Physical characterisation of near-Earth asteroids in search of the YORP effect

The Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect is a thermal recoil force that is experienced by small bodies when they re-emit energy absorbed from sunlight. Due to the complex shapes and surfaces of asteroids, this re-emission is anisotropic, causing a torque that can modify an asteroid's rotation rate and spin-axis orientation. This effect, which is loosely analogous to a windmill, has a profound influence on the evolution of the Solar System.

In order to better understand the YORP effect, it must be measured for a large number of asteroids. Almost 25 years since it was first predicted, the YORP effect has been observationally detected with twelve asteroids. Measuring the YORP effect requires that an asteroid is monitored over a significant period of time, and the asteroid must be well-characterised before it can be assessed for YORP. This means that the shapes, spin rates, and spin-axis orientations of asteroids must be measured, often requiring the use of both optical and radar observations.

This thesis presents the results of a project to characterise four asteroids and search for evidence of the YORP effect acting upon them. This work forms part of a larger research collaboration, originating with a European Southern Observatory (ESO) Large Programme (LP) to monitor 40 near-Earth asteroids. The asteroids considered in this work are (23187) 2000 PN9, (29075) 1950 DA, (85275) 1994 LY, and (159402) 1999 AP10. The results presented here include an independently-developed shape model for each of these asteroids, and a comprehensive search for signs of YORP-driven rotational acceleration.

For (23187) 2000 PN9, a high-resolution shape model was developed using a combination of optical lightcurves and planetary radar observations. A spin-state analysis shows that the asteroid is in a state of constant-period rotation, with any undetected YORP acceleration being significantly constrained. The asteroid is found to be YORP-evolved, and bears a striking resemblance to other asteroids with 'YORPoid' morphologies.

Following a previously reported indication of YORP acting upon (29075) 1950 DA, this work includes an extensive campaign to follow up and confirm a detection of rapid rotational deceleration. The previously reported measurement is verified and constrained by this work, using both previously published and newly developed shape models with a mix of old and new datasets. Two possible explanations are offered for the peculiar result: either there is a previously overlooked flaw in the tools and methods used to characterise asteroids, or this is the first detection of a hitherto unknown physical phenomenon.

Finally, analyses of (85275) 1994 LY and (159402) 1999 AP10 fail to detect YORP, but the asteroids are characterised and constraints are placed on any potential YORP acceleration. 1994 LY provides a good example of YORP false positives, and the results of this work will be beneficial to future studies of the asteroid system. The model of 1999 AP10 developed for this work has already contributed to significant results, and proves the viability of developing physical models using small optical telescopes over relatively short timescales.

Like many theses, this work ultimately raises more questions than it answers. The results for 2000 PN9 are conclusive, while an exhaustive analysis of 1950 DA has only deepened the mystery surrounding its rapid spin-down. 1994 LY and 1999 AP10 have been analysed as far as is sensible with existing data, but are shown to be worthy of further study when they are next observable.