Asteroid with two faces

Astronomers discover that the small planet Itokawa experienced an earlier collision

February 06, 2014

Partially compact, partially rather porous: the internal structure of the elongated asteroid Itokawa is not consistent – and points towards an earlier collision of two separate bodies. While one part of the peanut-shaped body shows a density comparable to that of granite, the other is built considerably far more porously, like densely packed sand. These are the findings of an international team of scientists, including researchers from the Max Planck Institute for Solar System Research in Göttingen. This is the first time that scientists managed to catch a glimpse into the inner life of an asteroid.

Cosmic peanut: the density of the smaller one (red) of these two lumps of the asteroid Itokawa amounts to approx. 2850 kilogram per cubic metre, that of the larger part (blue) to about 1750 kilogram per cubic metre.

Few asteroids in our solar system have been studied as thoroughly as Itokawa. The nearly 600 meter long body penetrates Earth's orbit every 556 days and can thus be classified as a Near Earth Object. Also, in 2005 Itokawa was the destination of the space mission Hayabusa. The Japanese space probe accurately surveyed the asteroid and brought back material from its surface. However, as with all other asteroids, it remained largely unkown what is hidden beneath this outer crust. In the past, scientists could only determine average densities that – for simplicity – were often assumed to characterize the body as a whole.

"In our new study, we have now for the first time been able to pin down properties from the interior of an asteroid ," said Dr. Colin Snodgrass from MPS. Itokawa's inner works proved to be far from homogeneous. “Itokawa appears to consist of two very different parts," says the asteroid and comet researcher, who is also part of ESA's current Rosetta mission to comet Churyumov-Gerasimenko. One of these parts coincides roughly with the smaller nub of the peanut-shaped body, referred to as its "head". It has a density of about 2800 kilograms per cubic meter, which corresponds to that of granite. The other, larger part (the "body") is quite different: with 1750 kilograms per cubic meter its density is comparable to that of firmly packed sand.
The asteroid's rotation period offered the crucial clue to these findings. In the case of small and irregularly shaped bodies, this indicator can change under the influence of the Sun. The body absorbs photons from the Sun and re-emitts this energy in the form of heat back into space. Because of its irregular shape, this emission is not uniform thus creating a tiny torque, known as the YORP effect. 

Portrait of the asteroid: the Japanese space probe Hayabusa visited Itokawa in the year 2005, snapping this picture of the small planet.

In the case of Itokawa, this torque makes the asteroid rotate faster and faster. But not to worry: the time it needs for one complete rotation decreases by only 45 milliseconds per year. The researchers were able to track down this tiny effect by analyzing telescope data taken from 2001 to 2013. Eight telescopes from the U.S., from Spain, and Chile were involved in this study and in this time recorded changes in brightness with high precision. These can help determine its rotational properties.

The significantly more detailed observations performed by Hayabusa also proved to be valuable. Based on  the exact shape of the asteroid as measured by the spacecraft, the researchers were able to determine the effective torque in theoretical calculations. These calculations require assumptions about the density of the body. “Both our approaches could only be reconciled by assuming a body structured in two parts with very different densities”, concludes Snodgrass.

How Itokawa came to this differentiated internal structure is still unclear. The researchers suspect that the elongated body arose from the collision of two smaller bodies. For example, Itokawa could have emerged from a system of two asteroids that originally orbited around a common balance point and that then collapsed.


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