YORP: Rock and Roll

HEIC 1405b (ESA)

“This is a rock. Seeing it fall apart before our eyes is pretty amazing.”

David Jewitt

For months, the Hubble Space Telescope has been checking in on asteroid P/2013 R3. First observed by Catalina and Pan-STARRS, observations at Keck two weeks later revealed what might be a once in a lifetime spectacle.

“Keck showed us that this thing was worth looking at with Hubble,” [David] Jewitt [of UCLA] said. With its superior resolution, the space-based Hubble observations soon showed that there were really ten distinct objects, each with comet-like dust tails. The four largest rocky fragments are up to 200 metres in radius, about twice the length of a football pitch.

The Hubble data showed that the fragments are drifting away from each other at a leisurely 1.5 kilometres per hour—slower than the speed of a strolling human. The asteroid began coming apart early last year, but the latest images show that pieces continue to emerge ….

…. The ongoing discovery of more fragments makes it unlikely that the asteroid is disintegrating due to a collision with another asteroid, which would be instantaneous and violent in comparison to what has been observed. Some of the debris from such a high-velocity smash-up would also be expected to travel much faster than has been observed.

It is also unlikely that the asteroid is breaking apart due to the pressure of interior ices warming and vaporising. The object is too cold for ices to significantly sublimate, and it has presumably maintained its nearly 480-million-kilometre distance from the Sun for much of the age of the Solar System.

This leaves a scenario in which the asteroid is disintegrating due to a subtle effect of sunlight that causes the rotation rate to slowly increase over time. Eventually, its component pieces gently pull apart due to centrifugal force. The possibility of disruption by this phenomenon—known as the YORP effect—has been discussed by scientists for several years but, so far, never reliably observed.

For break-up to occur, P/2013 R3 must have a weak, fractured interior, probably the result of numerous ancient and non-destructive collisions with other asteroids. Most small asteroids are thought to have been severely damaged in this way, giving them a “rubble pile” internal structure. P/2013 R3 itself is probably the product of collisional shattering of a bigger body some time in the last billion years.

It is a fascinating notion, that the rock fragments hurtling through space might wander apart at such a leisurely pace. But the YORP effect, as intuitive as it seems, is somewhat elusive.

As the European Southern Observatory explains:

The spin of an asteroid and other small bodies in space can be affected by sunlight. This phenomenon, known as the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect, occurs when absorbed light from the Sun is re-emitted from the surface of the object in the form of heat. When the shape of the asteroid is very irregular the heat is not radiated evenly and this creates a tiny, but continuous, torque on the body and changes its spin rate.

Lowry’s team measured that the YORP effect was slowly accelerating the rate at which Itokawa spins. The change in rotation period is tiny—a mere 0.045 seconds per year. But this was very different from what was expected and can only be explained if the two parts of the asteroid’s peanut shape have different densities.

This is the first time that astronomers have found evidence for the highly varied internal structure of asteroids. Up until now, the properties of asteroid interiors could only be inferred using rough overall density measurements. This rare glimpse into the diverse innards of Itokawa has led to much speculation regarding its formation. One possibility is that it formed from the two components of a double asteroid after they bumped together and merged.

Lowry added, “Finding that asteroids don’t have homogeneous interiors has far-reaching implications, particularly for models of binary asteroid formation. It could also help with work on reducing the danger of asteroid collisions with Earth, or with plans for future trips to these rocky bodies.”

Those who like the technical details should take note; a preprint of Jewett et al., “Disintegrating Asteroid P/2013 R3” is available via arXiv; the paper published this month in Astrophysical Journal Letters.

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