Asteroid Bennu: Changes in Rotation Rate

Tuesday’s post on asteroids and what it would take to deflect or destroy one has been usefully reinforced by a new paper from Mike Nolan (Lunar and Planetary Laboratory, University of Arizona) and colleagues, who discuss their findings in Geophysical Research Letters. Here we’re looking at observations of the near-Earth asteroid (101955) Bennu, both archival (extending back to 1999) and current, drawing on the OSIRIS-REx mission.

You’ll recall that OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) is in operation around the asteroid, its observations helping us understand the object’s rotation, structure and composition, with a sample return planned for 2023. The Nolan paper fills us in on observed changes in rotation, which are apparent on the order of about 1 second per century. The asteroid’s rotation is speeding up.

Exactly what’s going on here is something we can hope OSIRIS-REx can help nail down. One possibility is a process like the Yarkovsky‐O’Keefe‐Radzievskii‐Paddack effect (YORP), by which asteroids are known to be affected because of the uneven distribution of solar heating across their surfaces. The effects of YORP depend on the shape and orientation of the individual asteroid and can cause either a slowdown or uptick in the object’s spin rate.

Or are there other processes at work here? Even boulders on the surface and their relative positions can play into changes in asteroid spin. It’s important to find out because over astronomical time periods, a faster spinning asteroid could eventually shed some of its mass. One thing, then, that OSIRIS-REx will be looking for is the presence of landslides or other surface evidence of such changes. Nolan points to the possibilities:

“As it speeds up, things ought to change, and so we’re going to be looking for those things and detecting this speed up gives us some clues as to the kinds of things we should be looking for. We should be looking for evidence that something was different in the fairly recent past and it’s conceivable things may be changing as we go.”

Image: This series of MapCam images was taken over the course of about four hours and 19 minutes on Dec. 4, 2018, as OSIRIS-REx made its first pass over Bennu’s north pole. The images were captured as the spacecraft was inbound toward Bennu, shortly before its closest approach of the asteroid’s pole. As the asteroid rotates and grows larger in the field of view, the range to the center of Bennu shrinks from about 11.4 to 9.3 km (7.1 to 5.8 miles). This first pass was one of five flyovers of Bennu’s poles and equator that OSIRIS-REx conducted during its Preliminary Survey of the asteroid. Credit: NASA/Goddard/University of Arizona.

We’re fortunate in having data from ground-based telescopes as well as Hubble to study the object over time. 110 million kilometers from Earth, the spinning Bennu completes a full rotation every 4.3 hours. The Hubble data on rotation rate showed a slight mismatch with the predictions of the earlier observations. And while Nolan and team point out that a change in the asteroid’s shape could account for its change in rotation, they clearly favor the YORP hypothesis. Having OSIRIS-REx at Bennu offers the opportunity to put YORP ideas to a close-up test.

The increase in Bennu’s rotation over the past two decades does not fit some earlier analyses of the YORP effect, making the spacecraft’s work all the more important. As the paper notes:

The OSIRIS-REx science team will independently measure the rotational acceleration during its 2-years of proximity operations. The precise shape determination, surface boulder distribution, gravity measurements, and thermal property determinations will allow for a better connection between the dominant drivers of the YORP effect (if confirmed) and their relative importance. The OSIRIS-REx team can measure the stability of the rotation state, to confirm whether this acceleration is a steady increase due to the YORP effect, or some other (likely episodic) process such as mass movement. Thus, our observations form a critical baseline for future work.

Within two years, we should have the OSIRIS-REx data independently providing Bennu’s rotation rate, which should help to identify the cause. We’ll also be looking at Bennu with other instruments for the next several decades to see whether further changes in rotation rate, consistent with YORP or not, emerge. Usefully, work like this allows us to compare and contrast in situ measurements with ground-based observations, giving us the chance to hone our skills at asteroid analysis for application to the larger population.

The paper is Nolan et al., “Detection of Rotational Acceleration of Bennu Using HST Light Curve Observations,” Geophysical Research Letters 31 January 2019 (abstract).