Planetary science

In planetary system research, our focus is on asteroids and comets as well as on small particles in planetary regoliths and atmospheres, including the atmosphere of the Earth. In 2015, our activity has culminated in the peer-reviewed publication of much of the research initially reported at "Asteroids, Comets, Meteors 2014" organized by us in Helsinki on June 30 - July 4, 2014 (http://www.helsinki.fi/acm2014/).

We have developed algorithms for asteroid lightcurve and surface material analyses in an ESA-funded project, together with Space Systems Finland Ltd. The algorithms are to be deployed in an open computing environment called Gaia Added Value Interface for the Gaia space mission data.  The lightcurve analysis employs the recent advances, on one hand, in the analytical Lommel-Seeliger ellipsoid brightness integration, and, on the other hand, in the development of Markov-chain Monte Carlo methods in inverting the period, pole, and shape of an asteroid. The material analysis can be used to suggest a Bus-DeMeo taxonomic classification for an asteroid from spectral observations by Gaia, and to invert the observed spectra in terms of an asteroid's surface-material optical constants.

A joint proposal (ASPECT, Asteroid Spectrometer) by us, VTT, and Aalto University for ESA's AIM Cubesat Opportunity Payload (COPINS) was selected for Phase A study by ESA (among four proposals in total). The ESA AIM mission is part of the AIDA mission plan that also includes the NASA counterpart DART. The two spacecraft would encounter the Didymos binary asteroid system in 2022, and the DART spacecraft would hit the secondary body of the system. The Cubesat that we are proposing in the study would be transported by the AIM spacecraft, and once deployed close to the system, would observe with a VTT-designed spectral camera. Our responsibility is the scientific planning of the spectral observations.

    Solar superstorm

The AIM and CubeSats watch the impact. Courtesy of ESA.

Laboratory experiments and their analysis were conducted for the so-called space-weathering effects on iron-bearing silicate minerals typical in asteroid surface regolith. Space weathering is altering the mineral composition by creating nano- or micron-sized Fe inclusions in the shallow rim in the top layer of the regolith grains. This will, in turn, affect the spectral properties observed by these minerals. The Siris code developed here was used to model the spectral effects, and the results were disseminated in international conferences and partly discussed in the M.Sc. thesis by Julia Martikainen and in the B.Sc. thesis by Timo Väisänen

Studies on electromagnetic scattering by small particles and random media composed of such particles have continued with funding from the ERC Advanced Grant project entitled "Scattering and absorption of electromagnetic waves in particulate media" (SAEMPL) and the Academy of Finland Consortium project entitled "Electromagnetic Wave Scattering in Complex random Media".

The primary goal of the SAEMPL project is theoretical, that is, the development of an unprecedented numerical method for multiple scattering by close-packed media of small particles. For the validation of the method, the SAEMPL project includes an experimental component including an ultrasonic levitator, developed and constructed together with the Electronics Research Laboratory led by Prof. Edward Haeggström.

In light scattering, we have assessed the scattering evolution from particles to regolith using our radiative-transfer coherent-backscattering method.  We have studied scattering by large Gaussian random particles using the combined invariant-imbedding T-matrix and geometric-optics approach. We have unveiled the polarization mechanism responsible for the resonance-band structure in scattering by clusters of spherical particles and published the result in Optics Letters, one of the topmost optics journals. We have further studied the radar scattering by boulders using geometric optics.

For comets, we have offered a state-of-the-art inhomogeneous particle model with exact computations using the volume-integral-equation method. For dark asteroids, we have provided an efficient numerical model that can be utilized in scientific analyses of massive amounts of imaging data. We have also utilized the model in validating lightcurve inversion methods.

As to ESA’s astrometry mission Gaia (launch in December 20, 2013), our software for statistical inversion of asteroid orbits is ready in the Gaia Solar-System-Object Short-Term processing pipeline and we expect the daily processing to start in spring 2016.  We continue our involvement in the BepiColombo mission to planet Mercury and we also assess the risk of near-Earth asteroid collisions and participate in the Canadian NEOSSat mission.

We utilize state-of-the-art sky surveys such as the Panoramic Survey Telescope And Rapid Response System and the Catalina Sky Survey to understand population-level properties of main-belt and near-Earth objects. These efforts have led to, e.g., a new model describing the sizes and orbits of near-Earth objects and the realization that asteroids undergo a non-trivial disruption when approaching the Sun. This latter result indicates that models describing population statistics for asteroids can be used to constrain their physical properties.

Finally, we have contributed to Asteroids IV, the next milestone Space Science Series book on the state of the art in asteroid research, as well as to Polarimetry of Stars and Planetary Systems, a Cambridge University Press book collecting the present knowledge of stellar and planetary polarimetry.