This illustration of a bright comet over Mars was created by artist Kim Poor.
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.
Two comets will buzz Mars over the course of the next year, prompting excitement as well as some concern that cometary particles could hit the spacecraft orbiting the Red Planet and exploring its surface.
Three operational spacecraft currently circle Mars: NASA's Odyssey and Mars Reconnaissance Orbiter (MRO), as well as Europe's Mars Express. NASA also has two functioning rovers, Curiosity and Opportunity, on the ground on Mars.
All of these spacecraft will have ringside seats as Comet ISON cruises by Mars this year, followed by Comet 2013 A1 (Siding Spring) in 2014. (Photos of ISON, a Potentially Great Comet)
Crossing the sublime line
The MRO spacecraft has been on the lookout for Comet ISON, said Richard Zurek, MRO project scientist and chief scientist in the Mars Program Office at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif.
On Aug. 20, MRO looked for Comet ISON, which experts say could put on a dazzling sky show here on Earth shortly after the icy wanderer zips a scant 724,000 miles (1.16 million kilometers) above the surface of the sun on Nov. 28.
During last month's observation by MRO, ISON was 1 astronomical unit (AU) from Mars and 2.5 AU from the sun. (One AU is the distance from Earth to the sun — about 93 million miles, or 1.5 million km.)
Given ISON's distance from the sun, the comet should have crossed the solar system's "snow line" by that time, Zurek told SPACE.com. At the snow line, many comets brighten as ice more rapidly sublimes into gas due to increasing solar radiation.
"The MRO instruments did not see anything," Zurek said, and evidence suggests the instruments "were pointed accurately. Thus, the current conclusion is that the comet had not brightened quite enough to be seen at that range with the MRO instruments."
Comet ISON's current luminosity is a topic of much discussion among astronomers and skywatchers alike. The icy wanderer was branded a "comet of the century" candidate almost immediately after its discovery in September 2012, but recent observations suggest that it's not brightening as much as expected or hoped on its trek toward the sun.
More observations ahead
MRO will look at ISON again, Zurek said, with observations scheduled for Sept. 29, Oct. 1 and Oct. 2 (when the comet will be closest to Mars). At those times, ISON will be roughly 14 times closer and will likely be relatively easy to detect. (Comet of the Century? Sun-Grazing Comet ISON Explained (Infographic))
"At the closest passage distance, there is no concern that cometary particles from ISON will affect the orbiters or Mars," he said.
NASA's 1-ton Curiosity rover and its smaller, older cousin, Opportunity, will also image ISON from the Martian surface later this month, Zurek said. However, those plans are still being formulated.
The spacecraft in orbit around Mars and on the planet will give scientists a better chance of investigating Comet ISON, though that is not their primary function, said Michael Meyer, lead scientist for NASA's Mars Exploration Program at the agency's headquarters in Washington, D.C.
"Mars has a better view than Earth does right now," Meyer said. However, it is "challenging for orbital and landed assets as they are not really designed to do this sort of thing. They are supposed to be looking at Mars."
Meyer spoke via Skype Aug. 25 during a New Media Practitioners Professional Development Workshop on the upcoming launch of NASA's Mars Atmosphere and Volatile Evolution orbiter (or Maven for short). The workshop took place at the University of Colorado Boulder's Laboratory for Atmospheric and Space Physics (LASP).
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).NASA, ESA, J.-Y. Li (Planetary Science Institute), and the Hubble Comet ISON Imaging Science Team
Another comet coming
After ISON, scientists will look forward to Comet Siding Spring, Meyer said. That comet will make a very close approach to Mars in October 2014, skirting just 76,428 miles (123,000 km) from the planet, according to the current best estimates.
"That promises to be pretty exciting," Meyer said. "Right now, in all honesty, what we know about it and what sort of calculations can be done ... the error bars are extremely large."
The comet poses risks to orbiters circling Mars, Meyer said, a prospect that may lead to re-orienting and maneuvering of the craft to protect them from comet particle strikes. But whether it's a 10 percent, 1 percent or 0.1 percent risk remains unknown at the moment, he said.
"You can't get too worked up about it until you get some measurements as the comet gets closer," Meyer said. "It promises to be quite a show — if we're able to look at it." (Photos: Amazing Comets of 2013)
In early August, JPL issued a request for proposals to help characterize the cometary environment of Comet Siding Spring, with proposals due on Sept. 11.
"The intent is to provide data products useful for risk assessment and mitigation-strategy development for the Mars orbiter missions, due to possible impacts from dust and ion tail particles as this comet encounters Mars," the JPL request stated.
Model simulations are needed to characterize the evolving dust and ion particle distributions around Comet Siding Spring, as well as their motions with respect to Mars, as this comet approaches the Red Planet.
Because Comet Siding Spring will come so close to Mars, it's likely that the planet, along with its associated spacecraft, will pass through the coma of the comet, Zurek said. But NASA's rovers will probably be relatively well protected, he said.
"As thin as the Mars atmosphere is, it should still shield the rovers from infalling particles," Zurek said, "so the risks to be assessed are to the orbiters."
Zurek told SPACE.com that scientists won't have an idea of how big a risk the comet environment will pose until they make more observations of the comet's variability as it nears the sun.
"We have put out a call for modeling of the cometary environment," Zurek said. Such modeling, he said, is dependent on the developing comet activity.
Passage through the comet's coma could result in a wide range of effects: anything from a modest enhancement to the background meteoritic flux experienced by the spacecraft, which is deemed most likely, to something more substantial, Zurek said.
"So stay tuned!" he said.
Siding Spring's Mars visitation will also overlap with that of another Mars newcomer, NASA's Maven spacecraft. To be launched this November, it will arrive at the Red Planet in late September 2014, with Comet Siding Spring set to make its closest approach to Mars on Oct. 19.
However, that will likely be too soon for the Maven orbiter to analyze the comet.
"Maven will still be in the middle of its commissioning phase at that time and will not be ready to take regular measurements," said Bruce Jakosky, principal investigator for Maven at LASP.
"Although we'd like to be able to observe the comet as it passes by and how it affects the upper atmosphere, our first priority will be spacecraft and instrument health and safety," Jakosky told SPACE.com.
The Maven team is working with the Mars Program Office (MPO) to predict the likely dust environment as the comet passes by, and how impacts from the dust might affect the spacecraft, Jakosky said. The MPO is coordinating the activities, he said, as that group is concerned about Mars Odyssey and MRO, in addition to Maven.
Minimizing the risk
"After Maven's launch, we'll be looking in detail at what mitigations we can take to minimize any risk," Jakosky said. "We'll look at things such as turning the least-vulnerable face into the flow of the dust, putting the solar panels edge-on to the flow and so on."
Jakosky said that, at this point, the best analysis indicates a minimal risk to Maven.
The number of dust impacts expected, and the effect they'll have on the spacecraft should be within the range of what Maven researchers anticipated for a normal mission run, Jakosky said. That is, the dust impacts should not exceed what researchers had already planned to absorb just from interplanetary dust over the lifetime of the mission, he said.
"I do expect that telescopic observations of the comet in the spring and more-detailed modeling of the dust environment in the coma and tail will help us to refine our analysis," Jakosky said.
Leonard David has been reporting on the space industry for more than five decades. He is former director of research for the National Commission on Space and is co-author of Buzz Aldrin's new book "Mission to Mars – My Vision for Space Exploration," published by National Geographic.
This article originally appeared on Space.com. More from Space.com:
Best Close Encounters of the Comet Kind
Comet ISON's Mars 'Buzz' - Orbiter and Rovers on Lookout | Video
Comet ISON And Mars Pair Up In The September Night Sky | Video
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