When we think about asteroids, we imagine a large potato-shaped rock covered in craters. But a new analysis of data from Hayabusa, the plucky Japanese spacecraft that could, reveals a more interesting picture. It seems that asteroid Itokawa is not a simple space rock, but has a complex structure, with different internal regions having different densities. Hayabusa was launched by the Japanese space agency (JAXA) back in 2003.  It’s mission was to travel to a near-Earth asteroid, collect samples, and bring them back to Earth for study.  Unfortunately, Hayabusa seemed doomed almost from the start when a solar flare damaged its solar panels, affecting the efficiency of its ion engines and delaying its arrival at the asteroid.  As the mission proceeded, Hayabusa continued to be plagued with mechanical problems, forcing the flight engineers to conjure some inspired make-shift solutions.  It’s a credit to their ingenuity that the mission was completed successfully, even if the sample canister was finally returned to Earth several years behind schedule.

The destination for the mission was asteroid 25143 Itokawa.  To ensure a successful contact, Hayabusa recorded a large number of images of the asteroid as it approached.  These images were used to record the motions of the asteroid as precisely as possible and to study the surface to find a suitable landing site.  It is these images that have now been used to study the composition of the asteroid in detail.

Any object in space, when lit up by the Sun, will warm up and radiate heat away.  Unless it has a perfectly smooth and symmetrical surface, the heat will be radiated out unevenly.  Since photons exert a minuscule pressure, this causes a slight torque on the object.  In the case of Itokawa, the torque is enough to slow the rotation of the asteroid by a tiny amount.  But when Stephen Lowry (University of Kent, UK) and his colleagues used the mission photographs to measure the time Itokawa takes to rotate, they found that it was slowing by 0.045 seconds per year.  As small as this change is, it’s very significant because it differs from what was expected.  It shows that the mass of the asteroid is not distributed in the way we would have expected if it was just a solid lump of rock.

“This is the first time we have ever been able to to determine what it is like inside an asteroid,” explains Lowry. “We can see that Itokawa has a highly varied structure — this finding is a significant step forward in our understanding of rocky bodies in the Solar System.”

Itokawa is shaped roughly like a peanut.  The models used to calculate its expected spin rate were based on it being a homogeneous lump of rock.  But the anomalous rate of change shows that the two sides of the structure have different densities, suggesting that they are made from different materials.  The question remains how such an object could form.  Did the original material accreted from the protoplanetary disk accumulate unevenly, or is Itokawa formed by two differently structured asteroids colliding and merging?  Hopefully further research will answer the question.

The discovery was described in a paper titled The Internal Structure of Asteroid (25143) Itokawa as Revealed by Detection of YORP Spin-up