A small amount of the ashes of American astronomer Clyde Tombaugh are on board the New Horizons spacecraft, which has spent more than nine years traveling to the outer reaches of the solar system.
The probe is set to pass within 6,200 miles (9,978 kilometers) of Pluto, the closest any spacecraft has got to the icy world, beaming back data and images.
Rewind to 1930, and Pluto wasn't even on the solar system map.
Tombaugh, who grew up on a farm in Kansas, was working at Lowell Observatory in Arizona. Previously an amateur astronomer, he had been hired to help find a planet beyond Neptune.
Studied millions of images
His work involved thousands of hours spent pouring over millions of images of stars, according to the Johns Hopkins University Applied Physics Laboratory, which is managing the New Horizons mission for NASA.
Tombaugh made the landmark discovery on February 18. He was 24 years old at the time.
The breakthrough helped advance human understanding of the solar system. Scientists now think that there are thousands of other icy objects beyond Pluto in a region known as the Kuiper Belt.
That's where the New Horizons spacecraft is headed after Tuesday's flyby of Pluto.
Tombaugh died in 1997, at the age of 90.
'He would have been astounded'
Nine years later, New Horizons began its epic voyage.
Some of his ashes were put in a canister about two inches wide and half-an-inch tall that was attached to the inside of the piano-sized spacecraft's upper deck.
"When he looked at Pluto, it was just a speck of light," Annette Tombaugh, his daughter, said earlier this year. "To actually see the planet that he had discovered and find out more about its atmosphere, find out more of what it is and actually get to see the moons of Pluto, he would have been astounded."
The canister bears an inscription from Alan Stern, the head of the New Horizons mission: "Interned herein are remains of American Clyde W. Tombaugh, discoverer of Pluto and the solar system's "third zone." Adelle and Muron's boy, Patricia's husband, Annette and Alden's father, astronomer, teacher, punster, and friend: Clyde W. Tombaugh (1906-1997)."