Comet ISON's last flare-up before breaking up in 2013, as seen by the NASA/ESA Solar and Heliospheric Observatory (SOHO).
ESA/NASA/SOHO/GSFC
7 Intimate Close Encounters with Comets: Photos
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On Aug. 6, 2014, the European Space Agency's Rosetta spacecraft completed its decade-long journey to reach Comet 67P/Churyumov-Gerasimenko, becoming the first spacecraft to ever orbit a comet. The mission will reach its epic climax when it releases a small robotic lander, called Philae, onto the cometary surface in November. The lander will drill into the surface while Rosetta tags along with the comet's orbit as Churyumov-Gerasimenko makes close approach of the sun. Although Rosetta is unprecedented in that no other mission has achieved orbital insertion around a comet, it's certainly not the first robotic probe to make an intimate cometary encounter. So here's a rundown of 7 encounter of 6 comets by 5 spacecraft since the first close encounter with Halley's Comet in 1986.
NASA/ESA
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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.
ESA
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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.
NASA
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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 was granted a mission extension (dubbed New Exploration of Tempel 1 -- NExT). In 2011 it rendezvoused with its second 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.
NASA
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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.
NASA
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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 and 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.
NASA
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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 provided scientists with an unprecedented opportunity to study the same comet after six years of orbiting the sun. Preliminary findings suggested 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 appeared to match the size and shape predicted after the 2005 impact. However, the crater appeared smoother than expected, so work is ongoing to analyze the 72 photographs taken by the flyby to understand the processes shaping the comet's nucleus.
NASA
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At 5:29 a.m. EDT (9:29 a.m. GMT) on Aug. 6, 2014, the European Rosetta spacecraft completed a 6.5 minute-long engine burn to insert itself into orbit around Comet 67P/Churyumov-Gerasimenko. Once under the influence of the comet's weak gravity, the spacecraft began to carry out a series of triangular loops, taking several days to complete. The long-duration mission is the first of its kind, where the spacecraft will study the comet from orbit, watching for surface changes as it approaches the sun, making perihelion (the point of closest solar approach). In November, a small lander called Philae will touch down on the surface to drill into the comet's material, revealing its small-scale composition. This photograph from Rosetta was captured on Aug. 3 when the probe was fast approaching the comet at a distance of less than 300 kilometers. READ MORE: Rosetta Probe Makes Historic Comet Rendezvous
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SOHO wasn't meant to search for comets. That's clear even in its name, which stands for the Solar and Heliospheric Observatory; it was designed to study the heliosphere , or the sun's extended atmosphere.
On Earth, the sun usually looks like an orange blob or a yellow blob, depending on the time of day. Fascinating, sure, but - a blob. If you want to take the boring out of the sun, you need NASA images. Amy explains how the agency gets its cool shots.
But in nearly 20 years of gazing at the sun, the joint NASA and European Space Agency mission has served as an accidental spy for comets on a death plunge -- Comet Lovejoy (in 2011), Comet ISON (in 2013) and nearly 3,000 others.
The unofficial count as of Monday morning (Aug. 17) is in the low 2,990s, Karl Battams told Discovery News. Battams is the sungrazer project co-ordinator at the Naval Research Laboratory and works on many missions, including SOHO. He estimates that the 3,000th comet will be found in the next couple of weeks.
"The overwhelming majority (of comets) have been found by amateur astronomers in their spare time; it is a volunteering effort on their part," Battams said, adding that five or six people have together discovered roughly one-third of SOHO's comets. These volunteers scour SOHO's site for new pictures. The rules are that all comets are named after SOHO, but Battams and others acknowledge the volunteer efforts in publications.
What makes SOHO unique among sun-gazing spacecraft is its coronograph (an instrument including an occulter that blocks out the sun's glare) is directly in the line between the Earth and the sun, allowing for a perfect view of sungrazing comets.
Most of them hail from a single group known as Kreutz comets, which are likely fragments of a huge comet that broke up in 1106. Kreutz comets have an extremely short perihelion (distance to the sun); due to their tiny size and closeness to the sun, only one comet has survived a scorching close-pass in SOHO's observations -- and that 2011 comet, Lovejoy, broke up within two weeks.
SOHO remains in excellent operational shape and has enough fuel on board to last for decades, Battams added. The 2016 NASA budget request asked for $2.2 million in funding annually for SOHO through 2020, out of a total budget request of $18.6 billion in 2016.
Partner European Space Agency is prepared to stay on board for at least a few more years, Battams said. But a successor comet-hunting mission isn't on the books, at least yet.
"It is unique and extremely valuable," Battams said. "It’s operating now at really low cost; they’ve managed to scrape by not on pennies, but figuratively speaking they have done it. My own speculation is they will continue funding it for as long as its operational, or until it gets replaced by something else."