Two-period lightcurves of 1996 FG(3), 1998 PG, and (5407) 1992 AX: One probable and two possible binary asteroids

We present the results of photometric observations of 1996 FG(3), 1998 PG, and (5407) 1992 AX. Each asteroid has a complex lightcurve that can be desc ribed as the coaddition of two independent components, one with a short syn odic period P-1 of several hours, and another with a longer synodic period P-2. For 1996 FG(3), the shape of the long-period component strongly sugges ts a system of two bodies in bound orbits about their common center of mass , displaying both mutual eclipses and mutual occultations. We estimate the following parameters of the binary system: the primary's rotation period is P-1 = (3.5942 +/- 0.0002) h, the system's orbital period is P-2 = (16.135 +/- 0.005) h, the system's semimajor axis a = (1.7 +/- 0.3) d(p), the eccen tricity is 0.05 +/- 0.05, and the secondary-to-primary diameter ratio d(s)/ d(p) = 0.31 +/- 0.02. The measured color indices suggest that 1996 FG(3) be longs to the C taxonomic class; assuming the geometric albedo p = 0.06 typi cal for the class, we estimate the diameter of the primary d(p) = 1.4 km. F or 1998 PG and (5407), the evidence for binary systems is less conclusive. However, assuming a binary model, we estimate that both systems have a seco ndary-to-primary diameter ratio d(s)/d(p) greater than or equal to 0.3. For 1998 PG, the primary and secondary rotation periods are P-1 = (2.51620 +/- 0.00003) h and 2P(2) = (14.007 +/- 0.0014) h, respectively. (If the second ary's rotation is tidally coupled to the orbital motion, the latter is also the system's orbital period.) For (5407), the primary's rotation period is P-1 = (2.54878 +/- 0.00007) h, and the orbital period is 2P(2) = (13.5196 +/- 0.0016) h. The color indices of 1998 PG are consistent with an S classi fication. Assuming p = 0.16, the primary's diameters are d(p) = 0.9 and 4.0 km for 1998 PG and (5407), respectively. The finding of the three objects doubles the known sample of probable/possible binary asteroids. The six kno wn objects are so similar in some of their characteristics that we expect t hat most or all of them have been formed by the same mechanism, such as the tidal disruptions during close encounters with Earth proposed by Bottke an d Melosh (Nature 281, 51, 1996), The population of binary asteroids among s mall, especially near-Earth, asteroids seems to be significant. Our new obs ervations support an earlier estimate that 17% of NEAs are binary (uncertai nty factor of 2). (C) 2000 Academic Press.