Hubble Space Telescope fails to solve mystery of Sedna's missing moon

Planet's discoverer is baffled by failure

April 15, 2004|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

A search with the Hubble Space Telescope has failed to spot the tiny moon that astronomers were confident they'd find circling Sedna, the most remote body ever spotted circling our sun.

"I'm completely baffled," said Sedna's discoverer, California Institute of Technology astronomer Mike Brown. "I simply don't know what it means."

Sedna, named for an Inuit sea goddess, orbits the sun at a distance of 8 billion miles, more than twice as far out as Pluto.

When its discovery was announced last month, Brown said careful measurements of its reddish light showed changes that seemed to repeat about every 20 days. That was a strong hint that Sedna was spinning once in 20 days and in a gravitational "lock" with a sizable orbiting moon.

Most solo objects the size of Sedna rotate once every few hours. Pluto, a third larger than Sedna, rotates once every six days, a pace that puzzled astronomers until 1978, when they discovered a moon, Charon, circling the planet, also every six days.

To spot Sedna's moon, scientists needed the power and stability of the Hubble telescope. But when they aimed it in Sedna's direction March 16, there was no moon in the picture.

Brown said there's a slight chance the little moon was passing directly behind or in front of Sedna when the Hubble telescope snapped its pictures. Or, the initial light measurements might have been flawed. More observations are planned to solve the puzzle.

Even with the Hubble's power, the Sedna photos failed to show any detail on the surface of the distant sphere. It remained but a single pixel of light. Brown said that means Sedna is no larger that 1,000 miles across - roughly the diameter of Texas but colder - minus 400 degrees Fahrenheit.

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