Online Astronomy eText: The Planets / The Sky
The Rotation of Pluto
(also see Pictures of Pluto / The Discovery of Pluto / The Satellites (Moons) of Pluto)
      This page will discuss the synchronous rotation of Pluto, which causes it to always keep one face toward its moon, Charon. For now, here is a summary of how Pluto's unusual rotation affects its atmosphere.

      Like Uranus, Pluto rotates more on its side than anything else; and like Uranus' moons, which orbit in the equatorial plane of the planet, Charon rotates in Pluto's equatorial plane, so that its orbit is more perpendicular to the orbital plane of the planet than not. As shown in the first diagram (above), Pluto's orbit is fairly eccentric, allowing it to actually be closer to the Sun than Neptune for about 20 years out of its 248 year orbital period. The last time it was at perihelion was in 1989. At almost the same time, as shown in the second diagram, the plane of Pluto's rotation (and Charon's orbit) was parallel to our line of sight, so that Charon and Pluto eclipsed and occulted each other every 3.2 days (half of the 6.4 day rotation/orbital period, since there is one eclipse and one occultation in each period). Since the two dates were nearly the same, Pluto must be rotating nearly sideways to the Sun at occultation, and its axis of rotation must be pointing nearly at the Sun (and away from it) about a quarter of an orbital period earlier and later -- in other words, 1988/9 plus or minus 62 years, or 1926/7 and 2050/51. This means that when Pluto was discovered in 1930, one hemisphere was more or less perpetually sunlit (the hemisphere we saw at the time, which happened to be the Northern hemisphere of Pluto), and the other (Southern) hemisphere was more or less perpetually dark. During the following 58 years, as Pluto neared perihelion, it gradually became more and more sideways relative to the Sun (and the Earth), and every part of the planet had more or less equal day and night (each day and each night being about 3.2 Earth days).
      When discovered Pluto had no atmosphere, but as it approached the Sun dark areas absorbed sunlight and warmed up, and nitrogen and methane ices gradually evaporated, forming a very rarefied atmosphere (perhaps 1/100th of one percent of an Earth atmosphere). This process is still going on, as Pluto is still considerably closer to the Sun than usual, but as the planet moves away from perihelion it is pointing less and less at the Sun, and more and more pole-on. As it becomes more nearly exactly pole-on, toward the middle of this century, any gas present on the night side of the planet will freeze and become part of the icy surface again. Since perihelion and sideways rotation was 26 years ago (as of late 2014) and pole-on rotation is 36 years away, it will take a while for the atmosphere to gradually return to an icy state and completely disappear; but if we want to study Pluto's atmosphere, we need to do so fairly soon, which ws the rationale behind the Pluto Express project.
      The Pluto Express project was the tentative launch of two spacecraft in 2006, to arrive at Pluto in 2016, about six months apart. At that time, 27 years after perihelion, Pluto will still be relatively warm by Plutonian standards, and still possess a very thin atmosphere; and about 3/4 of the planet's surface will still be visible at one time or another, only about half of the Northern (winter) hemisphere being already semi-permanently pointed away from the Sun, while all of the Southern hemisphere will be visible throughout Pluto's rotation.
     The Pluto Express project was abandoned because of the cost of two spacecraft, and replaced by the single New Horizons spacecraft, which was launched on January 19, 2006. The fastest spacecraft ever launched, it took just over a year to reach Jupiter; and by taking advantage of Jupiter's gravitational effects to further increase its speed (and reduce its travel time by nearly three years), it will reach Pluto in 2015. It will study that planet and its moons for 24 hours before continuing onward and outward through the Kuiper Belt/Disk, before eventually (thousands of years later) leaving the Solar System. (Unfortunately, due to the short time near Pluto, it will only be able to study a little over half the surface of the planet, instead of the 3/4 originally planned.)

View of Pluto's orbit, rotation and seasons (NASA, JPL, New Horizons)
Another view of Pluto's orbit and its rotation. The central diagram shows the orbit of Pluto with perihelion at the top and aphelion at the bottom, with the rotation of the planet and the orbital motion of its moon at the Equinoxes and Solstices. The insets show the view from the Earth (more accurately from the Sun, but our view is almost identical) at each of those times. Note the apparent orbital motion of Charon at the Northern Spring Equinox, which was in 1988, as discussed above.