Posted: July 10, 2008
Binary asteroids – asteroids with moons – are common throughout the Solar System, but questions surrounding their formation have long been speculated. According to new research conducted by scientists from the University of Maryland and the Cote d’Azur Observatory, the answer lies in the Sun.
Around 15 percent of near-Earth and main belt asteroids with diameters less than 10 kilometres have satellites, and scientists had already deduced that these binary pairs were formed after the start of the Solar System, suggesting that some process still at work must have created them.
"It was at first thought the moons in these asteroid pairs probably formed through collisions and/or close encounters with planets," says Derek Richardson of the University of Maryland. "However, it was found that these mechanisms could not account for the large number of binary asteroids present among near-Earth and inner main belt asteroids."
One recent idea, known as the YORP effect (after the scientists Yarkovsky, O’Keefe, Radzievskii and Paddack who pioneered the idea) describes a phenomenon whereby energy from the Sun is absorbed by an asteroid and reradiated as heat, resulting in a measurable change in the asteroid’s motion by spinning it up or down over millions of years. For a typical one-kilometre near Earth asteroid, the time needed for YORP to accumulate its effect so that the asteroid’s period of rotation halves is about a million years. Given that the average lifetime of these bodies is about 10 million years – before meeting their demise in the Sun or via planetary impact – YORP has plenty of time to take effect.
Click image for link to animation that shows the spin-up and binary formation from two views. The left shows an overhead view and looks down onto the spin axis of the primary body. The right pane of the movie looks at the equator of the primary body, which is also the plane in which the asteroid's satellite is formed. Note that the time between each frame of the movie is long compared to the rotation period of the asteroid, giving a kind of 'strobe' effect, which is why the satellite sometimes appears to orbit backwards! Image: Walsh, Richardson and Michel.
This forms the basis for a new study presented in this week’s issue of the journal Nature whereby scientists Derek Richardson, Kevin Walsh and Patrick Michel have performed computer simulations to show that if the asteroid is spun up to a critical speed, it will morph into a slightly squashed object with a conspicuously fat equatorial belt, from which material is ejected into a circular orbit around the asteroid, which eventually coalesces into a moon roughly one-third the size of the parent body.
“The speed that the asteroid must spin in order to throw off material depends on the bulk density and structure of the body,” Michel tells Astronomy Now, “but the first mass loss occurs when the asteroid makes one turn around itself over 2-4 hours.”
“It’s a slow re-accumulation process though,” he adds. “The mass escapes little by little and those escaping materials collide together to form a satellite. But the asteroids have time to have mass loss over the course of their lifetime, and thus form binaries before they ‘die’, otherwise they (binaries) would not exist.”
The computer models also show that fresh material is exposed at the poles of the asteroid. “The material at the poles actually flows ‘down hill’ to the equator when you spin-up an aggregate,” says Michel. “So, by this mechanism, you sort of ‘shave’ the head of the body, revealing fresh material.”
A test of the spin-up model for binary formation is the ability to reproduce features of the radar-observed binary near Earth asteroid 1999 KW4. The primary body has a diameter of 1.5 kilometres and its moon has a diameter of 0.5 kilometres residing in a nearly circular orbit. Asteroid 1999 KW4 also has a density of just two grams per cubic centimetres and a porosity of 50 percent, placing it into the category of loosely packed ‘rubble-pile’ asteroids on which the spin-up model for binary formation depends. And understanding the composition of an asteroid will have implications for defending a potential Earth-bound asteroid, the authors say.
“The porous nature of these asteroids has strong implications for defensive strategies if faced with an impact risk to Earth from such objects, because the energy required to deflect an asteroid depends sensitively on its internal structure," says Michel. “This is one of the reasons why it is extremely important to improve our knowledge of the physical properties of these bodies. Better know your enemy before fighting it!”
Conceptual design of the JAXA mother spacecraft and the ESA lander that will make up the Marco Polo mission. Image: ESA.
This knowledge could come in the form of the Marco Polo mission, which is currently under study by ESA and JAXA, the Japanese Space Agency, the aim of which is to return a sample of untouched asteroidal material to the Earth that could shed new light on the Solar System’s early history. The oldest material in an asteroid is thought to lie underneath its surface, so the process of spinning off this material should uncover even older samples. "Thus a mission to collect and return a sample from the primary body of such a binary asteroid could give us information about the older, more pristine material inside an asteroid, just as the University of Maryland-led Deep Impact mission gave us information about the more pristine material inside a comet," says Richardson. Marco Polo would likely be launched in 2017 or 2018.