have kind of a
curvy tail that's not the only kind of
tail you have the second kind of tail is
called an ion tail and this is the
interaction of particles that are being
pushed off the the comet's nucleus and
they're interacting with charged
particles from the solar wind and so if
this is the Sun here the solar wind is
pushing these this is this ion tail out
in a straight line behind the comet and
this is formed from interactions between
material coming off the comet gas and
the solar wind the stream of charged
particles that is going out in all
directions from the Sun and so this
solar wind is going to push this ion
tail back and it's going to be in a
straight line the ion tail is always
going to be pointing away from the Sun
because of the solar wind the dust tail
is can be slightly curved and is going
to streak out behind the comet in in its
orbit and an interesting thing about the
dust tail is that that material can stay
there in orbit around the Sun for a long
time and our orbit on earth can
intersect that remnant of dust and when
that happens that's when we see these
periodic meteor showers every year so
the Perseids in August is a famous
meteor shower that happens every year
and that's because every year at that
time the Earth's orbit sweeps through
this leftover material from a comet and
those tiny little particles burn up in
the atmosphere and give us a great
meteor show comet Ison was discovered in
September 2012 and as its orbit was
this is it was way out by the orbit of
Jupiter's it was before it had any sort
of spectacular tail or anything like
that and as observations were taken and
its orbit was plotted we started to
realize that it was going to be one of
these sungrazing comets and in fact it
was going to come within about 1.8
million kilometers of the Sun at its
perihelion its distance of closest
approach that's three times the solar
radius that's really really close at the
time of filming I think nine different
spacecraft have actually observed and
will continue to observe comet Ison as
it approaches the inner solar system so
we're early November now 2013 we're a
few weeks away from perihelion the big
question is what's going to happen to
Ison it may well be that it
disintegrates before it even gets close
to the Sun that's a possibility
once it gets round the Sun it has to
contend with temperatures of 5000
degrees it has to contend with the tidal
gravitational field of the Sun which
might may well tear it apart it's going
to be traveling at 400 kilometres per
second at that time to put that in
perspective if we were to travel from
the University of Nottingham campus here
and not am to the University of
Nottingham vez in Ningbo China if we did
it that fast 400 kilometres per second
it would only take us 23 seconds so it's
going to whip around the Sun and if it
makes it out the other side that's when
we may well be in for a treat because
then it will feel the full force of the
solar wind of its orbital journey and
that's when it may well brighten very
very bright may be visible to the naked
eye may if we're lucky have a beautiful
long tail and we will be able to observe
it very well
it's not the most romantic of names
Iceland stands for the international
scientific optical network which was the
coordinated survey of people using
ground-based telescopes to look for
comments