A satellite orbit drift in binary near-Earth asteroids (66391) 1999 KW4 and (88710) 2001 SL9 -- Indication of the BYORP effect

We obtained thorough photometric observations of two binary near-Earth asteroids (66391) Moshup = 1999 KW4 and (88710) 2001 SL9 taken from 2000 to 2019 and derived physical and dynamical properties of the binary systems. We found that the data for 1999 KW4 are inconsistent with a constant orbital period and we obtained unique solution with a quadratic drift of the mean anomaly of the satellite of -0.65 +/- 0.16 deg/yr2 (all quoted uncertainties are 3sigma). This means that the semimajor axis of the mutual orbit of the components of this binary system increases in time with a mean rate of 1.2 +/- 0.3 cm/yr. The data for 2001 SL9 are also inconsistent with a constant orbital period and we obtained two solutions for the quadratic drift of the mean anomaly: 2.8 +/- 0.2 and 5.2 +/- 0.2 deg/yr2, implying that the semimajor axis of the mutual orbit of the components decreases in time with a mean rate of -2.8 +/- 0.2 or -5.1 +/- 0.2 cm/yr for the two solutions, respectively. The expanding orbit of 1999 KW4 may be explained by mutual tides interplaying with binary YORP (BYORP) effect (McMahon and Scheeres, 2010). However, a modeling of the BYORP drift using radar-derived shapes of the binary components predicted a much higher value of the orbital drift than the observed one. It suggests that either the radar-derived shape model of the secondary is inadequate for computing the BYORP effect, or the present theory of BYORP overestimates it. It is possible that the BYORP coefficient has instead an opposite sign than predicted; in that case, the system may be moving into an equilibrium between the BYORP and the tides. In the case of 2001 SL9, the BYORP effect is the only known physical mechanism that can cause the inward drift of its mutual orbit.