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article imageNASA gears up for close encounter between Mars and Comet

By Karla Lant     Oct 10, 2014 in Science
A mountain-sized comet will hurtle past Mars next weekend, providing not only a historically close encounter, but also a dramatic opportunity. More than one dozen NASA assets will observe Comet Siding Spring from different angles and in varying lights.
A mountain-sized comet will hurtle past Mars next weekend, providing not only a historically close encounter, but also a dramatic opportunity. In a press conference Thursday, NASA illustrated that its team is going to be ready for the cosmic event. More than one dozen NASA assets, the two Mars rovers and three Mars orbiters among them, will observe Comet Siding Spring from different angles and in varying lights.
The comet will be at a distance of only 87,000 miles from Mars on Oct. 19. This may seem like a long way, but it is a “near miss” in space terms, only about one-third of the distance between Earth and the moon. According to NASA’s Jet Propulsion Laboratory (JPL), that's also approximately 10 times nearer than any identified comet has ever passed by Earth. Fortunately, NASA's existing fleet of Mars orbiters and rovers are already positioned perfectly to capture the best possible view of the event and gather data on its influence on the atmosphere of the Red Planet.
In NASA's October 9th news conference, Jim Green, director of NASA's planetary science division, stated: “On October 19, we're going to observe an event that happens maybe once every million years. We're getting ready for a spectacular set of observations.”
Comet Siding Spring
Also called C/2013 A1, astronomer Rob McNaught discovered the comet in 2013 at the Siding Spring Observatory in Australia. It is making its first journey to our solar system from the Oort Cloud, a very distant spherical cluster of frozen bodies of “leftovers” from the time our solar system was formed, situated around 50,000 astronomical units (AU) from our sun. Each astronomical unit is about 93 million miles, so the comet has been on a long trip; it was probably formed 4.6 billion years ago.
Oort cloud comets are special because they are like time capsules from our past; researchers say they contain primordial matter that has been frozen for billions of years. But as comets like Siding Spring move toward a star like our sun, the deep freeze wears off, and the time capsules release their ancient gaseous matter into space forever.
Comet Siding Spring has never been “heat-treated,” which means that it should be mostly the same as it was when it was formed so long ago. NASA scientists hope that as we observe how this time capsule behaves and study its composition as it passes by this month it will provide us with information about conditions that may have existed at the time our solar system began.
John Grunsfield, Associate Administrator for NASA's Science Mission Directorate, clarified this directive in a statement: “This particular comet has never before entered the inner solar system, so it will provide a fresh source of clues to our solar system's earliest days.”
Although this is its first visit, a number of NASA missions have provided data on Comet Siding Spring, including NASA's Hubble, NEOWISE, Spitzer, and Swift spacecraft. Based on existing data researchers believe that the diameter of the comet's core is between 0.5 miles and 5 miles, and that the coma surrounding the comet's nucleus is approximately 100,000 miles wide. Scientists also think that the comet's tail is about 300,000 miles long.
Just how big is that in context? The highest mountain on Earth is Mount Everest which is 5.4979 miles tall at its peak. This means that at its widest point the comet's diameter might be similar in size. Meanwhile, the tail of the comet stretches about the same distance as the space between the Earth and the sun.
Just the Right Spot
While we do have some data, comparatively speaking, we haven't seen anything yet. The real views will happen on and near Mars on October 19 at 11:27 a.m. PDT. NASA's vehicles are going to be in the right places to capture the event as fully as possible. NASA researchers say that their goals for the data these vehicles will gather is to illuminate more about the comet's activity, composition, rotation speed, and size.
With those goals in mind, NASA's three Mars orbiters will watch the comet's passage from space, and the two rovers will see the flyby from the surface of Mars. Each of the vehicles is especially suited to fulfill one of the agency's goals.
The Mars Reconnaissance Orbiter (MRO), for example, is slated to photograph the comet's nucleus. If it does so successfully it will produce the first good pictures of any Oort Cloud comet nucleus in existence.
The newer Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft was designed to study the upper atmosphere of Mars. Based on this specialized function, NASA scientists are hoping it will be able to help them learn more about the Red Planet's atmosphere as it gathers data about the way comet particles and the atmosphere interact.
Researchers are also hoping that the Curiosity and Opportunity rovers are able to capture the first images of a comet from the surface of another planet. This will obviously depend in large part on conditions in the atmosphere which is often clouded by dust storms.
Telescopes, both Earth-based and space-bound, will observe the event. The Hubble Space Telescope and the Mauna Kea, Hawaii Infrared Telescope Facility as well as NASA's astrophysics space observatories—Chandra, Kepler, Spitzer, and Swift—will all track the comet.
Other NASA assets will also be trained on Comet Siding Springs. NASA's Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) asteroid hunter, has been and will continue to image the comet within its normal protocol. Both of NASA's Heliophysics spacecraft (STEREO and SOHO) will also image the event.
Protecting NASA's Assets
Not only is Comet Siding Spring big—it is also moving incredibly fast. When it moves by Mars it will be going about 126,000 miles per hour. This is fast enough that even tiny particles of dust from the comet could cause major harm to spacecraft in the vicinity. This is a major part of the impetus behind the large-scale planning in which NASA is engaging.
The comet itself is narrower at the approaching end, and widens with the tail. Rich Zurek, chief scientist for the Mars Exploration Program at JPL, explains: “The hazard is not an impact of the comet nucleus itself, but the trail of debris coming from it. Using constraints provided by Earth-based observations, the modeling results indicate that the hazard is not as great as first anticipated. Mars will be right at the edge of the debris cloud, so it might encounter some of the particles—or it might not.”
The plan is to ensure that the spacecraft are protected by the planet by the time the tail gets close enough to be harmful; the rovers are protected by the atmosphere. Jim Green explains: “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. So the planet will provide the additional protection we believe we need to be able to make these observations safely.”
Although most of the data will take at least a day or two to reach scientists and probably eighteen months to be completely studied, NASA researchers hope to release the first images from this historic flyby on October 20. They will also be providing information before October 19 on how to follow the event using social media. Regardless, all experts acknowledge that comets are unpredictable, so even the experts don't know exactly what will happen during the Mars flyby.
But experts are optimistic and excited about this opportunity. “This is a cosmic science gift that could potentially keep on giving, and the agency's diverse science missions will be in full receive mode,” said John Grunsfeld.
“Normally, you would send spacecraft to a comet. In this case, the comet is coming to the spacecraft,” said Kelly Fast, a program scientist at NASA headquarters.
“You've got all these spacecraft that are designed to study Mars, but they are repurposing themselves in order to take advantage of this amazing opportunity to study the comet and study what happens when the comet interacts with Mars, when material is deposited in the atmosphere, interaction with the comet's gas coma, is there heating of the atmosphere, an expansion, are there meteors? It's a fantastic opportunity.”
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