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Celestial flybys set to thrill in 2013

PARIS: Astronomers are gearing for thrills this year when Earth gets buzzed by two rogue asteroids and two comets, including a wanderer last seen by the forerunners of mankind, blaze across the sky.
This week, the guardians who scour the skies for dangerous space rocks will be closely tracking an asteroid called 99942 Apophis. Apophis measures around 270 meters across, a mass able to deliver more energy than 25,000 Hiroshima bombs if it ever smashed into Earth.
Apophis sparked some heart-stopping moments when it was first detected in 2004.
Early calculations suggested a 2.7-percent probability of a collision in 2029, the highest ever seen for an asteroid, but the risk was swiftly downgraded after more observations.
Even so, for April 13, 2036, “there is still a tiny chance of an impact,” says NASA’s fabled Jet Propulsion Laboratory (JPL), which puts the risk at about one in 250,000.
One of the big unknowns is the Yarkovsky effect, a phenomenon discovered by a Russian engineer at the start of the 20th century.
A slowly rotating body that orbits close to the Sun experiences heating on one side of its body that then cools at “night” as it turns over. This alternate heating and cooling can cause a tiny momentum, depending on the body’s spin and amount of area that warms. The question is whether, over time, the Yarkovsky effect is accelerating Apophis, thus skewing estimates for future approaches.
Seeking clues, NASA’s deep-space radars at Goldstone, in California’s Mojave desert, and at Arecibo in Puerto Rico will be scanning Apophis, which on Jan. 9 will pass by at some 14.5 million km.
“Using new measurements of the asteroid’s distance and line-of-sight velocity, we hope to reduce the orbital uncertainties and extend the interval over which we can compute the motion into the future,” JPL’s Lance Benner said in an e-mail.
“It’s possible that the new measurements improve the orbit to the point that we can completely rule out an impact.”
On Feb. 15, a 57-meter asteroid, 2012 DA14, will skim the planet at just 34,500 km. In other words, it will spookily fly by inside the orbit of geostationary satellites. “It’s going to be the closest predicted flyby of an asteroid,” says Mark Bailey, director of the Armagh Observatory in Northern Ireland.

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