Pluto

Pluto is seen in stunning detail from the one and only spacecraft flyby of this distant dwarf planet. Pluto is so far away that it was difficult to discover and was assumed to be a featureless small object until the images were seen in 2015.

Editor’s note: Astronomy Update is published on the third Saturday of the month, is provided by the Chippewa Valley Astronomical Society and is compiled by Lauren Likkel of the UW-Eau Claire physics and astronomy department.

Clyde Tombaugh was a Kansas farm boy. He was also an amateur astronomer. He built his own telescopes and took meticulous notes and sketches of what he saw. He had finished high school but had not gone on any further.

He had read about the work being done at Lowell Observatory in Flagstaff, Ariz., and sent some of his work. The people at Lowell Observatory were so impressed that they invited him to come to Flagstaff.

When he arrived, he was put to work with a small telescope, which could take long exposure pictures on large photographic plates, in search of the hypothetical “planet X” beyond Neptune. He took several pictures a night and repeated them several nights later.

He would later compare the pictures of the same patch of sky to see if anything had moved. This was tedious work, so he used a device called a “blink comparator,” which could rapidly switch between two pictures.

In February 1930, Tombaugh saw a dim star-like point that had moved in the photos and by about the right amount for something out near Neptune. After double-checking, he walked down the hall to the director’s office and said, “I have found Planet X.”

He had just turned 24 years old. After more detailed observations, the discovery was announced to the world.

Now all Planet X needed was a name. Many people sent letters and telegrams with suggestions, but the best one came from an 11-year old English school girl named Venetia Burney: Pluto.

In Greek and Roman mythology, Pluto was the god of the underworld, a cold and dark place where the spirits of the dead reside. This is very appropriate since from Pluto the sun is no more than a bright star, and it is so cold on Pluto that any atmosphere would freeze and cover the ground with ice and frost.

Fast forward to 2006. After many years of delays, the New Horizons space probe was launched. Its window of opportunity was running out; Earth, Jupiter and Pluto had to be lined up just right to allow a speed boost from a Jupiter flyby. Also, Pluto was moving on from its closest approach to the sun, and scientists were worried that any atmosphere would soon freeze.

When New Horizons arrived in 2015, the first thing it did was to send back a high resolution photograph. Then all the other instruments began sending back data. When it had flown past Pluto, it looked back and photographed a thin blue wisp of atmosphere. It is now on its way to another flyby with an object even further out from the sun.

Ironically, Pluto was dethroned as a planet in 2006, just months after New Horizons was launched, and was no longer considered a “planet” when New Horizons finally arrived in 2015. It had joined the new class of “dwarf planets” out past Neptune. But that is another story.