Charon at 40: Four Decades of Discovery on Pluto’s Largest Moon

Charon at 40: Four Decades of Discovery on Pluto’s Largest Moon

It’s been forty years since the largest moon, Charon, in the solar system relative to its parent planet was discovered.

Pluto has always been a topic of debate as there is a difference of opinion whether it is a planet or not. It enjoyed the status of the 9th planet of our solar system for more than 75 years after its discovery in 1930 but the International Astronomical Union (IAU) labeled it as a ‘Dwarf Planet’ in 2006. According to IAU, Pluto fails to fulfill all the three criteria to be called a Planet.

A lot of scientists including Alan Stern, the Principal Investigator of New Horizons Mission of NASA, objected this decision and raised voice against the criterion which said that a heavenly body can only be called a planet if its surrounding areas are clear. Stern joined forces with David Grinspoon, a Planetary Scientist, and wrote a book called ‘Chasing New Horizons: Inside the Epic First Mission to Pluto’. In this book, they criticized that criterion by saying,

This leads to many bizarre and absurd conclusions. For example, it would mean that Earth was not a planet for its first 500 million years of history, because it orbited among a swarm of debris until that time, and also that if you took Earth today and moved it somewhere else, say out to the asteroid belt, it would cease being a planet.

As people on sides have their reasons, this fight for giving Pluto the status of a planet will go on. Meanwhile, the 40th anniversary of the magnificent discovery of its largest moon, called Charon, was celebrated on 22nd of June. Robert Harrington and James Christy of the U.S. Naval Observatory located Charon all by surprise as they were not aiming to find the satellites of the Pluto. They were working on refining Pluto’s orbit around the Sun when Christy observed a small bump on one of the sides. He analyzed a lot of images and found that it was present in all of them but its position was changing all the time. Detailed examination revealed that it was revolving around Pluto. It cycled back and forth every 6.39 days, which is the Rotation Period of Pluto.

He concluded that either this planet has a huge mountain thousands of miles above its surface or it has a moon circling around it. It was a historic moment because no signs of a moon around Pluto were ever detected since its discovery in 1930. On exploring the image archives of the observatory, Christy found a number of cases where Pluto looked strangely elongated. He researched further on that and measured the angle where these elongations were spotted. On the other hand, Harrington calculated what should be the answer to Christy’s research if there was a moon around Pluto.

The Naval Observatory shared this story in 1998 on the 20th anniversary of the discovery and told the world that their calculations matched precisely. However, both of these researchers didn’t announce their findings immediately as they waited for the 61-inch telescope of the observatory to be 100% sure about the discovery. The images, taken on 2nd July 1978, from that telescope sealed the deal as the elongation was found exactly where they calculated it to be. The official announcement of the discovery came 5 days later.

Scientists of New Horizons Mission had a perception that Charon will be a monotonous world with a lot of craters on it but the visit of New Horizons Spacecraft to Pluto system in July 2015 revealed a lot more than craters. The landscape of Charon had a lot of variety to offer as images showed vast canyons, surface color variations, giant mountains, landslides, and a polar cap. It was a pleasant surprise for the researchers as acknowledged by Will Grundy, a Co-investigator of the New Horizons Science Team at Lowell Observatory. He said,

Even if Pluto wasn’t there, Charon would have been a great flyby target by itself. It’s a far more exciting world than we imagined.”

This discovery is considered the starting point after which a lot of work was done on exploring Pluto and its surroundings. Stern summed up the importance of this discovery beautifully in the following words:

The importance of the discovery of Charon really cannot be underestimated. We on the New Horizons team owe a big debt of gratitude to Jim Christy for his landmark discovery.”

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