Mars robotic rovers and orbiters are set to have a front row seat for a comet that will be flying past the Red Planet so close it will be less than one-tenth the distance of any known comet flyby of Earth.
NASA scientists are just trying to make sure it's not dangerously close.
What astronomers are describing as a "once-in-a-lifetime" comet flyby is expected to zoom within about 87,000 miles of Mars on Sunday, Oct. 19.
While that distance may seem large, it is less than half the distance between Earth and our moon.
The comet, known as C/2013 A1 or Siding Spring, should travel past Mars at approximately 126,000 mph with its nucleus coming closest to the planet at 2:27 p.m. ET, according to NASA. While the nucleus will miss the orbiters working around Mars, the comet will be shedding material as it goes by. That debris is expected to hurtle toward Mars at 35 miles per second.
NASA noted that at that velocity, even a particle only one-fiftieth of an inch across could cause significant damage to a spacecraft and could be disastrous for the Mars orbiters.
NASA now has three spacecraft -- the Mars Odyssey, the Mars Reconnaissance Orbiter and the Mars Atmosphere and Volatile EvolutioN orbiter -- working above the surface of Mars. And they won't have the protection the rovers do.
To keep them safe, NASA will maneuver all three orbiters to the opposite side of the planet for the duration of the flyby.
"The hazard is not an impact of the comet nucleus itself, but the trail of debris coming from it," said Rich Zurek, chief scientist for the Mars Exploration Program at NASA's Jet Propulsion Laboratory. "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."
With the rovers and orbiters out of danger, NASA scientists can focus on the fact that the proximity of the comet's approach to Mars will provide an unprecedented opportunity for researchers to gather data on both the comet and its effect on the Martian atmosphere.
"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, astronaut and associate administrator for NASA's Science Mission Directorate. "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."
The Siding Spring comet -- which came from the Oort Cloud, a spherical region of space surrounding our sun -- is made up of a giant swarm of icy objects believed to be material left over from the formation of the solar system.
It will be the first comet from that region scientists have been able to study up close, giving them a chance to learn more about the materials, including water and carbon compounds, that existed during the formation of the solar system 4.6 billion years ago, NASA noted.
The space agency said its orbiters are set to 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.
NASA won't just be relying on its machines and spacecraft around Mars to monitor the comet's flyby.
Scientists will be studying it, using Earth-based and space telescopes, including the Hubble Space Telescope, along with the Kepler, Swift, Spitzer and Chandra telescopes. The ground-based Infrared Telescope Facility on Mauna Kea, Hawaii also will be tracking the comet.
NASA's asteroid hunter, the Near-Earth Object Wide-field Infrared Survey Explorer, has been imaging the comet as it approaches Mars and will continue to track it as it passes by the planet.