On October 19, a comet that has travelled many billions of miles will come within about 87,000 miles of Mars -- about one-third of the distance of the Moon from Earth. Comet Siding Spring comes from the Oort Cloud, material left over from the formation of the solar system. "This comet is coming into the solar system straight from the Oort Cloud. It's likely this is its first time this close to the sun," said space scientist David Humm, of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland.

Oort Cloud material, including comets, is scattered through a vast region that begins outside the orbits of Neptune and Pluto and extends a substantial fraction of the distance to Proxima Centauri, the closest neighboring star. Oort Cloud comets can tell scientists about the materials -- including water and carbon compounds -- that existed during the formation of the solar system some 4.6 billion years ago.

Studying this close encounter will be the largest fleet of orbiting scientific observatories ever flown to another world, orbiting around (and rolling on the ground of) Mars. These instruments will, for the first time ever, have the chance to make close-up observations of a comet new to the inner solar system. And though it will not be the easiest task, the teams operating these instruments and spacecraft have developed plans to take advantage of this rare opportunity.

"The close fly-by of Mars by Comet Siding Spring is unique, unexpected, and lucky for us," said Humm, who serves as instrument scientist for the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), built by APL, and one of the instruments on board NASA's Mars Reconnaissance Orbiter (MRO) that will observe Siding Spring. Two other MRO instruments observing the comet will be the High Resolution Imaging Science Experiment (HiRISE), a very high-definition camera, and the Context Imager (CTX). Together, all three imagers will attempt to capture data about Siding Spring that is unobtainable from Earth. Though Earth-based observations of Siding Spring will reveal a great deal of information, "CRISM has a significant advantage due to its proximity to the comet at closest approach," he said.

"CRISM is both a spectrometer and a camera," Humm explained. "It can identify molecules by the light they emit and characterize minerals by the light they reflect. We can then make an image of any material we identify, and see its distribution. If we're fortunate, CRISM will be able to detect some features in the comet gas and dust, and we can make images of the distribution of different gases detected and learn something about the nature of the dust."

There are some challenges. First, though the chances of any comet dust impacting the spacecraft are thought to be very minimal, the decision was made to "hide" the spacecraft in the shadow of Mars after the comet passes, to let the planet absorb any potentially damaging high-speed dust particles that may trail the comet as it passes by.

The greater problem, explained Humm, is that "these instruments are designed for looking at the surface of Mars during daytime, not at a far dimmer comet in the night sky." But the teams have overcome that challenge as well, and now have full observation plans for Siding Spring.

Siding Spring is small (the nucleus is less than a mile in diameter) and fast (it will pass Mars at about 34 miles per second). CRISM, HiRISE, and CTX were built to study a slowly-moving planet, so they will use MRO's ability to rotate in order to capture images as the comet speeds by Mars. The instruments will observe the comet repeatedly for two and a half days as it gets closer and closer to Mars before it makes its closest approach. The peak density of comet dust at Mars is expected 98 minutes after closest approach of the nucleus, and the MRO spacecraft will position itself behind the planet at that time.

Last year's October 1 encounter with Comet ISON -- a sungrazing comet that passed within 6.5 million miles of Mars -- gave the teams on CRISM and the other Mars observatories a good chance to practice looking at an object hurtling past the planet.

The instruments on MRO are unique in their abilities to study the comet. "HiRISE is the only instrument that can image the nucleus of Comet Siding Spring with more than one pixel, and CRISM will have the best signal-to-noise ratio of any spectrometer that will observe the comet from close up," Humm said.

Still, comet behaviors are peculiar and may seem random; sometimes they fizzle out, sometimes they get very bright, sometimes they do both. "Comets are very unpredictable," said Humm. "No two ever seem to be the same. A new comet like Siding Spring will bring surprises."

Images from Earth-based observatories and the Hubble Space Telescope have shown a typical coma of gas and dust develop as Comet Siding Spring has come closer to the sun. The coma may be larger or smaller when the comet flies by Mars. "If the comet is really active, then we will get good spectra of the coma," Humm explained. "If the comet is inactive, then some of the compositional results could be in question but we may still see broad color differences."

For Humm and other scientists on the CRISM and other MRO teams, an event like the encounter with Siding Spring was not even considered during construction and launch of the orbiter and its instruments back in 2005.

"I would have been very surprised if you had told me we were going to use CRISM to look at a comet," Humm said. "The likelihood of being this close to a new comet is really very small, and we're operating well beyond our design lifetime, so this exciting an opportunity is completely unexpected."