Pluto and Charon as observed by the repaired Hubble Space Telescope at maximum apparent separation (0.9 arc seconds apart) (R. Albrecht (ESA/ESO), NASA, apod990213). Pluto is on the left, 2390 km (= 1485 mi) diameter, Charon on right, 1190 km (= 740 mi) diameter. The bodies orbit each other at a distance of 19,600 km (= 12,200 mi). At the time this image was taken, in 1994, Pluto was inside the orbit of Neptune -- a location it occupies for 20 years out of each 250 year orbital period. It was closer to the Sun than Neptune from 1979 to 1999.

     Color image of the side of Pluto which always faces Charon, showing details determined from occultations and eclipses of Pluto and Charon, in the mid to late 1980's. The brownish color is thought to be caused by darkening of methane ice as a result of exposure to ultraviolet radiation from the Sun. The light and dark areas represent just that -- light and dark areas -- and are unlikely to be related to actual surface features, which will remain unknown until the New Horizons spacecraft, launched in 2006, reaches Pluto in 2015. (Eliot Young (SwRI) et al., NASA, apod010319)

     The small images (inset at upper left of the larger images) are Hubble Space Telescope pictures of opposite hemispheres of Pluto taken in late June and early July of 1994, when Pluto was around 3 billion miles from Earth. Below each is a map based on a number of HST observations, taken in blue light to provide increased resolution (the shorter the wavelength of light used to observe an object, the sharper the image can be). Although some features may correspond to basins or other topographic features, it is most likely, as in the case of the Earth-based (color) image above, that the light and dark features simply represent differences in the brightness of the surface, caused by differences in the ices deposited on the surface. Lighter areas are probably freshly deposited ices of nitrogen, carbon dioxide and methane, resulting from the re-freezing of gases which were produced as Pluto neared the Sun over the last century, and gradually developed a thin atmosphere of (primarily) nitrogen and methane. Dark areas are probably ices contaminated by hydrocarbon deposits formed by the interaction of methane and other ices with ultraviolet radiation from the Sun. As Pluto gradually draws away from the Sun, over the next century, seasonal and orbital effects may well produce noticeable changes in the patterns of light and dark ices shown here. (A. Stern (SwRI), M. Buie (Lowell Observatory), NASA, ESA, apod960311)

     Map of Pluto constructed from four HST images, as Pluto went through one 6.5 day rotation in 1994. The map shows a region 7500 km or 4700 miles wide (E to W) and half that height (N to S), save for a black strip at the bottom, where the South Polar region could not be imaged, because it is pointing away from the Sun and Earth. (The rectilinear projection used here shows meridians of longitude and parallels of latitude as straight lines, and as a result, shows the single point at the North Pole just as wide as Pluto's Equator, grossly exaggerating the size of the polar regions.) The polar areas are brighter, and the equatorial area darker, as might be expected if there are more, or fresher, ices at the poles, but the actual nature of the dark and light regions will remain a matter of speculation for years or decades to come. (Alan Stern (Southwest Research Institute), Marc Buie (Lowell Observatory), ESA, NASA)

Study of the Plutonian atmosphere by the occultation of a star on June 9, 1988. If Pluto had no atmosphere, the light curve would have vertical sides, but instead, it has sloped sides, whose exact shape can be used to determine the density profile of the atmosphere (how the scale height changes at different altitudes). Based on the density profile and the low gravity of the planet, the average molecular weight can be determined, and is about 28, which suggests that (diatomic) nitrogen molecules are the primary component of the atmosphere. Absorption at specific wavelengths also suggests the presence of small (1%?) amounts of methane. The atmosphere appears to consist of an isothermal (constant temperature) outer atmosphere, and a complex (variable temperature) lower atmosphere, possibly including an inversion layer (a region where temperature decreases with altitude, as in our stratosphere). If so, that would make Pluto the only planet other than the Earth known to have an inversion layer. (Elliot, Dunham, Bosh, Slivan, Young, Wasserman, Millis, Kuiper Airborne Observatory, Icarus)

Pluto and Charon as they passed by members of the triple star system P126, in 2002. Pluto passed in front of P126A, casting a shadow the size of Pluto across parts of South America, providing more detailed information about its size and atmospheric structure. Among other things, it was discovered that the atmospheric pressure had doubled (from 1.5 microbars to 3 microbars) since 1988, suggesting a warming of about 2 1/2 Fahrenheit degrees, presumably caused by the fact that although it is now moving away from the Sun, Pluto is still considerably closer than its average distance, and is still warming up. However, as it moves away from the Sun, it is also turning one pole more directly away from it, which will soon cause gases forming on the sunward side to freeze out on the night side; as a result, it is expected that the atmospheric pressure will soon begin to decrease. (NACO Team, 8.2-meter VLT (Yepun), ESO, apod020911)

     Another model of the internal structure of Pluto (Lunar and Planetary Laboratory, NASA Gallery) is totally different from the previous one, showing the uncertainty involved in calculating the structures of objects when accurate data are scarce.