One week from today, NASA’s Red Planet armada is primed and ready with front row seats for the once-in-a-lifetime opportunity to study an Oort cloud comet, as it makes an extremely close flyby of Mars on Sunday, Oct. 19. It misses the planet by a mere 87,000 miles (139,500 kilometers) on its first passage ever through the inner Solar System during its millions-year-long orbital trek from the distant fringes of our Sun’s realm.
As comet Siding Spring hurtles past Mars at about 35 miles (56 kilometers) per second, an international fleet of seven spacecraft, including five NASA orbiters and rovers and two additional orbiters from Europe and India, will gather unprecedented, up-close scientific observations of the comet and its huge coma and tail, before and after closest approach, and simultaneously will study its effect on the tenuous Martian atmosphere.
The Oort Cloud is a giant swarm of icy objects some 50,000 AU from the Sun and is believed to be material left over from the formation of the Solar System. No spacecraft has ever visited one before.
Since comet Siding Spring has never flown past the Sun before; the Mars encounter with a record-breaking seven spacecraft thus provides researchers with an unparalleled opportunity to learn vastly more about the nature and composition of its pristine materials, including water and carbon compounds, that existed during the formation of the Solar System 4.6 billion years ago and have not been blown off by heating effects from a prior passage.
“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 at NASA Headquarters, Washington, said during a media briefing on Oct. 9.
“This is where a comet coming from the farthest reaches of the sun’s gravity will come to the inner part of our solar system. This comet will fly right in front of the planet Mars. Mars will be blanketed in cometary material.
“We’re getting ready for a spectacular set of observations. The science observations have been planned well in advance,” Green explained.
The comet is about the size of a mountain. The nucleus is estimated to be between 0.5 mile and 5 miles in diameter.
It will sweep nearest to Mars at around 2:27 p.m. EDT (18:27 GMT) on Oct. 19 at a distance of only about 82,000 miles. That translates to barely one-third the distance from the Earth to the Moon.
The ultra close flyby of Comet Siding Spring (Comet C/2013 A1) will be about 16 times closer to Mars than the closest Earth approach by a comet ever recorded in human history.
By comparison, two and a half centuries ago, on July 1, 1770, the now-defunct comet Lexell sped by Earth at a distance of about 1.4 million miles (2.3 million km), or about six times farther away than the Moon.
Because the comet is moving at the ferocious speed of about 125,000 mph (56 km/sec), NASA has taken great care to protect its priceless orbiting armada from “Gravity” like destruction from the trail of hurtling space debris.
NASA says that when Siding Spring flies by Mars, “even the smallest particle — estimated to be about one-fiftieth of an inch (half a millimeter) across — could cause significant damage to a spacecraft.”
On top of that the comet is moving retrograde, or in the opposite direction with respect to Mars, therefore tremendously magnifying the impact velocity. It will approach from below Mars.
“So anything that comes off the comet that hits Mars or a spacecraft is going to pack a large amount of kinetic energy, a real wallop,” said Carey Lisse, senior astrophysicist, Johns Hopkins University Applied Physics Laboratory, Laurel, Md.
“That’s one of the things we’ve been really worried about.”
So even before the comet’s swingby, NASA repositioned its three orbiters currently operating at Mars via a “duck and cover” maneuver to minimize their risk of impact with high-velocity dust particles streaming from the comet and impinging on the spacecraft.
NASA’s trio of orbiters comprises the Mars Odyssey (MO) orbiter, Mars Reconnaissance Orbiter (MRO), and the recently arrived Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter.
The orbiters will all be traveling on the opposite side of Mars at closest approach, effectively using the planet as a shield to block the enormous cloud of dust particles emanating from the nucleus.
Together they will “gather information before, during and after the flyby about the size, rotation and activity of the comet’s nucleus, the variability and gas composition of the coma around the nucleus, and the size and distribution of dust particles in the comet’s tail,” according to a NASA statement.
“Observations of the Martian atmosphere are designed to check for possible meteor trails, changes in distribution of neutral and charged particles, and effects of the comet on air temperature and clouds.”
MRO will observe the comet with three instruments: the High Resolution Imaging Science Experiment (HiRISE) , the Context Imager (CTX), and the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM).
The high-resolution HiRISE camera will attempt to image the comet’s nucleus and study its coma, looking at the shape, rotation, brightness or darkness, and composition of the nucleus. CRISM will attempt to identify elements and minerals.
MO will study the coma and tail in infrared and visible images.
MAVEN is especially wellssuited for the comet observations. It was specifically designed to investigate Mars’ upper atmosphere and will study the interaction of the comet with the atmosphere and use its unique ultraviolet imaging spectrometer (IUVS) to map the comet’s composition. It will look for changes and any heating and expansion of the upper atmosphere.
The European Space Agency’s Mars Express orbiter and India’s recently arrived Mars Orbiter Mission (MOM) spacecraft will also conduct spectroscopic and imaging observations.
The period of greatest spacecraft risk starts about 90 minutes after closest approach and lasts about 20 minutes, when Mars will come closest to the center of the widening dust trail spewing from the comet’s nucleus.
The comets coma extends about 100,000 miles across. The tail extends the distance from the Earth to the Moon, roughly 300,000 miles.
Mars’ extremely-thin atmosphere is expected to provide plenty of protection for NASA’s sister act of surface rovers, Curiosity and Opportunity. Both rovers will look up and attempt to capture images of the comet crossing the Red Planet’s sky, as they have the Martian moons Phobos and Deimos. They will also monitor the atmosphere for potential meteors in the wake of the dust trail.
“Opportunity will be coming out of dawn and Curiosity will be going into dusk,” Green elaborated at the briefing.
“These will be the first images of a comet taken from the surface of another planet,” said Lisse.
“Normally, you would send spacecraft to a comet. In this case, the comet is coming to the spacecraft because we happen to have multiple missions at Mars,” said Kelly Fast, program scientist of NASA’s planetary science division. “So it’s a fantastic opportunity.
“The spacecraft are repurposing themselves to study the comet.”
“The comet is coming to us. And we can’t get to an Oort cloud comet with our current rocket technology. So it’s a free flyby and a fantastic event for us to study,” noted Lisse.
NASA has also enlisted an armada of Earth- and space-based telescopes to observe the comet in the months leading up to the flyby and as it is happening.
So we’ll see if there is any change to the comet and Mars due to the close approach.
Observers on Earth may glimpse the comet flyby from the southern hemisphere with binoculars and telescopes.
Comet C/2013 A1 Siding Spring was discovered by Robert H. McNaught on Jan. 3, 2013, using the .05-meter (20-inch) Uppsala Schmidt Telescope, at Siding Spring Observatory in New South Wales, Australia. It was 7.2 AU from the Sun and located in the constellation Lepus.
McNaught has discovered numerous comets and asteroids, including Comet Mcnaught, or C/2006 P1, which was widely visible to the naked eye in the Southern Hemisphere during 2007 and was the brightest in four decades.
Meanwhile, ESA’s history-making Rosetta orbiter continues gathering stunning imagery and data of comet 67P/Churyumov-Gerasimenko in preparation for humanity’s first attempt to touchdown on a comet with the piggybacked Philae lander.
Stay tuned here for continuing cometary and space updates.
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Video Caption: Animation of Siding Spring Mars encounter courtesy of Solarsystemscope.com. Credit: Michal Sadlon
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