Rotation of Near-Earth Asteroids

Faculty: Tom Statler
Graduate Students: Desiree Cotto-Figueroa & David Riethmiller

We have an ongoing observational program to study the spin rates of small, near-Earth asteroids (NEAs). The animation above shows a simulated NEA, typical of the sort of shapes one fnds among asteroids that are about the size of shopping malls. These rocks are too far away to resolve their real shapes with telescopes, but we can observe their brightnesses change as they turn in the sunlight.

The distribution of asteroids in the diameter-period diagram is the basis for our understanding that large asteroids are not solid objects, but "rubble piles", bound by weak self-gravity.Nearly all asteroids with diameters > 150 meters spin more slowly than once in 2 hours, which corresponds to the dynamical timescale for an object with the expected density of porous rock.  This period is therefore regarded as the spin limit for rubble piles. An object made to spin faster would shed material from its equator, break apart, or reconfigure itself into a shape with larger moment of inertia and slower spin.

At diameters < 150 meters, a population of objects appears with substantially faster rotation rates. As these objects are spinning too fast to be gravitationally bound rubble piles, some workers leapt to the conclusion that they are chunks of solid rock and dubbed them  Monolithic Fast Rotating Asteroids (MFRAs).

Image of Period-Size Diagram
2006CL9 lightcurve However, the MFRAs need not necessarily be "M". They could still be rubble-piles if there is weak cohesion between the constituent particles or chunks. The distribution of objects in the transition region, between 150 meters and 1 km diameter, is a sensitive constraint on cohesive rubble-pile models.

We are working on determining the distribution of rotation rates of NEAs in this transition region by obtaining optical light curves at the 2.4 meter telescope at MDM.   An example of a light curve for a fast-rotating object is shown at left, where we have plotted the brightness of the asteroid as a function of time. 2006 CL9 has a nominal diameter of 90 meters (based on an assumed albedo of 0.17). The blue and yellow points represent observations on two different nights, and the data have been folded to the best-fit period of  8 minutes and 47 seconds.

To date (June 2007), we have observed about 2 dozen NEAs in, above, and below the transition region. Reduction and analysis of these data are about one-third completed. Some preliminary results are shown in context in the figure below. So far, we do not find a population of fast rotators in the transition region. (The light-blue squares in the figure are not ours, and, for reasons not worth detailing here, probably should not be believed.)

Our future goals include efforts to push our sensitivity to shorter periods, which is a technical challenge. The reradiation-recoil torque produced by the YORP effect is predicted to cause a  spin-up of some small NEAs, limited only by the eventual fissioning of, or material shedding by, the object. Determining the envelope of maximum spin rates will be a sensitive test of both the theoretical predictions of the YORP effect and the material strength of potential Earth impactors.

Current results in context

Two for the price of one?

Despite the emptiness of space, there are a lot of rocks out there. While observing the NEA 2007 DF8,  we unexpectedly caught the main-belt object 111242 in the same field. Here is a movie showing two asteroids at once, over the course of about 3 hours. 2007 DF8 has a probable diameter around 270 meters, and at the time of the observation it was 0.33 AU (31 million miles or 50 million km) from Earth.  111242 was 6 times farther away, and is a larger object, probably between 4 and 5 km in diameter. It's a matter of luck that the two happened to appear about the same brightness. Of course, all the stars and galaxies in the field are in the deep, deep background compared to these little Solar System objects.

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