By Marcia Dunn ,AP
CAPE CANAVERAL, Florida — The heavens are hosting an event this weekend that occurs once in a million years or so.
A comet as hefty as a small mountain will pass mind-bogglingly close to Mars on Sunday, approaching within 140,006 kilometers (87,000 miles) at a speed of 202,767 kph (126,000 mph).
NASA’s five robotic explorers at Mars �X three orbiters and two rovers �X are being repurposed to witness a comet named Siding Spring make its first known visit to the inner solar system. So are a European and an Indian spacecraft circling the Red Planet.
The orbiting craft will attempt to observe the incoming iceball, then hide behind Mars for protection from potentially dangerous dusty debris in the comet tail.
Shielded by the Martian atmosphere, the Opportunity and Curiosity rovers may well have the best seats in the house, although a dust storm on Mars could obscure the view.
��We certainly have fingers crossed for the first images of a comet from the surface of another world,�� said NASA program scientist Kelly Fast.
Spacecraft farther afield, including the Hubble Space Telescope, already are keeping a sharp lookout, as are ground observatories and research balloons.
��We’re getting ready for a spectacular set of observations,�� said Jim Green, head of NASA’s planetary science division.
Named for the Australian observatory used to detect it in January 2013, Siding Spring will approach Mars from beneath and zoom right in front Sunday afternoon, Eastern Time.
On Earth, the best viewing, via binoculars or telescope, will be from the Southern Hemisphere �X South Africa and Australia will be in prime position. In the Northern Hemisphere, it will be difficult to see Siding Spring slide by Mars.
The comet �X with a nucleus estimated to be at least a half mile (.8 kilometer) in diameter �X hails from the Oort Cloud on the extreme fringe of the solar system. It formed during the first million or two years of the solar system’s birth 4.6 billion years ago and, until now, ventured no closer to the sun than perhaps the orbits of Jupiter, Saturn, Uranus or Neptune. It comes around every one or more million years.
It will be the first Oort Cloud comet to be studied up close in detail.
For comparison, the flyby distance of 140,006 kilometers (87,000 miles) is about one-third of the way from here to the moon. Siding Spring’s tail could extend from Earth all the way to our moon. Its gaseous coma, the fuzzy head surrounding the nucleus, might stretch halfway to the moon.
No comet has come anywhere near this close to Earth in recorded history.
��We can’t get to an Oort Cloud comet with our current rockets … so this comet is coming to us,�� said Carey Lisse, senior astrophysicist at Johns Hopkins University’s applied physics laboratory.