Siltala, Lauri
(Helsingin yliopisto, 2021)
The mass of an asteroid is considered one of its fundamental properties. Knowledge of an asteroid's mass is, by itself, useful for spacecraft navigation particularly for space missions planned to the asteroid in question. The gravity of massive asteroids causes small yet measurable perturbations on the orbits of the Solar System's planets such as Earth and Mars and thus knowledge of asteroid masses also contributes to the development of accurate planetary ephemerides.
However, the mass of an asteroid gives us little scientifically interesting information on the asteroid by itself. Instead, the main scientific motivation for asteroid mass estimation is that the mass, alongside the volume, is one of the two critical parameters required to compute the asteroid's bulk density: when both parameters are known, the bulk density can be trivially computed with a simple division operation. The bulk density, in turn, may be combined with other compositional information of the asteroid, obtained mainly through spectroscopic observations of the asteroid in question, and compared to spectra, compositions, and densities of similar meteorites found on Earth. Such studies, in turn, allow for constraining the bulk composition and macroporosity, and, by extension, the structure of the asteroid. Thus, it is clear that knowledge of an asteroid's mass is critical for all detailed studies of the characteristics of the asteroid's interior. Besides scientific interest, such studies may also have future practical applications for characterizing potential targets for asteroid mining and for planning asteroid deflection in the event of an impact threat.
Asteroid mass estimation is traditionally performed by analyzing an asteroid's
gravitational interaction with another object, such as a spacecraft, Mars and/or Earth, or a separate asteroid during an asteroid-asteroid close encounter, or, in the case of binary asteroids, the orbits of the component asteroids. Recently, an alternative approach of direct density estimation through detection and modeling of radiative forces, particularly the Yarkovsky effect, has also begun to see use.
This thesis deals with asteroid mass estimation based on asteroid-asteroid close encounters. It begins with a general overview of the asteroids followed by a more detailed discussion on the different approaches for the estimation of asteroid masses and densities. Next, I describe our novel application of Markov-chain Monte Carlo techniques to the mass estimation problem in greater detail. To demonstrate the progress achieved with each consecutive paper, I highlight mass estimates for the asteroid (52) Europa beginning with results from my Master's thesis obtained with the initial version of the MCMC algorithm, followed by updated results from the first, third and finally the fifth and final paper included in this thesis. Clear improvements are seen throughout; in particular, the usage of astrometry from the second Gaia data release in Paper V leads to a significant order-of-magnitude reduction of the uncertainty of the mass. Finally, I briefly discuss future prospects, particularly in regards to the forthcoming third Gaia data release.