Hawai'i researchers find dancing asteroids in solar system

By Jan TenBruggencate
Advertiser Science Writer

The discovery by Hawai'i researchers of two asteroids spinning around each other has provided astronomy with new insight into the makeup of asteroids.

In the case of the binary system in question, known as 1998 WW31, they probably contain a fair amount of ice, said University of Hawai'i planetary astronomer Dave Tholen.

While 1998 WW31 was first noticed in late 1998, it was not until two years later that it was discovered to be a binary system — meaning it involves two objects. The discovery was made when two groups of astronomers working independently took images of it from the Canada-France-Hawai'i Telescope atop Mauna Kea. They included Tholen, University of Hawai'i graduate student Michael Connelley, Canada-France-Hawai'i Telescope staff astronomer Christian Veillet and others.

Their discovery was reported in yesterday's edition of the journal Nature.

"It was somewhat accidental," Tholen said. Researchers were actually trying to determine asteroid density within different areas of the Kuiper Belt, a region of asteroids that lies beyond the main planets of our solar system. He said Veillet first noticed the twin points of light, and Tholen's team confirmed it.

The two bodies in 1998 WW31 are orbiting each other roughly 12,500 miles apart — less than a tenth of the distance between the Earth and the moon. They are roughly the same size — between 60 and 100 miles in diameter.

They circle each other every 570 days, and astronomers were able to calculate their mass from their distance apart and speed of orbit. Using their relative brightness, their rough surface size could be judged. With that information, astronomers made calculations of their density.

"Then we can calculate what it's made of," Tholen said. He concedes that there's a fair amount of uncertainty built into the calculations. The biggest question is the accuracy of the surface size, since changes in the character of the surface could dramatically affect brightness.

Tholen said the research suggests 1998 WW31's two asteroids are of low density. That could mean they are extremely porous or that they contain frozen gases such as methane, nitrogen, carbon dioxide or carbon monoxide.

But Tholen said the research team feels another cosmically abundant compound — water — is more likely.

"Ice fits the bill," he said.