A comet making a close fly-by of Mars in what astronomers are calling a "once-in-a-lifetime event" will have their full attention, they say.
Comet C/2013 A1, also known as Comet Siding Spring after the observatory in Australia that first spotted it, will pass with just 87,000 miles of Mars.
That is 16 times as close as an approach any comet has ever made to the Earth, astronomers note, just half of the distance from the Earth to our moon.
The comet, as it makes its closest approach to Mars at around 2:30 p.m. Eastern time on Oct. 19, will be watched by spacecraft in orbit around the Red Planet and by rovers on its surface, as well as by telescopes here on Earth.
"Normally we go to the comets to observe them but this time the comet is coming to us," says Kelly Fast of NASA's Planetary Science Division, which has spent most of the past year preparing various spacecraft and space telescopes for observation.
Five orbiters at Mars, three belonging to NASA and one each from the European Space Agency and India, have been positioned to capture images of the icy body as it streams past.
Meanwhile, space telescopes including the Hubble, Spitzer, Kepler, Chandra and Swift instruments will turn their observing eyes toward the Red Planet.
Some earthbound sky watchers may get a chance to witness the event, experts say, but they'll need good equipment and possibly some luck.
"From the Earth it is what we call an 11th magnitude object, which means it would take a good telescope at least 6 inches wide and a very dark sky to find the comet," says Carey Lisse, principal scientists at Johns Hopkins University Applied Physics Laboratory.
Alternatively, a live feed of the comet flyby will by carried online by the SLOOH Community Observatory starting at 2:15 Eastern time.
Comet Siding Spring is approaching the sun at the end of a long journey from the Oort cloud, a region of icy bodies at the very edge of our solar system. The material in the Oort cloud is believed left over from the formation of the solar system's planets about 4.6 billion years ago.
Since this is the comet's first journey into our neighborhood from its deep-freeze "storage" in the cloud, scientists say they will be studying it closely.
"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," says John Grunsfeld, associate administrator of NASA's Science Mission Directorate in Washington, D.C.
