Photometry and model of near-Earth asteroid 2021 DW1 from one apparition

Aims. Very small asteroids (VSAs, objects with diameters smaller than about 150 m) can be spun up by the YORP effect to rotation periods as short as tens of seconds. This effect has been observed for many of them. It is also hypothesised, that in the same process their spin axes are asymptotically drawn to the position perpendicular to the orbital plane. So far this effect has been observed only for one VSA and needs further verification. For that, spin axes of several other VSAs should be determined by observing their brightness variations at many different positions on the sky. Methods. On 4 March 2021 at 9 UTC a 30-m in diameter near-Earth asteroid 2021 DW1 passed the Earth at a distance of 570 000 km, reaching the maximum brightness of V = 14.6 mag. We observed it photometrically from 2 March, when it was visible at V = 16.5 mag, until 7 March (V = 18.2 mag). During that time 2021 DW1 swept a 170 degrees long arc in the northern sky, spanning solar phase angles in the range from 36 degrees to 86 degrees. This made it an excellent target for physical characterisation, including spin axis and shape derivation. Results. Convex inversion of the asteroid lightcurves gives a sidereal period of rotation P-sid = 0.013760 +/- 0.000001 h, and two solutions for the spin axis ecliptic coordinates: (A) lambda(1) = 57 degrees +/- 10 degrees, beta(1) = 29 degrees +/- 10 degrees and (B) lambda(2) = 67 degrees +/- 10 degrees; beta(2) = 40 degrees +/- 10 degrees. The magnitude-phase curve can be fitted with a standard H, G function with H = 24.8 +/- 0.5 mag and an assumed G = 0.24. The asteroid colour indices are g - i = 0.79 +/- 0.01 mag, and i - z = 0.01 +/- 0.02 mag which indicates an S taxonomic class, with an average geometric albedo p(V) = 0.23 +/- 0.02. The asteroid effective diameter, derived from H and p(V), is D-eff = 30 +/- 10m. Conclusions. It was found that the inclination of the spin axis of 2021 DW1 is not perpendicular to the orbital plane (obliquity epsilon = 54 degrees +/- 10 degrees or epsilon = 123 degrees +/- 10 degrees). More spin axes of VSAs should be determined to check, if 2021 DW1 is an exception or a typical case.