Pluto was discovered 90 years ago this week. Controversy about its identity rages on.
When Tombaugh discovered Pluto (the first frame of the animation below is a digital zoom-in on the image he captured), he was using 1930's most cutting-edge technology: a "blink comparator."
Pluto orbits the sun at a distance of about 3.6 billion miles (5.8 billion kilometers) on average — about 40 times as far as Earth's orbit.
Pluto is 5.5 times smaller than Earth, at 1,400 miles (2,380 kilometers) wide. That's about half as wide as the US, or two-thirds the width of our moon.
After Tombaugh's discovery, astronomers learned that Pluto's orbit is more elliptical than the eight other planets' paths and not on the same plane as theirs.
In 1978, US Naval Observatory astronomer James Christy discovered a "small bump" on Pluto's side. He realized the bump was an object orbiting Pluto: a moon.
The large moon is called Charon, and it's unusual because the time it takes to orbit Pluto is the same amount of time it takes Pluto to rotate, so the moon always hovers over the same spot.
In 1992, astronomers David Jewitt and Janet Luu discovered the first Kuiper Belt object aside from Pluto, called 1992 QB1. It was evidence that more objects lurk in the distant solar system.
Then in 2003, astronomers found a Kuiper Belt object that became known as Eris. It's larger than Pluto, so gave rise to discussions about what it really means to be a planet.
"Just so you know, in my view, Pluto is a planet," NASA administrator Jim Bridenstine said during a tour of the aerospace lab at the University of Colorado Boulder in August 2019.
NASA's New Horizons mission launched in 2006 to study Pluto, its moons, and other distant objects.
It was the first spacecraft to visit the dwarf planet.
Charon's size and proximity to Pluto were two of the main reasons astronomers wanted to send New Horizons there.
The mission sent back new data about Pluto's surface and moons, and indicated that the system was more complex than astronomers had realized.
New Horizons is still in space collecting data.
Its images revealed Arrokoth to be an icy, snowman-shaped object.
The animation of Arrokoth below was released last week, compiled from the data New Horizons gathered during its flyby.
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