Scientists Celebrate Pluto's Discovery With A Retrospective Of Its Greatest Images

Pluto and Charon, in enhanced color, thanks to observations from New Horizons’ Ralph/Multispectral Visual Imaging Camera (MVIC). Pluto’s frozen surface and intricate features are only part of its fascinating story; an ocean of subsurface water lurks far beneath the ice.

Pluto, first discovered in 1930, was no more than a distant dot in our most powerful telescopes.

Clyde Tombaugh’s original images identifying Pluto in 1930. The tiny, faint dot moves very slightly relative to the background stars, but sufficiently so that we’ve been able to successfully reconstruct its orbit.

Moving against the backdrop of fixed stars, its orbit was constructed after years of observations, revolving like no other planet.

The planets of the Solar System, along with the asteroids in the asteroid belt, all orbit in almost the same plane, making elliptical, nearly circular orbits. Beyond Neptune, things get progressively less reliable. Pluto was the oddball for decades, with its highly eccentric, out-of-the-plane orbital parameters.

Pluto orbits out of the Solar System’s plane, highly inclined, and even approaches within Neptune’s orbit for a time.

In 1978, measurements of Pluto over time showed an unresolved bulge on one side that would appear and disappear periodically. That was its largest moon: Charon. The discovery of Charon has led to a much better understanding of just how tiny Pluto is. Its diameter is about 2274 km (1413 miles), and its mass is 0.25% of the mass of the Earth. Charon has a diameter of about 1172 kilometers (728 miles) and a mass of about 22% that of Pluto. The two worlds circle their common center of mass with a period of 6.387 days.

In 1978, our telescopes had advanced enough to determine that it had a large satellite: the giant moon Charon.

Pluto, shown as the brightest spot, against a field of background stars. When Pluto passes in front of a background star, the light filtering through can deliver information about the contents of its atmosphere. We determined this was a nitrogen-rich world from occultations like this in the late 1980s and early 1990s.

Through occultations of distant stars, we determined Pluto had an atmosphere that changed over time, growing larger near perihelion.

After the Hubble Space Telescope’s primary mirror had its optics corrected, both Pluto and Charon could be clearly imaged and resolved as separate, independent worlds. Note the brighter color on Pluto, corresponding to its greater reflectivity (by a factor of approximately 3) owing to the vast amounts of ices on its surface.

1994 saw the first optically-corrected pictures of the Pluto-Charon system by Hubble, the first image to resolve these worlds independently.

The Hubble telescope’s snapshots of nearly the entire surface of Pluto, taken with the ESA’s Faint Object Camera as the planet rotated through a 6.4-day period in 1996, showed that Pluto is a complex object with more large-scale contrast than any planet other than Earth.

By 1996, observations of Pluto led to our first-ever map of this distant world from the Kuiper belt.

Pluto, shown as imaged with Hubble in a composite mosaic, along with its five moons. Charon, its largest, must be imaged with Pluto in an entirely different filter due to their brightnesses. The four smaller moons orbit this binary system with a factor of 1,000 greater exposure time in order to bring them out. Nix and Hydra were discovered in 2005, with Kerberos discovered in 2011 and Styx in 2012.

In 2005, two additional moons were discovered, followed by two more in 2011 and 2012, bringing Pluto’s total to five.

The full Hubble Space Telescope photomap of Pluto, released in 2010, shows the best-ever views of this distant world as obtained remotely. These features are all consistent with everything New Horizons saw, but at the optical limit of the Hubble Space Telescope.

Observations continued to improve, but were fundamentally limited by the great interplanetary distances.

New Horizons was about 3.7 million miles (6 million kilometers) from Pluto and Charon when it snapped this portrait late on July 8, 2015. Most of the bright features around Pluto’s edge are a result of image processing, but the bright sliver below the dark “whale,” which is also visible in unprocessed images, is real. Color information from the Ralph instrument was added in. The Pluto-Charon system is shown to scale.

The New Horizons mission, upon visiting Pluto, changed everything.

Pluto and its five moons, including the monster moon Charon and the four smaller satellites, are highly processed due to the collisions and other interactions that gave rise to the Plutonian system in its current form. These are the highest-resolution views of these moons, shown to actual relative size, ever taken. They are Pluto’s only moons.

Flying by in 2015, its views were unlike anything else.

A computer simulation of the New Horizons flyby of Pluto, based on the full suite of data obtained by the spacecraft and reconstructed based on its trajectory past this distant world.

Pluto is the Kuiper belt’s largest world.

Pluto’s atmosphere, as imaged by New Horizons when it flew into the distant world’s eclipse shadow. The atmospheric hazes are clearly visible, and these clouds lead to periodic snow on this outer, cold world. Pluto’s atmosphere changes as it moves from perihelion to aphelion, and can continue to be monitored through periodic occultations.

Celebrate its 89th anniversary in style.

This animation combines various observations of Pluto over the course of several decades. The first frame is a digital zoom-in on Pluto as it appeared upon its discovery by Clyde Tombaugh in 1930 (image courtesy Lowell Observatory Archives). The other images show various views of Pluto as seen by NASA’s Hubble Space Telescope beginning in the 1990s and NASA’s New Horizons spacecraft in 2015. The final sequence zooms in to a close-up frame of Pluto released on July 15, 2015.

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Mostly Mute Monday tells the astronomical story of an object or phenomenon in images, visuals, and no more than 200 words. Talk less; smile more.