NORMAL - Mars makes its closest approach to Earth during the month of December this year, which explains why the Twin City Amateur Astronomers club has scheduled a Mars observing session in Normal tonight.

But the bigger story involving the Red Planet may occur after the New Year. Scientists announced last week they think there's a 1-in-75 chance an asteroid will hit Mars on Jan. 30. They won't know for certain for a while; asteroids move slowly in space terms, so tracking their exact trajectory takes time, said Carl Wenning, an officer in the astronomy club and former director of the Illinois State University planetarium.

Odds in favor of a collision may rise as scientists continue to watch, he said.

The asteroid, dubbed 2007 WD5, was discovered in late November, said Steve Chesley, an astronomer with the Near Earth Object Program at NASA's Jet Propulsion Laboratory in Pasadena, Calif. If the asteroid strikes Mars, it will hit on the side visible to Earth and would be the first time in history astronomers could witness an asteroid's impact on a planet as it happens, Chesley said.

The asteroid is traveling at eight miles per second and is similar in size to an object that hit remote Siberia in 1908, Chesley said. The energy unleashed in that collision was equivalent to a 15-megaton nuclear bomb.

A collision on Mars would make a hole the size of the Meteor Crater in Arizona. The asteroid could hit near Mars' equator, near where the rover Opportunity has been exploring since 2004.

The robot is thought to be out of harm's way.

Any collision would not be visible from Bloomington-Normal, but people further west - in places such as Alaska, Hawaii and Asia - could witness the event in real time, Wenning said.

Still, he noted projections made far in advance could be off a few hours.

If so, Central Illinois may have a front-row seat for a sky show more spectacular than the one that occurred nearly 14 years ago, when the planet Jupiter was struck by 20 fragments from a comet, Wenning said.

The impacts occurred on the far side of Jupiter, and the damage became visible only after the planet spun on its axis.

"Who can forget July 16 to 22, 1994?" asked Wenning.

The fact Mars is involved captures even more interest, he said. More than 100 years ago, at least a handful of astronomers speculated channels on the planet's surface could have been made by intelligent life, and changing surface colors seen seemed to indicate the presence of plants.

Explanations today lean more toward science than science fiction, but Mars' polar ice caps and a robot's discovery of water crystals a few years ago mean water - an essential building block of life - was present.

"Apparently, it has sub-surface water that has played a role in the history of Mars," he said.

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Mars viewing

What: Mars viewing session

When: 8 to 10 p.m. today

Where: Fairview Park, Normal

Why: Mars is at its closest to Earth in December. Its nearest approach this year, 54.8 million miles, was on Dec. 18. It will be 56 million miles from earth today. Mars will rise in the northeastern sky at sunset and travel across the sky until it sets at sunrise in the northwestern sky.

Sponsors: Twin City Amateur Astronomers; Normal Parks & Recreation Department

Info: (309) 830-4085.