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IRON AND ICE
Is the 'Christmas Comet' cracking up?
by Staff Writers
Paris (AFP) Oct 22, 2013


Out-of-fuel European satellite to come crashing down
Paris (AFP) Oct 18, 2013 - A satellite monitoring Earth's gravity field since 2009 will run out of fuel "in the coming days" and eventually crash, with little risk to humans, the European Space Agency said Friday.

About 40 to 50 fragments with a combined mass of 250 kilogrammes (550 pounds) are projected to hit our planet within weeks of the GOCE satellite running out of fuel, according to spacecraft operations manager Christoph Steiger.

"We are very close to the end," he told AFP on Friday, when the pressure in the satellite's fuel tank dropped below 2.5 bar -- the minimum required for full operation.

Not yet known is when and where the fragments will impact -- over the ocean or on land.

The pressure in the orbiter's tank is expected to drop to zero no later than October 26 but the engine will likely stop working before then, said Steiger.

"Right now, it is not possible to predict where it will happen, it could be anywhere. The closer we get to the reentry point, the more precisely we will be able to say.

"Roughly one day before (impact), one can exclude certain regions of the Earth. A few hours before, we will be able to tell with... a few thousand kilometres of precision."

The Gravity Ocean Circulation Explorer (GOCE) was launched into orbit in March 2009 at an altitude of just 260 kilometres (160 miles).

It has stayed aloft thanks to its unusual aerodynamic shape and an ion propulsion system.

GOCE's stock of 41 kg of fuel stood at about 350 grammes on Friday. When it runs out completely, the satellite will start losing altitude, become unstable and eventually de-orbit.

Most of the 5.3-metre-long (17.2-foot) spacecraft will break up at an altitude of about 80 kilometres (50 miles), said Steiger.

But about a quarter of its mass will survive, hitting the surface in a trail of fragments over an area of a few hundred kilometres.

"The risk is very small, but it is not zero," said Fernand Alby, in charge of space debris and space surveillance at France's National Centre of Space Studies (CNES) in Toulouse.

"Per year, it is estimated that about 100 tonnes of manmade space debris is reentering (the atmosphere), out of which between 20 and 40 tonnes survive reentry... and impact somewhere on Earth," Steiger said.

In 50 years of spaceflight, there have been no casualties from manmade space debris reentering, he insisted.

"The risk of getting hit by such a reentering, manmade space debris is 65,000 times lower than getting hit by lighting. It is 1.5 million times lower than being killed in a home accident -- falling down the stairs or something like that."

The 350-million-euro ($465-million) mission has lasted twice as long as its initially scheduled 20 months.

Scientists say it has returned reams of data on Earth's gravity field and ocean circulation.

GOCE was designed and built before 2008, when international recommendations were adopted that a scientific satellite must be able to execute a controlled reentry, or burn up completely after its mission.

Steiger said a global space debris coordinating committee was monitoring the satellite to predict its point of reentry, "and we are passing on the information to national authorities".

An incoming comet that skygazers had hoped would provide one of the greatest celestial shows of the century, could be a fizzle.

So say astronomers tracking the eagerly-awaited Comet ISON as it races to a searing encounter with the Sun.

Formally known as C/2012 S1 (ISON), the comet was spotted by a pair of hard-working amateur Russian astronomers, Vitaly Nevski and Artyom Novichonok, on September 21, 2012.

It is called ISON because they used a telescope called the International Scientific Optical Network near Kislovodsk, in the northern Caucasus.

After the discovery was validated by the International Astronomical Union (IAU), interest in the enigmatic wanderer became huge.

Calculations showed that after looping around the Sun, the comet would become a blaze of glory towards the end of the year -- a timing that gave it the tabloid title of "Christmas Comet" or even "Comet of the Century."

But fears are multiplying that the great show will be cancelled.

Light signatures from ISON, which has just streaked past Mars, indicate the comet is about to break up, says Ignacio Ferrin, an astrophysicist at the University of Antioquia in Medellin, Colombia.

"This disintegration will take place before it reaches perihelion," Ferrin told AFP in an email. Perihelion is an orbit's closest point to the Sun, which ISON is supposed to reach on November 28.

"There are also predictions for disintegration at perihelion. But based on the evidence, the comet will not get there," said Ferrin.

He explained that comets typically brighten as they get closer to the Sun, crossing a temperature threshold that causes their icy surfaces to evaporate, depositing water vapour, other gases and dust in their wake.

But, said Ferrin, the light curve from ISON slowed down and then remained practically constant, with no sign of greater brightness, as it raced forward.

This is a signature that matches four previous comets that have broken up catastrophically, he said.

"Comets in general appear to be quite fragile, and are observed to fragment or split," said Duncan Steel, a visiting astronomer at Armagh Observatory in Northern Ireland.

"It has always been a good bet that ISON would do this, and there is now evidence that this may be now occurring."

Cursed to wander Solar System

Comets are believed to be huge clusters of primeval dust and frozen ices, including water and organic molecules that, say some, delivered the building blocks of life to the infant Earth.

Doomed to orbit the Sun in periods that can range from years to many millennia, comets undergo thermal stress as they near the star.

Veterans that make short-period flybys, such as Halley's Comet, appear to have a crust of silicates and "tarry" carbon molecules to insulate them from the heat.

But rare visitors such as ISON have no such protection, said Steel. Internal gases start to expand in the heat, stressing the crumbly "dirty snowball" structure.

Comets can also be torn apart by gravitational forces if they cross the path of a planet.

This famously happened with Comet Shoemaker-Levy 9, whose fate was dictated by Jupiter. Fragments of the comet smashed into the jovian giant in 1994.

Even if the gloomy predictions are wrong, ISON still has to survive the climax of its ordeal by fire.

A "Sun-grazing" comet, at perihelion, it will be less than 1.2 million kilometres (730,000 miles) from the surface of the star -- just three times the distance between the Earth and Moon -- and subjected to temperatures of 2,800 degrees Celsius (5,000 degrees Fahrenheit).

According to preliminary estimates by the Lowell Observatory and Southwest Research Institute in Arizona, ISON has a good chance of surviving the solar furnace and gravitational rip at perihelion.

Comets smaller than 200 metres (650 feet) across almost always are destroyed when passing at such a distance. ISON appears be between 1,000 and 4,000 metres (1,000 and 4,000 yards) across.

What, though, will be left of ISON after it has kissed the Sun?

Will enough remain for it to be a real comet? Or will it be just a sad, shrivelled lump?

"We have absolutely no idea," said Patrick Rocher, of the Institute of Celestial Mechanics at the Paris Observatory.

.


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IRON AND ICE
Comet ISON Appears Intact
Baltimore MD (SPX) Oct 18, 2013
A new image of the sunward plunging Comet ISON suggests that the comet is intact despite some predictions that the fragile icy nucleus might disintegrate as the Sun warms it. The comet will pass closest to the Sun on November 28. In this NASA Hubble Space Telescope image taken on October 9, the comet's solid nucleus is unresolved because it is so small. If the nucleus broke apart then Hubb ... read more


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