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J. Torppa (University of Helsinki), K. Aksnes, Z. Dai (University of Oslo), T. Grav (University of Hawaii), G. Hahn (Deutschen Zentrum fur Luft- und Raumfahrt (DLR)), T. Laakso (University of Helsinki), C.-I. Lagerkvist (University of Uppsala), K. Muinonen, J Niemelaa, J. Naaraanen (University of Helsinki), H. Rickman (University of Uppsala), J. Virtanen (University of Helsinki)
Asteroids whose orbits lie close to that of the Earth (called near-Earth objects) are a threat to our planet. Though none of the known asteroids are currently on a collision course with the Earth, there is a class called potentially hazardous asteroids (PHA's) whose orbits may in a few tens of years be perturbed to make a collision possible. In such case, we should have thorough knowledge of the physical properties of these objects because, unlike most other natural disasters, such a hazard is in principle avoidable. Thus in the fall of 2003, a group of asteroid and comet researchers from the Nordic countries have set started an observing program called Nordic NEON (Nordic Near-Earth Object Network). Both photometric and astrometric observations of PHA's have been carried out since April 2004. Photometry is used for determining the spin state and shape of PHA's and astrometry for computing more accurate orbits. In this DPS meeting we will concentrate on the photometric part of the program.
We use methods developed in the University of Helsinki for analysing both kinds of data. Spin state and convex shape solution are obtained using the convex inversion method described in [1] and information of the nonconvex shape features are obtained with the spherical harmonics method (Muinonen and Torppa, in preparation). For initial period estimation only a few nights of observations is usually enough, while pole and shape determination requires more abundant data. Previously observed photometric data exists for about 35 PHA's, but only an approximate period is known for most of these. Properties of well over 500 PHA's are totally unknown. So far we have obtained period estimates for 4 new PHA's, and improved spin and shape solution for 3 targets.
A number of foundations and universities from all the participating countries have taken part in funding this project.
[1] M. Kaasalainen, J. Torppa and K. Muinonen, Icarus 153 (2001) 37.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.