Κυριακή, 24 Νοεμβρίου 2013

What Should You Know About Comet ISON? - Photos, Video

type="html">The icy wanderer has traveled trillions of miles over a million years to reach the interior of our solar system. Comet ISON might become the brightest comet of the century as it draws closer to the Sun. Heat from the sun will vaporize the ice and frozen gases that comprise the body of the comet, hopefully meaning that it will be visible with the naked eye throughout December and January. If it stays intact it will slingshot around the sun and never be seen by humans from earth again. 




NASA Releases Comet ISON Images from STEREO; A Comet Race With Encke and ISON


Comet ISON entered the field of view of the HI-1 camera on NASA's Solar Terrestrial Relations Observatory, or STEREO, on Nov. 21, 2013, and the comet shows up clearly.

NASA spotted Comet ISON coming on strong on Thursday. An awesome set of images shows Comet Encke, its tail wriggling, along with Mercury and Earth. Then ISON enters the field. It looks like a comet race. It's evidence, the space agency says, that ISON is still intact and hasn't splintered as it approaches the sun. A camera on NASA's Solar Terrestrial Relations Observatory captured the action Thursday. Watch the tails of the comets above. Solar wind, the particles that stream outward from the sun at 1 million miles per hour, make the comets' tails move.

Newcomer to the Inner Solar System


Scientists think ISON is making its first trip to the inner solar system from the huge comet repository known as the Oort Cloud, which lies about 1 light-year from the sun.

This explains some of the uncertainty in the forecasts of ISON's performance. It's difficult to predict how any comet will behave during a close solar passage, experts say, and especially tough to do so for "dynamically new" comets like ISON.



Once ISON Is Gone, It's Gone

Some comets are on long, elliptical orbits dropping them in to the inner solar system before sailing them back out to the depths of space. There, they slow, stop, then fall once again back into the warmth and light. Comet Halley, for example, is on a 75-year orbit that takes it out past Neptune.

But some are more extreme. If they get an extra kick on their way in — perhaps from a collision, or a boost by a planet’s gravity — their elliptical orbit gets turned into an open-ended hyperbola: they have more than enough energy to leave the solar system forever.
 Once they’re gone, they’re gone.

ISON is a hyperbolic comet, which means this is it: These next few weeks are our only chance to see it. After it swings back out, it ain’t coming back. This is likely its first tour of the inner solar system as well, which is why scientists are so excited about it; we’re seeing a pristine comet, billions of years old, a relic of the ancient solar system. It’s a time capsule, letting us study what conditions were like when the Sun and planets were young.



Observers Perplexed by Comet ISON's Erratic Behavior



The highly regarded comet observer John Bortle is just as perplexed by the comet's recent appearance, commenting that the recent images along with his own visual impression, is "downright weird." He adds that, "There is a bright, miniature, long-tailed comet situated within a much larger, but very much fainter and diffuse halo of a coma."


"Those visual people using larger telescope also often remark about the odd way the comet looks, while those using relatively small scopes and big binoculars report seeing a larger, more-or-less faint but uniform cometary mass," he added. "This comet is currently at a distance from the sun where it should no longer exhibit such a dichotomy of appearance.”




ISON Is A Sun-Diver


The orbit of ISON takes it very, very close to the Sun’s surface. Next week, on Nov. 28, it will skim a mere 1.1 million kilometers (about 700,000 miles) above the Sun’s surface. Given that the Sun is 1.4 million km across, that’s a mighty close shave! The heat it feels will be intense, and it may not survive the encounter



The Total Mass of the Comet is about 2 - 3 Billion Tons.


Ice isn’t terribly dense; it floats on water! If ISON is a typical mix of ice and rock, it has a density of about 600 kg per cubic meters. Assuming it’s a sphere two km across, that gives it a mass of roughly 2 - 3 billion tons. That may sounds like a lot, but remember, ice is far less dense than rock. A small rocky mountain would be far more massive.



The Solid Part of ISON is Only About Two Kilometers Across.



Comets are actually lumps of rock, gravel, and ice mixed together. This solid part of the comet is called the nucleus, and some are huge; Hale-Bopp had a nucleus about 30 km (20 miles) across.

ISON, though, is tiny, only about 2 km (1.2 miles) across. Heck, plop it down in the middle of the Rocky Mountains and you’d hardly notice it! The size has been estimated using images taken from the Hubble Space Telescope, which in reality only give us an upper limit. It might even be smaller.

Still, that’s enough to make the comet visible to the naked eye even from a distance of a hundred million kilometers! How can that be? 




The Coma is Well Over 100,000 km in Size. 


When you look at a picture of ISON (or any comet), you’re not seeing the nucleus. You’re seeing the gas surrounding it that was once frozen beneath the surface. When the comet gets near the Sun this ice warms and turns directly into a gas. It escapes the weak gravity of the nucleus, forming the fuzzy coma around it.

Since the coma isn’t solid, it doesn't have a sharp edge. But on Nov. 15, the coma for ISON was estimated to appear about 3 arcminutes across (that’s a size on the sky; the Moon is 30 arcminutes across for comparison). Since ISON was about 140 million km (90 million miles) from Earth at the time, that would put the coma at a size of about 120,000 km (80,000 miles). That’s ten times the diameter of Earth!




ISON Will Not Hit Planet Earth



Earthlings have nothing to fear from Comet ISON, no matter how it behaves as it approaches the sun.

ISON will miss Earth by 40 million miles (64 million km) during its swing through the inner solar system. And so will its bits and pieces, if the sun's gravity happens to tear the comet apart along the way.

"During a breakup, comet fragments don't fly off in different directions like shards in a cinematic explosion," explains a new Comet ISON video released by the operators of the Hubble Space Telescope. "They break off but continue to travel along the path of their parent body. So any pieces would remain far from us, millions of kilometers away."

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