Sedna's Slow Rotation a Mystery
http://www.washingtonpost.com/wp-dyn/articles/A12950-2004Apr14.html

Sedna, the most distant known object in the solar system, appears to rotate about every 20 days, so slowly that scientists thought it had to have a moon, but a month of searching since its discovery has failed to find one, and astronomers are scratching their heads in bewilderment.




"There is no other explanation for something to rotate so slowly," said the California Institute of Technology's Michael Brown, leader of the team that first observed Sedna. "But no satellite whatsoever was visible in our telescope."

Brown, speaking in a NASA-sponsored telephone news conference yesterday, said the likeliest explanations for the anomaly were either that the moon was there, and the Hubble Space telescope had not seen it, or that Sedna is actually turning much more rapidly and "fooled us" into thinking that it was rotating slowly.

Sedna, a bright red, icy, planetlike object, revolves around the sun in an elliptical orbit and is currently about 8 billion miles away -- nearly three times as far as Pluto. Sedna has a diameter of between 800 and 1,000 miles -- at most about one-seventh the size of Earth.

The researchers discovered Sedna using California's Palomar Observatory. Co-researcher David Rabinowitz, of the Yale Center for Astronomy and Astrophysics, said the team observed "a long-term change" in Sedna's brightness, which repeated itself about a month later.

"The only way to explain it is a slow rotation," Rabinowitz said. Sedna was rotating once about every 20 Earth days, he added. "We thought that Sedna had a companion moon that slowed it." To find it, the team switched to Hubble, better able to focus on distant small objects because it is unencumbered by the Earth's atmosphere.

Ordinarily, planets, asteroids and comets rotate rapidly -- a few hours, or, in Earth's case -- 24 hours. "When these bodies were formed, they collided with each other," Southwest Research Institute senior space scientist Clark Chapman explained in a telephone interview. "The collisions imparted spin."

In the vacuum of space, spin, once imparted, does not go away -- unless something interferes. So when scientists see a heavenly knuckleball, it usually means there is another object whose gravity is tugging on it to slow it down. Mercury and Venus turn very slowly because they are so close to the sun that it brakes them. Pluto moves in lockstep with Charon, its companion moon, rotating once every six days. Earth's gravity grips the moon, and vice versa. But Hubble observations failed to reveal a moon, and while Brown and Rabinowitz offered several explanations, they made clear that none satisfied them:

"There's always a possibility that the satellite was too close to Sedna to be seen -- that at the moment we were looking at it, it was just in front of Sedna or just in back," Brown said, but he said the odds of this were one in 100.

He said he favored the theory that there is a satellite that is "quite large" but "quite dark," and could not be seen by Hubble, which had been configured to look at Sedna and look for a bright moon. Brown said the team could reprogram the telescope to pick up darker objects.

Brown said some sort of cataclysm could have caused the moon to be lost or had destroyed it. Rabinowitz noted that Sedna was too far away to be examined for the surface cratering that such an event would cause.

Finally, Brown said, the data could be wrong, or the planet could be rotating much faster. Brown suggested that if Sedna rotated every 25 hours, Hubble observations every 24 hours would detect only a small movement, when in fact Sedna had made a full rotation plus a little more.

But Chapman noted that "there are quite a few mysteries out there," and rejected quick theories: "I wouldn't agree that either there is a moon out there, or the data's wrong. I would say there's a third choice -- that there is something out there we don't understand."