by David Whitehouse
27 December 2012
'Brighter Than a Full Moon'
A comet discovered by two Russian astronomers will be visible from
Earth next year.
Get ready for a once-in-a lifetime light
At the moment it is a faint object, visible only in sophisticated
telescopes as a point of light moving slowly against the background
It doesn't seem much - a frozen chunk of
rock and ice - one of many moving in the depths of space. But this
one is being tracked with eager anticipation by astronomers from
around the world, and in a year everyone could know its name.
Comet ISON could draw millions out into the dark to witness what
could be the brightest comet seen in many generations - brighter
even than the full Moon.
It was found as a blur on an electronic image of the night sky taken
through a telescope at the Kislovodsk Observatory in Russia as part
of a project to survey the sky looking for comets and asteroids -
chunks of rock and ice that litter space.
Astronomers Vitali Nevski and
Artyom Novichonok were expecting to use the International
Scientific Optical Network's (ISON) 40cm telescope on the night
of 20 September but clouds halted their plans.
It was a frustrating night but about half an hour prior to the
beginning of morning twilight, they noticed the sky was clearing and
got the telescope and camera up and running to obtain some survey
images in the constellations of Gemini and Cancer.
When the images were obtained Nevski loaded them into a computer
program designed to detect asteroids and comets moving between
images. He noticed a rather bright object with unusually slow
movement, which he thought could only mean it was situated way
beyond the orbit of Jupiter.
But he couldn't tell if the object was a
comet, so Novichonok booked time on a larger telescope to take
another look. Less than a day later the new images revealed that
Nevski and Novichonok had discovered a comet, which was named
A database search
showed it has been seen in images taken by other telescopes earlier
that year and in late 2011. These observations allowed its orbit to
be calculated, and when astronomers did that they let out a
Comet ISON has taken millions of years to reach us travelling from
Oort cloud - a reservoir of trillions and trillions of
chunks of rock and ice, leftovers from the birth of the planets. It
reaches out more than a light-year - a quarter of the way to the
nearest star. In the Oort cloud the Sun is but a distant point of
light whose feeble gravity is just enough to hold onto the cloud.
Every once in a while a tiny tug of gravity, perhaps from a nearby
star or wandering object, disturbs the cloud sending some of its
comets out into interstellar space to be lost forever and a few are
Comet ISON is making its first, and perhaps only
visit to us. Its life has been cold, frozen hard and unchanging, but
it is moving closer to the Sun, and getting warmer.
ISON's surface is very dark - darker than asphalt - pockmarked and
dusty with ice beneath the surface. It's a small body, a few tens of
miles across, with a tiny pull of gravity. If you stood upon it you
could leap 20 miles into space taking over a week to come down
again, watching as the comet rotated beneath you.
You could walk to
the equator, kneel down and gather up handfuls of comet material to
make snowballs, throw them in a direction against the comet's spin
and watch them hang motionless in front of you.
But it will not
remain quiet on Comet ISON for the Sun's heat will bring it to life.
By the end of summer it will become visible in small telescopes and
binoculars. By October it will pass close to Mars and things will
begin to stir. The surface will shift as the ice responds to the
thermal shock, cracks will appear in the crust, tiny puffs of gas
will rise from it as it is warmed. The comet's tail is forming.
Slowly at first but with increasing vigor, as it passes the orbit of
Earth, the gas and dust geysers will gather force.
The space around
the comet becomes brilliant as the ice below the surface turns into
gas and erupts, reflecting the light of the Sun. Now ISON is
surrounded by a cloud of gas called the coma, hundreds of thousands
of miles from side to side. The comet's rotation curves these jets
into space as they trail into spirals behind it.
As they move out
the gas trails are stopped and blown backwards by the Solar Wind.
By late November it will be visible to the unaided eye just after
dark in the same direction as the setting Sun. Its tail could
stretch like a searchlight into the sky above the horizon. Then it
will swing rapidly around the Sun, passing within two million miles
of it, far closer than any planet ever does, to emerge visible in
the evening sky heading northward towards the pole star. It could be
an "unaided eye" object for months.
When it is close in its approach
to the Sun it could become intensely brilliant but at that stage it
would be difficult and dangerous to see without special
instrumentation as it would be only a degree from the sun.
Remarkably ISON might not be the only spectacular comet visible next
Another comet, called 2014 L4 (PanSTARRS), was discovered last
year and in March and April it could also be a magnificent object in
the evening sky. 2013 could be the year of the great comets.
As Comet ISON heads back to deep space in 2014 the sky above it
would begin to clear as the dust and gas geysers loose their energy.
Returning to the place where the Sun is a distant point of light,
Comet ISON may never return.
Its tail points outward now as the
solar wind is at its back, and it fades and the comet falls quiet
once more, this time forever.