THE NUCLEUS OF COMET-P ENCKE

We present charge-coupled device (CCD) images and spectra of P/Encke taken near aphelion in August and September 1988. The CCD photometry shows the brightness of the comet varying in a cyclic fashion about a mean apparent red magnitude mR - 19.8, with a photometric range Δm = 0.62 ± 0.04 mag. The periods T0 = 15.08 ± 0.08 hr, T0/2, 3T0/2 are consistent with the photometry; we argue that the nucleus synodic rotation period is equal to T0. The cyclic variations, the conformation of the photometry to the inverse square law, and the absence of resolvable coma are all consistent with the detection of an aspherical rotating nucleus. CCD spectra of the nucleus reveal a reddened continuum with a reflectivity gradient S′ = 11 ± 2%/103 A ̊. The compatibility of the observations with the published pole directions of P/Encke is discussed. © 1990.