In 1930, Clyde Tombaugh, a 25-year-old amateur astronomer, spied a small, dim object in the night sky.
He’d been working at the Lowell Observatory in Flagstaff, Arizona, for about a year when he used a blink comparator—a special kind of microscope that can examine and compare images—to glimpse what was for a time considered to be the ninth planet in our solar system: Pluto.
By all accounts, Pluto was—well—weird. At one point, astronomers believed it could potentially be bigger than Mars (it’s not). Its unusual 248-year orbit has been known to cross Neptune’s path. Today, Pluto is recognized as the largest object in the Kuiper Belt—but it’s no longer considered a planet.
In 2006, the International Astronomical Union voted to downgrade Pluto, defining a planet as a body that orbits the sun, is round in shape, and has “cleared the neighborhood around its orbit”—meaning it has become gravitationally dominant, so that there are no bodies in its orbital zone besides its own moons. Since Pluto did not check that third box, it was deemed a dwarf planet.
Now a new concept mission submitted to NASA aims to take a close look at Pluto and its nearby systems. Proposed in late 2020, Persephone would explore whether Pluto has an ocean and how the planet’s surface and atmosphere have evolved.
Persephone would send a spacecraft armed with high-resolution cameras to orbit Pluto for three years and map its surface as well as that of its largest moon, Charon.
But why is Pluto worth visiting?
The same year Pluto was shoved from its planetary pedestal, NASA sent the New Horizons mission to Pluto and the Kuiper Belt to better understand the outer edge of our solar system.
After reaching Pluto in 2015, New Horizons struck what amounted to scientific treasure. Close-ups of Pluto revealed potentially active mountain ranges, flowing ice, and a surprising record of geologic history on its surface.
Carly Howett, a planetary physicist and the principal investigator for Persephone, says New Horizons showed us just how complex that part of space really is.
“It wasn’t that New Horizons fundamentally had technology that is new, but it kind of gave people an insight into what the Pluto system might be like,” Howett says. “The world, for the first time, saw Pluto.”