Amateur astronomer Bruce Gary captured this view of the brightening Comet ISON on Nov. 14, 2013, from Hereford, Ariz.
Image: A series of photographs of comet Hartl
6 Intimate Comet Encounters
Feb. 14, 2011 will go down in history as the Valentine's Day when a comet was visited a second time. Comet Tempel 1 has now played host to two different NASA spacecraft; Deep Impact in 2005 and Stardust-NExT in 2011. This amazing scientific feat comes hot on the heels of another cometary encounter only a few months ago. The NASA mission called EPOXI flew past comet Hartley 2 on Nov. 4, 2010 coming within 700 kilometers (435 miles) of the icy body. Both Stardust-NExT and EPOXI (formerly known as Deep Impact) are recycled comet missions and both have seen Tempel 1 up-close. EPOXI and Stardust-NExT may be the first two missions to be recycled for two comet flybys, but they certainly are not the first mission to rendezvous with these mysterious "dirty snowballs." So far, with the help of our robotic space explorers, humanity has had a close-up look at six cometary nuclei in the aim of unraveling their secrets. Let's take a look at each encounter with imagery from other space probes.
Image: Giotto's view of Halley's nucleus (ESA
Unquestionably the most famous comet in history, Halley's Comet was a prime target for space agencies in 1986 during its 75- to 76-year orbit through the inner solar system. Comet science is still a developing field, but in 1986, very little was known about the composition of these interplanetary vagabonds. In October of that year, the 15-kilometer-long Halley's Comet was visited by the European Space Agency's Giotto mission. The half-ton probe came within 600 kilometers (373 miles) of the comet's nucleus, taking the first photographs of the outgassing vapor from discrete areas of the surface producing its tail and coma (the gas surrounding the nucleus). It was this mission that confirmed the "dirty snowball" theory of cometary composition: a mix of volatile ices and dust. However, Giotto was only able to get so close to the famous comet with the help of the "Halley Armada," a number of international spacecraft all tasked with observing this rare event. Giotto captured the closest imagery, but two Russia/France probes (Vega 1 and 2) and two Japanese craft (Suisei and Sakigake) observed from afar.
Image: Comet Borrelly just before Deep Space
At roughly half the size of Halley's comet, Comet Borrelly was found to have similar attributes to its famous cousin. The nucleus was also potato-shaped and blackened. Outgassing vapor was also observed coming from cracks in the nucleus crust where volatiles were exposed to sunlight, sublimating ices into space. NASA's Deep Space 1 probe flew past the comet with a close approach of 3,417 kilometers on Sept. 22, 2001.
Image: A Stardust image of Wild 2 during its
Comet Wild 2 -- pronounced "Vilt" after its Swiss discoverer Paul Wild who spotted it in 1978 -- underwent a dramatic alteration in 1974. It is calculated that due to a close pass of Jupiter in 1974, the 5 kilometer-wide comet now orbits the sun every 6 years as opposed to its leisurely 43 years before the gas giant bullied it. The orbital modification meant that Wild 2 was an ideal target for NASA's Stardust mission to lock onto. On Jan. 4, 2004, the Stardust probe gave chase, getting so close to the comet that it was able to collect particles from Wild 2's coma. This image was taken at a distance of less than 240 kilometers (149 miles). The Stardust sample return canister came back to Earth safely, landing in Utah on Jan. 15, 2006. The microscopic particles captured from the comet continue to provide a valuable insight into the organic compounds comets contain. Interestingly, the Stardust spacecraft has been granted a mission extension (dubbed New Exploration of Tempel 1 -- NExT). In 2011 it will rendezvous with comet Tempel 1 -- the scene of NASA's 2005 Deep Impact mission -- to analyze the crater that Deep Impact's impactor left behind on the cometary surface.
Image: The view from Deep Impact's impactor b
NASA's Deep Impact mission reached the eight-kilometer-wide (five-mile-wide) comet Tempel 1 in 2005. On July 4, the probe deliberately smashed its impactor into the comet's nucleus, producing a cloud of fine material. A crater -- 100 meters wide (328 feet) by 30 meters (98 feet) deep -- was left behind. A treasure trove of compounds were spotted by the Deep Impact spacecraft and the explosion could be observed from Earth. In 2011, the recycled Stardust-NExT mission visited comet Tempel 1 for the second time.
Image: A close-up of comet Hartley 2 (NASA)
The fifth space probe encounter with a comet happened on Nov. 4, 2010. NASA's recycled Deep Impact probe -- now the EPOXI mission -- visited comet Hartley 2, examining its strange-shaped nucleus. Described as a "peanut" or "chicken drumstick," this comet is an oddity. During its close approach of under 700 kilometers (435 miles), EPOXI photographed the comet's irregular topography: two rough lobes connected by a smooth center. Jets of gas could be seen being ejected from discrete locations. During the Hartley 2 flyby press conference at NASA's Jet Propulsion Laboratory (JPL), mission scientists expressed their surprise that these jets of vapor are being emitted from sun-facing
shaded regions on the comet surface. Needless to say, analysis of the Hartley 2 flyby data will keep scientists busy for some time to come. "This is an exploration moment," remarked Ed Weiler, NASA's Associate Administrator for the Science Mission Directorate, during the conference.
Image: Tempel 1 as seen by Stardust-NExT at c
Most recently, on Feb. 14, 2011, the veteran Stardust-NExT (New Exploration of Tempel) mission made history by visiting a comet for the second time. Comet Tempel 1 was first encountered by NASA's Deep Impact mission in 2005 after smashing the cometary nucleus with an impactor. This second encounter provides scientists with an unprecedented opportunity to study the same comet after six years of orbiting the sun. Preliminary findings suggest Tempel 1 has undergone some erosion during those six years in deep space. Also, the impact crater left behind by Deep Impact was imaged during the Stardust-NExT flyby and it appears to match the size and shape predicted after the 2005 impact. However, the crater appears to be smoother than expected, so further work will need to be done to analyze the 72 photographs taken by this most recent flyby to understand the processes shaping the comet's nucleus.
Get ready for a stellar show. The much-anticipated Comet ISON is now visible to the naked eye according to reports from many observers.
Comet ISON -- the potential "comet of the century" -- has suddenly brightened in an outburst of activity with just two weeks to go before it literally grazes the surface of the sun.
In recent months, Comet ISON has repeatedly befuddled forecasters trying to anticipate just how bright it will ultimately become. But earlier this week, the comet's brightening trend again seemed to sputtering and stalling, but more recent observations suggest a sudden and radical upsurge in brightness. [Photos of Comet ISON: A Potentially Great Comet]
Comet ISON Lightens Up, Literally
Comet ISON is now in full outburst mode, becoming many times brighter over just the past few days. Astronomers measure the brightness of objects in the night sky as magnitude, in which the brighter an object is, the lower its magnitude number. The human eye can perceive objects as faint as magnitude +6.5.
According to veteran comet observer, John Bortle, Comet ISON was shining only at magnitude +8.5 on Monday (Nov. 11) morning -- more than six times too dim to be visible to the unaided eye. But by Wednesday morning, the comet’s brightness had increased three-fold brightening to +7.3. [8 Essential Facts About Comet ISON]
If that was a surprise, an even bigger one was waiting for Bortle on Thursday morning (Nov. 14).
"Ready to go at 4:45 a.m. but I couldn't figure out what the funny-looking, blotted, star that came into view was," Bortle said. "(Was my) seeing that bad? But, no, the 'blotted star' was, in fact, at the comet's position! Getting to the point, the little but intensely condensed, globular cluster-looking comet was a whopping magnitude 5.4 -- two full magnitudes brighter than just 24 hours ago! This makes for a three magnitude total rise since my observation on Monday."
In just 72 hours, Comet ISON increased nearly 16 times in brightness.
Carl Hergenrother, acting co-coordinator of the comet section of the Association of Lunar and Planetary Observers, has confirmed Bortle's observations.
"ISON has dramatically brightened over the past few days," Hergenrother told SPACE.com via email. "The latest observations put the comet around magnitude 5.7 to 6.1 which is a 2+ magnitude increase from this weekend. My own observations from this morning in 10x50 and 30x125 binoculars show a nice 'lollipop' comet with a very condensed blue-green head and a long narrow tail. The tail was over 1 degree in length even in the 10x50s. The comet may continue to brighten as the outburst is still in its early stages."
Unmistakable Comet Outburst
Long Island amateur Dennis Wilde was also impressed by ISON’s appearance in the predawn sky Thursday morning.
"ISON, while not as large as the full moon, was an impressive sight in the eyepiece," Wilde said. "The coma was compact with a very bright apparent nucleus, very bright green in color. The tail was very thin and bright near the coma and widened slightly as it extended out to almost 3.5 degrees as seen in the 15". It wasn't huge or extraordinarily bright but it was a great view nonetheless. I viewed the coma at up to 490x and it was uniformly dense and bright. There was no indication of the start of any breakup. After finding it with the telescope it was quite easy to pick out the coma with the ."
This outburst is not completely unusual since ISON has demonstrated short "spurts" of brightening over the past few weeks, but they were quickly followed by abrupt slow-down in its brightening trend.
The Hubble Space Telescope imaged the incoming Comet ISON on Oct. 9 to find the nucleus is likely intact.NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
So will the current outburst persist until the comet arrives at the sun on Thanksgiving Day (Nov. 28)?
"Whether by chance we have caught the comet at the peak of the outburst is certainly debatable (to me rather improbable) and it may well still brighten further," Bortle said.
"The comet may continue to brighten as the outburst is still in its early stages," Hergenrother said. "Whether this outburst will be a short-lived event or the beginning of a more active phase is still to be seen."
This sudden upsurge in brightness is certainly very good for a comet that until now seemed to be running well behind in brightness predictions. It seems now that we can feel a little more optimistic about this enigmatic object putting on show for us later this month on into early December.
Comet ISON was first discovered by Russian amateur astronomers Vitali Nevski and Artyom Novichonok in September 2012. The comet is officially designated C/2012 S1 (ISON), with ISON standing for International Scientific Optical Network.
The comet is rapidly approaching its Nov. 28 perihelion and as a result it is becoming more and more difficult to observe low near the east-southeast horizon in the dawn sky. Still, observers with access to a clear horizon may be able to follow ISON for about another week.
Next Monday morning (Nov. 18), ISON will be passing close to the bright 1st magnitude star Spica in Virgo. Using the handle of the Big Dipper, sweep an arc to the brilliant orange star Arcturus. Then continue that arc on to Spica. Using binoculars, ISON should still be readily be visible as a fuzzy star with a short tail.
Will it still be visible to the unaided eye? Check it out for yourself!
More from SPACE.com:
How To See Comet ISON In November 2013 | Video
Comet Quiz: Test Your Cosmic Knowledge
Best Telescopes for Beginners
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