'Separated asteroids could collide with earth'

From Jerusalem Post: 

International researchers, including Israelis at Tel Aviv University, have revealed the nature of “separated asteroids” – an understanding that could help the most powerful nations to find a way to divert these small minor planets orbiting around the sun from one day smashing into Earth.
An article on the research, published in the August 26 edition of the prestigious journal Nature, was written by researchers from institutions in the Czech Republic, the US, Chile, Slovakia, Ukraine, Spain and France, in addition to Israel.
The most thorough of the celestial observations – 320 hours of stargazing – leading to the discovery were conducted over three years with two telescopes at Mitzpe Ramon’s TAU Wise Observatory, headed by Dr. Noah Brosch, and doctoral student David Polishook of the university’s Department of Geophysics and Planetary Sciences and a co-author of the study. The team director was Dr. Petr Pravec of the Astronomical Institute of the Czech Republic.
Polishook was supported by an Ilan Ramon scholarship from the Ministry of Science and Technology, granted to excellent PhD students. TAU was chosen by the Israel Space Agency and the Science and Technology Ministry to host Israel’s Knowledge Center for Near-Earth Objects (asteroids).
The researchers found evidence supporting a theory of “asteroid fissioning,” in which asteroids break apart due to their fast rotation rate. It is now clear, they said, that bodies in the solar system – instead of being “just rocks floating in space” – are really “rubble piles” of small stones only “glued together” by gravity. When such bodies have enough spin, they fission into two separate asteroids initiating their own lives around the sun, they suggested.
The asteroids that populate the solar system are mainly concentrated in the main asteroid belt between Mars and Jupiter some 300 million kilometers from the sun, but they also extend all the way down into the inner solar system, some even getting very close to the Earth.
The team observed 35 “asteroid pairs” – each having two asteroids that may be millions of kilometers apart but circle the sun on the same orbit.
“Observing brightness changes of these asteroids allows us to derive their rotation rates,” Polishook noted. “This is an essential property when studying the origin and formation mechanism of these interesting objects.”
The research showed that all the asteroid pairs in the study had a specific relationship between the larger and smaller members, with the smaller one always less than 60 percent of the size of its companion asteroid.
The measurement fits precisely the prediction of a theory developed in 2007 by a Colorado University-Boulder aerospace engineering sciences professor and Nature paper co-author, Daniel Scheeres. His theory predicts that if a binary asteroid forms by rotational fission, the two can escape from each other only if the smaller one is less than 60% the size of the larger asteroid. During rotational fission, the asteroids separate gently from each other at relatively low velocities. The smaller one steals rotational energy from the bigger one, causing the bigger to rotate more slowly and the size of the orbit of the two bodies to expand, Scheeres said.
“If the second asteroid is small enough, there is enough excess energy for the pair to escape from each other and go into their own orbits around the sun,” he added.

While asteroid pairs were first discovered in 2008 by Nature paper co-author David Vokrouhlicky of Prague’s Charles University, their formation process remained a mystery until the new study.
The study makes the clear connection between asteroids spinning up and breaking into pieces, showing that asteroids are not static, monolithic bodies,” said Scheeres. “Instead, they are little worlds that may be constantly changing as they grow older, sometimes giving birth to smaller asteroids that then start their own life in orbit around the sun,” he said.
“By now it seems that most of the jigsaw puzzle is solved,” Polishook added. “The sunlight is responsible for the spinning of the asteroids, as if they were windmills reacting to the wind; the fragile nature of asteroids, the rubble pile structure, allows the centrifugal force to disrupt them; and there is sufficient time for this process to recur again and again making asteroids smaller and rounder.”

“Asteroids are important for the understanding of life on Earth,” said Scheeres. He pointed to the Chicxulub asteroid, believed to have impacted the Earth near the Yucatan Peninsula 65 million years ago, causing dinosaurs to become extinct and thus essentially resetting the evolutionary clock on Earth. Some asteroids have even been found to contain amino acids – the building blocks of life – causing some scientists to speculate that life on Earth could have originated from asteroids pelting the planet.
Brosch noted that these separated asteroids can be observed not from Earth but from the Hubble Space Telescope. “It is upsetting. We want to know whether any will eventually hit us. We have been learning asteroid processes, and if one is actually aimed at Earth, the rich countries will have to find a way to divert it with a spaceship or other means. It could take 20 years to build such a thing, so the US, Russia, China, India, Japan and other powers will have to prepare in advance for such an eventuality.
“In Israel,” he added with a smile, “we are not in that league of space capability, but we can pray.”