NASA, ESA, J.-Y. Li (Planetary Science Institute), and the Hubble Comet ISON Imaging Science Team
This NASA Hubble Space Telescope image of Comet C/2012 S1 (ISON) was photographed on April 10, when the comet was slightly closer than Jupiter's orbit at a distance of 386 million miles from the sun (394 million miles from Earth).
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 small but incredibly bright comet heading toward the sun could do more than dazzle Earth’s skies when it arrives later this year. Scientists say Comet ISON, already shedding dust at the prodigious rate of about 112,000 pounds per minute, could spark an unusual meteor shower.
Computer simulations predicting the location and movement of the comet’s dust trail show Earth will be passing through the fine-grained stream around Jan. 12, 2014.
Some of the particles, which are smaller in diameter than a red blood cell, should be pushed back by the pressure of sunlight, allowing them to be captured by Earth’s gravity when the planet plows through the largely invisible stream.
“As the comet passes Earth’s orbit going into the sun, you’ll have particles trailing behind it. But since it’s passing so close to the sun, you’re also going to have particles pushed away by the pressure of the sunlight. That means we’ll have particles coming outward and also falling inward. We don’t often deal with particles that come both directions,” said Bill Cooke, lead scientist at NASA's Meteoroid Environment Office at the Marshall Space Flight Center in Huntsville, Ala.
The particles are so small that even though they will hit the atmosphere at about 125,000 mph, instead of burning up, triggering so-called “shooting stars,” they will be stopped entirely, predicts astronomer Paul Wiegert, with the University of Western Ontario in Canada.
The only visible and detectable sign of the comet dust might be a proliferation of bright blue clouds at the edge of space. Scientists suspect these so-called noctilucent, or “night-shining” clouds are be seeded by dust in the upper atmosphere.
Eventually, the trapped comet dust will make its way -- silently and invisibly -- to the planet’s surface.
Comet ISON, which was discovered in September 2012 by amateur astronomers in Russia, is believed to be making its first swing into the inner solar system, so unlike repeat fliers, it hasn’t laid down a rich dust trail from previous orbits for Earth to fly through.
ISON is an acronym for the telescope the astronomers were using, the International Scientific Optical Network.
If the comet survives -- and that’s a big if -- the comet will about 700,000 miles above the surface of the sun when it makes its closest approach on Nov. 28. The closest it will come to Earth will be about 40 million miles on Dec. 26.
A comet in the 1970s passed 10 times farther away from the sun than ISON's orbit and partially disintegrated, noted Cooke.
“ISON may very well not survive. I guess we won’t know for sure until we look for it to come out from behind the sun,” Cooke told Discovery News.
Currently the comet is about 280 million miles away from Earth and approaching the outer part of the asteroid belt.