Tel Aviv University astronomers reveal the nature of "separated asteroids"

A multi-year research project, performed at Tel Aviv University's Florence & George S. Wise Observatory, yielded information due to appear in the Aug. 26 issue of the journal Nature supporting a theory of "asteroid fissioning": Asteroids break apart due to their fast rotation rate. It is now clear that Solar System bodies, previously thought of as being "floating rocks" in space are really "rubble piles" of small stones only glued together by gravity. When such bodies acquire sufficient spin they fission into two separate asteroids initiating their own lives around the Sun.

Tel Aviv University was selected by the Israel Space Agency and the Ministry of Science and Technology to host the Israeli Knowledge Center for Near-Earth Objects (asteroids). The asteroids that populate the Solar System are primarily concentrated in the main asteroid belt between Mars and Jupiter some 300 million kilometers from the Sun, but also extend all the way down into the inner Solar System, some even getting very close to the Earth.

The study of asteroids was part of the PhD thesis research of Tel Aviv University's Mr. David Polishook, from the Department of Geophysics and Planetary Sciences and a co-author of the Nature paper, who was supported by an Ilan Ramon scholarship from the Ministry of Science and Technology, granted to excellent PhD students.

The present discovery, obtained by the participation of the Wise Observatory in an international network of 15 institutions around the world, led by Petr Pravec from the Astronomical Institute of the Czech Republic, followed 35 "asteroid pairs"; these are 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," said Polishook, "this is an essential property when studying the origin and formation mechanism of these interesting objects.


The pair asteroid 10484 Hecht moving in front of the background stars." Copy rights: David Polishook.

The research team 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.

Scheeres theory predicts that if a binary asteroid forms by rotational fission, the two can only escape from each other if the smaller one is less than 60 percent the size of the larger asteroid. During rotational fission the asteroids separate gently from each other at relatively low velocities. The smaller guy steals rotational energy from the bigger guy, causing the bigger guy 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.


An artist impression of a binary asteroid." Copy rights: W. M. Keck Observatory/Lynette Cook.

While asteroid pairs were first discovered in 2008 by Nature paper co-author David Vokrouhlicky of Charles University in Prague, their formation process remained a mystery prior to the new Nature study.

Several telescopes around the world were used for the study, with the most thorough observations, of more than 320 hours, being made with the 0.46-meter and 1-meter telescopes at Wise Observatory in the Negev desert in Israel and the Danish 1.54-meter telescope at La Silla, Chile. This 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.


The 1-meter telescope of the Wise Observatory,
Tel-Aviv University's astronomical facility." Image by Iair Arcavi.

"By now it seems that most of the jigsaw puzzle is solved," Polishook said. "The sunlight is responsible for the spinning-up 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."


An illustration of the 'separated pairs of asteroids' formation mechanism." Copy rights: David Polishook.

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.


An artist impression of a binary asteroid." Copy rights: ESO.

Other co-authors of the Nature paper are from institutions in the United States, Chile, the Czech Republic, Slovakia, the Ukraine, Spain and France.