An artist's concept depicts Comet Siding Spring (2013 A1) hurtling towards Mars. Closest approach to Mars comes on October 19, 2014.
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.
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.
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.
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'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.
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.
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.
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
A comet will give Mars a historically close shave next weekend, and NASA aims to be ready for the dramatic cosmic event.
The space agency has already trained a number of its science assets on Comet Siding Spring, which will zoom within 87,000 miles (139,500 kilometers) of Mars on Oct. 19 — about one-third the distance between Earth and the moon. And NASA's fleet of Red Planet orbiters and rovers will be watching on the big day, studying the comet and its influence on Mars' atmosphere.
"On October 19, we're going to observe an event that happens maybe once every million years," Jim Green, director of NASA's planetary science division, said during a news conference today (Oct. 9). "We're getting ready for a spectacular set of observations." [See photos of Comet Siding Spring]
Comet Siding Spring, also known as C/2013 A1, was discovered in 2013 by astronomer Rob McNaught using Australia's Siding Spring Observatory. The comet is making its first trip through the inner solar system from the frigid, faraway Oort Cloud, which lies about 50,000 astronomical units from the sun. (One astronomical unit, or AU, is the average distance between Earth and the sun — about 93 million miles, or 150 million km).
Because Siding Spring has never been "heat-treated" before, the incoming comet likely remains largely unchanged since its formation 4.6 billion years ago, researchers said. So studying its composition and behavior should provide clues about the conditions that existed at the birth of the solar system.
"That's one of the reasons we study comets — they're the remnants of our solar system's formation," said ?Padma Yanamandra-Fisher, a senior research scientist at the Space Science Institute's Rancho Cucamonga branch in California.
Observations by a number of missions, including NASA's Hubble, Swift, Spitzer and NEOWISE spacecraft, have already returned some data on Siding Spring. For example, researchers think the comet's core is between 0.5 miles and 5 miles (0.8 to 8 km) in diameter. Further, the fuzzy cloud (or coma) surrounding Siding Spring's nucleus is about 100,000 miles (160,000 km) wide at this point, and its tail stretches for about 300,000 miles (480,000 km), scientists said.
But the real show will begin Oct. 19. NASA's three Mars orbiters — Mars Odyssey, the Mars Reconnaissance Orbiter (MRO) and the newly arrived MAVEN spacecraft — will observe Siding Spring's flyby from space, while the agency's Opportunity and Curiosity rovers will watch from the Red Planet's surface.
Artist's impression of a cometary nucleus.NASA
The goal is to learn more about the comet's size, rotation speed, activity and composition, researchers said. The interactions between comet particles and Mars' atmosphere could also help scientists better understand the Red Planet's air. MAVEN is particularly well suited to perform this latter task, since the mission was designed to study Mars' upper atmosphere (MAVEN is short for Mars Atmosphere and Volatile Evolution).
If all goes according to plan, MRO will take the first-ever good pictures of an Oort Cloud comet's nucleus. And Opportunity and Curiosity could make some history as well, if Martian dust storms don't cloud up the atmosphere too much.
"We certainly have fingers crossed for the first images of a comet from the surface of another world," said Kelly Fast, program scientist at NASA's planetary science division. "That would be really exciting."
Various instruments and assets will continue to watch Siding Spring after the flyby, following the comet as it recedes into space.
Protecting the Spacecraft
Siding Spring will barrel past Mars at about 126,000 mph (203,000 km/h) on Oct. 19 — so fast that even tiny particles shed by the comet could do some serious damage to an orbiting spacecraft. So NASA has taken pains to maneuver its Red Planet orbiters out of harm's way.
"When Mars gets very close to the dust tail, which is about 100 minutes after closest approach, all our spacecraft will be on the opposite side of the planet," Green said. "So the planet will provide the additional protection we believe we need to be able to make these observations safely."
Curiosity and Opportunity aren't in any danger, researchers said; the Mars atmosphere, while just 1 percent as dense as that of Earth, is still substantial enough to protect the rovers from incoming comet material.
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