New worlds in our solar system

Asteroids and comets form the largest and, perhaps paradoxically, the least well known population of celestial bodies in our solar system. The shortage of detailed information is mainly due to the fact that, because of the large interplanetary distances, disk-resolved images can be obtained only of a limited number of these targets. Nevertheless, our view of this population has started to change dramatically. Spacecraft images and radar observations, though few and far between, have already revealed to us that asteroids come in just about all possible shapes, configurations, and structures.The need for a large number of detailed asteroid models is now pressing, so all possible data sources are valuable. A hitherto much unused but major and easily available source of information on small solar system bodies consists of their photometric lightcurves, i.e., measurements of their total brightnesses that vary as the viewing/illumination geometry changes. The inverse problem of determining the object's shape, its rotational state, and the scattering properties of its surface from lightcurves is notoriously demanding.

We have shown that advanced methods can produce
detailed asteroid models that are even
comparable to those obtained with radar imaging.
Our results also match the `ground truth'
from the few space probe fly-by missions very well, so the inversion
method has been well tested. Indeed, we can now well say that
**the resolving capacity of lightcurve inversion lies between
space telescope and radar, and its
range extends from near-Earth to main-belt asteroids**. Our approach has
also opened up a whole new possibility of including
complementary data in multidatainversion
from various sources that would otherwise
be insufficient alone: combined with photometry,
these will allow the building of increasingly detailed models.

Besides their obvious part in completing the picture of our solar system at present, asteroids and comets are also important sources of cosmogonical information. Their rotation states and physical structure offer direct views of the evolution and primordial stages of the solar system.

*Several modelling results and software are available at our
asteroid model site. Software is also available at Brian Warner's
LCInvert site.*

**Some references:**

**M. Kaasalainen, L. Lamberg, K. Lumme and E. Bowell (1992)**:
Interpretation of lightcurves of atmosphereless bodies. I.
General theory and new inversion schemes.
Astron. Astrophys. 259, 318.

**L. Lamberg and M. Kaasalainen (2001)**:
Numerical solution of the Minkowski problem.
J. Comp. Appl. Math. 137, 213.

**M. Kaasalainen and J. Torppa (2001)**:
Optimization methods for asteroid lightcurve inversion. I.
Shape determination.
Icarus 153, 24.

**M. Kaasalainen, J. Torppa, and K. Muinonen (2001)**:
Optimization methods for asteroid lightcurve inversion. II.
The complete inverse problem.
Icarus 153, 37.

**M. Kaasalainen (2001)**:
Interpretation of lightcurves of precessing asteroids.
Astron. Astrophys. 376, 302.

**M. Kaasalainen, J. Torppa, and J. Piironen (2002)**:
Binary structures among large asteroids.
Astron. Astrophys. 383, L19.

**M. Kaasalainen, J. Torppa, and J. Piironen (2002)**:
Models of twenty asteroids from photometric data.
Icarus 159, 369.

**M. Kaasalainen, S. Mottola and M. Fulchignoni (2002)**:
Asteroid models from disk-integrated data.
In "Asteroids III", eds. W. Bottke, R. Binzel, P. Paolicchi, and
A. Cellino, Space Science Series, Univ. of Arizona Press,
Tucson, USA, pp. 139.

**S. Slivan, R. Binzel, L. da Silva, M. Kaasalainen, M. Lyndaker,
and M. Krco (2003)**:
Spin vectors in the Koronis family:
Comprehensive results
from two independent analyses of 213 rotation lightcurves.
Icarus 162, 285.

**J. Torppa, M. Kaasalainen, T. Michalowski, T. Kwiatkowski, A. Kryszczynska,
P. Denchev, and R. Kowalski (2003)**:
Shapes and rotational properties of thirty asteroids from
photometric data. Icarus 164, 346.

**J. Durech and M. Kaasalainen (2003)**:
Photometric signatures of highly nonconvex and binary
asteroids. Astron. Astrophys. 404, 709.

**M. Kaasalainen, T. Kwiatkowski, M. Abe and 14 colleagues (2003)**:
CCD photometry and model of MUSES-C target (25143) 1998 SF36. Astron. Astrophys. 405, L29.

**M. Kaasalainen (2003)**:
Unveiling asteroids:
international observing project and amateur-professional connection.
J. Roy. Ast. Soc. Can. 97, 283 (inv. rev.)

**M. Kaasalainen, P. Pravec, Yu. Krugly and 19 colleagues (2004)**:
Photometry and models of eight near-Earth asteroids. Icarus 167, 178.

**D. Vokrouhlicky, D. Capek, M. Kaasalainen and S.J. Ostro (2004)**:
Detectability of YORP rotational slowing of
asteroid 25143 Itokawa. Astron. Astrophys. 414, L21.

**M. Kaasalainen and P. Tanga (2004)**:
Photocentre offset
in ultraprecise astrometry: implications to barycentre
determination and asteroid modelling. Astron. Astrophys. 416, 367.

**T. Michalowski, T. Kwiatkowski, M. Kaasalainen and 8 colleagues (2004)**:
Photometry and models of selected main-belt asteroids (I): 52 Europa,
115 Thyra, and 382 Dodona. Astron. Astrophys. 416, 353.

**M. Kaasalainen (2004)**:
Physical models of large number of asteroids from calibrated photometry sparse in time. Astron. Astrophys. 422, L39.

**M. Kaasalainen, D. Hestroffer and P. Tanga (2005)**:
Physical models and refined orbits for asteroids from
Gaia photo- and astrometry. In "The Three Dimensional Universe with Gaia",
ESA (SP-576), 301.

**A. Nathues, S. Mottola, M. Kaasalainen and G. Neukum (2005)**:
Spectral study of the Eunomia family. I. Eunomia.
Icarus 175, 452.

**S. Kaasalainen, M. Kaasalainen and J. Piironen (2005)**:
Ground reference for space remote sensing: Laboratory photometry of an asteroid model.
Astron. Astrophys. 440, 1177.

**T. Michalowski, M. Kaasalainen and 9 colleagues (2005)**:
Photometry and models of selected main-belt asteroids (II): 173 Ino,
376 Geometria, and 451 Patientia. Astron. Astrophys. 443, 329.

**T. Mueller, T. Sekiguchi, M. Kaasalainen, M. Abe and S. Hasekawa (2005)**:
Thermal infrared observations of the Hayabusa
spacecraft target asteroid 25143 Itokawa.
Astron. Astrophys. 443, 347.

**M. Kaasalainen and L. Lamberg (2006)**:
Inverse problems of generalized projection operators. Inverse Problems 22,
749.

**J. Durech, T. Grav, R. Jedicke, M. Kaasalainen and L. Denneau (2006)**:
Asteroid models from Pan-STARRS photometry. Earth, Moon and Planets 97, 179.

**T. Michalowski, M. Kaasalainen and 5 colleagues (2006)**:
Photometry and models of selected main-belt asteroids (III): 283 Emma,
665 Sabine, and 690 Wratislavia. Astron. Astrophys. 459, 663.

**F. Marchis, M. Kaasalainen and 6 colleagues (2006)**:
Shape, size and multiplicity of main-belt asteroids:
I. Keck adaptive optics. Icarus 185, 39.

**M. Kaasalainen and J. Durech (2007)**:
Inverse problems of NEO photometry: Imaging the NEO
population. In IAU Symposium 236, eds. A. Milani, G. Valsecchi and
D. Vokrouhlicky, 151.

**J. Durech, P. Scheirich, M. Kaasalainen, T. Grav, R. Jedicke and
L. Denneau (2007)**:
Physical models of asteroids from sparse
photometric data. In IAU Symposium 236, eds. A. Milani, G. Valsecchi and
D. Vokrouhlicky, 191.

**J. Durech, M. Kaasalainen, A. Marciniak, and 39 colleagues (2007)**:
Physical models of ten asteroids from an observers'
collaboration network. Astron. Astrophys. 465, 331.

**M. Kaasalainen, J. Durech, B. Warner, Y. Krugly and N. Gaftonyuk (2007)**:
Acceleration of the rotation of asteroid 1862 Apollo
by radiation torques. Nature 446, 420.

[see also:
Supplementary Information and News and Views by Bill Bottke]

**J. Durech, D. Vokrouhlicky, M. Kaasalainen, and 16 colleagues (2008)**:
New photometric observations of asteroids (1862)
Apollo and (25143) Itokawa -- an analysis of YORP effect. Astron.
Astrophys. 488, 345.

**J. Durech, D. Vokrouhlicky, M. Kaasalainen, and 9 colleagues (2008)**:
Detection of the YORP effect in asteroid (1620)
Geographos. Astron. Astrophys. 489, L25.

**J. Durech, M. Kaasalainen, B. Warner, and 8 colleagues (2009)**:
Asteroid models from combined sparse and dense
photometric data. Astron. Astrophys., 493, 291.