Thế giới của Sao: Comet

Hiển thị các bài đăng có nhãn Comet. Hiển thị tất cả bài đăng

Chủ Nhật, 5 tháng 5, 2013

Meteor shower from Halley's Comet peaks this weekend

An image of Halley's Comet taken in 1986.NASA
An Eta Aquarid meteor streaks over northern Georgia on April 29, 2012.NASA/MSFC/B. Cooke

Early Sunday, just before dawn, we'll have an opportunity to see some of the remnants of the most famous of comets briefly light up the early morning sky.

The famed Halley's Comet made its last pass through the inner solar system in 1986 and is not due back until the summer of 2061. But each time Halley sweeps around the sun, it leaves behind a dusty trail — call it "cosmic litter" — that is responsible for two meteor showers on Earth each year. The first of those "shooting stars" displays, the >Eta Aquarid meteor shower, will peak on Sunday.

The Eta Aquarid meteor shower occurs each year in early May because the orbit of Halley's Comet closely approaches the orbit of Earth in two places. The first is the May timeframe, which leads to the Eta Aquarids. The other point occurs in mid-October, producing the Orionid meteor shower. [Meteor Shower Quiz: Do You Wish on Shooting Stars?]

When and where to watch
The Eta Aquarid meteor shower is predicted to peak early on Sunday morning, but dark skies and clear weather are vital to observe them. Under ideal conditions (a dark, moonless sky) about 55 to 60 of these very swift meteors might be seen per hour.

The meteor shower appears at about one-quarter of its peak strength for several days before and after May 5. This is a very good year to look out for the Eta Aquarid meteors from Halley's Comet because the moon will be at a thin, waning crescent phase, and just 20-percent illuminated and providing little interference for viewing these swift streaks of light.

From places south of the equator, the Eta Aquarids typically put on very good show. Australian stargazers consider them to be the best meteor display of the year.

But for those watching from north of the equator, it's a much different story.

Where to look
The apparent origin point of a meteor shower in the night sky is known as its radiant. For the Eta Aquarids, the radiant is found within the "Water Jar" of the constellation Aquarius, which begins to rise above the eastern horizon around 3 a.m. your local time. Unfortunately, this constellation never really gets very high as seen from north temperate latitudes. And soon after 4 a.m. local time, the morning twilight will begin to brighten the sky.

So if you are hoping to see up to 60 meteors per hour, forget it. With the radiant so low above the horizon, the majority of those meteors will be streaking below the horizon and out of sight.

In fact, from North America, typical Aquarid rates are only 10 meteors per hour at 26 degrees north latitude (Miami, Fla., or Brownsville, Texas), 5 per hour at around 35 degrees latitude (Los Angeles, Calif., or Cape Hatteras, N.C.) and practically zero to the north of 40 degrees (New York, Chicago, and Philadelphia).

"So," you may be thinking, "What's the point of getting up before dawn to watch?"

The answer: Despite the long odds, you still just might see something spectacular.

Catch an Earthgrazer, or comet crumb
For most, perhaps the best hope is perhaps catching a glimpse of a meteor emerging from the radiant that will skim the atmosphere horizontally — much like a bug skimming the side window of an automobile.

Meteor watchers call such shooting stars "earthgrazers" and they are known for spectacularly long, colorful, long-lasting trails.

"These meteors are extremely long," Robert Lunsford of the International Meteor Organization explained. "They tend to hug the horizon rather than shooting overhead where most cameras are aimed."

"Earthgrazers are rarely numerous," NASA meteor expert Bill Cooke, a member of the Space Environments team at the Marshall Space Flight Center has said. "But even if you only see a few, you're likely to remember them."

If you do catch sight of one early these next few mornings, remember that you are likely seeing the incandescent streak produced by material which originated from the core of Halley's Comet.

So it is that the shooting stars that we have come to call the Eta Aquarids are really an encounter with the traces of a famous visitor from the depths of space and from the dawn of creation.

View the original article here

Thứ Hai, 1 tháng 4, 2013

NASA spacecraft snaps new photo of potential 'comet of the century'

The Ultraviolet/Optical Telescope aboard NASA's Swift imaged comet ISON (center) on Jan. 30, when it was located about 3.3 degrees from the bright star Castor in the constellation Gemini. At the time of this 5.5-minute optical exposure, ISON waNASA/Swift/D. Bodewits, UMCP
From now through October, comet ISON tracks through the constellations Gemini, Cancer and Leo as it falls toward the sun. Image released March 29, 2013.NASA's Goddard Space Flight Center/Axel Mellinger

Next Slide Previous Slide

A NASA spacecraft scanning for the most powerful explosions in the universe has captured a photo of Comet ISON, an icy wanderer that could potentially dazzle stargazers when it swings close to the sun later this year.

NASA's Swift satellite, which is typically used to track intense gamma-ray bursts from distant stars, photographed Comet ISON on Jan. 30, with the space agency unveiling the photo today (March 29). By tracking the comet over the last two months, Swift has allowed astronomers to learn new details about how large the comet is and how fast it is spewing out gas and dust.

"Comet ISON has the potential to be among the brightest comets of the last 50 years, which gives us a rare opportunity to observe its changes in great detail and over an extended period," said Lead Investigator Dennis Bodewits, an astronomer with University of Maryland at College Park (UMCP) who helped obtain the new image.

Some astronomers have predicted that ISON could be the "Comet of the Century" when it makes its closest approach to the sun in late November. But a recent analysis found that the comet is not brightening as expected, and may have a ways to go to meet such expectations.

Comet ISON was first discovered in September 2012 by Russian astronomers Vitali Nevski and Artyom Novichonok using the International Scientific Optical Network (ISON) located near Kislovodsk. The comet's official designation is Comet C/2012 S1 (ISON).

Swift's Comet ISON view
Bodewits and his university colleagues teamed up with the Lowell Observatory in Flagstaff, Ariz., to capture new views of Comet ISON using the Swift spacecraft. The satellite's Jan. 30 photo shows the comet as a bright, fuzzy white ball. At the time, Comet ISON was about 375 million miles from Earth and 460 million miles from the sun.

"Using images acquired over the last two months from Swift's Ultraviolet/Optical Telescope (UVOT), the team has made initial estimates of the comet's water and dust production and used them to infer the size of its icy nucleus," NASA officials wrote in a statement.

Swift's observations revealed that Comet ISON is currently shedding about 112,000 pounds of dust and about 130 pounds of water every minute, an odd mismatch for such an anticipated comet.

All comets are made of dust and frozen gases that mix together to form a sort of "dirty snowball" in space, NASA officials explained. Water ice in comets typically stays frozen until the comet approaches within three times the Earth's distance to the sun, at which time the water ice heats up and changes directly into gas (a process called sublimation), creating jets of material that can brighten the comet.

"The mismatch we detect between the amount of dust and water produced tells us that ISON's water sublimation is not yet powering its jets because the comet is still too far from the sun," Bodewits said. "Other more volatile materials, such as carbon dioxide or carbon monoxide ice, evaporate at greater distances and are now fueling ISON's activity."

Currently, Comet ISON is about 5,000 times fainter than the dimmest object the human eye can see.

NASA's Comet ISON campaign
NASA's Swift spacecraft is one of several satellites and probes to observe Comet ISON as it passes through the inner solar system. The space agency has formed the Comet ISON Observing Campaign (CIOC) to make the most of the comet's visit.

On Oct. 1, the comet will pass within 6.7 million miles of Mars, and may be spotted by orbiters around the Red Planet. [Comet ISON's Path Through Solar System (Video)]

"During this close encounter, Comet ISON may be observable to NASA and ESA spacecraft now working at Mars," said Michael Kelley, a UMCP astronomer and also a Swift and CIOC team member. "Personally, I'm hoping we'll see a dramatic postcard image taken by NASA's latest Mars explorer, the Curiosity rover."

On Nov. 28 — 58 days after swing close by Mars — Comet ISON will make its closest approach to the sun, flying within 730,000 miles of the star's surface during the encounter. Several sun-watching observatories will be tracking the comet at that time, and ISON may even become visible in the daytime sky to observers who block the sun's light with their hand, NASA officials said.

The comet will make its closest approach to Earth on Dec. 26, coming with 39.9 million miles of the planet as a late Christmas present.

But whether Comet ISON will live up to its celestial hype or fizzle out in a whimper still remains to be seen, astronomers warned. The comet must still survive the approach into the inner solar system, as well as its close encounter with the sun.

"It looks promising, but that's all we can say for sure now," Matthew Knight, an astronomer at Lowell Observatory said. "Past comets have failed to live up to expectations once they reached the inner solar system, and only observations over the next few months will improve our knowledge of how ISON will perform."

View the original article here

Thứ Ba, 19 tháng 3, 2013

Astronomers gearing up for possible 'comet of the century'

The comet that may put on a spectacular light show during a November date with the Sun, was observed by the Deep Impact mission. The spacecraft has also had close fly-bys of comet's Tempel 1 and Hartley 2 and scientific observations of Garradd.NASA/JPL-Caltech/UMD (Tony Farnham)
This is the orbital trajectory of comet C/2012 S1 (ISON). The comet is currently located just inside the orbit of Jupiter. In November 2013, ISON will pass less than 1.1 million miles from the sun's surface.NASA/JPL-Caltech

Next Slide Previous Slide

Astronomers are already getting set for the arrival of Comet ISON, which may become one of the brightest comets ever seen when it cruises through the inner solar system this fall.

NASA has brought together a small team of experts to organize an observing campaign for Comet ISON, which could potentially shine as brightly as the moon when it makes its closest pass by the sun in late November if the most optimistic scenarios play out.

Coordinating the efforts of observatories on the ground and in space should help wring as much quality science as possible out of the comet's solar flyby, Comet ISON Observing Campaign (CIOC) officials said.

'We have to go forward with optimism and prepare for something really special, something really spectacular.'

- Karl Battams, a scientist at the U.S. Naval Research Lab in Washington

"It's a rare opportunity that we've got such a long heads-up time, so we actually have time to organize a campaign like this," said Karl Battams, a scientist at the U.S. Naval Research Lab in Washington and a member of the eight-person CIOC Team. "There's a lot of new science that we could get from this." [Photos of Comet ISON in Night Sky]

Bringing everyone together
Russian amateur astronomers Vitali Nevski and Artyom Novichonok discovered Comet ISON in September 2012 after poring over photographs taken by a telescope run by the International Scientific Optical Network. Hence the comet's official name, which is C/2012 S1 (ISON).

ISON is a sungrazing comet, meaning its long, looping path through space takes it extremely close to our star. Indeed, ISON is forecast to zoom just 680,000 miles above the solar surface on its closest approach, which will take place on Nov. 28.

Comet ISON could put on a spectacular show around this time, experts say, potentially glowing so brightly that it's visible in the daytime sky. (The comet poses no impact threat to Earth.)

Skywatchers aren't the only people looking forward to the icy wanderer's solar encounter. Astronomers are excited as well, for ISON's flyby could give them a rare window into comet composition.

"Sungrazers experience the most intense thermal and gravitational stresses of any comet. There's a lot of sublimation of material that doesn't normally sublimate," Battams told SPACE.com. "Once [ISON] gets really close in to the sun, then we could start to see some of the composition that you wouldn't normally get."

So Battams and his colleagues have contacted major ground-based observatories, raising awareness of the comet's flyby and encouraging them to solicit ISON-observing proposals. The response has been warm, with telescope operators typically voicing enthusiasm about the campaign, Battams said.

The CIOC crew has also reached out to some spacecraft mission teams, asking them to consider taking a look at the comet at some point. And a number of them are already on board.

"Observing campaigns are planned by the SOHO, STEREO and SDO solar missions; by Spitzer, Chandra and Hubble space telescopes; and by the Deep Impact, JUNO, Mercury MESSENGER, Mars Odyssey and Mars Reconnaisance Orbiter missions," the CIOC website states. "Other missions at or on Mars are looking into observing ISON, as are a handful of other NASA Planetary missions. We welcome and encourage our international partners to contact us and join in the fun!"

In fact, Deep Impact has already begun its ISON work. The NASA probe snapped its first photos of the comet in January.

Sizzle or fizzle?
There's no guarantee that ISON will live up to the hype; it may fall apart before even making its closest solar approach. Comets are notoriously unpredictable and sometimes fizzle out despite great expectations, as Comet Kahoutek did in 1973.

The wild-card factor with ISON is especially high, Battams said, since this is apparently the comet's first trip through the inner solar system from the distant, icy Oort cloud. Astronomers and skywatchers probably won't get a real sense of how the comet is going to behave until early August, when water ice should start sublimating in earnest, he added.

But the CIOC Team is proceeding as if Comet ISON will put on a dazzling show, because that's the scientifically prudent thing to do.

"We have to go forward with optimism and prepare for something really special, something really spectacular, and hope that it happens," Battams said. "If it does, we're absolutely going to be in place to take an unprecedented and comprehensive set of observations of the comet."

View the original article here

Chủ Nhật, 10 tháng 3, 2013

Comet Pan-STARRS visible tonight with naked eye

The first of two bright comets predicted for 2013 will soon be appearing in the evening sky for observers in the Northern Hemisphere. The comet — while not as bright as first predicted — will still be more brilliant than any comet seen in recent years, so here are some tips to spot the celestial wanderer.

The Comet Pan-STARRS, known officially as C/2012 L4 (PANSTARRS) will be readily visible with the unaided eye, though binoculars will be a help for spotting it against the twilight sky. Weather permitting, the comet could be as bright as a first-magnitude star on the brightness scale of night sky objects (the lower the number, the brighter the object).

After months appearing in the Southern Hemisphere night sky, Comet Pan-STARRS will make its first appearance in the Northern Hemisphere sky on Thursday, March 7.

How to see Comet Pan-STARRS
To see the comet, look for it in the west, just to the left of the point on your horizon where the sun sets, shortly after sunset beginning in the next few days.

If this advice sounds a little vague, it is because comets are slightly unpredictable, in particular their brightness. Also, observing locations vary widely in their suitability to observe objects close to the sun.

Try to find a site with a very low western horizon to improve your chances of catching sight of the comet early. The farther south you live on Earth, the more likely you are to see it early. Any haze in the air will make it harder to see the comet.

Comet Pan-STARRS made its closest approach to Earth on Tuesday (March 5), when it passed by at a range of about 102 million miles. The only large objects closer to Earth on that date were the moon (at a range of 228,442 miles), the planet Mercury (58 million miles) and the sun (92 million miles).

Comet at its best next week
Comet Pan-STARRS will be at its brightest on Sunday (March 10) when it makes its closest approach to the sun. At that time the comet will be about 28 million miles from the sun — a bit closer to the star than Mercury, which will be about 37 million miles from the sun. [Comet Pan-STARRS in Night Sky Explained (Infographic)]

The comet will still be low in the western sky on Sunday, and may be lost in the sun's glare. It will fade in brightness over the next few days, but at the same time will be higher in the sky at sunset. SPACE.com will provide up-to-date information on sighting opportunities for the comet.

Two comet-watching dates to prepare for now will be next Tuesday and Wednesday (March 12 and 13). On those evenings, the thin crescent moon will be close to the comet in the sky.

Depending on the size and direction of the comet's tail, the moon may actually be silhouetted against the tail on March 13.

On April 3, Comet Pan-STARRS will pass within a few degrees of the Andromeda Galaxy, making for a great photo opportunity for stargazers with telescopes. The comet and the galaxy will both shine at about 5th-magnitude on that date.

Comet's name explained
We are used to comets being named for their discoverers, but in recent years there have been many comets discovered by teams rather than individuals. When first discovered, comets are assigned unique designations based on the year and month of their discovery.

For Comet Pan-STARRS, that designation is C/2012 L4 (PANSTARRS).

The year marker is easy to figure out, but the month is a bit complicated, because astronomers use a letter based on the half-month of the discovery. For example: the letter 'A' denotes the first half of January, 'B' is for the second half of January, 'C' for the first half of February, and so on.

There is also a name in parentheses indicating the discoverer. In this case, the comet was discovered by a team project named PANSTARRS, short for Panoramic Survey Telescope and Rapid Response System, which actively searches for asteroids and comets using a telescope in Hawaii.

The problem with this name is that the PANSTARRS project has so far discovered a number of comets, which makes this only one of quite a few "Comets PANSTARRS" in the sky.

Whatever we call it, this comet should be a pretty sight in the early evening in March and it is not the only comet to grace the night sky this year.

Another southern night sky comet, Comet Lemmon, has wowed some stargazers who have managed to observe it from the Southern Hemisphere and in November, the promising Comet ISON could put on a dazzling display when it passes closest to the sun.

View the original article here