Photometric Studies Of Near-Earth Asteroids

Near-Earth asteroids?NEAs?are a class of small bodies in the solar system with orbits of a perihelion distance less than 1.3 AU and an aphelion distance more than 0.983 AU.They are subdivided into Apollos,Atens and Amors according to their orbits.The study of physical parameters of near-Earth asteroids can provide basic information for predicting risk of impact events and making reply strategies for the NEAs'catastrophic collisions.Also,near-Earth asteroids are the optimal targets for space exploration projects.Therefore,the study of NEAs'physical parameters is of great significance.Although the success of a series of space projects has greatly enhanced our understand-ing of near-Earth asteroids,ground-based equipments will continue to play the protagonist of near-Earth asteroid research.Among many ground-based observational ways?such as occul-tation,photometry,spectrometry,radar echo,etc.?,photometry is the main source to obtain asteroid's physical characteristics.The main content of this thesis is to study the shapes and rotation parameters of near-Earth asteroids using photometric data,which are obtained by a joint international observation project with 45 cm,1.0 m and 2.4 m telescopes of the Yun-nan Observatories and the 0.6 m,0.91 m and 0.94 m telescopes of the SARA?Southeastern Association for Research in Astronomy?.As for the inversion method,we used the Lommel-Seeliger ellipsoid method(Muinonen et al.?2015?^[1])or the convex method(Kaasalainen et al.?2001?^[2,3]).In order to have a more intuitive understanding for the inversion model,we tested on two main-belt asteroids??585?Bilkis and?103?Hera?with a large amount of data.From 2012 to2014,we performed 7 nights photometric observations on?585?Bilkis.Based on this data,we used the Lommel-Seeliger ellipsoid model^[1]to invert the shape of the asteroid?585?Bilkis and its spin parameters.From 2015 to 2017,we performed 11 nights photometric observations on?103?Hera.Then we updated the convex modeling results of this asteroid,and the two sets of solutions?including the mirror solution?have a rotation period of 23.74264 hours and23.74267 hours,respectively,and the corresponding orientation of spin are?83.0^?,39.0^??and?269.7^?,56.8^??.For comparison,we also did lightcurve inversion with the Lommel-Seeliger ellipsoid modeling for?103?Hera,the spin parameters is consistent with that derived by convex modeling considering the uncertainties.In addition,we fit the phase function of?103?Hera,the absolute magnitude of this target is H=8.92,the phase function parameters are G₁=0.13,G₂=0.45.Next,we studied the shapes and rotation parameters of several near-Earth asteroid targets??3122?Florence,?3200?Phaethon,?143404?2003 BD44,?333888?1998 ST4,and?1627?Ivar?.We conducted one night of observation on the near-Earth asteroid?3122?Florence in February 2016.The lightcurves of?3122?Florence are analysed with the Lommel-Seeliger method and we derived a spin period of 2.358 hours,a pole of?181.1^?,-50.0^??and the triaxial ellipsoid axis ratio b/a=0.91,c/a=0.88.Following?3122?Florence,we did two nights of observation on the near-Earth asteroid?3200?Phaethon on October 18 and 19,2016.We added new data to the work of Hanu?s et al.?2016?^[4]and re-performed the convex modeling for?3200?Phaethon.Our results is more supportive of Ansdell et al.?2014?^[5].The near-Earth asteroid?143404?2003 BD44 was observed on March 22 and 23,2017.The photometric data of?143404?2003 BD44 show that it is an asteroid of a long rotation period.The near-Earth asteroid?333888?1998 ST4 was observed on October 29,2017.From the photometric data,this target has a short rotation period?about 5.3 hours?and a regular shape.So we used the Lommel-Seeliger ellipsoid model^[1]to inverse its shape and spin parameters.Observations of the near-earth asteroid?1627?Ivar are the result of international cooperation.We conducted one night of observation on this target on May 22,2018.After adding new data,we performed convex modeling on?1627?Ivar and the results is consistent with previous related studies.In addition,we used the data from US Naval Observatory and Catalina's survey to fit the phase function of this near-Earth asteroid and the obtained phase function parameters are G₁=0.49,G₂=0.30.