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, and a brief discussion of the New Horizons mission.
Comparison of Pluto and Neptune's orbits, shown as if the same orbital plane, to show how Pluto is sometimes even closer to the Sun than Neptune ever gets

Diagram of how Charon's orbital motion around Pluto appears from the Earth when it is near the time when it can pass directly in front of or behind Pluto
      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 27 years ago (as of late 2015) and pole-on rotation 35 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 was 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 would still be relatively warm by Plutonian standards, and still possess a very thin atmosphere; and about 3/4 of the planet's surface have been 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 would have been 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 reached Pluto on July 14, 2015. Although the period of nearest approach lasted only a few minutes (due to the spacecraft's 30 thousand miles per hour motion past the planet) the spacecraft spent close to a day in relatively close proximity to Pluto and its moons, and was able to study the half of the planet facing the Sun in great detail. Because its entire power supply is only 28 watts, it can return information to Earth at only a few tens of bits per second, so it will take about 16 months for the craft to transmit the results of all its near-Pluto observations to Earth. After that it will continue onward and outward through the Kuiper Disk (possibly visiting another Kuiper Belt object about 2019) before eventually, thousands of years later, leaving the Solar System.

Diagram of the relationship between Pluto's orbit, rotation and seasons
View of Pluto's orbit, rotation and seasons (NASA, JPL, New Horizons; no longer online)
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.