P. Pravec et al., Two-period lightcurves of 1996 FG(3), 1998 PG, and (5407) 1992 AX: One probable and two possible binary asteroids, ICARUS, 146(1), 2000, pp. 190-203
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.