Pluto's lightcurve: Results from four oppositions

The rotational lightcurve, phase function, and orbital lightcurve of the Pluto-Charon system were measured in blue light over four consecutive oppositions spanning 1980 to 1983. Compared with observations made in the three previous decades, our lightcurve shows a higher amplitude of 0.29 mag, as well as a fainter rotationally averaged brightness, which provide constraints on the surface albedo distribution of the two bodies. The synodic rotational period of 6.38726 +/- 0.00007 days is consistent with the orbital period of Charon, which provides evidence for a completely tidally evolved system. The phase coefficient is 0.0372 +/- 0.0016 mag/deg, indicating a very shallow opposition surge compared with asteroids, but consistent with a high albedo surface. The orbital lightcurve shows substantially less fading than the earlier observations, which suggests that there is not a gross difference in average albedo between the southern and northern hemispheres.