Astronomy Education Services/Gingin Observatory
Close-up of comet C/2011 L4 PANSTARRS as seen from Mount Dale, Western Australia.
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.
A comet sailing through the inner solar system make its closest approach to the sun and will be at its brightest at sunset tonight, but the glare of twilight may make it tricky to see, NASA says.
The Comet Pan-STARRS will be 28 million miles from the surface of the sun when it swings around the star today, and should be bright enough to see without the aid of telescopes or binoculars, weather permitting. But the comet is also appearing low on the western horizon at sunset so some planning is needed to spot the celestial wanderer with the naked eye tonight.
"Look too early and the sky will be too bright," said Rachel Stevenson, a NASA Postdoctoral Fellow at the agency's Jet Propulsion Laboratory in Pasadena, Calif. "Look too late, the comet will be too low and obstructed by the horizon. This comet has a relatively small window."
A good time to look is about 40 minutes after sunset. The comet may appear as a sort of exclamation point in the evening sky, with the point being the comet itself and its diffuse tail stretching nearly straight up from the horizon, JPL officials added. (How to see the comet)
Comet Pan-STARRS, officially known as comet C/2011 L4 Pan-STARRS, was discovered in June 2011 by astronomers using the Panoramic Survey Telescope and Rapid Response System (or PAN-STARRS), a telescope atop the Haleakala volcano in Hawaii. The comet takes more than 100 million years to orbit the sun and appears to come from the Oort cloud, a vast halo of comets and icy objects at the outer edge of the solar system.
While the comet is making its closest pass by the sun tonight, the best views of the object are still to come. NASA scientists said the comet's proximity to the sun may make it too difficult to spot tonight, but that will change over the next few days.
"As it continues its nightly trek across the sky, the comet may get lost in the sun's glare but should return and be visible to the naked eye by March 12," JPL officials explained. "As time marches on in the month of March, the comet will begin to fade away slowly, becoming difficult to view (even with binoculars or small telescopes) by month's end."
And there is another reason to look for comet Pan-STARRS later this week. On Tuesday (March 12), the moon rises into the cosmic display.
"The comet will be joined in the western sky after dark by the slender crescent moon on March 12, 13, and 14," the editors of StarDate Magazine, a stargazing publication of the McDonald Observatory at the University of Texas. "Good naked-eye views of the comet should continue for several nights, with the comet remaining visible through binoculars into April."
Comet Pan-STARRS is one of three comets capturing the attention of stargazers this year. The Comet Lemmon C/2012 F6 is currently visible to observers in the Southern Hemisphere and at times was in the night sky at the same time, allowing stargazers to capture rare photos of two comets in the sky together.
Meanwhile, the Comet ISON is making its way into the inner solar system and could put on a spectacular cosmic display later this year. Officially designated C/2012 S1 (ISON), Comet ISON was discovered in September 2012 by Russian amateur astronomers Vitali Nevski and Artyom Novichonok using a remotely operated International Scientific Optical Network (ISON) telescope.
Some astronomers have billed Comet ISON as a potential "comet of the century" since it could be brighter than the full moon in daylight when it makes its closest approach to the sun on Nov. 28. At that time, Comet ISON will be much closer to the sun than Comet Pan-STARRS is now. ISON will approach within 800,000 miles (1.2 million km) of the star, making it a true sungrazing comet.
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