This artist's impression depicts Comet Siding Spring narrowly miss Mars in 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
When Comet C/2013 A1 (Siding Spring) swung past the Red Planet in October 2014, it was an unprecedented opportunity for an armada of Mars robots to have a ringside seat of the interplanetary spectacle. But as dazzling as the flyby was, the real drama wasn’t seen by the cameras of Mars orbiters or rovers; it was detected by a magnetometer. And that magnetometer, located 100 miles above the Martian surface, detected chaos.
“Comet Siding Spring plunged the magnetic field around Mars into chaos,” said Jared Espley, of NASA’s Goddard Space Flight Center in Greenbelt, Md. and science team member of NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, in a NASA press release. “We think the encounter blew away part of Mars’ upper atmosphere, much like a strong solar storm would.”
Although Mars’ magnetic field is weak and patchy (unlike Earth’s strong, global magnetosphere), MAVEN’s sensitive magnetometer detected a huge upheaval in orbit as Siding Spring’s own magnetism rattled the planet’s magnetic field.
The comet’s nucleus may only be a third of a mile wide, but the atmosphere surrounding the nucleus (known as the coma) was as wide as 600,000 miles when it encountered Mars. (The coma is formed through solar heating — the ices contained within a comet’s nucleus sublimate into space, pumping the coma with gas.) Through interactions with the solar wind, comets also generate their own magnetic fields that loop their way through the coma. So when Siding Spring buzzed Mars, coming as close as 87,000 miles, the cometary magnetism punched Mars’ weak magnetic field, sending it into violent turmoil for several hours.
MAVEN scientists likened the effect as a magnetic curtain flapping in the wind.
“The main action took place during the comet’s closest approach,” said Espley, “but the planet’s magnetosphere began to feel some effects as soon as it entered the outer edge of the comet’s coma.”
These magnetic observations were very lucky as MAVEN had only just arrived in Mars orbit weeks before the close encounter. Although most of the spacecraft’s instruments were powered down at that time (to protect them from possible damage by comet dust), the magnetometer remained on for the duration, carrying out unique observations of two magnetic fields slamming into one another.
“With MAVEN, we’re trying to understand how the sun and solar wind interact with Mars,” added Bruce Jakosky, MAVEN’s principal investigator from the University of Colorado in Boulder. “By looking at how the magnetospheres of the comet and of Mars interact with each other, we’re getting a better understanding of the detailed processes that control each one.”
We now know that Mars’ atmosphere is slowly leaking into space as its magnetic field isn’t strong enough to completely protect it from solar wind erosion. And, from these MAVEN measurement, it looks like this magnetic “hit and run” by Siding Spring also ripped a chunk out of the Red Planet’s atmosphere, giving us a profound look at what happens when comets and planets (almost) collide.
The close encounter between comet Siding Spring and Mars flooded the planet with an invisible tide of charged particles from the comet’s coma. The dense inner coma reached the surface of the planet, or nearly so. The comet’s powerful magnetic field temporarily merged with, and overwhelmed, the planet’s weak field, as shown in this artist’s depiction. NASA/Goddard