Asteroid properties from SuperWASP light curves

The SuperWASP project, primarily an ultra-wide angle search for extra-solar planetary transits, has serendipitously observed bright asteroids for over 10 years as they pass through the survey fields. The multiple wide fields (Northern and southern hemisphere sites, each with 8 cameras with 7.8 x 7.8 fields), with observations every minute for several hours per night over several weeks at a time, provide an ideal combination to obtain rotational light curves of asteroids brighter than magnitude 15. Software tools to identify asteroid detections and perform photometry have already been developed [1]. Data from limited observations in 2004 and 2006 alone have provided 570 light curves for 270 asteroids, plus phase curves and absolute magnitudes for 123 asteroids, 93 of which had no previous values [2].
This project will involve completion of the data reduction (from 2010 onwards) and exploitation of this massive, unbiased, magnitude-limited asteroid light curve dataset through one or more of the following:
- Determination of spin pole orientations: The wide range of viewing geometries over the 10 year timespan allows simultaneous extraction of spin period, shape and spin pole orientation [3,4]. While periods are generally well-characterised for bright asteroids, the dataset will provide a several-fold increase in spin orientations, allowing investigation of size-dependent evolutionary effects.
- Identification of binaries: The wide range of viewing geometries also maximises the possibility of observing eclipses of binary asteroids [review, 5], from which asteroid densities can be derived. We will undertake a feasibility study to determine if binaries can be detected using the SuperWASP dataset.
- An alternative technique for density determination is the use of equilibrium shapes [6]. The method appears to work well for large objects with large light curve amplitudes (the objects best recorded in SuperWASP data), perhaps because impact jostling of rubble pile objects temporarily removes pressure-induced shear strength.
- Detection of activity: In rare cases, asteroids have exhibited one-off or sporadic brightening events, due to collisions [e.g. 7] or transient outgassing. The large temporal coverage will allow a search for anomalous brightening.
- Phase curves (brightness variation as a function of solar phase angle) are related in a complex way to surface scattering properties. Fitting with the new IAU H-G1-G2 system [8] requires good phase coverage, particularly near zero phase angle. Unlike many published phase curves, the continuous, long term coverage provided by SuperWASP fulfils this criterion. The significant increase in reliable phase curves will be used to investigate possible correlations with spectral type [review in 9], albedo, size or spin.
References:
1) Parley N et al. (2005) Serendipitous asteroid lightcurve survey using SuperWASP. Earth, Moon & Planets, 97, 261-268.
2) Parley N (2008) Serendipitous surveys of the Solar System using SuperWASP. PhD Thesis, The Open University.
3) Kaasalainen M. & Torppa J. (2001) Optimisation methods for asteroid lightcurve inversion I: Shape determination. Icarus 153, 24-36.
4) Kaasalainen M, Torppa J & Piironen J (2001) Optimisation methods for asteroid lightcurve inversion II. The complete inverse problem. Icarus 153, 37-51.
5) Margot J-L et al. (2016) Asteroid systems: Binaries, triples and pairs. In Asteroids IV, Univ. Arizona Press, p355-373.
6) Chandrasekhar S (1963) The Equilibrium and the stability of the Roche ellipsoids. Ap. J. 138, 1182-1213.
7) Snodgrass C et al. (2010) A collision in 2009 as the origin of the debris trail of asteroid P/2010 A2. Nature, 467, 814-816
8) Muinonen K. et al., (2010) A three-parameter magnitude phase function for asteroids. Icarus 209, 542-555.
9) Li et al. (2016) Asteroid photometry. In Asteroids IV, Univ. Arizona Press, p129-150.