Understanding the Influence of Non-gravitational Forces on the Physical Evolution of Near-Earth Asteroids and Comets

Near-Earth asteroids are the small rocky bodies orbiting the Sun in the vicinity of Earth's orbit. They are remnants of the planetesimals formed in the young Solar System, which repeatedly collided and underwent disruption. They form loosely-bound aggregates dubbed 'rubble piles'. Their dynamical and physical evolution is expected to be affected by a nongravitational torque called the YORP effect.

The YORP effect is a torque due to the anisotropic emission of thermal photons on minor bodies in the Solar System. For small asteroids the radiation recoil torques can systematically modify rotational rates or shift spin axis orientations (Rubincam, 2000). The effect is crucial to understanding the dynamical and physical evolution of near-Earth asteroids, like the alignment of spin-axes (Slivan, 2002), the peculiar spin-top shapes observed for a few targets (Ostro et al., 2006; Scheeres et al., 2006), or rotational fission and evolution of asteroid binaries (Walsh et al., 2008; Pravec et al., 2010; Jacobson and Scheeres, 2011; Jacobson et al., 2016). The first direct detection of the asteroidal YORP effect on asteroid (54509) 2000 PH5 was possible thanks to the combination of radar and photometric lightcurve observations (Lowry et al., 2007; Taylor et al., 2007). Since then, YORP spin-up has also been detected on several other asteroids. However, the sample is still very small, and further observational data is needed to refine the YORP theories.

The asteroids (1917) Cuyo and (85990) 1999 JV6, discussed here, were selected from a sample of nearly 40 YORP-detection candidates that were monitored photometrically, and in infra-red, through an ESO Large Programme (ESO LP) led by S. C. Lowry at the ESO New Technology Telescope and Very Large Telescope telescopes, and at other facilities with associated programmes.

The ESO LP has been used to acquire photometric lightcurves of the asteroid (1917) Cuyo spanning the period between 2010 and 2013, which, combined with the 1989-2008 archive lightcurves, should provide a large enough time-base to constrain a possible YORP strength. However, the distribution of observations in time results in effectively having observations from just two epochs. This produces potential YORP values in the range of ?0.7 × 10?8 rad/d² (radians per day squared) and 1.5 × 10?8 rad/d². The rotation pole of the object is most likely located at l = 46°, b = -62°. The sidereal period was refined relative to earlier lightcurve estimates, to be (2.6897642 ± 0.0000035) h (hours). The shape of the object suggests the presence of an 'equatorial bulge', typical for an evolved system close to shedding mass due to fast rotation.

For asteroid (85990) 1999 JV6, the data in the ESO LP span the period between 2007 and 2016. Additionally, the author has secured radar spectra and imaging observations with Arecibo and Goldstone planetary radars. Having radar observations permitted additional constraints on the shape and spin-state, but YORP spin-up was not detected. The asteroid is shown to have a bi-lobed shape, likely a result of two ellipsoidal components collapsing onto each other. The smaller lobe is close to spherical and has diameters (345 ± 9) m, (281 ± 8) m and (291 ± 9)m, and the larger is more elongated, with (580 ± 10) m, (322 ± 5) m and (332 ± 7) m. The rotation pole resides at negative latitudes in a circle of a 10° radius, close to the southern pole of the celestial sphere. The refined sidereal rotation period is (6.536787 ± 0.000006) h. No YORP-induced change in period was detected using the phase offset measurement using the radar model, however the global lightcurve-only analysis shows the object could be experiencing a spin-up of up to 7 × 10?8 rad/d².

The shapes and spin-states developed here were used in further studies, beyond the scope of this thesis. Combined with the infra red observations the outcome of this work was used for thermophysical analysis by ESO LP collaborator B. Rozitis to constrain physical properties of both targets. The shape and rotation state of (1917) Cuyo can be used to investigate cohesive forces as a way to explain why some targets survive rotation rates faster than the fission limit. The detection of non-gravitational acceleration in the orbital motion of the asteroid (85990) 1999 JV6 combined with thermophysical modelling suggest a low, cometary-like density.

The shape modelling and spin-state analysis tools were also applied to a Jupiter family comet, and the Rosetta mission target, 67P/Churyumov Gerasimenko. The author contributed to the confirmation of the seminal measurement of spin-rate change between previous perihelion approach and the arrival of Rosetta (Mottola et al., 2014, incl. A. Rozek). The detected 20 min decrease in the sidereal period, from ?12.7 h to ?12.4 h, was later linked to cometary activity (Keller et al., 2015b; Bertaux, 2015). Tools were also developed to assess the mean insolation of the comet’s surface, useful in calculations of nucleus dust production rates (Guilbert-Lepoutre et al., 2014, incl. A. Rozek), establish jet-activity source regions on the surface of the nucleus (Lara et al., 2015; Lin et al., 2015, 2016, incl. A. Rozek), and calibrate ground-based photometry using the spacecraft shape model (Snodgrass et al., 2016, incl. A. Rozek).