The Mars Odyssey orbiter will be forced to change its orbit to avoid possible dangers when Comet Siding Spring makes a close encounter with the Red Planet. Orbital correction happens on Aug. 5, in anticipation of the celestial encounter taking place on Oct. 19.

There is no danger of the comet colliding with the $297 million orbiter, but dust from the comet, shot out at tremendous speed by heating from the Sun, could be hazardous to the mission.

The Mars Reconnaissance Orbiter, launched to the Red Planet in 2005, has already undergone one course change in response to the arriving comet. Another orbital correction is scheduled to be undertaken on Aug. 27.

Comet Siding Spring will come within 82,000 miles of Mars at its closest approach. At that time, dust will be rushing away from the icy body at more than 126,000 miles per hour. These velocities, more than three times faster than the Voyager 1 spacecraft, provide enough energy to small particles to damage the Mars orbiters. Particles as small as 1/50th of an inch could prove hazards at these speeds.

The Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft is due to arrive just one month before the comet makes its close encounter. NASA mission planners are hoping to have all three spacecrafts positioned safely on the opposite side of Mars during the encounter. An orbital adjustment for the new spacecraft will be completed on Oct. 9, before the main science mission begins.

"Researchers plan to use several instruments on the Mars orbiters to study the nucleus, the coma surrounding the nucleus, and the tail of Siding Spring, as well as the possible effects on the Martian atmosphere. 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," NASA officials wrote in a press release on the maneuvers.

Mission planners will use the Mars Reconnaissance Orbiter and the Odyssey to study the coma of the comet - the region surrounding the nucleus. Spectral and thermal recordings will be made, to better understand the composition of the comet, coma and tail. Surface features and rotation rate may be detected by these craft. MAVEN will study the interaction between the comet and solar wind emanating from the Sun as well as the effect of the gas on the Martian atmosphere.

The greatest danger to the spacecraft will last about 20 minutes, starting around an hour-and-a-half after the closest encounter. This is the period in which the planet will encounter the greatest concentrations of dust and debris from the comet.