Comets – summary of appearance, history and composition
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Comets – summary of appearance, history and composition

October 16, 2019

The appearance of a comet in the sky is one
of the most spectacular and beautiful natural events that can be witnessed.
A Great Comet will feature a large bright head and a long tail streaming across the
sky. It can be seen each night for weeks on end,
showing changes in brightness and the length of its tail.
Often the tail will split into two or more sections. Observing a bright comet is a rare
and unforgettable experience. Comets are thought to come from a large cloud
of debris — which was left over after the formation of the Solar System — and which
is known as the Oort cloud. The cloud lies well beyond the furthest planet,
about eight thousand billion kilometres from the Sun.
The Oort cloud is made up of billions of chunks of rock and ice up to several kilometres across.
It is thought that the gravitational effect of an occasional passing star will cause a
large number of these icy chunks to move in towards the Sun and become comets.
These will all have orbital periods of thousands, or even millions of years.
However, sometimes one of these comets will pass by a large planet — such as Jupiter
— whose gravitational pull will slow down the comet
— and hence change its orbit. Such a comet will remain within the Solar System and have
an orbital period of less than a hundred years. It will then have become a short period comet.
During most of a comet’s orbit when it is a long way from the Sun, it will not have
a head or tail but will consist solely of a small chunk of rock and ice
— about 5km across, which is known as the nucleus of the comet.
The surface is basically ice with a few rocks that have become prominent due to the surrounding
ice having evaporated. As the nucleus approaches the Sun, solar heating
will cause the frozen gasses and ice to melt and then evaporate into space
— forming a large diffuse cloud, known as the coma or head of the comet which will also
contain as small amount of dust from the nucleus. The coma is enormous when compared with the
nucleus — being about 100 thousand kilometres across.
However, the coma is in turn small compared with the immense tail that streams out from
the head, which is typically tens of millions of kilometres in length.
The tail is produced when matter in the coma is blown away into space.
It was once thought that the tail of a comet trailed behind the head as the comet sped
through space. Now it is known that the tail always points
away from the Sun — so that on the comet’s outward journey, the tail will actually precede
the head. There are in fact two types of cometary tails,
and each is due to a different solar effect. One type of tail — here seen veering to
the right — is due to the pressure of sunlight itself, which slowly pushes dust particles
from the comet’s head away into space. The resultant tail is diffuse and curved and
is known as a dust tail. The other type of tail is due to a much more
powerful solar effect; a stream of highly energetic particles
— from the Sun’s outer atmosphere speeding out into space at 500km a second called “the
solar wind”. The solar wind energizes the gas particles
in the head of the comet, causing them to fly off in a direction exactly opposite to
the Sun. The long straight streamer that results is
known as a gas tail. Observing a bright comet is a rare and unforgettable
experience. Since leaving Earth in 2004, the spacecraft
has travelled three billion kilometres — a third of its journey towards Comet 67P/Churyumov-Gerasimenko.
Circling the Sun, it has so far swung by Earth twice and Mars once, each time accelerating
to reach its target. These planetary flybys are exciting events
mobilising everyone’s attention, however they are only steps towards the final goal
— and everyone imagines the historic moment in January 2014 when Rosetta reaches its destination.
“For the first time we will fly, not just fly by a comet for a few hours but flying
in the vicinity of a comet for more than 1½ year.
“So this will allow the scientists to collect data over an entire cycle of a lifetime of
a comet from very far distance to the Sun down to its closest distance.”
The mission scientists will of course be the most thrilled at the prospect of studying
at close quarters — an object which probably has not changed
since the formation of the Solar System 4½ billion years ago.
Rosetta will investigate how the comet losses water and dust as it approaches the Sun.
But the mission’s greatest challenge will be to deploy a small lander called ‘Philae’
— to analyse the comet’s surface composition and do drill into the icy nucleus to collect
and analyse samples. Rosetta truly will be making the most of this
ten-year historic odyssey. [All quotes] The Deep Impact mission was a
mission to Comet Tempel 1 to deliver an impactor in 2005.
The instruments on the Deep Impact spacecraft were designed to be diagnostic in a flyby
of a comet. We got some fascinating results from Comet
Tempel 1… [Music] But once we got past Tempel 1, we had plenty
of fuel left, the spacecraft was healthy, then immediately everybody set to work on
figuring out what new bodies we could get to.
That’s what led to the proposal to go to Comet Hartley 2.
We were able to retarget the spacecraft using a few flybys of Earth, take advantage of the
gravity assist from Earth to retarget ourselves — change our trajectory just enough so that
now we’re able to get to Comet Hartley 2 in November.
We flew by it at a speed of about 27,000 miles per hour, and the spacecraft was slightly
below the Comet, in the Sun plane. Who would have thought that we’d actually
get to see a comet close up like we just did? And when we first saw this, our mouths just
dropped… the whole team, just dropped… Because you can begin to see, if you look
very close to the nucleus, you can see things that are slowly moving…
But then as you go farther away, they are really migrating. To me this whole thing looks
like a snow globe that you’ve just simply shaken, and watching it fly.
When we saw the images come down, even in real time, in the raw data, and realized we
had a cloud of snow around the nucleus, we were astounded.
Those are not stars! Those are all chunks of ice. We think the biggest ones are at least
the size of a golf ball, and possibly up to the size of a basketball.
They’re akin more to maybe a dandelion, so what that means is that the snowballs are
not what we might have thought to begin with — we’re not seeing softballs or even ice
cubes… What we’re seeing are fluffy aggregates
of very small pieces of ice. While it’s true that water is where the
ice is, in fact it’s everywhere, particularly on the sunward side of the image…
To our great surprise, there’s a tremendous enhancement of water vapour coming out of
the “waist” of this body. We wouldn’t expect this at all!
And so what we are seeing is an indication that here the ice is still on the inside,
it’s being heated up by the Sun, and that drives the water off.

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