University of Arizona scientists have their eyes on Mars for the fly-by of comet Siding Spring, which will pass the red planet on Oct. 19, closer than any comet has ever zoomed past the Earth in recorded history.

"We expect Mars to be bathed in the comet's coma, the gas and dust clouds that make for their famous tails," said Roger Yelle, a professor of planetary science in the UA's Lunar and Planetary Laboratory, who is on the science team of NASA's MAVEN spacecraft, which went into orbit at Mars on Sept. 21.

"The probability of an encounter like this is one in a million." MAVEN - short for NASA's Mars Atmosphere and Volatile Evolution mission - is the latest addition to an armada of seven spacecraft currently studying Mars, either observing from high above or roving and digging on the surface.

During the comet fly-by, NASA has programmed its orbiters to take measurements and images, then "duck and cover" behind the planet, just in case.

"It only takes a half-a-millimeter-size particle traveling at 56 kilometers per second to injure one of these spacecraft," said Don Yeomans, manager of NASA's Near Earth Object Program Office.

Yelle and his colleagues anxiously await the arrival of the city-block-size chunk of ice, rock and dust on its first-ever journey toward the sun. Unlike so-called short period comets whose journey around the sun takes them into the inner parts of the solar system every few years or decades, Siding Spring is a long period comet, visiting the solar system for the first time from the far reaches of space.

The comet originated in the Oort Cloud, a vast region of space surrounding the solar system speckled with billions of far-and-few-between comets, some of which embark on journeys that bring them back into our system of planets from which they originated billions of years ago during the early evolution of the solar system.

"Those comets are especially interesting because they are pristine," Yelle explained. "Comets are leftovers from the birth of the solar system, but unlike short-period comets, which have been altered by the sun's heat and solar wind, Siding Spring has been in deep freeze, in deep space, for billions of years."

Alfred McEwen, a professor in the UA Department of Planetary Sciences, leads the High Resolution Imaging Science Experiment, known as HiRISE, on NASA's Mars Reconnaissance Orbiter, or MRO.

"This is the first time the nucleus of a long-period comet can actually be resolved by a telescope, either in space or on the ground," McEwen said. "Planning a spacecraft mission to these types of comets is nearly impossible because there is typically only about a year's notice between discovery and passage into the inner solar system."

Because comets such as Siding Spring are difficult to study, scientists know very little about them.

"We want to know the shape of its nucleus, rotation period, its brightness, and hopefully observe the inner coma for jets and outbursts," McEwen said.

All previously resolved comet nuclei are nearly black on their surfaces, despite being rich in ices. A key unanswered question is whether comets are formed black, become black from exposure to galactic cosmic rays, or are blackened over frequent visits to the inner solar system.

In hopes of lifting some of Siding Spring's secrets, the UA-led HiRISE camera team will interrupt its daily routine of photographing the Martian surface.

"We will roll the spacecraft and point HiRISE at the approaching comet," McEwen said. "The tricky part is to predict where the camera has to look, because the comet will be close and traveling fast. Photographing the comet's nucleus at its closest approach is like trying to photograph a speeding bullet while riding a roller coaster."

"Over the past month the comet has been observed to fade in brightness compared to standard comet models, but we should still get a good look at the nucleus even if the coma is not very active."

Comet Siding Spring first appeared on images taken by the UA's Catalina Sky Survey but was not identified as an Oort Cloud comet until independent discovery observations were made approximately four weeks later, by Robert McNaught at the Siding Spring Survey. The survey was one of three telescopes operated by CSS - and the only full-time asteroid survey in the Southern Hemisphere.

On Oct. 19, Siding Spring will race past Mars within 88,000 miles, less than a third of the Earth-moon distance, closer than any comet has ever passed the Earth in recorded history. Traveling at 35 miles per second, the comet - less than half a mile in diameter - would shoot over Los Angeles and out into the Pacific Ocean only one minute after it appeared over Manhattan.

Siding Spring will never get close to the Earth, Yelle said.

"After its pass by the orbit of Mars, it will go back to the Oort Cloud and not come back for many millions of years, if ever," he said.

Scientists are not sure of what to expect when Siding Spring zooms by Mars. What is certain, though, is that there is no chance of an impact.

"Earlier on, there was some concern the dust trail could endanger the spacecraft, but that no longer seem to be a possibility," Yelle said. "Nevertheless, we are taking mitigation strategies to be cautious. When the comet is coming, we'll be hiding on the other side of Mars, and when it goes by, we'll turn the MAVEN spacecraft so that the least sensitive surfaces are pointing to the comet and can't damage instruments.

"After about an hour, we'll come out of hiding. MAVEN will be observing the comet for about three days before and two days after the fly-by."

In contrast, MRO will observe the comet during its closest pass to Mars, although the orbiter will be hiding behind Mars when the dust trail will pass, if it extends that far.

Over eons, Mars has been losing its atmosphere to space, and MAVEN is a mission designed to study the physical and chemical aspects of that process.

"The atmosphere escape process happens from the upper parts of the atmosphere, close to the region that will be perturbed by the coma of the comet, mostly by water molecules," Yelle said. "They will hit the Martian atmosphere about 250 kilometers above the surface and heat it through their impact momentum, which will in turn tell us about the escape process. We will also study the comet itself - for example, ions that stream from its coma."