On the horizon
Asteroids get a kick out of sunlight
Two independent teams of astronomers have monitored separate asteroids with radar and telescopes and uncovered the first direct evidence that sunlight speeds up an asteroid's spin and changes the direction of its axis. This is dubbed the YORP effect – an acronym based on the last names of four scientists who proposed and refined the idea. In essence, these four suggested that after a warm, sunlit region of an asteroid rotates into night, the heat it radiates provides a tiny amount of thrust. Over time, this thrust can boost an asteroid's spin rate.
For asteroids less than six miles across, this acceleration may eventually break up the object as the force from its spin overpowers the weak gravity that holds these piles of rubble together.
One team, led by Finnish mathematician Mikko Kaasalainen, found that over 40 years, the YORP effect added an extra full rotation to asteroid 1862 Apollo. The team's results appear in Thursday's issue of the journal Nature. A second team of US and European astronomers reports similar effects for asteroid 2000 PH5. By combing radar and telescope data, they found that each day, the YORP effect is spinning 2000 PH5 by another 0.0002 degrees per day. Their results appear in Friday's issue of the journal Science.
Scientists have long noted that a small eye region indicates a strong hurricane and a wide eye suggests a weaker storm. The size can vary several times in a storm's lifetime. Now researchers say they have found the first direct evidence that clouds outside a storm's eye wall can trigger rapid changes in storm strength as they form a new eye wall.
The team, led by the University of Washington's Robert Houze Jr., flew three aircraft equipped with Doppler radar through hurricane Rita in 2005. In less than a day, Rita went from a Category 1 hurricane to a Category 5, the most powerful type. The team found that as the storm revved up, a "moat" of dry air formed around the outside of the eye wall. This moat cut into the old eye wall and eventually merged with the air at the center of the storm. As a new eye wall took shape from the bands of rain clouds spiraling away from the outside edge of the moat and contracted, the storm picked up strength. The team concludes that the moats play an active role in clearing the way for a new eye wall to form. The results appear in the current issue of the journal Science.
Astronomers using a suite of observatories on Earth and in orbit have gained new insights into how older, elliptical galaxies run low on gas.
For years, astronomers have noted that elliptical galaxies with few new stars tend to appear in the middle of dense clusters of galaxies. Spirals like the Milky Way tend to be more solitary. So how do spirals become ellipticals? A team of scientists turned the Hubble Space Telescope on the Comet Galaxy, 3.2 billion light-years away. It is hurtling through a cluster's interior at just over 2 million miles an hour. Its accelerator: The combined gravity of the other galaxies, dark matter, and the tenuous hot gas that permeate the cluster. Tidal forces from the luminous and dark matter it encounters and pressure from the hot gas it passes through strip the galaxy of stars and the matter needed to form new suns. Ripped free of the Comet Galaxy's grip, millions of these stars become homeless intergalactic waifs.
The Comet Galaxy is substantially more massive than the Milky Way, and the team estimates that it takes about a billion years to transform a spiral galaxy like the Comet into a gas-poor elliptical filled with aging stars. So by the time you read this, the galaxy will have been running on empty for nearly 2.5 billion years.