Pluto is the only planet in our solar system still unvisited by a spacecraft. Its status as the only planet in our Sun's family still studied purely by telescope is unique—and frustrating—to planetary scientists trying to uncover its secrets.
Clyde Tombaugh discovered Pluto in 1930. Despite astronomers' best efforts, Pluto's faintness and star-like appearance allowed the planet to keep most of its secrets. For 25 years, all that could be done was to refine our knowledge of its strange orbit, finding it on old photographs and taking new ones. Pluto's orbit is more eccentric (e=0.2482) and more inclined (i=17°) than that of any other planet, taking 248.8 years to make one trip around the Sun. At perihelion (closest approach, which occurred in 1996), it is only 60% as far from the Sun as at aphelion-closer to the Sun than Neptune gets. Yet, the planets cannot collide for two reasons. First, the relative inclination of the two orbits means their paths do not intersect. Second, Pluto is in a 2:3 orbit-orbit resonance with Neptune. For each two trips Pluto makes around the Sun, Neptune makes exactly three. When Pluto is at perihelion, Neptune is on the other side of the Sun.
In 1955, photometry (brightness measurements) of Pluto showed a repetition of 6.38 days-the length of Pluto's day. Two trends to the evolution of the brightness have since been found. First, its amplitude has increased from about 10% to a current value of 30%. This tells us that the sub-solar point has been moving equatorward, and that the planet's spin axis must be severely tilted. Second, the average brightness has faded over the years, evidence that Pluto's poles are likely brighter than its equator. Decades of photometry have been interpreted to derive maps of Pluto's surface reflectance, or albedo. These are comparable in detail with what the Hubble Space Telescope has been able to reveal.
Little regarding Pluto's size or composition was known until recently. In 1976, the absorption of methane was discovered in Pluto's spectrum. This implied a bright, icy planet, and therefore a small radius. In 1978, James Christy, then an astronomer at the United States Naval Observatory, discovered Pluto's satellite, chich was named Charon. Orbiting Pluto with the same 6.38726-day period as Pluto's spin, Charon was the key to unlocking Pluto's secrets. By timing the orbital period and measuring the estimated separation between the two, we can compute the total mass of the system: about 0.002 Earth masses. Charon orbits retrograde, and Pluto spins backwards (just like Venus and Uranus).
Charon's orbital plane above Pluto's equator was seen edge-on in 1988. This produced a series of occultations and eclipses of and by the satellite, each half-orbit, from 1985 to 1992. Timing these “mutual events” allows calculation of the radii for both bodies-approximately 1153 km for Pluto and 640 km for Charon-the sum is about the radius of the Moon. When Charon hid behind the planet, Pluto's spectrum could be observed uncontaminated by its moon. This spectrum, when subtracted from a combined spectrum of the pair taken a few hours before or after, yields the spectrum of Charon. Pluto's spectrum showed methane frost: the gas we use for cooking is frozen solid on its surface! Charon's spectrum revealed nothing but dirty water ice. (Independent measurements show the amount of methane on Pluto varies with longitude. Bright regions have more methane than dark.) When Charon passed between Pluto and Earth, it (and its shadow) selectively hides different portions of its primary. Interpretation of these measurements is complicated, but has allowed refined albedo (or reflectivity) maps of one hemisphere of Pluto to be extracted.
The surface temperature of Pluto currently is under debate. Two results have been published: ~40 °K and ~55 °K. The first value is Triton-like; the latter is more consistent with Pluto's lower albedo (0.44-0.61). In either case, it's cold. Water ice there is harder than steel is at room temperature! Misconceptions exist about how dark it would seem for an astronaut on Pluto. Despite the planet's remote distance, the Sun would appear to have the brightness of ~70 full Moon on Earth. Combine this with the bright, icy surface and one would have no problems navigating the surface.
On 9 June 1985, Pluto passed in front of a star. Rather than blinking out, the starlight gradually dimmed due to refraction by an atmosphere. Too dense to be methane alone, nitrogen and carbon monoxide were suspected. Both have since been identified on Pluto's surface, with nitrogen comprising about 97% of the ground material. From details of precisely how the starlight faded, we think there is a temperature increase close to the surface, much like on Earth. Pluto's atmospheric pressure is only a few millionths that of Earth, and the atmosphere actually may “frost out”with increasing distance from the Sun.
The Hubble Space Telescope has been used to measure the size of Charon's orbital radius, about 19,500 km (approximately 1.5 Earth diameters). The mass ratio of satellite/planet has been constrained as 0.122±0.005 by watching both bodies orbit around the center-of-mass. With sizes and masses determined, densities can be calculated, 1.8-2.0 gm cm-3 for Pluto and 1.6-1.8 gm cm-3 for Charon. Density tells us about internal composition: roughly a 50:50 mix of rock and ice.
Efforts to learn more continue. New large earth-based telescopes equipped with adaptive optics and fast computers will allow the blurring effects of our atmosphere to be nullified, surpassing the resolution of Hubble's rather small 2.4-meter mirror. On the other hand, NASA's “faster, better, cheaper” policy has led to a halt of the Pluto-Kuiper Express spacecraft. A new mission profile, called the New Horizons Pluto-Kuiper Belt Mission, was approved by Congress in 2001. However, funding for this mission is not in the President's proposed budget for the current year. Launch must happen by 2006, or Jupiter will no longer be in position to slingshot the craft Pluto-ward with a gravity assist. The trip to Pluto will take years longer. We will have to wait the better part of a Jupiter orbit (11.8 years) until the geometry repeats itself. By then, Pluto's atmosphere may have frozen out. Until the task is taken seriously, Pluto will remain the only planet unvisited by a spacecraft.
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Robert L. Marcialis, Ph.D.
Senior Research Specialist
Lunar and Planetary Laboratory
University of Arizona
1629 E. University Blvd.
Tucson, AZ 85721
http://www.lpl.arizona.edu/~umpire
Bibliography
Binzel, Richard P. “Pluto.” Scientific American 262, no. 6 (1990): 50-58.
Marcialis, Robert L. “The first 50 years of Pluto-Charon research.” In Pluto and Charon, eds. S. Alan Stern and David J. Tholen. Tucson: University of Arizona Press, Tucson, 1997, pp. 27-83.
PLUBIB: A Pluto-Charon Bibliography. Ed. Robert L. Maricalis. Jan. 2001. University of Arizona. 05 Jan. 2001 http://www.lpl.arizona.edu/~umpire/science/plubib_home.html
Is Pluto a planet? You be the judge. Pluto is spherical. It has an atmosphere, a satellite, and polar caps.